Department in the News
NASA's Webb Space Telescope to Inspect Atmospheres of Gas Giant Exoplanets
July 11, 2018
NASA, by Christine Pulliam
In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger "super-Earths" orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA's James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.
Some of Webb's first observations of gas giant exoplanets will be conducted through the Director's Discretionary Early Release Science program. The transiting exoplanet project team at Webb's science operations center is planning to conduct three different types of observations that will provide both new scientific knowledge and a better understanding of the performance of Webb's science instruments.
"We have two main goals. The first is to get transiting exoplanet datasets from Webb to the astronomical community as soon as possible. The second is to do some great science so that astronomers and the public can see how powerful this observatory is," said Jacob Bean of the University of Chicago, a co-principal investigator on the transiting exoplanet project.
"Our team's goal is to provide critical knowledge and insights to the astronomical community that will help to catalyze exoplanet research and make the best use of Webb in the limited time we have available," added Natalie Batalha of NASA Ames Research Center, the project's principal investigator.
Transit - An atmospheric spectrum
When a planet crosses in front of, or transits, its host star, the star's light is filtered through the planet's atmosphere. Molecules within the atmosphere absorb certain wavelengths, or colors, of light. By splitting the star's light into a rainbow spectrum, astronomers can detect those sections of missing light and determine what molecules are in the planet's atmosphere.
For these observations, the project team selected WASP-79b, a Jupiter-sized planet located about 780 light-years from Earth. The team expects to detect and measure the abundances of water, carbon monoxide, and carbon dioxide in WASP-79b. Webb also might detect new molecules not yet seen in exoplanet atmospheres.
Phase curve - A weather map
Planets that orbit very close to their stars tend to become tidally locked. One side of the planet permanently faces the star while the other side faces away, just as one side of the Moon always faces the Earth. When the planet is in front of the star, we see its cooler backside. But as it orbits the star, more and more of the hot day-side comes into view. By observing an entire orbit, astronomers can observe those variations (called a phase curve) and use the data to map the planet's temperature, clouds, and chemistry as a function of longitude.
The team will observe a phase curve of the "hot Jupiter" known as WASP-43b, which orbits its star in less than 20 hours. By looking at different wavelengths of light, they can sample the atmosphere to different depths and obtain a more complete picture of its structure. "We have already seen dramatic and unexpected variations for this planet with Hubble and Spitzer. With Webb we will reveal these variations in significantly greater detail to understand the physical processes that are responsible," said Bean.
Eclipse - A planet's glow
The greatest challenge when observing an exoplanet is that the star's light is much brighter, swamping the faint light of the planet. To get around this problem, one method is to observe a transiting planet when it disappears behind the star, not when it crosses in front of the star. By comparing the two measurements, one taken when both star and planet are visible, and the other when only the star is in view, astronomers can calculate how much light is coming from the planet alone.
This technique works best for very hot planets that glow brightly in infrared light. The team plans to study WASP-18b, a planet that is baked to a temperature of almost 4,800 degrees Fahrenheit (2,900 K). Among other questions, they hope to determine whether the planet's stratosphere exists due to the presence of titanium oxide, vanadium oxide, or some other molecule.
Ultimately, astronomers want to use Webb to study potentially habitable planets. In particular, Webb will target planets orbiting red dwarf stars since those stars are smaller and dimmer, making it easier to tease out the signal from an orbiting planet. Red dwarfs are also the most common stars in our galaxy.
"TESS should locate more than a dozen planets orbiting in the habitable zones of red dwarfs, a few of which might actually be habitable. We want to learn whether those planets have atmospheres and Webb will be the one to tell us," said Kevin Stevenson of the Space Telescope Science Institute, a co-principal investigator on the project. "The results will go a long way towards answering the question of whether conditions favorable to life are common in our galaxy."
The James Webb Space Telescope will be the world's premier space science observatory. Webb will solve mysteries of our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international project led by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Department members: Jacob L. Bean
Doyal "Al" Harper has received a Norman Maclean Faculty Award
June 7, 2018
UChicago News, by Ryan Goodwin
The Norman Maclean Faculty Award recognizes emeritus or senior faculty for extraordinary contributions to teaching and to the student experience of life within the University community. This year's recipient is Doyal "Al" Harper, professor in the Department of Astronomy and Astrophysics and the College.
Prof. Doyal "Al" Harper has mentored and supported countless students in the Department of Astronomy and Astrophysics and in the College and has been an enthusiastic research adviser to both undergraduate and graduate students at UChicago. Students have benefited in particular from the wide range of research opportunities he provides--they are able to do hands-on work in the laboratory, design and build instrumentation, carry out observational programs at ground-based and airborne observatories, and develop and employ computational tools for data analysis.
Harper's research addresses problems in the formation of galaxies, stars and planetary systems; the physics of the interstellar medium; and the properties of interstellar dust. His experimental research group constructs and operates infrared instrumentation and pioneered airborne infrared astronomy and astrophysical observations from the South Pole.
Department members: Doyal ''Al'' Harper
Angela Olinto named dean of Physical Sciences Division
June 7, 2018
Angela V. Olinto, the Albert A. Michelson Distinguished Service Professor in the Department of Astronomy and Astrophysics, has been appointed dean of the Division of the Physical Sciences at the University of Chicago.
Olinto is a leading scholar in astroparticle physics and cosmology, focusing on understanding the origin of high-energy cosmic rays, gamma rays and neutrinos. Her appointment as dean is effective July 1.
"Angela brings depth of University experience and scholarly expertise to this leadership role, making her an excellent choice as dean," wrote President Robert J. Zimmer and Provost Daniel Diermeier in announcing her appointment.
Olinto's research includes important contributions to the physics of quark stars, inflationary theory and cosmic magnetic fields. She currently leads NASA sub-orbital and space missions to discover the origins of high-energy cosmic rays and neutrinos. This includes a NASA-funded balloon mission planned for 2022 that will use an ultra-sensitive telescope to detect cosmic rays and neutrinos coming from deep space.
"I am thrilled and humbled to be appointed to lead this historic and dynamic division, home to visionary scholars who constantly redefine the boundaries of the physical and mathematical sciences. I look forward to collaborating with faculty, students and staff to advance the important work of the division," Olinto said.
Olinto joined the UChicago faculty in 1996 and served as chair of the Department of Astronomy and Astrophysics from 2003 to 2006 and from 2012 to 2017. She is the leader of the POEMMA and EUSO space missions and a member of the Pierre Auger Observatory, which are international projects designed to discover the origin of high-energy cosmic rays. She is a fellow of the American Physical Society, was a trustee of the Aspen Center for Physics, and serves on advisory committees for the National Academy of Sciences, U.S. Department of Energy, the National Science Foundation and NASA.
Olinto's awards and honors include the Chaire d'Excellence Award of the French Agence Nationale de la Recherche in 2006, the University's Llewellyn John and Harriet Manchester Quantrell Award for Excellence in Undergraduate Teaching in 2011, and the Faculty Award for Excellence in Graduate Teaching and Mentoring in 2015. Olinto received her undergraduate degree from Pontificia Universidade Catolica in Rio de Janeiro, Brazil and her doctoral degree from the Massachusetts Institute of Technology.
Olinto succeeds Edward "Rocky" Kolb, the Arthur Holly Compton Distinguished Service Professor of Astronomy & Astrophysics, whose work over the last five years enhanced the division's historic strengths as a leading center of scientific discovery. Kolb will return to his full-time work on the faculty next month.
The selection of the new dean by Zimmer and Diermeier was informed by the recommendations of an elected faculty committee chaired by Stuart A. Kurtz, professor in the Department of Computer Science.
Department members: Edward ''Rocky'' W. Kolb, Angela V. Olinto
Scientific projects: Extreme Universe Space Observatory at the Japanese Module, Extreme Universe Space Observatory on a Super Pressure Balloon, Pierre Auger Observatory
NASA Spacecraft Discovers New Magnetic Process in Turbulent Space
May 24, 2018
In a new discovery reported in the journal Nature, scientists working with NASA's Magnetospheric Multiscale spacecraft -- MMS -- have uncovered a new type of magnetic event in our near-Earth environment by using an innovative technique to squeeze extra information out of the data. Astronomy & Astrophysics postdoctoral scholar Colby Haggerty was part of the team of scientists who made this exciting discovery.
Department members: Colby Haggerty
Big Brains podcast explores how world's largest telescope might glimpse universe's birth
May 15, 2018
UChicago News, by Andrew Bauld
Cosmologist Wendy Freedman on how new technology could lead to major discoveries
Prof. Wendy Freedman spent much of her career measuring the age of the universe. Now she's working on a project that may very well give scientists a chance to glimpse into its birth.
Freedman, the John & Marion Sullivan University Professor of Astronomy & Astrophysics, works in the field of observational cosmology, measuring the expansion rate of the universe. In 2001, she and a team of scientists found that the universe is around 13.7 billion years old -- far more precise than the previous estimate in the 10- to 20-billion-year-old range.
Freedman was the founding leader from 2003 until 2015 of an international consortium of researchers and universities (including UChicago) to build the world's largest telescope high in the mountains of Chile. The Giant Magellan Telescope will be as tall as the Statue of Liberty when complete, and ten times more powerful than the Hubble Space Telescope -- with the ability to look back at the dawn of the cosmos.
"In our field, the new developments have come with new technology," Freedman said. "Without exception, from the time that Galileo first turned a telescope to the sky in 1609, every time we've built a new capability we've made new discoveries, which is why we're so excited about this."
The telescope, 80 feet in diameter and weighing more than 20 tons, will be the first of its kind to see fine details like a planet's atmosphere, which could one day help discover life on other planets. The telescope is expected to be operational starting in 2024.
"If we really were able to show that there's life on a planet outside of our own solar system, that will be one of the discoveries that will not only be exciting for astronomers but will change human kind's perspective on our place in the universe," Freedman said.
On this episode of Big Brains, Freedman discusses her research on measuring the age of the universe, her leadership of the Giant Magellan Telescope and the search for life outside our solar system.
Department members: Wendy L. Freedman
Scientific projects: Giant Magellan Telescope
ALMA follow up of SPT sources discovers massive conglomerations of Star Forming Galaxies in early Universe
May 1, 2018
National Radio Astronomy Observatory
Peering deep into space - an astounding 90 percent of the way across the observable universe - astronomers have witnessed the beginnings of a gargantuan cosmic pileup, the impending collision of 14 young, starbursting galaxies. This ancient megamerger is destined to evolve into one of the most massive structures in the known universe: a cluster of galaxies, gravitationally bound by dark matter and swimming in a sea of hot, ionized gas.
Cosmologists Can't Agree on the Hubble Constant
April 23, 2018
American Physical Society, by David Ehrenstein
The discrepancy in measures of the Hubble constant, which quantifies the expansion of the Universe, has only grown in recent years.
The Hubble constant H0 tells us the speed at which galaxies are receding from us as the Universe expands. Over the past five years, cosmologists have recognized that there is a discrepancy between different measurements of this fundamental parameter. Three speakers in a session at the April Meeting of the American Physical Society in Columbus, Ohio, discussed the status of this "crisis in cosmology." The field has now accepted that the problem is real, and some researchers are optimistic that it could lead to important discoveries.
The problem began in 2013, when the first results were reported from the Planck satellite, which had measured the cosmic microwave background (CMB). The Planck team's value for H0 was 67.±1.2 kilometers per second per megaparsec (km/s/Mpc), lower than previous measurements, which had been between 70 and 75 km/s/Mpc. The result also had error bars small enough that even this slight difference was a potential problem. Planck's 2015 result was not very different, though it came with even smaller error bars.
Prior to the Planck announcement, the Supernova H0 for the Equation of State (SH0ES) Collaboration, led by Adam Riess of Johns Hopkins University in Baltimore, had already set out to make a measurement ofH0 in our cosmic neighborhood with higher precision than previous efforts. The researchers focused on re-calibrating three of the standard distance-measuring techniques from scratch - the motion of stars due to Earth's orbit (parallax), pulsating stars known as Cepheid variables, and Type Ia supernovae, said team member David Jones of the University of California at Santa Cruz. Based on their improved distance measures, SH0ES reported an H0 value of 73.2 ± 1.7 km/s/Mpc in 2016. This result differed from Planck's by more than 3 standard deviations, a highly statistically significant difference that could not easily be explained.
Re-analyses of the SH0ES results confirmed the 2016 finding, as did additional measurements of H0 in the local Universe. But an independent H0 determination in 2016 based on so-called baryon acoustic oscillations - the sloshing of matter in the early Universe that produced the characteristic CMB patterns - lined up with the Planck result.
Stephen Feeney of the Flatiron Institute in New York said that despite quite a bit of attention to the issue, no one has found any problems with the measurements that could have a large enough effect to close the gap. Cosmologists have also been discussing whether the standard cosmological model, known as ΛCDM, may require modification. This theory is used for the CMB-based determinations of H0. But the proposed adjustments to ΛCDM all introduce at least some conflicts with other types of data. Feeney estimates that the odds are 60:1 that all of the data could be explained by statistical flukes and ΛCDM alone.
Last year, the Planck team performed a more detailed analysis of their data and found that the CMB fluctuations on the smallest angular scales had the largest effect on lowering H0. Describing these results, Bradford Benson of Fermilab said that when the team used only their data from larger angular scales (above about 0.2o), they derived an H0 value consistent with the SH0ES result.
According to Benson, the smaller angular scales provide a more sensitive test of a particular parameter in ΛCDM than larger scales. The parameter is the density of neutrinos in the Universe, which should be proportional to the number of neutrino species (there are three in the standard model of particle physics). Increasing the number of neutrino types is one of the few ways to reasonably tweak ΛCDM and increase Planck's H0 enough to close the gap with local measurements. However, this solution would also require more massive neutrinos to avoid disagreements with other cosmological data sets, said Benson. And of course, there isn't much evidence for a fourth neutrino type.
Jones, Feeney, and Benson agreed that the discrepancy isn't going away and that more data are essential to explain it. The expected future trove of gravitational waves from binary neutron star mergers, for example, will provide independent estimates for H0. (Last year's event led to a value somewhere between those of Planck and SH0ES but with much larger error bars.) In addition, the South Pole Telescope and the Atacama Cosmology Telescope have upgraded equipment that will soon provide better CMB maps, and the Gaia satellite will provide a new level of precision parallax measurements.
Benson thinks there's a good chance that a "benign" explanation will solve the problem. However, in a different session, Riess pointed out that problems with the value of H0 have led to great discoveries in the past, including the existence of dark energy.
Department members: Bradford A. Benson
Scientific projects: South Pole Telescope
2018: Norman Maclean Faculty Award - Doyal "Al" Harper
April 12, 2018
University of Chicago, Alumni Association
Al Harper is a professor in the Department of Astronomy and Astrophysics and director of Yerkes Observatory in Williams Bay, Wisconsin. His research addresses problems in the formation of galaxies, stars, and planetary systems; the physics of the interstellar medium; and the properties of interstellar dust. His experimental research group constructs and operates infrared instrumentation and pioneered airborne infrared astronomy and astrophysical observations from the South Pole.
