Department in the News
Scientists seize rare chance to watch faraway star system evolve
March 4, 2020
Young stars are surrounded by dense disks of gas and dust the raw materials for creating planets. Over time, the disk scatters and disappears, making new planets visible to outside observers.
Credit: NASA/JPL-Caltech
Click on the image to enlarge
Phys.org, by Sherry Landow
At only 1% the age of the sun, the DS Tuc binary system shows us how a planet might naturally develop before its orbit is disturbed by external forces.

A young planet located 150 light-years away has given UNSW Sydney astrophysicists a rare chance to study a planetary system in the making.

"Finding young planets is challenging. We really need to understand the behavior of the parent star to be able to find the shallow signals of these planets which can be overwhelmed by starspots and flares," says Adina Feinstein, a National Science Foundation Graduate Research Fellow at the University of Chicago and co-author of the study.

Related:
Department students: Adina Feinstein

Leftover Big Bang light helps calculate how massive faraway galaxies are
March 3, 2020
The South Pole Telescope and Dark Energy Camera provided key data for scientists to create a new method to weigh galaxy clusters.

Photo by Daniel Michalik
Click on the image to enlarge
UChicago News
Fermilab, UChicago scientists tap South Pole Telescope data to shed light on universe

A team of scientists have demonstrated how to "weigh" galaxy clusters using light from the earliest moments of the universe - a new method that could help shed light on dark matter, dark energy and other mysteries of the cosmos, such as how the universe formed.

The new method calculates the bending of light around galaxy clusters using the orientation of light from shortly after the Big Bang - data taken by the South Pole Telescope and the Dark Energy Camera.

"Gravitational lensing," a phenomenon in which light distorts as it's affected by the gravity of big objects like galaxies, can function as a kind of magnifying glass. It's helped scientists discover key information about the universe - but it's always been done by looking for the smearing of light around distant objects like stars.

In a study published in Physical Review Letters, Fermilab and University of Chicago scientist Brad Benson and colleagues use a different method to calculate the masses of distant galaxies: the polarization, or orientation, of the light left over from the moments after the Big Bang.

"Making this estimate is important because most of the mass of galaxy clusters isn't even visible - it's dark matter, which does not emit light but interacts through gravity and makes up about 85% of the matter in our universe," said Benson, an assistant professor in the Department of Astronomy and Astrophysics. "Since photons from the cosmic microwave background have literally traveled across the entire observable universe, this method has the potential to more accurately measure the dark matter mass in the most distant galaxy clusters."

Related:
Department members: Bradford A. Benson, John E. Carlstrom
Scientific projects: South Pole Telescope

New Solar Telescope Reveals Sun's Surface in More Detail than Ever Before
February 13, 2020
WTTW News, by Paul Caine
A new solar telescope in Hawaii has captured images of the sun unlike any seen before. Professor Robert Rosner, an astrophysicist at the University of Chicago and one of the lead investigators on the project, says he's been waiting for almost 40 years to see images like the ones recently captured.
"Those of us fascinated by the sun - and who would not be? - have been waiting since the 1980s to see the sun at high resolution, to see if the amazing structuring of the surface, from the grand sunspots down to the 'salt and pepper' smaller magnetic structure, continues to yet smaller spatial scales. And it does!" said Rosner. "The physics to be mined from this result, these remarkable images, are just so exciting."

Related:
Department members: Robert Rosner

Prof. Eugene Parker wins prestigious Crafoord Prize in Astronomy
January 31, 2020
Prof. Emeritus Eugene Parker, 92, laid the foundations for a new field of astrophysics with his research on stars and magnetic fields.
Photo by Jean Lachat
Click on the image to enlarge
UChicago News, by Louise Lerner
Swedish Royal Academy of Sciences honors UChicago scientist's pioneering work.

The Royal Swedish Academy of Sciences announced Jan. 30 that University of Chicago Prof. Emeritus Eugene Parker has been awarded the 2020 Crafoord Prize in Astronomy.