In addition to his scientific research contributions, Professor Harper has mentored and supported countless students and has been an enthusiastic research adviser to both undergraduate and graduate students at UChicago. Students have benefited in particular from the wide range of research opportunities he provides--they are able to do hands-on work in the laboratory, design and build instrumentation, carry out observational programs at ground-based and airborne observatories, and develop and employ computational tools for data analysis.
Professor Harper's commitment to the students at the University of Chicago has been an incredible gift to those he has taught and mentored. One current project is the renovation and modernization of a telescope at Yerkes Observatory so that it can be used for teaching and student projects at the undergraduate level. This is just one example of his dedication to the students in the University community. Professor Harper embraces teaching and one-on-one guidance as central to his career, and all his students can attest to his generosity and leadership capabilities.
Department members: Doyal ''Al'' Harper
Lab experiments mimic the origin and growth of astrophysical magnetic fields
April 3, 2018
Physics Today, by Rachel Berkowitz
A turbulent, laser-generated plasma can amplify magnetic fields to cosmic scales.
Magnetic fields permeate the space between objects throughout the universe, from galaxy clusters to protogalaxies to nebulae. It's not difficult to explain how those fields might be initiated: Weak fields originate in astrophysical plasmas through various mechanisms - for example, temperature or density gradients in the plasma that alter electron trajectories.
But it remains challenging to demonstrate how seed fields of 10−21 gauss in intergalactic space grow to their measured astronomical values of several microgauss. Theory suggests that turbulent motion in an astrophysical plasma amplifies tiny magnetic fields by converting kinetic energy into magnetic energy. Such a self-sustaining energy conversion mechanism is called a dynamo.
Dynamos are often distinguished between large scale and small scale. The familiar large-scale geodynamo relies on Earth's symmetry-breaking rotation to generate a magnetic field that grows at scales larger than those of the liquid-core motion. But in a small-scale dynamo, the magnetic field grows below the length scales of fluid motion. There, turbulence operates first at the smallest length scales, where it rapidly amplifies the fields. The mechanism works even in isotropic conditions.
Now Gianluca Gregori (Oxford University), Petros Tzeferacos (University of Chicago), and colleagues have measured the amplification of a magnetic field in a turbulent laboratory plasma and provided the first physical demonstration of a dynamo resulting from turbulent motion.1 The study shows that the turbulent dynamo could be a viable mechanism for magnetic field amplification in the lab and in astrophysical settings.
Galaxies far, far away
To sustain a dynamo, a fluid needs to be electrically conductive, and its motion cannot be too symmetric. Dynamo conditions also require that the magnetic field lines stay in the plasma, rather than diffusing away. Those conditions are easily satisfied by the turbulent, high-temperature, high-velocity plasma that fills intergalactic space.
Even though conditions favorable for dynamos are common in astrophysical settings, the strong turbulence is extremely difficult to replicate in the lab. (See the article by Daniel Lathrop and Cary Forest, Physics Today, July 2011, page 40.) Astronomical x-ray observations provide information about the temperature, density, and composition of hot plasmas in the universe. But observations cannot convey the geometry and topology of the magnetic field lines, both of which strongly affect basic plasma processes such as thermal conduction. Most knowledge of how astrophysical plasmas behave is based on numerical simulations and modeling.2
Joseph Larmor first invoked the dynamo mechanism a hundred years ago to explain solar magnetic fields,3 and theoretical predictions of how the turbulent dynamo should operate go back more than half a century. In 2001 Agris Gailitis and colleagues achieved the first laboratory amplification of a self-exciting magnetic field in a turbulent swirling flow of liquid sodium.4 In 2012 Gregori generated shock waves in a laboratory plasma with a laser and showed that asymmetric shocks, like those occurring in supernovae remnants and gamma-ray bursts, produced seed magnetic fields.5 But the plasma did not reach high enough temperature and velocity to trigger and sustain the dynamo effect. The advent of high-power lasers has now opened a new field of research in which astrophysical environments can be studied in the laboratory.
Galaxy in a lab
At the Omega Laser Facility at the University of Rochester, Tzeferacos and his colleagues irradiated two fixed targets of penny-sized pieces of chlorine-doped polystyrene foil with a series of progressively stronger UV laser pulses to deliver a total of 5 kJ to each target. The foils faced each other 8 mm apart. The laser energy quickly stripped and ionized each foil's atoms and generated two hot plasma jets directed toward each other. Along the way the jets each passed through a grid of holes. The grids were offset from each other and created interpenetrating fingers of plasma that sheared when they collided and generated a strongly turbulent region. Colliding plasma jets offer a laboratory representation of merging galaxy clusters.
"We've tried similar experiments at smaller-scale facilities, but generating plasma conditions where a dynamo can operate was impossible. We needed the power and diagnostics of Omega," says Tzeferacos. The Omega laser experiments were based on earlier efforts by Tzeferacos, Gregori, and colleagues that used the IR Vulcan laser at the UK's Rutherford Appleton Laboratory but that did not have sufficient power to achieve dynamo conditions. (See Physics Today, September 2015, page 16.)
Images taken by an x-ray framing camera showed that the experiments reached the turbulent regime. Inhomogeneities developed in a region more than 1 mm across after the plasma flows collided (see figure 2a). The intensity fluctuations are directly related to electron density fluctuations, which in turn indicate the plasma turbulence. Results from numerical simulations, seen in figure 2b, verify that x-ray images capture the electron density fluctuations expected in a turbulent plasma.
"The big advance was getting the laboratory plasmas to high enough temperature and velocity," says Christopher Reynolds (University of Cambridge). "We have good simulations, but at the end of the day, lab work is real gas doing real things."
Tzeferacos and colleagues used numerical simulations to develop two methods for measuring the magnetic field produced by the plasma. Simulations were necessary to determine when to run the diagnostic tools since the experiments lasted tens of nanoseconds, but the strongly amplified magnetic fields persisted for only a fraction of that time.
In one method, the researchers measured the polarization of light passing through the plasma. They based their approach on a technique astronomers use to measure the magnetic fields of distant objects. In the other method, they imaged protons fired through the plasma. Protons are focused into concentrated regions by a magnetic field that bends the protons' trajectories. By analyzing the concentration of protons, the researchers could reconstruct the magnetic field at the moment the protons were fired through the plasma.
Both methods determined the magnetic field in the turbulent region at several times during the experiment and found that in less than 10 ns, the field amplified by a factor of 25-30. On galactic spatial and temporal scales, the same amplification mechanism could grow a tiny seed field to the observed values.
The initially weak magnetic fields in turbulent dynamo theory evolve according to Maxwell's equations. When the magnetic field becomes strong enough to deflect the moving plasma, the magnetic energy grows linearly until the plasma motion can no longer amplify the magnetic field. The ratio between magnetic and kinetic energy saturates at a value that depends on the individual fluid properties.
From the lab measurements, the team determined that the magnetic field energy of the plasma saturated at about 4% of its kinetic energy. "It's reasonable that the mean magnetic energy is quantitatively smaller than the kinetic energy," says Tzeferacos. Larger and denser plasmas would give the same result, and the measured 4% value is consistent with models in the literature.
Numerical simulations supported the case that the precollision plasmas became primed with tiny seed magnetic fields as a result of density and temperature gradients produced by the laser.6 Simulations also showed that the fields grew exponentially, but the experiments did not temporally resolve the magnetic field growth rate. Follow-up experiments will address the rate at which the magnetic field grows and other, more detailed questions.
From lab to universe
"The fact that the dynamo effect is seen in an experiment validates simulations and provides very strong evidence that this is indeed what happens in the universe at large," Gregori says. The equations that describe fluids are scale independent - waves in a water glass have the same properties as waves in the ocean.
For magnetic fields, the condition for scale invariance is that the magnetized fluid remains a good conductor. Mathematically, the condition means that a dimensionless parameter called the magnetic Reynolds number Rm is above a critical threshold value of 200. The higher the Rm, the more closely the magnetic field lines move with the plasma. The experiments achieved an Rm of 600, consistent with modeled intergalactic plasmas.
Reynolds observes that "lab work has often not had the prominence it deserves in astrophysics, because it's difficult to get gases to have meaningful properties. Experiments like this are important for providing fundamental verification of an important theory."
Atoms in a laboratory plasma frequently collide with each other. That tends to homogenize the plasma, making laboratory plasmas simple to analyze. Unfortunately, the same homogenization does not occur in many less dense astrophysical plasmas, which are harder to understand. However, laboratory plasmas are beginning to approach the collision rate of their astrophysical brethren.
The laboratory platform offers a new opportunity to test theories about magnetic turbulent plasmas in situations relevant to astrophysics, and to analyze how charged particle acceleration develops in a plasma. Future experiments could provide insight into how quickly the magnetic field increases in strength, how strong it can get, and how it alters the turbulence that amplified it.
Department members: Petros Tzeferacos
Scientific projects: Flash Center for Computational Science
2018 APS Medal for Exceptional Achievement in Research
March 20, 2018
The 2018 APS Medal for Exceptional Achievement in Research was awarded on February 1 to Eugene Parker, professor emeritus at the University of Chicago, for his "many fundamental contributions to space physics, plasma physics, solar physics, and astrophysics during the past 60 plus years." (Top Left) The medal was presented to Parker by 2018 APS President Roger Falcone along with APS CEO Kate Kirby. (Top Right) Family members and colleagues joined in the celebration: from left to right, Eric Parker, Susan Kane-Parker, Niesje Parker, Eugene Parker (seated); Michael Turner, Rocky Kolb, and Young-Kee Kim (University of Chicago), and Timothy Gay (University of Nebraska-Lincoln, APS Speaker of the Council, and University of Chicago Ph.D. graduate). APS is accepting nominations for the 2019 APS Medal now through May 1.
Department members: Edward ''Rocky'' W. Kolb, Eugene N. Parker, Michael S. Turner
Stephen Hawking: A physicist's appreciation
March 16, 2018
Bulletin of the Atomic Scientists, by Daniel Holz
Stephen Hawking was a singular individual, revered both by his fellow scientists and by the public. His books were bestsellers, his public lectures were always standing-room-only, and as a sign of his broad appeal, he appeared in Star Trek, The Simpsons, and The Big Bang Theory. He became a household name. Hawking was passionate about his science, but also passionate about sharing his unique insights with the world.
His scientific legacy is assured. I believe his discovery that black holes have a temperature is one of the most beautiful and profound accomplishments of humankind, bar none. Hawking presented his results in a paper titled "Particle creation by black holes." This relatively innocuous title hides a revolution in physics. The paper is staggeringly perfect, combining physical insight with technical mastery, and is thrilling to read even 40 years later and will remain so for the foreseeable future.
Black holes are the most extreme objects in the universe, regions where the gravity is so strong that nothing can escape. Hawking showed that when you add quantum mechanics, the uncertainty principle results in particles appearing to leak out of the black holes, and thus black holes have a temperature. Black holes aren't truly black! This is a beautiful result, vividly demonstrating the unity of physics, while hinting at some underlying, fundamental processes that we have yet to understand. Because black holes radiate particles, they must lose mass, in a process known as black hole evaporation. As the black holes radiate they get smaller, and thus hotter, meaning that they radiate energy faster, and thus get even smaller and even hotter, and so on. The result is that all black holes eventually explode! Hawking radiation is still at the very forefront of theoretical physics. There are raging debates about how Hawking radiation actually works, and whether it somehow carries away the information of how the black hole was formed. One of the greatest mysteries in physics is what is left behind after Hawking radiation causes a black hole to fully evaporate.
I met Hawking a few times over the years, including at the University of Cambridge and the University of Chicago. In addition to scientific discussions, I participated in Stephen Hawking's Universe, a six-part TV documentary about the birth and evolution of the universe, black holes, and other important aspects of modern cosmology. One of my most memorable interactions with him was from when I was a post-doctoral fellow at the Kavli Institute for Cosmological Physics in Santa Barbara. Hawking visited and sat across the hall from me for two weeks. It was wonderful to spend an extended period of time with him and appreciate his insights, intuition, and, especially, his humor.
One memory is particularly vivid: I described a project I had been working on for months; it had to do with how light travels in the universe. After listening politely, he responded with several statements I initially found inscrutable. I went back to my office somewhat deflated; it seemed clear he hadn't understood what I had been describing. But I continued to contemplate his comments and over the course of the ensuing weeks it slowly dawned on me that not only had he understood, but he had jumped way past me, providing signposts, far off in the distance, that pointed toward a deeper understanding of the subject. It took me months to fully discover and appreciate the entire meaning of his comments.
When he wasn't exploring the far reaches of the universe, Hawking actively engaged with the here and now. His was a voice that transcended politics, urging us to fully appreciate the world around us and to maintain a healthy perspective, given the immensity of the cosmos. In his later years, Hawking became increasingly alarmed about the potential for human-induced global catastrophe. He helped found the Breakthrough Starshot initiative to try to develop prototypes for interstellar travel; part of his motivation was to develop a back-up plan if the Earth becomes inhospitable. He also joined the Board of Sponsors of the Bulletin of Atomic Scientists and was a particularly vital and valued member. When the Bulletin moved the minute hand of the Doomsday Clock closer to midnight in 2007, Hawking said, "As scientists, we understand the dangers of nuclear weapons and their devastating effects, and we are learning how human activities and technologies are affecting climate systems in ways that may forever change life on Earth. As citizens of the world, we have a duty to alert the public to the unnecessary risks that we live with every day, and to the perils we foresee if governments and societies do not take action now to render nuclear weapons obsolete and to prevent further climate change."
The ensuing decade has made Hawking's exhortation ever more relevant and urgent. We ignore him at our peril.
Hawking has left the universe a much more interesting place than he found it.
Daniel Holz is an Associate Professor in Physics, Astronomy & Astrophysics, the Enrico Fermi Institute, and the Kavli Institute for Cosmological Physics, at the University of Chicago. His research focuses on general relativity in the context of astrophysics and cosmology. He is a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration, and was part of the team that announced the first detection of gravitational waves in early 2016. He received a 2012 National Science Foundation CAREER Award, the 2015 Quantrell Award for Excellence in Undergraduate Teaching, and the Breakthrough Prize in Fundamental Physics in 2016, and was selected as a Kavli Fellow of the National Academy of Sciences in 2017. Holz received his PhD in physics from the University of Chicago and his AB in physics from Princeton University.
Department members: Daniel E. Holz
Astrophysicists settle century-old cosmic debate on magnetism of planets and stars
February 9, 2018
UChicago News, by Rob Mitchum
Laser experiments verify 'turbulent dynamo' theory of how cosmic magnetic fields are created
The universe is highly magnetic, with everything from stars to planets to galaxies producing their own magnetic fields. Astrophysicists have long puzzled over these surprisingly strong and long-lived fields, with theories and simulations seeking a mechanism that explains their generation.
Using one of the world's most powerful laser facilities, a team led by University of Chicago scientists experimentally confirmed one of the most popular theories for cosmic magnetic field generation: the turbulent dynamo. By creating a hot turbulent plasma the size of a penny, which lasts a few billionths of a second, the researchers recorded how the turbulent motions can amplify a weak magnetic field to the strengths of those observed in our sun, distant stars and galaxies.
The paper, published this week in Nature Communications, is the first laboratory demonstration of a theory explaining the magnetic field of numerous cosmic bodies, which has been debated by physicists for nearly a century. Using the FLASH physics simulation code, developed by the Flash Center for Computational Science at UChicago, the researchers designed an experiment conducted at the OMEGA Laser Facility in Rochester, N.Y. to recreate turbulent dynamo conditions.
Confirming decades of numerical simulations, the experiment revealed that turbulent plasma could dramatically boost a weak magnetic field up to the magnitude observed by astronomers in stars and galaxies.
"We now know for sure that turbulent dynamo exists, and that it's one of the mechanisms that can actually explain magnetization of the universe," said Petros Tzeferacos, research assistant professor of astronomy and astrophysics at the University of Chicago and associate director of the Flash Center. "This is something that we hoped we knew, but now we do."
A mechanical dynamo produces an electric current by rotating coils through a magnetic field. In astrophysics, dynamo theory proposes the reverse: the motion of electrically-conducting fluid creates and maintains a magnetic field. In the early 20th century, physicist Joseph Larmor proposed that such a mechanism could explain the magnetism of the Earth and sun, inspiring decades of scientific debate and inquiry.
While numerical simulations demonstrated that turbulent plasma can generate magnetic fields at the scale of those observed in stars, planets and galaxies, creating a turbulent dynamo in the laboratory was far more difficult. Confirming the theory requires producing plasma at an extremely high temperature and volatility to produce the sufficient turbulence to fold, stretch and amplify the magnetic field.
To design an experiment that creates those conditions, Tzeferacos and colleagues at UChicago and the University of Oxford ran hundreds of two- and three-dimensional simulations with FLASH on the Mira supercomputer at Argonne National Laboratory. The final setup involved blasting two penny-sized pieces of foil with powerful lasers, propelling two jets of plasma through grids and into collision with each other, creating turbulent fluid motion.
"People have dreamed of doing this experiment with lasers for a long time, but it really took the ingenuity of this team to make this happen," said Donald Lamb, the Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy and Astrophysics and director of the Flash Center. "This is a huge breakthrough."
The team also used FLASH simulations to develop two independent methods for measuring the magnetic field produced by the plasma: proton radiography, the subject of a recent paper from the FLASH group, and polarized light, based on how astronomers measure the magnetic fields of distant objects. Both measurements tracked the growth in mere nanoseconds of the magnetic field from its weak initial state to over 100 kiloGauss - stronger than a high-resolution MRI scanner and a million times stronger than the magnetic field of the Earth.
"This work opens up the opportunity to experimentally verify ideas and concepts about the origin of magnetic fields in the universe that have been proposed and studied theoretically over the better part of a century," said Fausto Cattaneo, professor of astronomy and astrophysics at the University of Chicago and a co-author of the paper.
Now that a turbulent dynamo can be created in a laboratory, scientists can explore deeper questions about its function: How quickly does the magnetic field increase in strength? How strong can the field get? How does the magnetic field alter the turbulence that amplified it?
"It's one thing to have well-developed theories, but it's another thing to really demonstrate it in a controlled laboratory setting where you can make all these kinds of measurements about what's going on," Lamb said. "Now that we can do it, we can poke it and probe it."
In addition to Tzeferacos and Lamb, UChicago co-authors on the paper include Carlo Graziani and Gianluca Gregori, who is also professor of physics at the University of Oxford. The research was funded by the European Research Council and the U.S. Department of Energy.
Department members: Fausto Cattaneo, Carlo Graziani, Donald Q. Lamb, Petros Tzeferacos
Scientific projects: Flash Center for Computational Science
Award honors Prof. Eugene Parker's lifetime of physics research
February 1, 2018
UChicago News, by Louise Lerner
Prof. Emeritus Eugene Parker's ideas were once widely questioned in the physics world. This week, he will receive one of the field's highest honors.
Parker will receive the American Physical Society's Medal for Exceptional Achievement in Research at a Feb. 1 ceremony in Washington, D.C. The medal citation notes Parker's "fundamental contributions to space physics, plasma physics, solar physics and astrophysics for over 60 years."
"I've been a member of the APS since 1952, so this is a nice honor," said Parker, the S. Chandrasekhar Distinguished Service Professor Emeritus in Physics at the University of Chicago. "I'm very pleased, particularly since people were skeptical about these concepts for a long time."
Early in his career Parker proposed a theory that faced widespread skepticism - notably that a "solar wind" was carrying charged particles from the surface of the sun to the far reaches of the solar system. Beginning with the Mariner II space probe to Venus in 1962, however, measurements from spacecraft began to validate his predictions.
In addition to the solar wind, he has investigated magnetic fields, including the role played by cosmic rays in Milky Way magnetic fields and how cyclonic turbulence generates magnetic fields.
"Gene Parker has a wonderful and exceptional record of seminal contributions to solar, space and astrophysics over the many years of his distinguished career," said Roger Falcone, chair of the 2018 APS Medal selection committee. "It is remarkable to see so many effects that bear his name."
It's been an eventful year for Parker, whom NASA honored in May 2017 by naming its first mission to send a spacecraft through the sun's corona after the professor. The Parker Solar Probe, which recently embarked on its thermal testing phase to be frozen and then blasted with heat to simulate conditions on its journey, is scheduled to launch in July 2018. It is the first spacecraft to be named after a living person.
Scientists are eager to explore the surface of the sun, especially as flares, winds and ejections from the sun can affect electronics and infrastructure here on Earth.
Parker said he plans to travel to witness the probe's launch this summer. He's looking forward to it; he's never seen a rocket launch. "I imagine it's like the Taj Mahal," he said. "Everyone's seen a picture of it, but to see it in person is a completely different story."
Department members: Eugene N. Parker
Astronomy faculty member Bob Rosner participates in Atomic Bulletin of Scientists Doomsday Clock news conference
January 26, 2018
New York Times, by Sewall Chan
The Doomsday Clock, a potent symbol of scientific concerns about humanity's possible annihilation, was advanced by 30 seconds on Thursday, to 2 minutes to midnight, the Bulletin of the Atomic Scientists announced in Washington.
The last time the clock was moved so close to midnight was in 1953, during the Cold War.
"In 2017, world leaders failed to respond effectively to the looming threats of nuclear war and climate change, making the world security situation more dangerous than it was a year ago - and as dangerous as it has been since World War II," the bulletin's science and security board, which oversees the clock, said in a statement.
Department members: Robert Rosner
Dark Energy Survey finds remains of 11 galaxies eaten by the Milky Way
January 16, 2018
Scientific collaboration including UChicago and labs releases three years of data
Scientists have released the preliminary cosmological findings from the Dark Energy Survey - research on about 400 million astronomical objects, including distant galaxies as well as stars in our own galaxy.
Among the highlights of the first three years of survey data, presented Jan. 10 during the American Astronomical Society meeting in Washington, D.C., is the discovery of 11 new stellar streams - remnants of smaller galaxies torn apart and devoured by our Milky Way.
The results were announced by the Dark Energy Survey, an international collaboration of more than 400 members including scientists from UChicago, Argonne and Fermilab, that aims to reveal the nature of the mysterious force of dark energy.
The public release fulfills a commitment scientists on the survey made to share their findings with the astronomy community and the public. The data cover about 5,000 square degrees, or one-eighth of the entire sky, and include roughly 40,000 exposures taken with the Dark Energy Camera. The images correspond to hundreds of terabytes of data and are being released along with catalogs of hundreds of millions of galaxies and stars.
"There are all kinds of discoveries waiting to be found in the data," said Brian Yanny of Fermi National Accelerator Laboratory, Dark Energy Survey data management project scientist. "While DES scientists are focused on using it to learn about dark energy, we wanted to enable astronomers to explore these images in new ways, to improve our understanding of the universe."
The Dark Energy Camera, the primary observation tool of the Dark Energy Survey, is one of the most powerful digital imaging devices in existence. It was built and tested at UChicago-affiliated Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation's 4-meter Blanco telescope in Chile. The DES images are processed by a team at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
One new discovery enabled by the data set is the detection of 11 new streams of stars around our Milky Way. Our home galaxy is surrounded by a massive halo of dark matter, which exerts a powerful gravitational pull on smaller, nearby galaxies. The Milky Way grows by pulling in, ripping apart and absorbing these smaller systems. As stars are torn away, they form streams across the sky that can be detected using the Dark Energy Camera. Even so, stellar streams are extremely difficult to find since they are composed of relatively few stars spread out over a large area of sky.
"It's exciting that we found so many stellar streams," said astrophysicist Alex Drlica-Wagner of Fermilab and the Kavli Institute for Cosmological Physics at UChicago. "We can use these streams to measure the amount, distribution and 'clumpiness' of dark matter in the Milky Way. Studies of stellar streams will help constrain the fundamental properties of dark matter."
Prior to the new discoveries, only about two dozen stellar streams had been discovered. Many of them were found by the Sloan Digital Sky Survey, a precursor to the Dark Energy Survey. The effort to detect new stellar streams in the Dark Energy Survey was led by University of Chicago graduate student Nora Shipp.
"We're interested in these streams because they teach us about the formation and structure of the Milky Way and its dark matter halo. Stellar streams give us a snapshot of a larger galaxy being built out of smaller ones," said Shipp. "These discoveries are possible because DES is the widest, deepest and best-calibrated survey out there."
Since there is no universally accepted naming convention for stellar streams, the Dark Energy Survey has reached out to schools in Chile and Australia, asking young students to select names. Students and their teachers have worked together to name the streams after aquatic words in native languages from northern Chile and aboriginal Australia.
Funding: U.S. Department of Energy Office of Science, National Science Foundation
- This release was originally posted on the Fermi National Accelerator Laboratory website.
Department members: Joshua A. Frieman
Department students: Nora Shipp
Scientific projects: Dark Energy Survey
Computational astrophysics team uncloaks magnetic fields of cosmic events
January 4, 2018
UChicago News, by Rob Mitchum
New method enhances study of stars, black holes in laboratory settings
The development of ultra-intense lasers delivering the same power as the entire U.S. power grid has enabled the study of cosmic phenomena such as supernovae and black holes in earthbound laboratories. Now, a new method developed by computational astrophysicists at the University of Chicago allows scientists to analyze a key characteristic of these events: their powerful and complex magnetic fields.
In the field of high-energy density physics, or HEDP, scientists study a wide range of astrophysical objects -- stars, supermassive black holes at the center of galaxies and galaxy clusters -- with laboratory experiments as small as a penny and lasting only a few billionths of a second. By focusing powerful lasers on a carefully designed target, researchers can produce plasmas that reproduce conditions observed by astronomers in our sun and distant galaxies.
Planning these complex and expensive experiments requires large-scale, high-fidelity computer simulation beforehand. Since 2012, the Flash Center for Computational Science of the Department of Astronomy & Astrophysics at UChicago has provided the leading open computer code, called FLASH, for these HEDP simulations, enabling researchers to fine-tune experiments and develop analysis methods before execution at sites such as the National Ignition Facility at Lawrence Livermore National Laboratory or the OMEGA Laser Facility in Rochester, N.Y.
"As soon as FLASH became available, there was kind of a stampede to use it to design experiments," said Petros Tzeferacos, research assistant professor of astronomy and astrophysics and associate director of the Flash Center.
During these experiments, laser probe beams can provide researchers with information about the density and temperature of the plasma. But a key measurement, the magnetic field, has remained elusive. To try and tease out magnetic field measurements from extreme plasma conditions, scientists at MIT developed an experimental diagnostic technique that uses charged particles instead, called proton radiography.
In a new paper for the journal Review of Scientific Instruments, Flash Center scientists Carlo Graziani, Donald Lamb and Tzeferacos, with MIT's Chikang Li, describe a new method for acquiring quantitative, high-resolution information about these magnetic fields. Their discovery, refined using FLASH simulations and real experimental results, opens new doors for understanding cosmic phenomena.
"We chose to go after experiments motivated by astrophysics where magnetic fields were important," said Lamb, the Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy & Astrophysics and director of the Flash Center. "The creation of the code plus the need to try to figure out how to understand what magnetic fields are created caused us to build this software, that can for the first time quantitatively reconstruct the shape and strength of the magnetic field."
In proton radiography, energetic protons are shot through the magnetized plasma towards a detector on the other side. As the protons pass through the magnetic field, they are deflected from their path, forming a complex pattern on the detector. These patterns were difficult to interpret, and previous methods could only make general statements about the field's properties.
"Magnetic fields play important roles in essentially almost every astrophysical phenomena. If you aren't able to actually look at what's happening, or study them, you're missing a key part of almost every astrophysical object or process that you're interested in," said Tzeferacos.
By conducting simulated experiments with known magnetic fields, the Flash Center team constructed an algorithm that can reconstruct the field from the proton radiograph pattern. Once calibrated computationally, the method was applied to experimental data collected at laser facilities, revealing new insights about astrophysical events.
The combination of the FLASH code, the development of the proton radiography diagnostic, and the ability to reconstruct magnetic fields from the experimental data, are revolutionizing laboratory plasma astrophysics and HEDP. "The availability of these tools has caused the number of HEDP experiments that study magnetic fields to skyrocket," said Lamb.
The new software for magnetic field reconstruction, called PRaLine, will be shared with the community both as part of the next FLASH code release and as a separate component available on GitHub. Lamb and Tzeferacos said they expect it to be used for studying many astrophysics topics, such as the annihilation of magnetic fields in the solar corona; astrophysical jets produced by young stellar objects, the Crab Nebula pulsar, and the supermassive black holes at the center of galaxies; and the amplification of magnetic fields and acceleration of cosmic rays by shocks in supernova remnants.
"The types of experiments HEDP scientists perform now are very diverse," said Tzeferacos. "FLASH contributed to this diversity, because it enables you to think outside the box, try different simulations of different configurations, and see what plasma conditions you are able to achieve."
The paper, "Inferring Morphology and Strength of Magnetic Fields From Proton Radiographs," was published online by Review of Scientific Instruments. The work was funded by grants from the U.S. Department of Energy and the National Science Foundation.