Awarded every three years, the prestigious Crafoord Prize consists of a gold medal and a sum of six million Swedish krona (about $600,000) - one of the largest prizes in science.

The Academy, which is also responsible for selecting Nobel Prize winners, cited Parker for his "pioneering and fundamental studies of the solar wind and magnetic fields from stellar to galactic scales."

Parker is widely known for his proposal of the solar wind, which radically changed scientists' understanding of the solar system. He suggested, and spacecraft later confirmed, that the sun radiates an intense stream of charged particles that travel throughout the solar system at supersonic speeds. The discovery reshaped our view of space, stars and their surroundings, and it laid the foundations for a new field of astrophysics.

"I am humbled by the award of the Crafoord Prize," said Parker, now 92. "It gives strong incentive for maintaining the historic high level of research that merits the prize."

Related:
Department members: Eugene N. Parker

NASA's TESS spacecraft discovers its first habitable planet, first world with two stars
January 9, 2020
An illustration of what the climate of the newly discovered exoplanet TOI 700 d might look like.
Illustration courtesy NASA Goddard Space Flight Center/Chris Smith (USRA)
Click on the image to enlarge
UChicago News
Teams including UChicago scientists analyze Earth-size world and Tatooine-like planet

Scientists from the University of Chicago --- Emily Gilbert, Adina Feinstein, Daniel Fabrycky and Benjamin Montet --- and other institutions around the world have discovered multiple new interesting worlds beyond Earth --- including its first potentially habitable Earth-size world and another that is a 'Star Wars'-type system with two suns.

The new exoplanets --- planets beyond our solar system - were spotted with NASA's new TESS satellite and announced Jan. 6 at the 235th American Astronomical Society meeting. In both cases, high school students helped find them.

Related:
Department members: Daniel Fabrycky, Benjamin Montet
Department students: Adina Feinstein, Emily Gilbert

What will the next decade bring in science?
January 2, 2020
NASA, ESA, the Hubble Heritage Team (STScI/AURA) and J. Green (University of Colorado, Boulder)
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UChicago News
Daniel Holz, astrophysicist

What do you think might be the most exciting result of scientific or technological advances in the next decade?

Our gravitational wave detectors are getting more and more sensitive, so we'll learn a ton about the universe as we get more data. Then there's a few roll-of-the-dice things that could happen that would be amazing. Like if a supernova went off in our own galaxy --- with all the ways we have to detect astronomical events now, that would be an incredible way to learn about everything from the physics of stars to the history of the universe. Those happen about once every hundred years, so it could be anytime. Or maybe something unusual will pop up in our gravitational wave detectors --- we've picked up black hole collisions and neutron star collisions, but what if we detected waves from cosmic strings?

The most exciting thing is always something you haven't anticipated. In astronomy, whenever we've invented a new way to look at the sky, we discover something new that no one had ever thought of before. Our gravitational wave detectors haven't discovered anything profoundly unexpected, at least not yet.

What's a possible consequence of science or technology in the next decade that you worry about?

Two things that worry me about the future are nuclear annihilation and climate change --- both due to technological advancements. I'm a member of the Bulletin of the Atomic Scientists, which sets the Doomsday Clock, so that's something we think about a lot.

The next 10 years will be critical for climate. We can impact just how bad things will get decades from now. If we act now, we might avoid some of the worst, civilization-threatening outcomes. The danger of nuclear annihilation is on the rise, too. These are compounded by the deliberate erosion of facts and truth, which pose grave threats to society.

The number of ways in which we walk blithely into Armageddon is very high. But that's something all of us can help address. Agitate for change! It's not too late.

Related:
Department members: Daniel E. Holz
Scientific projects: Laser Interferometer Gravitational-wave Observatory

Parker Solar Probes first discoveries: Odd phenomena in space weather, solar wind
December 5, 2019
Prof. Emeritus Eugene Parker
UChicago News
NASA mission named for pioneering UChicago scientist produces landmark research

Last summer, NASA's Parker Solar Probe split the predawn skies in a blaze of light as it headed closer to the sun than any other spacecraft. Named for pioneering University of Chicago astrophysicist Eugene Parker, the probe has now made three of its 24 planned passes through the sun's corona - enough for scientists to announce their first discoveries.