Department members: Carlo Graziani, Donald Q. Lamb, Petros Tzeferacos
Scientific projects: Flash Center for Computational Science
The Solar System Could Have Formed in a Bubble Surrounding One of the Hottest Stars
December 22, 2017
Popular Mechanics, by Jay Bennett
A new theory of the solar system's formation provides an alternative to the idea our system formed from a supernova.
The prevailing theory of the formation of the solar system is that a large supernova ejected material that formed a dense cloud of interstellar dust, which then collapsed to form our sun - as well as other stars that have since drifted away - and the leftover material orbits our host star as planets, moons, asteroids and the rest. Astronomers developed this theory because of the high abundance the isotope aluminum-26, which is liberally ejected in a supernova explosion.
However, more recent measurements of meteorites from the early solar system suggest there is not as much iron-60 as in the rest of the galaxy, which is also created in high abundances in supernovae. "It begs the question of why one was injected into the solar system and the other was not," said coauthor Vikram Dwarkadas, a research associate professor in Astronomy and Astrophysics at the University of Chicago, in a press release.
Dwarkadas and his team have developed a new solar system formation theory in a study published in today in the Astrophysical Journal, one that could explain the apparent imbalance of aluminum-26 and iron-60 compared to what you would expect to see from supernova material.
The new theory suggests that the sun formed within the walls of a dense bubble of material surrounding one of the hottest-burning stars in the galaxy, a Wolf-Rayet star. Wolf-Rayet stars are rare and diverse stars full of heavy elements that tower about 50 times larger than our sun. Their surfaces burn at temperatures ranging from 30,000 K to more than 200,000 K (50,000 to 360,000F), making them hotter and brighter than almost all other stars.
The immense heat and volatile conditions cause Wolf-Rayet stars to hemorrhage heavy elements out into space. These stars with hundreds of times the mass of our sun are short-lived, only lasting for millions of years compared to the 4.6-billion-year-old sun, which still has about 5 billion years to go. They quickly burn through any hydrogen as Red Supergiants, and then begin fusing helium and other heavier elements as Wolf-Rayets, all the way up to iron by the end. These heavy elements bubble up to the surface in large amounts, where vicious stellar winds rip them up and eject them into space.
A bubble with a thin dense shell of heavy material forms around the raging star, inflated by stellar winds. These bubbles vary in size based on the initial mass of the star, but they can stretch as big as tens of parsecs in diameter with the volatile star in the middle. The new theory suggests that the sun could have formed within the walls of this immense cosmic bubble, where the high densities cause rapid star formation.
"These are really large, and the sun's radius is very small compared to the bubble size," Dwarkadas told Popular Mechanics in an email. "We postulate that a part of the shell will collapse to form the early solar system."
Department members: Vikram Dwarkadas
Scientists describe how solar system could have formed in bubble around giant star
December 22, 2017
UChicago News, by Louise Lerner
Despite the many impressive discoveries humans have made about the universe, scientists are still unsure about the birth story of our solar system.
Scientists with the University of Chicago have laid out a comprehensive theory for how our solar system could have formed in the wind-blown bubbles around a giant, long-dead star. Published Dec. 22 in the Astrophysical Journal, the study addresses a nagging cosmic mystery about the abundance of two elements in our solar system compared to the rest of the galaxy.
The general prevailing theory is that our solar system formed billions of years ago near a supernova. But the new scenario instead begins with a giant type of star called a Wolf-Rayet star, which is more than 40 to 50 times the size of our own sun. They burn the hottest of all stars, producing tons of elements which are flung off the surface in an intense stellar wind. As the Wolf-Rayet star sheds its mass, the stellar wind plows through the material that was around it, forming a bubble structure with a dense shell.
A simulation shows how stellar winds carry mass from a giant star over the course of millions of years, forming bubbles around it - which could have served as the origins of our solar system. Simulation by V. Dwarkadas/D. Rosenberg. Click here for more info.
"The shell of such a bubble is a good place to produce stars," because dust and gas become trapped inside where they can condense into stars, said coauthor Nicolas Dauphas, professor in the Department of Geophysical Sciences. The authors estimate that 1 percent to 16 percent of all sun-like stars could be formed in such stellar nurseries.
This setup differs from the supernova hypothesis in order to make sense of two isotopes that occur in strange proportions in the early solar system, compared to the rest of the galaxy. Meteorites left over from the early solar system tell us there was a lot of aluminium-26. In addition, studies, including a 2015 one by Dauphas and a former student, increasingly suggest we had less of the isotope iron-60.
This brings scientists up short, because supernovae produce both isotopes. "It begs the question of why one was injected into the solar system and the other was not," said coauthor Vikram Dwarkadas, a research associate professor in Astronomy and Astrophysics.
This brought them to Wolf-Rayet stars, which release lots of aluminium-26, but no iron-60.
"The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed - trapping the aluminum inside the shell," Dwarkadas said. Eventually, part of the shell collapses inward due to gravity, forming our solar system.
As for the fate of the giant Wolf-Rayet star that sheltered us: Its life ended long ago, likely in a supernova explosion or a direct collapse to a black hole. A direct collapse to a black hole would produce little iron-60; if it was a supernova, the iron-60 created in the explosion may not have penetrated the bubble walls, or was distributed unequally.
Other authors on the paper included UChicago undergraduate student Peter Boyajian and Michael Bojazi and Brad Meyer of Clemson University.
Citation: "Triggered star formation inside the shell of a Wolf-Rayet bubble as the origin of the solar system." The Astrophysical Journal, Dec. 22, 2017.
Department members: Vikram Dwarkadas
First multimessenger observation of a neutron-star merger is Physics World 2017 Breakthrough of the Year
December 18, 2017
The Physics World 2017 Breakthrough of the Year goes to the international team of astronomers and astrophysicists that ushered in a new era of astronomy by making the first ever multimessenger observation involving gravitational waves.
On 17 August 2017 the LIGO-Virgo gravitational-wave detectors, the Fermi Gamma-ray Space Telescope and the INTEGRAL gamma-ray space telescope detected nearly-simultaneous signals. They came from the merger of two neutron stars - an object now called GW 170817. This was the first time that LIGO-Virgo scientists had seen a neutron star merger, but five hours later they had already worked out the location of the source in the sky. Over the next hours and days, more than 70 telescopes were pointed at GW 170817 and a wealth of observations were made in the gamma-ray, X-ray, visible, infrared and radio portions of the electromagnetic spectrum. Astrophysicists also searched for neutrinos, but none were seen.
These coordinated observations have already provided a vast amount of information about what happens when neutron stars collide in what is called a "kilonova". The observations have yielded important clues about how heavy elements, such as gold, are produced in the universe. The ability to measure both gravitational waves and visible light from neutron-star mergers has also given a new and independent way of measuring the expansion rate of the universe. In addition, the observation settles a long-standing debate about the origin of short, high-energy, gamma-ray bursts.
Department members: Daniel E. Holz
Scientific projects: Laser Interferometer Gravitational-wave Observatory
Auger result in the list of scientific breakthroughs of the year
December 18, 2017
The Physics World top 10 breakthroughs of 2017 is awarded to the Pierre Auger Observatory collaboration for showing that ultra-high-energy cosmic rays come from outside the Milky Way. For decades, astrophysicists have believed that the sources of cosmic rays with energies greater than about 1 EeV (1018eV) could be worked out from the arrival directions of these particles. This is unlike lower energy cosmic rays, which appear to come from all directions after being deflected by the Milky Way's magnetic fields. Now, Pierre Auger's 1600 Cherenkov particle detectors in Argentina have revealed that the arrival rate of ultra-high-energy cosmic rays is greater in one half of the sky. What is more, the excess lies away from the centre of the Milky Way - suggesting that the cosmic rays have extra-galactic origins.
Department members: Angela V. Olinto, Paolo Privitera
Scientific projects: Pierre Auger Observatory
ALMA follow up of SPT discovered galaxies
December 12, 2017
ALMA finds massive primordial galaxies swimming in vast ocean of dark matter
Astronomers expect that the first galaxies, those that formed just a few hundred million years after the Big Bang, would share many similarities with some of the dwarf galaxies we see in the nearby universe today. These early agglomerations of a few billion stars would then become the building blocks of the larger galaxies that came to dominate the universe after the first few billion years.
Ongoing observations with the Atacama Large Millimeter/submillimeter Array (ALMA), however, have discovered surprising examples of massive, star-filled galaxies seen when the cosmos was less than a billion years old. This suggests that smaller galactic building blocks were able to assemble into large galaxies quite quickly.
The latest ALMA observations push back this epoch of massive-galaxy formation even further by identifying two giant galaxies seen when the universe was only 780 million years old, or about 5 percent its current age. ALMA also revealed that these uncommonly large galaxies are nestled inside an even-more-massive cosmic structure, a halo of dark matter several trillion times more massive than the sun.
The two galaxies are in such close proximity - less than the distance from the Earth to the center of our galaxy - that they will shortly merge to form the largest galaxy ever observed at that period in cosmic history. This discovery provides new details about the emergence of large galaxies and the role that dark matter plays in assembling the most massive structures in the universe.
The researchers report their findings in the journal Nature ("Galaxy growth in a massive halo in the first billion years of cosmic history").
"With these exquisite ALMA observations, astronomers are seeing the most massive galaxy known in the first billion years of the universe in the process of assembling itself," said Dan Marrone, associate professor of astronomy at the University of Arizona in Tucson and lead author on the paper.
Astronomers are seeing these galaxies during a period of cosmic history known as the Epoch of Reionization, when most of intergalactic space was suffused with an obscuring fog of cold hydrogen gas. As more stars and galaxies formed, their energy eventually ionized the hydrogen between the galaxies, revealing the universe as we see it today.
"We usually view that as the time of little galaxies working hard to chew away at the neutral intergalactic medium," said Marrone. "Mounting observational evidence with ALMA, however, has helped to reshape that story and continues to push back the time at which truly massive galaxies first emerged in the universe."
The galaxies that Marrone and his team studied, collectively known as SPT0311-58, were originally identified as a single source by the South Pole Telescope. These first observations indicated that this object was very distant and glowing brightly in infrared light, meaning that it was extremely dusty and likely going through a burst of star formation. Subsequent observations with ALMA revealed the distance and dual nature of the object, clearly resolving the pair of interacting galaxies.
To make this observation, ALMA had some help from a gravitational lens, which provided an observing boost to the telescope. Gravitational lenses form when an intervening massive object, like a galaxy or galaxy cluster, bends the light from more distant galaxies. They do, however, distort the appearance of the object being studied, requiring sophisticated computer models to reconstruct the image as it would appear in its unaltered state.
This "de-lensing" process provided intriguing details about the galaxies, showing that the larger of the two is forming stars at a rate of 2,900 solar masses per year. It also contains about 270 billion times the mass of our sun in gas and nearly 3 billion times the mass of our sun in dust. "That's a whopping large quantity of dust, considering the young age of the system," noted Justin Spilker, a recent graduate of the University of Arizona and now a postdoctoral fellow at the University of Texas at Austin.
The astronomers determined that this galaxy's rapid star formation was likely triggered by a close encounter with its slightly smaller companion, which already hosts about 35 billion solar masses of stars and is increasing its rate of starburst at the breakneck pace of 540 solar masses per year.
The researchers note that galaxies of this era are "messier" than the ones we see in the nearby universe. Their more jumbled shapes would be due to the vast stores of gas raining down on them and their ongoing interactions and mergers with their neighbors.
The new observations also allowed the researchers to infer the presence of a truly massive dark matter halo surrounding both galaxies. Dark matter provides the pull of gravity that causes the universe to collapse into structures (galaxies, groups and clusters of galaxies, etc.).
"If you want to see if a galaxy makes sense in our current understanding of cosmology, you want to look at the dark matter halo - the collapsed dark matter structure - in which it resides," said Chris Hayward, associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute in New York City. "Fortunately, we know very well the ratio between dark matter and normal matter in the universe, so we can estimate what the dark matter halo mass must be."
By comparing their calculations with current cosmological predictions, the researchers found that this halo is one of the most massive that should exist at that time.
"There are more galaxies discovered with the South Pole Telescope that we're following up on," said Joaquin Vieira of the University of Illinois at Urbana-Champaign, "and there is a lot more survey data that we are just starting to analyze. Our hope is to find more objects like this, possibly even more distant ones, to better understand this population of extreme dusty galaxies and especially their relation to the bulk population of galaxies at this epoch."
"In any case, our next round of ALMA observations should help us understand how quickly these galaxies came together and improve our understanding of massive galaxy formation during reionization," added Marrone.
Department members: John E. Carlstrom, Thomas Crawford
Scientific projects: South Pole Telescope
Three UChicago faculty members named AAAS fellows
November 21, 2017
UChicago News, by Louise Lerner
Three members of the University of Chicago faculty were named as 2017 fellows of the American Association for the Advancement of Science. Fellows are elected by AAAS members for their scientifically or socially distinguished efforts to advance science and its applications.
Don Q. Lamb, the Robert A. Millikan Distinguished Service Professor of Astronomy and Astrophysics and the College, was named "for outstanding contributions to theoretical astrophysics, especially for seminal contributions to the understanding of supernovae and for leadership in the Sloan Digital Sky Survey."
His research interests have included the properties of matter at high densities and temperatures, the evolution of white dwarfs and neutron stars, gamma-ray bursts, supernovae and most recently, experiments that use intense lasers to study the origin of the magnetic fields in the universe. He played a key role in founding the Sloan Digital Sky Survey and was the co-leader and Mission Scientist for the NASA High Energy Transient Explorer. Head of the Flash Center for Computational Science, Lamb is also affiliated with the Enrico Fermi Institute, the Energy Policy Institute of Chicago and the Harris School of Public Policy.
Department members: Donald Q. Lamb
Scientific projects: Flash Center for Computational Science
November 8, 2017
UChicago Magazine, by Maureen Searcy Sean Carr
From dark matter to gravitational waves to a balloon-borne telescope, scientists discuss how they handle setbacks.
Scientific progress follows a winding path, filled with detours and wrong turns - a natural result of exploring the unknown. Science makes headway by challenging itself, identifying mistakes, self-correcting, and persevering. That's how alchemy becomes chemistry, astrology becomes astronomy, and belief in the four humors leads to medicine.
UChicago scientists have seen their share of scientific wandering. One describes searching for something that no one is sure even exists, and how not finding it is in fact a discovery. Another explains how skepticism - of historical discoveries as well as his own team's data - leads to more reliable methods, sensitive instruments, and credible results. And one story is a study in resilience in the face of repeated misfortune, and in how catastrophe can give rise to creativity and improvisation.
Science is not a "lockstep march toward progress," says Edward "Rocky" Kolb, dean of the Physical Sciences Division. He compares the process to Brownian motion, with ideas bouncing around erratically but with a general direction toward deeper understanding and more correct results. "How do we know what the right direction is? We bump into a wall and say, 'Oops, that's the wrong way.'"
Angela Olinto improvises when her experiment crashes.