In four papers published Dec. 4 in Nature, researchers describe strange space phenomena and a flood of new data that will help us understand everything from the nature of stars to improving our forecasting of solar storms that can affect electronics on Earth.

Sidling up to the nearest star that humans can reach, the Parker Solar Probe learned new information about two types of major space weather events. It also saw the first signs of the zone around the sun where cosmic dust disappears - predicted decades ago, but never seen - as well as an entirely new phenomenon: bizarre "switchbacks" in the solar wind that flows off the surface of the sun. Scientists said it will dramatically change our theories of the corona and solar wind.

Scientists are eager to learn more about the the solar wind - a flow of charged particles off the surface of the sun which radically affects the Earth and the entire solar system, which University of Chicago Professor Emeritus Eugene Parker first proposed in 1958. NASA named the solar mission after the famed astrophysicist in 2017.

Related:
Department members: Eugene N. Parker

Astronomers Propose a Novel Method of Finding Atmospheres on Rocky Worlds
December 2, 2019
This artist's impression shows a rocky exoplanet with a wispy, cloudy atmosphere orbiting a red dwarf star. Astronomers have identified a new method that could allow Webb to detect an exoplanet's atmosphere in just a few hours of observing time.
Credits: L. Hustak and J. Olmsted (STScI)
Click on the image to enlarge
NASA News
When NASA's James Webb Space Telescope launches in 2021, one of its most anticipated contributions to astronomy will be the study of exoplanets - planets orbiting distant stars. Among the most pressing questions in exoplanet science is: Can a small, rocky exoplanet orbiting close to a red dwarf star hold onto an atmosphere?

In a series of four papers in the Astrophysical Journal, a team of astronomers proposes a new method of using Webb to determine whether a rocky exoplanet has an atmosphere. The technique, which involves measuring the planet's temperature as it passes behind its star and then comes back into view, is significantly faster than more traditional methods of atmospheric detection like transmission spectroscopy.

"We find that Webb could easily infer the presence or absence of an atmosphere around a dozen known rocky exoplanets with less than 10 hours of observing time per planet," said Jacob Bean of the University of Chicago, a co-author on three of the papers.

Astronomers are particularly interested in exoplanets orbiting red dwarf stars for a number of reasons. These stars, which are smaller and cooler than the Sun, are the most common type of star in our galaxy. Also, because a red dwarf is small, a planet passing in front of it will appear to block a larger fraction of the star's light than if the star were larger, like our Sun. This makes the planet orbiting a red dwarf easier to detect through this "transit" technique.

Red dwarfs also produce a lot less heat than our Sun, so to enjoy habitable temperatures, a planet would need to orbit quite close to a red dwarf star. In fact, to be in the habitable zone - the area around the star where liquid water could exist on a planet's surface - the planet has to orbit much closer to the star than Mercury is to the Sun. As a result, it will transit the star more frequently, making repeated observations easier.

But a planet orbiting so close to a red dwarf is subjected to harsh conditions. Young red dwarfs are very active, blasting out huge flares and plasma eruptions. The star also emits a strong wind of charged particles. All of these effects could potentially scour away a planet's atmosphere, leaving behind a bare rock.

"Atmospheric loss is the number one existential threat to the habitability of planets," said Bean.

Related:
Department members: Jacob L. Bean

NASA Instrument to Probe Planet Clouds on European Mission
November 11, 2019
Click on the image to enlarge
NASA
NASA will contribute an instrument to a European space mission that will explore the atmospheres of hundreds of planets orbiting stars beyond our Sun, or exoplanets, for the first time.

The instrument, called the Contribution to ARIEL Spectroscopy of Exoplanets, or CASE, adds scientific capabilities to ESA's (the European Space Agency's) Atmospheric Remote-sensing Infrared Exoplanet Large-survey, or ARIEL, mission.

Prof. Jacob Bean is the science team lead for the new CASE mission.

Related:
Department members: Jacob L. Bean

Fermilab and University of Chicago scientist Josh Frieman awarded $1 million by DOE Office of Science
October 18, 2019
Josh Frieman
Photo: Brandon Schulman
Fermilab, by Leah Hesla
The Department of Energy has awarded Fermilab and University of Chicago scientist Josh Frieman $1 million over three years as part of the inaugural Office of Science Distinguished Scientist Fellowship program.

Office of Science distinguished scientist fellows were chosen from nominations submitted by nine U.S. national laboratories. Frieman is one of only five scientists selected, chosen for his scientific leadership, engagement with the academic research community, scientific excellence and significant scientific achievement.

The Distinguished Scientist Fellowship was established to develop, sustain and promote excellence in Office of Science research through collaborations between institutions of higher education and national laboratories.

Frieman says he will use the funding to support his cosmic research program and to foster tighter connections in cosmic frontier research between Fermilab and the University of Chicago.

Related:
Department members: Joshua A. Frieman

Nearly a decade in the making, exoplanet-hunting instrument installed in Hawaii
October 2, 2019
MAROON-X team members and Gemini Observatory staff standing in front of the Gemini North telescope with the MAROON-X unit. (From left): Paul McBride, John Randrup, Rody Kawaihae, Harlan Uehara, and Eduardo Tapia of Gemini Observatory; MAROON-X team members Andreas Seifahrt, David Kasper and Julian Stuermer; as well as Alison Peck, and John White of Gemini Observatory.
Image courtesy of Andreas Seifahrt
Click on the image to enlarge
UChicago News, by Louise Lerner
Built by UChicago scientists, MAROON-X will search for worlds in other solar systems

Atop a dormant volcano in Hawaii, an extremely delicate instrument - designed to help scientists find distant worlds - is scattered across the floor in hundreds of pieces.

"Imagine trying to assemble one of those huge LEGO sets, except there's no instruction book; you've done it once before, but then you had to take it all apart and put it in little bags," said Jacob Bean, associate professor of astronomy and astrophysics at the University of Chicago. "Also you're at 14,000 feet, and when the air is that thin it impairs your judgment and thinking, and so here you are working 12-hour shifts lifting heavy things but also trying to put together a delicate instrument."

This was Bean's task as the head of a UChicago project to build and install an innovative instrument that will scan the skies for new exoplanets - worlds in other solar systems that could potentially host life. Over the past eight years, Bean and his team had designed and built the instrument, called MAROON-X; this summer they finally attached it to a telescope at the Gemini Observatory at the top of Mauna Kea, Hawaii.

"It has been a pretty intense six months for my team to commission this instrument," said Bean, an expert in faraway worlds whose research focuses on discovering and examining potentially habitable planets in other solar systems. "But in the next 10 years we're going to learn things about habitable worlds that we'd never known before. It's going to be really transformative."

However, we still have no confirmed Earth - like exoplanets with habitable surface conditions. The thing about Earth-like planets, which is why it is taking so long to be able to find and characterize them, is that they're extremely hard to see. Because these planets are circling around a star that is at least a million times brighter than they are, trying to look directly for them is like trying to see a lightning bug next to a lighthouse that is on the other side of the country. So scientists have to find indirect ways of finding them based on the effects they have on their stars.

MAROON-X does this by noticing the extremely tiny gravitational tug that an exoplanet (or two, or five, or seven) exerts on its star as it orbits around it. This tug causes the star to wobble just the slightest bit in its orbit. But that's enough motion to catch it.

Attached to the Gemini North telescope, MAROON-X takes all the light gathered by the 25-foot telescope and focuses it down to a spot that is the width of a human hair. Then it separates out that light into the different colors of the rainbow and reads the intensity of each band. The color of the light will change slightly as the star moves forward or back. "It's kind of like a radar gun for stars," Bean said.