On April 25, astrophysicist Angela Olinto let go of her balloon.
Launched from Wanaka, New Zealand, it rose more than 20 miles into the sky - a stadium - sized super pressure helium balloon, carrying a one-ton UV telescope and Olinto's hopes to discover the secrets of ultra-high-energy cosmic rays. "I find the most energetic particles exciting," says Olinto, the Albert A. Michelson Distinguished Service Professor of Astronomy and Astrophysics, "because they challenge our theories on how they became so energized."
The extremely rare charged particles strike Earth at a rate of one particle per square kilometer per century. When they collide with the atmosphere, they produce a cascade of secondary particles, including neutrinos. If astrophysicists can observe those particle showers, they can look backward and search for their origin.
The balloon's payload, an instrument called the Extreme Universe Space Observatory (EUSO), was designed to measure the UV light produced when nitrogen molecules in the atmosphere are energized by the cascade and then return to ground state. The balloon was scheduled to carry the fluorescence detector for 100 days, testing the equipment but "mostly collecting data," says Olinto.
Three days into the flight, the balloon sprang a leak. By day 12, it was at the bottom of the South Pacific Ocean. NASA planned for this possibility and sank the balloon, using a remote termination command to prevent a dangerous descent. NASA's 30-year-old balloon program had conducted an environmental analysis of an open-ocean landing and designed the payload to act as an anchor, pulling the entire balloon quickly to the ocean floor to protect marine life.
Olinto had no say over if or when the balloon should come down. "We are responsible for the payload," she says. "The balloon and the flight - that's all under NASA's control." Despite her disappointment, Olinto stays positive. "This was not my worst nightmare. That would have been completing the 100-day flight and finding our equipment doesn't work well."
The 13-country EUSO collaboration was able to collect some data, in part because after the leak the researchers changed their strategy to optimize what time they had left. "We had to improvise," says Olinto.
Normally they would collect data on moonless nights, when the particle shower lights are best observed, and download data when the moon is bright. When the leak was confirmed, they downloaded no matter the moon's state. Luckily their launch window opened during the new moon, and they collected about 60 gigabytes of data.
The balloon's leak is one of many setbacks the EUSO project has faced. A version of EUSO was originally designed for the International Space Station (ISS) in the early 2000s, but after the 2003 Space Shuttle Columbia disaster, NASA halted space shuttle missions for more than two years pending the investigation. The shuttle program was then phased out in 2011.
In 2012, when the detector was reconfigured for the Japanese Experiment Module of the ISS and became JEM-EUSO, Olinto was invited to lead the US branch of the 13-country collaboration. But several factors, compounded by the 2011 Fukushima meltdown, made the future of that project uncertain. So JEM-EUSO was broken into several projects, one of which was EUSO-SPB, aboard the super pressure balloon, whose launch was then delayed a month by weather concerns.
"I have been in many situations where it looked like the whole effort was about to dissolve into dust," says Olinto. Yet she finds those situations filled with creative energy, which she funnels into formulating new approaches. "The goals in research are flexible," she says, "so the alternate path and the final destination are redefined when challenges are overwhelming."
Olinto's new plan is to build another telescope and add a neutrino detector. The project's second generation, EUSO-SPB2, received a NASA award in September. "No one has seen ultra-high-energy neutrinos before," she says. The second flight will allow EUSO to collect more data and test the neutrino instrument's capabilities. "It will be easier to predict and prepare for what can go wrong, learning from the first flight, where lots of things went wrong."
Second time's the charm. And the fourth. And the fifth.
Daniel Holz, SM'94, PhD'98, explains fake gravitational waves.
On Monday, September 14, 2015, at 4:51 a.m. CDT, the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors - in Hanford, Washington, and Livingston, Louisiana - picked up the signal of gravitational waves. Produced by the collision and merging of two massive black holes, it was the first observation of the ripples in space-time that Albert Einstein had predicted a century earlier.
Five months after the detection - once scientists, including UChicago associate professor of physics Daniel Holz, SM'94, PhD'98, had checked, rechecked, and triple-checked the data - they announced their results to the world.
As it turns out, however, this wasn't the first time LIGO had been through this drill; it was just the first time that it turned out not to be a drill.
Five years earlier, before Holz joined the collaboration, a less sensitive previous incarnation of LIGO had picked up what appeared to be gravitational waves. The collaboration had gone through all the usual steps with the detected event: "It was studied, taken apart, everything, hundreds and hundreds of people involved" over several months, says Holz. A paper was drafted; the decision was made to submit it for publication. "We're talking about people arguing about the title of the paper," Holz says - it was that close to done.
There was just one problem: there had been no event.
The initial signal had been a "blind injection," a test designed by a sworn-to-secrecy team within LIGO to see if the equipment - and most important, the scientists interpreting the data - could distinguish between a false positive and an actual event.
"The answer," Holz says, "was, 'No, this isn't real.' The answer was, 'We're not publishing this. We haven't just detected gravitational waves, and no one's getting a Nobel Prize.'"
It might seem like "a complete waste of time," Holz says of the negated months of work, but it's "actually useful. It makes you go through the whole process" and ask, what went wrong, what did they get right, and how could everything be improved? It keeps the scientists on their toes.
Such tests are standard in the field of gravitational waves research, and an understandable precaution when you're working to confirm a key part of the general theory of relativity. The abundance of caution is part of the legacy of the first scientist to claim to have detected gravitational waves, Joe Weber of the University of Maryland - the "father of the field," Holz says, and "an absolutely brilliant experimenter." In 1969 Weber published a paper in Physical Review Letters that described what he had detected.
But the signal he had found was "at least five orders of magnitude too loud," Holz explains. Others "could not think of any way from the theory side that there really could be waves that were that loud." No one else was able to reproduce Weber's results. Nonetheless, he remained convinced and continued to make more "detections" throughout his life.
Weber's example "set a particular tone" to the search for gravitational waves, Holz says, and so the goal for LIGO was "to have our detection, especially our first detection, to be so clear, so impressive, that no one could possibly doubt what we've done."
After the false alarm of the blind injection, which came during the era of "initial" LIGO, improvements in the detectors made them far more sensitive. By September 2015 "advanced" LIGO was ready - or almost. In fact, at that point the new equipment was not officially online. "We were still fiddling with the machine," Holz says. "We were going to turn it on very soon."
So when the detection came through, everyone assumed it had to be an injection. That's when they received word from the top: the blind injection system was not yet up and running. And if such a "perfect event" wasn't an injection, it could be only one thing.
"We still ripped it apart," says Holz. Without the blind injection system up and running, it was even more important to make sure they weren't fooling themselves. "It was five months of a thousand people doing their very best to figure out how this might not be real." But it was real. "We couldn't make the event go away."
More gravitational waves have followed - confirmed detections in December 2015 and January 2017. Conservatism, however, still rules: an October 2015 detection is classified only as a "candidate" gravitational wave because it wasn't loud enough for the collaboration to be confident.
To this day, however, LIGO has yet to switch on its blind injection system. "Because we've seen real events, we know it's working," Holz says. So the last thing they need is fake signals to analyze. "At this point it's becoming difficult to keep up with the real events that keep showing up."
Process of elimination
Rocky Kolb searches for the mysterious particle.
Astrophysicists theorize that about 85 percent of the universe's mass is dark matter, which can be detected only through its gravitational effects. Galaxies and galaxy clusters spin so quickly that they should have torn themselves apart based on their observable matter. Something is holding them together, but no one knows what.
Scientists know much about what dark matter is not: It is not the visible stuff of stars and planets. It is not dark clouds of baryonic (ordinary atomic) matter, which can be observed absorbing radiation passing through them. And it's not antimatter, which would produce gamma rays when it annihilates with matter. So what is it?
One hypothetical candidate is WIMPs - weakly interacting massive particles that don't interact much with ordinary matter, proposed more than 30 years ago. As a graduate student at the University of Texas, Austin, in the 1970s, Kolb, now the Arthur Holly Compton Distinguished Service Professor of Astronomy and Astrophysics at UChicago, helped lay the foundations for WIMPs by exploring the limits to weak interaction.
WIMPs may bepart of the concept of supersymmetry, which fills gaps in astrophysicists' understanding of known particles and forces. The idea says that each fundamental particle has an as-yet-undiscovered superpartner. When scientists use the properties of the lightest supersymmetric particles - WIMPs - and calculate how many would still exist after the big bang, that number matches the amount of dark matter seen (or inferred) today.
But so far no detectors or colliders have been able to shed light on WIMPs. So does Kolb still think they're the answer? "I think we'll be surprised, that the answer will come out of left field," he says.
What's advantageous about the WIMP hypothesis says Kolb, is that it's falsifiable. British philosopher Karl Popper's concept of falsifiability states that theories are scientific only if it is possible, in principle, to prove them false, and that empirical science is never confirmed, only incrementally corroborated through absence of disconfirming evidence.
Another dark matter candidate - ordinary matter in the form of black holes, neutron stars, or brown dwarfs called MAssive Compact Halo Objects, or MACHOs - was falsified in 2004 through the discovery of a galaxy cluster that doesn't behave in accordance with the hypothesis.
"Maybe we're on the verge of falsifying WIMPs," says Kolb, which would be a form of discovery.
He cites the famous failed experiment of Albert Michelson, founder of UChicago's physics department, and Edward Morley to establish the existence of "ether," the medium they believed filled space and was required to transmit light. In the process of failing, they established the speed of light as a fundamental constant, and their work eventually led to the theory of relativity.
So discovering that WIMPs arent the explanation for dark matter would point astrophysicists in other directions. But scientists "should completely exhaust the possibilities," Kolb says, before making that call.
Department members: Daniel E. Holz, Edward ''Rocky'' W. Kolb, Angela V. Olinto
Scientific projects: Extreme Universe Space Observatory at the Japanese Module, Extreme Universe Space Observatory on a Super Pressure Balloon, Laser Interferometer Gravitational-wave Observatory
NASA solar probe named for U. of C. astrophysicist from Flossmoor to launch next summer
November 6, 2017
Chicago Tribune, by Donna Vickroy
In the meeting room of Homewood's Izaak Walton Preserve on a recent Thursday evening, Eugene Parker led an audience of scientists, children, janitors and curious suburbanites through the complex theory that has come to make him a "star" in the world of astrophysics.
Once criticized for his 1958 theory on "solar wind," Parker, a distinguished University of Chicago professor emeritus and resident of Flossmoor, recently learned that because of that work, conducted nearly 60 years ago, his name will be on the first probe to explore the outer corona of the sun.
NASA will launch the Parker Solar Probe, the first spacecraft named for a living individual, on July 31, 2018 (with a 20-day window for weather).
Parker, now 90, recalled the day earlier this spring when he got the news.
"I was sitting at home doing some work when the phone rang and it was a guy at NASA that I know," he said.
He said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in Washington, told him NASA's plan to name the solar probe after him and asked if he had any objections.
"Needless to say," Parker said, "I did not have any objections."
Parker said though initially "taken aback because it never occurred to me that the day would come that they could send a rocket into the sun's atmosphere," being recognized on such a universal scale "always makes one happy."
The six decades that have passed since Parker wrote the seminal paper on the solar wind, spurred by his observation that a comet's tail always points away from the sun, have been filled with other important work in the field of physics, but it is his recognition by NASA that is now attracting the attention of the public.
The Homewood facility was packed to hear the free lecture. Extra chairs were brought in and a smattering of standing-room-only fans gathered in the back.
Through diagrams, mathematical formulas and the occasional wisecrack, Parker, who was elected to the National Academy of Sciences in 1967, showed his work.
Belying a resume that includes a host of top science awards, including the George Ellery Hale Prize, the National Medal of Science, the Gold Medal of the Royal Astronomical Society and the Kyoto Prize, Parker spoke in layman's terms, even recalling a response he gave to critics so many years ago: "Solve the goddamn equation."
A sketch of a comet orbiting the sun revealed that the common assumption that the tail always follows behind was wrong.
"Your instinct tells you it's moving this way and that the tail is behind. But the interesting thing is it doesn't work that way. The tail of the comet always points straight away from the sun, in antisolar direction. Sometimes the tail trails behind but sometimes it's perpendicular, and sometimes it's ahead of the comet like a headlight on a railroad locomotive," he explained.
"People said, 'That's impossible.' But since it happens it evidently is not impossible," he added.
Department members: Eugene N. Parker
Chicago Professor Leads NASA Balloon Mission to Study 'Ghost Particles'
November 3, 2017
Chicago Tonight, by Alex Ruppenthal
What is Angela Olinto hoping to learn about the universe with a football field-sized balloon and a 3,000-pound telescope? To start, how about whether there are extra dimensions of space?
The entire field encompassing Olinto's career began with a balloon ride in 1912, when Austrian physicist Victor Hess ascended 3-plus miles during a near total solar eclipse and discovered cosmic rays, or high-energy particles coming from beyond our own galaxy.
Now, Olinto, a professor in the University of Chicago's Department of Astronomy and Astrophysics, is leading a first-of-its kind mission to launch a $7 million telescope on a super pressure balloon with the goal of learning more about these mysterious subatomic particles, the origins of which are still unknown.
"We don't really have a clue how the universe produces these ultra high-energy cosmic rays," Olinto said. "We know they come from outside of our galaxy, probably galaxies very far away from us."
Planned for launch in 2022, the NASA-built Extreme Universe Space Observatory Super Pressure Balloon, or EUSO-SPB1, will take off from New Zealand and ride the polar jet stream that circles the bottom part of the globe. Researchers hope the balloon will make several trips around the Antarctic over 100 or more days.
Traveling at 20 miles into the atmosphere, the balloon will carry an ultra-sensitive telescope that will feed information to Olinto's team. The balloon will include three mirrors that direct light toward two types of detectors on the telescope: One built to pick up trails of nitrogen as cosmic ray showers cross the atmosphere; and one designed to capture the radiation from high-energy neutrinos, often called "ghost particles," coming from the Earth below.
Cosmic rays and neutrinos are known to pass through Earth without being affected, Olinto said. But scientists don't know much about these particles because they are extremely rare. They're also small and very fast, moving at the speed of light.
"Just like we see meteorites, we can see these particles in the atmosphere," Olinto said. "The difference is that meteorites are larger and much slower."
By taking measurements from space using the balloon, Olinto and other researchers will have a significantly wider field of view to catch the particles. They might even be able to observe neutrinos traveling upward from Earth, which has never been witnessed.
"It's going to be so much fun if we see them," Olinto said.
The balloon mission will serve as a proof of concept for another planned NASA mission that could provide groundbreaking new insights about the universe.
A team of scientists and NASA engineers led by Olinto is already designing the larger mission - the Probe of Extreme Multi-Messenger Astrophysics, or POEMMA - which will send a pair of orbiting satellites into space to study cosmic rays with even more sensitive telescopes.