Several decades ago, advances in technology allowed scientists to begin detecting the very faint signatures from planets orbiting other stars in faraway solar systems. There's been an explosion of discoveries; currently, NASA lists 4,000 confirmed exoplanets and thousands more candidates.

Related:
Department members: Jacob L. Bean, David Kasper, Andreas Seifahrt, Julian Stuermer
Scientific projects: MAROON-X: M dwarf Advanced Radial velocity Observer Of Neighboring eXoplanets

Ambitious project to map the Big Bang's afterglow earns NSF funding
September 30, 2019
The UChicago-led South Pole Telescope maps the leftover light from the earliest age of the universe.
Photo by Jason Gallicchio
Click on the image to enlarge
UChicago News, by Louise Lerner
UChicago-led proposal receives NSF funding for pioneering sky measurements

The National Science Foundation has awarded $4 million to the University of Chicago to host the development of an ambitious multi-institutional program to map the leftover light from the Big Bang in greater detail than ever before.

Called CMB-S4, the groundbreaking project will allow us to see back in time to the earliest epoch of the universe. Remnant light from this period, called the cosmic microwave background, is still visible in the skies and holds clues to many of the most pressing mysteries about the universe - from its earliest moments to how it evolved to produce the wondrous structure of galaxies, stars and planets that we see today.

"The history of the universe-and the physics that govern its evolution - are encoded in the cosmic microwave background, and rigorous, precise measurements will allows us to unlock this information and will likely lead to new discoveries," said renowned UChicago cosmologist John Carlstrom, principal investigator for this initial phase of the project and co-spokesperson of the CMB-S4 collaboration, who also holds a joint appointment at Argonne National Laboratory. "These are big, big topics in physics, many of which we don't know how to get at any other way."

The CMB-S4 project builds on the success of pioneering observations of the cosmic microwave background from telescopes located in the high Chilean Atacama plateau and at the South Pole, including the 10-meter diameter South Pole Telescope - an international collaboration led by Carlstrom for more than a decade.

But the next phase, which is projected to see first light in the late 2020s, will provide much more sensitivity - requiring new telescopes working in concert, as well as a much larger science team.

The new CMB-S4 telescopes will be deployed to the South Pole and to the Atacama Desert in Chile - two places whose cold, dry climates offer the best sky viewing on Earth. They will allow CMB-S4 to boost the precision of its map by more than an order of magnitude past the best readings we have today.

The initial $4 million from the National Science Foundation will complement funding from the U.S. Department of Energy to support the design of the project. CMB-S4 will be a massive, multi-institutional effort over the next decade, with a total cost of the order of half a billion dollars. The project will involve multiple universities as well as the Department of Energy's national laboratories. The CMB-S4 collaboration consists of 200 scientists from over 70 institutions spread across 12 countries.

This incredible precision will enable physicists to look for evidence of ancient gravitational waves - ripples in the fabric of space-time created by the Big Bang - as well as insights into the elusive question of how to tie the force of gravity and the theory of quantum mechanics together. It also may provide fodder for the search for dark matter, a mysterious substance that pervades the universe, but has yet to be detected directly.

Astronomers will also find a trove of information about stars, galaxies and clusters in the CMB-S4 data - for example, cosmic microwave background observations from the South Pole Telescope and the Chilean Atacama Cosmology Telescope have made numerous discoveries, including record-breaking massive clusters of galaxies and ancient stellar nurseries.

"The history of the universe - and the physics that govern its evolution - are encoded in the cosmic microwave background."
-Prof. John Carlstrom


The University of Chicago has a long history of working at the Amundsen-Scott South Pole Station, which is managed by the National Science Foundation. The design team will also work with Associated University, Inc., which will coordinate the CMB-S4 plans for Chile.

"We've learned an incredible amount from measurements of the cosmic microwave background, including discoveries that have pointed the way to new physics, but we have really only begun to tap the information that's encoded there," said CMB-S4 co-spokesperson Julian Borrill from DOE's Lawrence Berkeley National Laboratory. "This will be a major leap forward for the entire physics and astronomy community."