"It's much cheaper to try to do these things from a balloon than from space," Olinto said. "The balloon is one way to show that we can do this from space, which I'm pretty sure we can."
Although years away, POEMMA has the potential to produce some out-of-this-world discoveries, Olinto said.
"For example, if there are extra dimensions of space, then the cross-section of neutrinos will change," she said, "and we should be able to see that."
Department members: Angela V. Olinto
Scientific projects: Extreme Universe Space Observatory on a Super Pressure Balloon, Probe of Extreme Multi-Messenger Astrophysics
Colliding Neutron Stars Could Settle Cosmology's Biggest Controversy
October 26, 2017
Quanta Magazine, by Natalie Wolchover
Newly discovered "standard sirens" provide an independent, clean way to measure how fast the universe is expanding.
To many cosmologists, the best thing about neutron-star mergers is that these events scream into space an otherwise close-kept secret of the universe. Scientists combined the gravitational and electromagnetic signals from the recently detected collision of two of these stars to determine, in a cleaner way than with other approaches, how fast the fabric of the universe is expanding -- a much-contested number called the Hubble constant.
In the days since the neutron-star collision was announced, Hubble experts have been surprised to find themselves discussing not whether events like it could settle the controversy, but how soon they might do so.
Scientists have hotly debated the cosmic expansion rate ever since 1929, when the American astronomer Edwin Hubble first established that the universe is expanding -- and that it therefore had a beginning. How fast it expands reflects what's in it (since matter, dark energy and radiation push and pull in different ways) and how old it is, making the value of the Hubble constant crucial for understanding the rest of cosmology.
And yet the two most precise ways of measuring it result in different answers, with a curious 8 percent discrepancy that "is currently the biggest tension in cosmology," said Dan Scolnic of the University of Chicago's Kavli Institute for Cosmological Physics. The mismatch could be a clue that cosmologists aren't taking into account important details that have affected the universe's evolution. But to see if that's the case, they need an independent check on the measurements.
Neutron-star collisions -- newly detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors -- seem to be just the thing.
"This first [collision] gives us a seat at the cosmology table," Daniel Holz, an astrophysicist with the University of Chicago and LIGO who was centrally involved in the new Hubble measurement, said in an email. "And as we get more, we can expect to play a major role in the field."
In an expanding universe, the farther away an astronomical object is, the faster it recedes. The Hubble constant says how much faster. Edwin Hubble himself estimated that galaxies move away from us 500 kilometers per second faster for each additional megaparsec of distance between us and them (a megaparsec is about 3.3 million light-years). This was a gross overestimate; by the 1970s, astrophysicists favored values for the Hubble constant around either 50 or 100 kilometers per second per megaparsec, depending on their methods. As errors were eliminated, these camps met near the middle. However, in the past year and a half, the Hubble trouble has reheated. This time, 67 stands off against 73.
The higher estimate of 73 comes from observing lots of astronomical objects and estimating both distance and velocity for each one. It's relatively easy to see how fast a star or galaxy is receding by looking at its "redshift" -- a reddening in color that happens for the same reason the sound of a receding ambulance's siren drops in pitch. Correct for an object's "peculiar velocity," caused by the gravitational pull of other objects in its neighborhood, and you're left with its recessional velocity due to cosmic expansion.
Historically, however, it has proven much, much harder to measure the distance to an object -- the other data point needed to calculate the Hubble constant.
To gauge how far away things are, astronomers build up rungs on a "cosmic distance ladder" in which each rung calibrates more-distant rungs. They start by deducing the distances to stars in the Milky Way using parallax -- the stars' apparent motion across the sky over the course of the year. With this information, astronomers can deduce the brightness of so-called Cepheid stars, which can be used as so-called "standard candles" because they all shine with a known intrinsic brightness. They then spot these Cepheid stars in nearby galaxies and use them to calculate how far away the galaxies must be. Next, the Cepheids are used to calibrate the distances to Type Ia supernovas -- even brighter (though rarer) standard candles that can be seen in faraway galaxies.
Each jump from one rung to the next risks miscalculation. And yet, in 2016, a team known as SH0ES used the cosmic distance ladder approach to peg the Hubble constant at 73.2 with an accuracy of 2.4 percent.
However, in a paper published the same year, a team used the Planck telescope's observations of the early universe to obtain a value of 67.8 for the current expansion rate -- supposedly with 1 percent accuracy.
The Planck team started from the faint drizzle of ancient light called the cosmic microwave background (CMB), which reveals the universe as it looked at a critical moment 380,000 years after the Big Bang. The CMB snapshot depicts a simple, nearly smooth, plasma-filled young universe. Pressure waves of all different wavelengths rippled through the plasma, squeezing and stretching it and creating subtle density variations on different length scales.
At the moment recorded in the CMB, pressure waves with particular wavelengths would have undergone just the right fraction of an undulation since the Big Bang to all reach zero amplitude, momentarily disappearing and creating smooth plasma densities at their associated length scale. Meanwhile, pressure waves with other wavelengths undulated just the right amount to exactly peak in amplitude at the critical moment, stretching and squeezing the plasma to the full extent possible and creating maximum density variations at their associated scales.
These peaks and troughs in density variations at different scales, which can be picked up by telescopes like Planck and plotted as the "CMB power spectrum," encode virtually everything about the young universe. The Hubble constant, in particular, can be reconstructed by measuring the distances between the peaks. "It's a geometric effect," explained Leo Stein, a theoretical physicist at the California Institute of Technology: The more the universe has expanded, the more the light from the CMB has curved through expanding space-time, and the closer together the peaks ought to appear to us.
Other properties of nature also affect how the peaks end up looking, such as the behavior of the invisible "dark energy" that infuses the fabric of the cosmos. The Planck scientists therefore had to make assumptions about all the other cosmological parameters in order to arrive at their estimate of 67 for the Hubble constant.
The similarity of the two Hubble measurements "is amazing" considering the vastly different approaches used to determine them, said Wendy Freedman, an astrophysicist at the University of Chicago and a pioneer of the cosmic distance ladder approach. And yet their margins of error don't overlap. "The universe looks like it's expanding about eight percent faster than you would have expected based on how it looked in its youth and how we expect it to evolve," Adam Riess of Johns Hopkins University, who led the SH0ES team, told Scientific American last year. "We have to take this pretty darn seriously."
The 67-versus-73 discrepancy could come down to an unknown error on one side or both. Or it might be real and significant -- an indication that the Planck team's extrapolation from the early universe to the present is missing a cosmic ingredient, one that changed the course of history and led to a faster expansion rate than otherwise expected. If a hypothesized fourth type of neutrino populated the infant universe, for instance, this would have increased the radiation pressure and affected the CMB peak widths. Or dark energy, whose repulsive pressure accelerates the universe's expansion, might be getting denser over time.
Suddenly, neutron-star collisions have materialized to cast the deciding vote.
The crashing stars serve as "standard sirens," as Holz and Scott Hughes of the Massachusetts Institute of Technology dubbed them in a 2005 paper, building on the work of Bernard Schutz 20 years earlier. They send rushes of ripples outward through space-time that are not dimmed by gas or dust. Because of this, the gravitational waves transmit a clean record of the strength of the collision, which allows scientists to "directly infer the distance to the source," Holz explained. "There is no distance ladder, and no poorly understood astronomical calibrations. You listen to how loud the [collision] is, and how the sound changes with time, and you directly infer how far away it is." Because astronomers can also detect electromagnetic light from neutron-star collisions, they can use redshift to determine how fast the merged stars are receding. Recessional velocity divided by distance gives the Hubble constant.
From the first neutron-star collision alone, Holz and hundreds of coauthors calculated the Hubble constant to be 70 kilometers per second per megaparsec, give or take 10. (The major source of uncertainty is the unknown angular orientation of the merging neutron stars relative to the LIGO detectors, which affects the measured amplitude of the signal.) Holz said, "I think it's just pure luck that we're smack in the middle," between the cosmic-distance-ladder and cosmic-microwave-background Hubble estimates. "We could easily shift to one side or the other."
The measurement's accuracy will steadily improve as more standard sirens are heard over the next few years, especially as LIGO continues to ramp up in sensitivity. According to Holz, "With roughly 10 more events like this one, we'll get to 1 percent [of error]," though he stresses that this is a preliminary and debatable estimate. Riess thinks it will take more like 30 standard sirens to reach that level. It all depends on how lucky LIGO and Virgo got with their first detection. "I do think the method has the potential to be a game changer," said Freedman. "How fast this will occur [or] what the rate of these objects will be ... we don't yet know."
Scolnic, who was part of SH0ES, said his team's tension with Planck's measurement is so large that "the standard siren approach doesn't need to get to 1 percent to be interesting."
As more standard sirens resound, they'll gradually home in on the Hubble constant once and for all and determine whether or not the expansion rate agrees with expectations based on the young universe. Holz, for one, is exhilarated. "I've dedicated the last decade of my life in the hopes of making one plot: a standard siren measurement of the Hubble. I got to make my Hubble plot, and it is beautiful."
Department members: Wendy L. Freedman, Daniel E. Holz
Scientific projects: Laser Interferometer Gravitational-wave Observatory
UChicago astrophysicists to catch particles from deep space on NASA balloon mission
October 26, 2017
UChicago News, by Louise Lerner
A team led by Prof. Angela Olinto was awarded NASA funding to fly an ultra-long duration balloon mission with an innovative ultra-sensitive telescope to pick up cosmic rays and neutrinos coming from deep space.
Planned for launch in 2022, the Extreme Universe Space Observatory on a Super Pressure Balloon is a major step toward a planned mission to send a probe to space. "This program will help us solve the great mystery of where in the universe these highly energetic particles are coming from, and how they could possibly be made," said Olinto, the Albert A. Michelson Distinguished Service in Astronomy and Astrophysics.
The Earth is constantly hit by particles from space. One type is cosmic rays: sub-atomic nuclei traveling from every direction in space, accelerated by supernovas and other unknown cosmic phenomena. Similarly mysterious are neutrinos, the "ghost particles" that pass through us all the time, mostly undetected.
There is much we don't know about them -- most pressingly where they come from, although studies at the Pierre Auger Observatory in Argentina recently confirmed that the most energetic cosmic rays that hit the Earth are coming from beyond our own galaxy.
The most extreme cosmic rays and neutrinos offer the most clues to their origins and travels, as they can resist the effects of magnetic fields in space that curve the paths of weaker particles. These are what the new NASA -- built balloon will be hunting.
"It's very difficult to explain some of these particles using our current model of the universe," Olinto said. "This balloon offers a truly unique opportunity to learn more about one of the great puzzles in astrophysics. In addition, once neutrinos are observed, we can test their interactions at energies well beyond what we can make in the laboratory."
Taking measurements from space offers a much wider field of view to catch these rare particles, Olinto said. The Earth's atmosphere makes these ghostly particles observable as faint flashes of light moving at ultra-high speeds. Hence the football-field-sized balloon, which can travel for months at 20 miles into the atmosphere, carrying the pioneering 3,000-pound telescope, which was built by an international team.
Three different mirrors will hang from the balloon, directing light toward two different types of detectors. One system is built to capture the radiation from extremely energetic neutrinos coming from the Earth below; the other is a fluorescence camera, which picks up the trails of excited nitrogen nuclei as cosmic ray showers cross the atmosphere.
The mission will launch from New Zealand, so that the balloon can catch a ride on the polar jet stream that circles the bottom part of the globe. The researchers hope the balloon will make several trips around the Antarctic over the course of 100 or more days.
The flight will provide proof of concept for the planned Probe of Extreme Multi-Messenger Astrophysics, a pair of orbiting satellites with the same capabilities, but with several orders of magnitude more sensitivity. A Olinto-led team of scientists and NASA engineers is designing the POEMMA mission for consideration by the 2020 Astronomy and Astrophysics Survey, which sets scientific priorities for the decade for NASA, the National Science Foundation and the U.S. Department of Energy.
POEMMA could offer some fundamental new insights about the universe, Olinto said. "For example, if there are extra dimensions of space, then the cross-section of neutrinos will change, and we should be able to see that."
This is the second balloon launch to prepare for the space mission. The first balloon, which lifted off in April 2017 from New Zealand, sprang a leak early in the mission and sank into the Pacific. This new iteration will have much expanded scientific capabilities, Olinto said. An overnight test flight is planned in 2020 in the United States before the official launch in 2022.
In addition to the University of Chicago, the team includes the Colorado School of Mines, the Lehman College (CUNY), the Marshall Space Flight Center and the University of Alabama-Huntsville.
Department members: Angela V. Olinto
Scientific projects: Extreme Universe Space Observatory at the Japanese Module, Extreme Universe Space Observatory on a Super Pressure Balloon, Pierre Auger Observatory, Probe of Extreme Multi-Messenger Astrophysics
LIGO announces detection of gravitational waves from colliding neutron stars
October 16, 2017
UChicago News, by Louise Lerner
UChicago physicists calculate expansion rate of universe using breakthrough research
About 130 million years ago, two incredibly heavy, dense neutron stars spiraled around each other. Their dance brought them closer to one another and made them spin faster, until they were circling more than 100 times per second. The ensuing collision sent a shockwave through the very fabric of spacetime, which traveled across the universe at the speed of light until it rippled through the Earth at 7:41 a.m. Central time on Aug. 17, 2017.
The U.S.-based Laser Interferometer Gravitational-Wave Observatory and the Virgo detector in Italy announced on Oct. 16 that all three of their detectors had picked up the ripples, or gravitational waves, from this event. Two seconds later, a satellite looking for gamma rays registered a burst from the same direction of the sky.
The event was the first time humans have directly observed two neutron stars, the collapsed cores of bigger stars, smashing into one another. Unlike the black holes that merged in LIGO's first detection of gravitational waves two years ago -- a breakthrough that earned this year's Nobel Prize in Physics -- the newly married neutron stars gave off a bright flash of light visible for days afterward. That allowed the world's most advanced telescopes to point in that direction of the sky, including the Dark Energy Camera in Chile and the Hubble Space Telescope and Chandra X-ray Observatory in orbit above the Earth.
The result is the first measurement of a gravitational wave event in multiple mediums -- optical, gamma ray and X-ray as well as gravitational waves -- and scientists said the combination opens a wealth of new scientific discovery.
This includes determining the precise location of the galaxy where the event happened, which no previous LIGO detection has been able to do. They also confirmed that gravitational waves travel at approximately the speed of light, verifying a century-old Einstein prediction. And they used gravitational waves to directly calculate the rate at which the universe is expanding.
"Any one of these findings would be groundbreaking on its own merits, and here we got all the pieces together in the span of 12 hours," said Daniel Holz, an associate professor of physics and astrophysics who led the UChicago team, which was involved in both the LIGO and Dark Energy Survey discoveries. "This is akin to seeing the lightning bolt and hearing the thunder. We have just witnessed the birth of a new field of astronomy. It's been an unbelievable few weeks."
The Hubble constant: Chasing a 'white whale'
Holz is a co-author on 12 papers published Oct. 16 on the event, including a leading role in one published in Nature announcing an entirely new measurement of the rate at which the universe is expanding.
Originally suggested by famed astronomer and UChicago alumnus Edwin Hubble, this number, called the Hubble constant, is important to such central questions in astrophysics as the age of the universe and the nature of dark matter and dark energy. It's also at the center of a raging controversy.
Everyone agrees on the ballpark number, but whether it's exactly 67 or 72 kilometers per second per megaparsec is hotly debated. Different methods of computing the constant spit out different results, and, Holz said, "they disagree by more than they should."
Gravitational waves should be one of the cleanest ways to compute the number, Holz said, because scientists understand the physics of what's happening very well. "Other ways involve many more steps and calibrations that we aren't sure about," he said, "but gravitational waves give you this very elegant way to perform this fundamental measurement."
The initial calculations show LIGO's number smack in the middle of other estimates, at 70 kilometers per second per megaparsec.
In 2006 Holz was the first to suggest the concept of calculating the Hubble constant via gravitational waves from a gamma-ray burst, calling it a "standard siren," a nod to the term used to describe certain types of supernovas used for the same calculation called "standard candles."
"Everyone has their white whale, and mine has been to detect the Hubble constant with gravitational waves," he said. "And now we've done it. A few hours after the discovery I sat down and made the plot, and there it was, the culmination of all those years, right in front of me. And it was beautiful."
A literal and figurative 'gold mine'
The neutron star merger is also the closest signal to be detected by gravitational waves, and the closest gamma-ray burst -- only about 130 million light-years away, as opposed to the first black hole merger, which was more than a billion light-years away. "That's really in our cosmological backyard," Holz said.
Neutron stars are unfathomably dense -- the weight of one-and-a-half suns packed into a ball just a dozen or so miles across. They give out a fainter gravitational wave signal than black holes, Holz said, so such proximity is necessary to capture them -- even for the extraordinary sensitivity of the detectors.
Most scientists, even optimists, predicted it would be a decade before they would see a neutron star collision and be able to take such a measurement in all mediums, he said.
"This event is a gold mine -- literally and figuratively," Holz said. "We're going to learn an incredible amount about astrophysics and cosmology from studying its properties. We're also watching the production of most of the gold in the universe," since initial studies of the event suggest that such star collisions are likely to be the origin of the heaviest elements in the universe, including gold. (Back-of-the-envelope calculations indicate that this single collision produced an amount of gold greater than the weight of the Earth, Holz said.) This solves a decades-long mystery of where about half of all elements heavier than iron are produced.
The researchers also noted the incredible good fortune of the detection's timing. There are three gravitational wave detectors in the world: two in the U.S. run by LIGO, located in Washington and Louisiana, and one in Italy. The Italian detector had just started up, and the Louisiana and Hanford locations were just a week from shutting down for a year of maintenance. The event took place in the brief three-week window when all three gravitational wave detectors happened to be on -- crucial for an accurate triangulation of the location.
Each detector has two identical arms several miles long, held at right angles to one another. Lasers run the length of each arm, perfectly calibrated to combine in tune with one another, unless one arm suddenly becomes slightly shorter or longer than the other -- as would only happen if the universe itself is rippling.
Aside from analyzing all of the data they already have, Holz said, they are still measuring the radio waves produced from the ejected material interacting with the surrounding environment.
"We'll be mining this data for a long time," he said.
"With this we truly open a new era of astronomy," he said. "We used to have only one way to look at the sky, but by combining existing telescopes and gravitational waves, we can learn staggeringly more about the universe."
Hundreds of scientists are now sorting through the results. The UChicago LIGO team included postdoctoral fellow Ben Farr (now a professor at the University of Oregon) and graduate students Hsin-Yu Chen (now at Harvard), Zoheyr Doctor and Maya Fischbach, as well as Reed Essick, who started this fall at UChicago as a Kavli Institute for Cosmological Physics Fellow.
The UChicago team works closely with colleagues at Fermi National Acceleratory Laboratory and elsewhere on the Dark Energy Survey, which captured optical pictures of the merger just hours after LIGO and Virgo detected the gravitational waves. The scientists looked by eye at the telescope's digital photographs for bright spots that hadn't been there before in the section of the sky LIGO indicated, and found a new source in the galaxy labeled NGC 4993.
"Because we're on the telescope nearly every night at that time of year, we were able to watch it peak and then fade very rapidly and could precisely map its brightness and color over time," said Josh Frieman, UChicago professor of astronomy and astrophysics and the director of the Dark Energy Survey. "This development is very exciting for us, because more data on the expansion rate of the universe will help us chart the billion-year history of the cosmic tug-of-war between gravity and dark energy."
Holz was on a plane from Hong Kong when the Aug. 17 gravitational wave event happened. He landed to dozens of texts and notifications. "I walked off the plane with my laptop held up to my face, and that's basically how I've been walking around ever since," he said. "Nature has given us these wonderful gifts. We're all sleep-deprived, but no one's complaining."
Citation: "A gravitational-wave standard siren measurement of the Hubble constant." Nature, Oct. 16, 2017.
Department members: Joshua A. Frieman, Daniel E. Holz
Department students: Hsin-Yu Chen, Maya Fishbach
Scientific projects: Laser Interferometer Gravitational-wave Observatory
Observatory detects extragalactic cosmic rays hitting the Earth
September 22, 2017
UChicago News, by Louise Lerner
Finding is an important step to understanding origin of mysterious particles
Fifty years ago, scientists discovered that the Earth is occasionally hit by cosmic rays of enormous energies. Since then, they have argued about the source of those ultra-high-energy cosmic rays -- whether they came from our galaxy or outside the Milky Way.
The answer lies in a galaxy or galaxies far, far away, according to a report published Sept. 22 in Science by the Pierre Auger Collaboration, which includes University of Chicago scientists. The internationally run observatory in Argentina, co-founded by the late UChicago Nobel laureate James Cronin, has been collecting data on such cosmic rays for a more than a decade.
The collaboration found that the rate of such cosmic particles, whose energies are a million times greater than that of the protons accelerated in the Large Hadron Collider, is about six percent greater from one side of the sky than the other, in a direction where the distribution of galaxies is relatively high.
"We are now considerably closer to solving the mystery of where and how these extraordinary particles are created -- a question of great interest to astrophysicists," said University of Wuppertal Prof. Karl-Heinz Kampert, spokesperson for the Auger Collaboration, which involves more than 400 scientists from 18 countries. "Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way."
Cosmic rays are the nuclei of elements from hydrogen to iron. The highest-energy cosmic rays, those of interest in this study, only strike about once per square kilometer per year -- equivalent to hitting the area of a soccer field about once per century.
Such rare particles are detectable because they create showers of secondary particles --including electrons, photons and muons -- as they interact with the nuclei in the atmosphere. These cosmic ray showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, like a dinner plate but several kilometers in diameter.
At the Auger Observatory, the shower particles are detected through the light they produce in several of 1,600 detectors, spread over 3,000 square kilometers of western Argentina -- an area comparable to that of Rhode Island -- and each containing 12 tons of water. Tracking these arrivals tells scientists the direction from which the cosmic rays came.
After racking up detections of more than 30,000 cosmic particles, the Auger Collaboration found one section of the sky was producing significantly more than its share. The probability of this happening by a random fluctuation is extremely small, the collaborators said: a chance of about two in ten million.
"This result unequivocally establishes that ultra-high-energy cosmic rays are not just random wanderers of our nearby universe," said Paolo Privitera, UChicago professor in astronomy and astrophysics, who heads the U.S. groups participating in the project.
Privitera credited Cronin, who died last year, with the original vision for the Auger observatory back in 1992.
"The imprint detected in their arrival directions -- a tantalizing evidence for extragalactic origin -- required several years of observations with a detector working, in Jim Cronin's words, 'like a Swiss clock.' It was a tribute to Jim's vision to build an observatory and unveil the mystery of the origin of the most energetic particles in the universe." Privitera said.
Even at these high energies, cosmic rays may be significantly deflected by magnetic fields in outer space; thus the excess found by the Auger Collaboration in a broad section of the sky cannot yet determine which extragalactic objects might be the specific sources, the authors said. The observatory is looking to examine even higher-energy cosmic rays -- rarer, but less likely to be deflected -- which may provide a clearer route to their sources. Work on this problem is targeted for the observatory's upgrade, scheduled to be completed in 2018.
Citation: "Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above 8x1018 eV." Science, Sept. 22, 2017. doi: 10.1126/science.aan4338
Funding: National Science Foundation
Department members: Angela V. Olinto, Paolo Privitera
Scientific projects: Pierre Auger Observatory
50 year-old mystery has been solved
September 22, 2017
From galaxies far far away!
In a paper to be published in Science on 22 September, the Pierre Auger Collaboration reports observational evidence demonstrating that cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider come from much further away than from our own Galaxy. Ever since the existence of cosmic rays with individual energies of several Joules (1 Joule = ~ 6x1018 eV) was established in the 1960s, speculation has raged as to whether such particles are created there or in distant extragalactic objects. The 50 year-old mystery has been solved using cosmic particles of mean energy of 2 Joules recorded with the largest cosmic-ray observatory ever built, the Pierre Auger Observatory in Argentina. It is found that at these energies the rate of arrival of cosmic rays is ~ 6% greater from one half of the sky than from the opposite one, with the excess lying 120 ̊ away from the Galactic centre.
In the view of Professor Karl-Heinz Kampert (University of Wuppertal), spokesperson for the Auger Collaboration, which involves over 400 scientists from 18 countries, "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way". Professor Alan Watson (University of Leeds), emeritus spokesperson, considers this result to be "one of the most exciting that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago".
Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules the rate of their arrival at the top of the atmosphere is only about 1 per sq km per year, equivalent to one hitting the area of a football pitch about once per century. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere. These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner-plate, several kilometres in diameter. They contain over ten billion particles and, at the Auger Observatory, are detected through the Cherenkov light they produce in a few of 1600 detectors, each containing 12 tonnes of water, spread over 3000 km2 of Western Argentina, an area comparable to that of Rhode Island. The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events to within ~ 1 ̊.
By studying the distribution of the arrival directions of more than 30000 cosmic particles the Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations (a chance of about two in ten million), in a direction where the distribution of galaxies is relatively high. Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down. The direction of the excess points to a broad area of sky rather than to specific sources as even particles as energetic as these are deflected by a few 10s of degrees in the magnetic field of our Galaxy. The direction, however, cannot be associated with putative sources in the plane or centre of our Galaxy for any realistic configuration of theGalactic magnetic field.
Cosmic rays of even higher energy than the bulk of those used in this study exist, some even with the kinetic energy of well-struck tennis ball. As the deflections of such particles are expected to be smaller, the arrival directions should point closer to their birthplaces. These cosmic rays are even rarer and further studies are underway using them to try to pin down which extragalactic objects are the sources. Knowledge of the nature of the particles will aid this identification and work on this problem is targeted in the upgrade of the Auger Observatory to be completed in 2018.
Department members: Angela V. Olinto, Paolo Privitera
Scientific projects: Pierre Auger Observatory
Fastest spinning star confirms Indian Nobel laureate's theory
September 20, 2017
Nearly seven decades after it was predicted that rapidly spinning stars would emit polarised light, astronomers have observed the phenomenon for the first time
More than 70 years after Indian astrophysicist and Nobel laureate Subrahmanyan Chandrasekhar first predicted the emission of polarised light from the edges of stars, a team of scientists have for the first time observed rapidly rotating stars emitting polarised light.
The researchers detected polarised light from Regulus, one of the brightest stars in the night sky, which is in the constellation Leo.
The research has provided unprecedented insights into the star, allowing the scientists to determine its rate of spinning and the orientation in space of the star's spin axis, according to the study published in the journal Nature Astronomy .
"We found Regulus is rotating so quickly it is close to flying apart, with a spin rate of 96.5 per cent of the angular velocity for break-up," said study first author Daniel Cotton from University of New South Wales (UNSW), Sydney, Australia.
"It is spinning at approximately 320 km per second - equivalent to travelling from Sydney to Canberra in less than a second," Cotton added.
Chandrasekhar's prediction in 1946 prompted the development of sensitive instruments called stellar polarimeters to try and detect this effect.
Optical polarisation is a measure of the orientation of the oscillations of a light beam to its direction of travel.
In 1968, other researchers built on Chandrasekhar's work to predict that the distorted, or squashed shape, of a rapidly rotating star would lead to the emission of polarised light, but its detection has eluded astronomers until now.
For this study, the researchers used a highly sensitive piece of equipment designed and built at UNSW Sydney and attached to the Anglo-Australian Telescope at Siding Spring Observatory in western New South Wales to detect the polarised light from Regulus.
"The instrument we have built, the High Precision Polarimetric Instrument, HIPPI, is the world's most sensitive astronomical polarimeter.
Its high precision has allowed us to detect polarised light from a rapidly spinning star for the first time," Cotton said.
"We have also been able to combine this new information about Regulus with sophisticated computer models we have developed at UNSW to determine the starís inclination and rotation rate," Cotton added.
UChicago scientists detect first X-rays from mystery supernovas
August 23, 2017
UChicago News, by Louise Lerner
Exploding stars carry cloak of dense material that puzzles astronomers
Exploding stars lit the way for our understanding of the universe, but researchers are still in the dark about many of their features.
A team of scientists, including scholars from the University of Chicago, appear to have found the first X-rays coming from type Ia supernovas. Their findings are published online Aug. 23 in the Monthly Notices of the Royal Astronomical Society.
Astronomers are fond of type Ia supernovas, created when a white dwarf star in a two-star system undergoes a thermonuclear explosion, because they burn at a specific brightness. This allows scientists to calculate how far away they are from Earth, and thus to map distances in the universe. But a few years ago, scientists began to find type Ia supernovas with a strange optical signature that suggested they carried a very dense cloak of circumstellar material surrounding them.
Such dense material is normally only seen from a different type of supernova called type II, and is created when massive stars start to lose mass. The ejected mass collects around the star; then, when the star collapses, the explosion sends a shockwave hurtling at supersonic speeds into this dense material, producing a shower of X-rays. Thus we regularly see X-rays from type II supernovas, but they have never been seen from type Ia supernovas.
When the UChicago-led team studied the supernova 2012ca, recorded by the Chandra X-ray Observatory, however, they detected X-ray photons coming from the scene.
"Although other type Ia's with circumstellar material were thought to have similarly high densities based on their optical spectra, we have never before detected them with X-rays," said study co-author Vikram Dwarkadas, research associate professor in the Department of Astronomy and Astrophysics.
The amounts of X-rays they found were small -- they counted 33 photons in the first observation a year and a half after the supernova exploded, and ten in another about 200 days later -- but present.
"This certainly appears to be a Ia supernova with substantial circumstellar material, and it looks as though it's very dense," he said. "What we saw suggests a density about a million times higher what we thought was the maximum around Ia's."
It's thought that white dwarfs don't lose mass before they explode. The usual explanation for the circumstellar material is that it would have come from a companion star in the system, but the amount of mass suggested by this measurement was very large, Dwarkadas said -- far larger than one could expect from most companion stars. "Even the most massive stars do not have such high mass-loss rates on a regular basis," he said. "This once again raises the question of how exactly these strange supernovas form."
"If it's truly a Ia, that's a very interesting development because we have no idea why it would have so much circumstellar material around it," he said.
"It is surprising what you can learn from so few photons," said lead author and Caltech graduate student Chris Bochenek; his work on the study formed his undergraduate thesis at UChicago. 'With only tens of them, we were able to infer that the dense gas around the supernova is likely clumpy or in a disk."
More studies to look for X-rays, and even radio waves coming off these anomalies, could open a new window to understanding such supernovas and how they form, the authors said.
Citation: "X-ray Emission from SN 2012ca: A Type Ia-CSM Supernova Explosion in a Dense Surrounding Medium." Bochenek et al, Monthly Notices of the Royal Astronomical Society. Aug. 23, 2017. https://doi.org/10.1093/mnras/stx2029
Funding: NASA, National Science Foundation, TABASGO Foundation, Miller Institute for Basic Research in Science, Christopher R. Redlich Fund.
Department members: Vikram Dwarkadas
2018 APS Medal for Exceptional Achievement in Research Awarded to Eugene Parker
August 17, 2017
APS News, by David Voss
Astrophysicist made fundamental contributions to solar and space plasma physics
The APS Council Steering Committee has voted to award the Societyís 2018 Medal for Exceptional Achievement in Research to Eugene Parker, professor emeritus at the University of Chicago. Parker, 90, is recognized for his "many fundamental contributions to space physics, plasma physics, solar physics and astrophysics for over 60 years."
"Eugene N. Parker is the Dean of the field of space and astrophysical plasma physics," commented Louis Lanzerotti of the New Jersey Institute of Technology. "Parker's seminal theoretical work beginning in the mid-1950s revolutionized understanding of the solar corona and its production of the interplanetary medium, and the effects of the medium on Earth's space environment."
Parker's theory predicted that the interplanetary magnetic field would be locked into the coronal plasma and would exhibit a spiral shape as the solar wind carried it into the region known as the heliosphere.
"There are very few scientists in the history of science of whom it can be said that they were responsible for the establishment of an entire scientific discipline," said Lennard Fisk of the University of Michigan. "In the late 1950s, as a young untenured professor at the University of Chicago, Gene Parker wrote his seminal paper on the acceleration of the solar wind, predicting that it would be a supersonic flow. This work was ridiculed by more senior, well-established astrophysicists." Parker's prediction was confirmed by the Mariner 2 spacecraft in 1962 and by the Voyager missions.
"Gene Parker has a wonderful and exceptional record of seminal contributions over the many years of his distinguished career," said Roger Falcone, chair of the 2018 APS Medal selection committee. "It is remarkable to see so many effects that bear his name."
"Focusing on our nearest star, Gene has taken on the incredibly difficult task of elucidating many of its complexities and has provided the world with new and better understanding of the sun," added APS Chief Executive Officer Kate Kirby.
Eugene Parker received his B.S. degree from Michigan State University in 1948 and his Ph.D. from the California Institute of Technology in 1951. After an assistant professorship at the University of Utah, he joined the faculty of the University of Chicago in 1957. Since then, he has published numerous seminal papers in solar magnetohydrodynamics, cosmic ray physics, and space plasma physics.
The APS Medal for Exceptional Research Achievement was initiated in 2016. The first medal was awarded to Edward Witten of the Institute for Advanced Study and the 2017 medal was awarded to Daniel Kleppner of the Massachusetts Institute of Technology. The medal, together with a prize of $50,000 will be presented to Parker at a special ceremony in Washington, DC, on February 1, 2018. Parker will also present a plenary lecture describing his award-winning work at the 2018 APS April Meeting.
Department members: Eugene N. Parker
Eclipse reflects sun's historic power
August 16, 2017
Eclipses have fascinated people since the earliest days of recorded history.
These rare astronomical events have helped explain the world around us -- from ancient Mesopotamia, where they were believed to foretell the deaths of kings, all the way to the 20th century, when they helped prove Einstein's theory of general relativity.
Such interest hasn't dimmed. People across the United States will have an opportunity on Aug. 21 to witness the first total solar eclipse from coast to coast in 99 years. UChicago faculty and students are among the hordes of enthusiasts traveling across the country toward the area of "totality," the 70-mile-wide stripe stretching from Oregon to South Carolina in which the moon will fully block the sun.
Ahead of this historic event, UChicago News asked scholars in fields ranging from theoretical cosmology to Islamic studies to discuss eclipses and their power.
The eclipse that proved Einstein was right
Michael Turner, Bruce V. & Diana M. Rauner Distinguished Service Professor in Physics
"Astronomers have learned a lot from eclipses, including one in 1919 that proved Einstein was right.
At the time, only a handful of people were aware of general relativity; Sir Arthur Eddington was one of them. He led an eclipse expedition into the Atlantic to find out whether gravity would bend starlight, as predicted by general relativity. What you want to do is look at stars very close to the sun, and see whether the light coming toward us is bent by the sun's gravity. With the moon blocking the sun, you can get that measurement, and it was exactly what Einstein predicted. The scientific community was agog. It instantly put general relativity on the map, and made Einstein a rockstar.
We're still learning things from eclipses. One thing people will study during this event is the corona of the sun, which is the glowing aura of gases that surrounds the sun. There are still things we don't understand about it -- such as exactly why it actually burns hundreds of times hotter than the surface of the sun itself.
A few years from now, NASA will launch a probe named after UChicago's own Eugene Parker that will explore the sun's corona -- closer than any probe has ever come to the sun."
Department members: Eugene N. Parker, Michael S. Turner
Prof. Jacob Bean has been awarded two NASA grants for the FINESSE and ARIEL/CASE missions
August 11, 2017
NASA has selected six astrophysics Explorers Program proposals for concept studies. The proposed missions would study gamma-ray and X-ray emissions from clusters of galaxies and neutron star systems, as well as infrared emissions from galaxies in the early universe and atmospheres of exoplanets, which are planets outside of our solar system.
The selected proposals, three Medium-Class Explorers missions and three Explorers Missions of Opportunity, call for focused scientific investigations and developments of instruments that fill the scientific gaps between the agency's larger missions.
"The Explorers Program brings out some of the most creative ideas for missions to help unravel the mysteries of the universe," said Thomas Zurbuchen, associate administrator of the agency's Science Mission Directorate in Washington. "The program has resulted in great missions that have returned transformational science, and these selections promise to continue that tradition."
The proposals were selected based on potential science value and feasibility of development plans. After concept studies and detailed evaluations, one of each mission type will be selected by 2019 to proceed with construction and launch. The earliest launch date would be in 2022. Medium-Class Explorer mission costs are capped at $250 million each, excluding the launch vehicle, and Mission of Opportunity costs are capped at $70 million each.
Each astrophysics Medium-Class Explorer mission will receive $2 million to conduct a nine-month mission concept study.
Department members: Jacob L. Bean
Hubble pushed beyond limits to spot clumps of new stars in distant galaxy
August 10, 2017
When it comes to the distant universe, even the keen vision of NASA's Hubble Space Telescope can only go so far. Teasing out finer details requires clever thinking and a little help from a cosmic alignment with a gravitational lens.
By applying a new computational analysis to a galaxy magnified by a gravitational lens, astronomers have obtained images ten times sharper than what Hubble could achieve on its own. The results show an edge-on disk galaxy studded with brilliant patches of newly formed stars.
"When we saw the reconstructed image we said, 'Wow, it looks like fireworks are going off everywhere,'" said astronomer Jane Rigby of NASA's Goddard Space Flight Center in Greenbelt, Md.
The galaxy in question is so far away that we see it as it appeared 11 billion years ago, only 2.7 billion years after the big bang. It is one of more than 70 strongly lensed galaxies studied by the Hubble Space Telescope, following up targets selected by the Sloan Giant Arcs Survey, which discovered hundreds of strongly lensed galaxies by searching Sloan Digital Sky Survey imaging data covering one-fourth of the sky.
"Mining the Sloan Digital Sky Survey has given us the opportunity to peer inside a distant star-forming galaxy with a sharpness of vision never before allowed," said Michael Gladders, associate professor in Astronomy and Astrophysics at the University of Chicago. "Admittedly, it's a highly distorted view -- like looking at your reflection in the famous Chicago 'Bean' -- but with some work we can and have reconstructed a detailed image of the distant galaxy."
The galaxy cluster shown here was discovered as part of the Sloan Giant Arcs Survey. It is located about 6 billion light-years from Earth and contains hundreds of galaxies
Researchers had been grappling with the gravity of a giant cluster of galaxies between the target galaxy and Earth that distorts the more distant galaxy's light, stretching it into an arc and also magnifying it almost 30 times. The team had to develop special computer code to remove the distortions caused by the gravitational lens, and reveal the distant galaxy as it would normally appear.
The resulting reconstructed image revealed two dozen clumps of newborn stars, each spanning about 200 to 300 light-years. This contradicted theories suggesting that star-forming regions in the distant, early universe were much larger: 3,000 light-years or more in size.
"There are star-forming knots as far down in size as we can see," said doctoral student Traci Johnson of the University of Michigan, lead author of two of the three papers describing the research.
Without the magnification boost of the gravitational lens, Johnson added, the disk galaxy would appear perfectly smooth and unremarkable to Hubble. This would give astronomers a very different picture of where stars are forming.
While Hubble highlighted new stars within the lensed galaxy, NASA's James Webb Space Telescope will uncover older, redder stars that formed even earlier in the galaxy's history. It will also peer through any obscuring dust within the galaxy.
"With the Webb Telescope, we'll be able to tell you what happened in this galaxy in the past, and what we missed with Hubble because of dust," said Rigby.
These findings appear in a paper published in The Astrophysical Journal Letters and two additional papers published in The Astrophysical Journal.
The Hubble Space Telescope, which is named after UChicago alumnus Edwin Hubble, is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute in Baltimore, Md., conducts Hubble science operations. The institute is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
Department members: Michael D. Gladders
Scientific projects: Sloan Digital Sky Survey
Dark Energy Survey reveals most accurate measurement of dark matter structure in the universe
August 3, 2017
Imagine planting a single seed and, with great precision, being able to predict the exact height of the tree that grows from it. Now imagine traveling to the future and snapping photographic proof that you were right.
If you think of the seed as the early universe, and the tree as the universe the way it looks now, you have an idea of what the Dark Energy Survey (DES) collaboration has just done. In a presentation today at the American Physical Society Division of Particles and Fields meeting at the U.S. Department of Energy's (DOE) Fermi National Accelerator Laboratory, DES scientists will unveil the most accurate measurement ever made of the present large-scale structure of the universe.
Department members: Scott Dodelson, Joshua A. Frieman
Scientific projects: Dark Energy Survey
Dark Energy Survey reveals most precise measure of universe's structure
August 3, 2017
Result supports view that dark matter, dark energy make up most of cosmos
Imagine planting a single seed and, with great precision, being able to predict the exact height of the tree that grows from it. Now imagine traveling to the future and snapping photographic proof that you were right.
If you think of the seed as the early universe, and the tree as the universe the way it looks now, you have an idea of what the international Dark Energy Survey collaboration has just done. Scientists unveiled their most accurate measurement of the present large-scale structure of the universe at a meeting Aug. 3 at the University of Chicago-affiliated Fermi National Accelerator Laboratory. UChicago, Argonne and Fermilab scientists are members of international Dark Energy Survey collaboration.
These measurements of the amount and "clumpiness" (or distribution) of dark matter in the present-day cosmos were made with a precision that, for the first time, rivals that of inferences from the early universe by the European Space Agency's orbiting Planck observatory. The new Dark Energy Survey result (the tree, in the above metaphor) is close to "forecasts" made from the Planck measurements of the distant past (the seed), allowing scientists to understand more about the ways the universe has evolved over 14 billion years.
"This result is beyond exciting," said Fermilab's Scott Dodelson, a professor in the Department of Astronomy and Astrophysics at UChicago and one of the lead scientists on this result, which was announced at the American Physical Society Division of Particles and Fields meeting. "For the first time, we're able to see the current structure of the universe with the same clarity that we can see its infancy, and we can follow the threads from one to the other, confirming many predictions along the way."
Most notably, this result supports the theory that 26 percent of the universe is in the form of mysterious dark matter and that space is filled with an also-unseen dark energy, which makes up 70 percent and is causing the accelerating expansion of the universe.
Paradoxically, it is easier to measure the large-scale clumpiness of the universe in the distant past than it is to measure it today. In the first 400,000 years following the Big Bang, the universe was filled with a glowing gas, the light from which survives to this day. The Planck observatory's map of this cosmic microwave background radiation gives us a snapshot of the universe at that very early time. Since then, the gravity of dark matter has pulled mass together and made the universe clumpier over time. But dark energy has been fighting back, pushing matter apart. Using the Planck map as a start, cosmologists can calculate precisely how this battle plays out over 14 billion years.
"These first major cosmology results are a tribute to the many people who have worked on the project since it began 14 years ago," said Dark Energy Survey Director Josh Frieman, a scientist at Fermilab and a professor in the Department of Astronomy and Astrophysics at UChicago. "It was an exciting moment when we unveiled the results to ourselves just last month, after carrying out a 'blind' analysis to avoid being influenced by our prejudices."
The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Its primary instrument is the 570-megapixel Dark Energy Camera, one of the most powerful in existence, which is able to capture digital images of light from galaxies eight billion light years from Earth. The camera was built and tested at Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation's four-meter Blanco telescope, part of the Cerro Tololo Inter-American Observatory in Chile. The DES data are processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
Scientists are using the camera to map an eighth of the sky in unprecedented detail over five years. The fifth year of observation will begin this month. The new results draw only from data collected during the survey's first year, which covers one-thirtieth of the sky.
Scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light years, using a technique called gravitational lensing.
To make these ultra-precise measurements, the team developed new ways to detect the tiny lensing distortions of galaxy images - an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals. In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn. The new dark matter map is ten times the size of the one that the Dark Energy Survey released in 2015 and will eventually be three times larger than it is now.
"The Dark Energy Survey has already delivered some remarkable discoveries and measurements, and they have barely scratched the surface of their data," said Fermilab Director Nigel Lockyer. "Today's world-leading results point forward to the great strides DES will make toward understanding dark energy in the coming years."
Department members: Scott Dodelson, Joshua A. Frieman
Scientific projects: Dark Energy Survey