The award is being made from a new NSF program for Mid-scale Research Infrastructure. "These awards represent the first in NSF's agency-wide effort to support the mid-range infrastructure that will be invaluable to strengthening the U.S. scientific research enterprise," said Jim Ulvestad, the National Science Foundation's chief officer for research facilities. "The funded projects include an impressive collection of new design efforts and advanced instrumentation. These projects fill gaps and provide unique research capabilities for the U.S. that will engage many early-career scientists and engineers in the pursuit of groundbreaking discoveries."

Related:
Department members: John E. Carlstrom
Scientific projects: Cosmic Microwave Background Stage 4, South Pole Telescope

Eugene Parker on first year of namesake NASA mission: "It's pretty exciting stuff"
August 13, 2019
A year after the launch of Parker Solar Probe, NASA scientist Nicola Fox sits down with the mission's namesake, Prof. Emeritus Eugene Parker, to discuss their findings so far.
Click on the image to enlarge
UChicago News, by Louise Lerner
Pioneering UChicago astrophysicist looks back at historic launch of solar probe

Prof. Eugene Parker had never seen a NASA launch in person until Aug. 12, 2018 - when a spacecraft honoring the pioneering University of Chicago astrophysicist blazed through the predawn skies, beginning its ambitious mission to the sun.

Six decades after Parker wrote a groundbreaking paper that shaped how we view the solar system, the launch of Parker Solar Probe was not only historic but also emotional for the 92-year-old scientist.

"So much has gone into this launch, and then to see it all disappear slowly - fading away into the night sky, knowing it will never come back - it was a moving experience," Parker said.

In the year since, Parker has been receiving updates on the NASA mission, which has already come closer to the sun than any spacecraft. Parker recently visited with Nicky Fox, director of NASA's Heliophysics Division at NASA Headquarters in Washington, at his Hyde Park apartment and learned that the spacecraft is running perfectly - and has gathered twice as much data as originally predicted.

"Parker Solar Probe is blazing a trail as she orbits through the corona; she's behaving beautifully," said Fox. "We're actually very excited because we have more than twice as much data as we had expected."

Parker Solar Probe has circled the sun twice thus far, and on Sept. 1 will reach its closest point to the star yet.

"It's pretty exciting stuff," said Parker, who is the only living person to have a NASA mission named after them.

The spacecraft, built by Johns Hopkins Applied Physics Laboratory to withstand the intense heat and radiation coming off the sun, is a marvel of engineering. It is the fastest-moving object built by humans, traveling at more than 150,000 miles per hour. It must constantly orient itself to keep its shield - a four-and-a-half inch-thick slab of carbon composite that can withstand the 2,500-degree Fahrenheit temperatures - between itself and the star. (Except for one especially tough instrument, built by UChicago alum Justin Kasper, which peeks around the edge of the craft to scoop up particles of the solar wind).

Fascinating every step of the way
Parker Solar Probe will come closer than any other spacecraft to the sun, seeking to understand multiple mysteries about our star - such as why the corona around the sun is actually hotter than the surface of the sun itself.

"You rarely have a space mission that doesn't come up with the unexpected, and it's actually going to get more exciting as the mission goes on and crosses into regions that spacecraft have never been in before," Parker said. "It's just fascinating every step of the way."

These questions have their origins in a revolutionary 1958 paper authored by Parker, which claimed that the sun was expelling waves of particles in every direction. Though the idea was roundly rejected by the scientific community at first, we now know this phenomenon, which Parker called the solar wind, shapes the entire solar system.

"A lot of people don't realize that space is not just empty space - it's full of this constantly active plasma that is carrying the magnetic field from the sun towards us and forming a protective bubble around the solar system," Fox said. "When something happens on the sun, we feel the impact here on Earth, and it was all predicted by Eugene Parker."

Scientists also hope to use Parker Solar Probe's observations to better predict solar flares and to understand how to protect future astronauts traveling through space who would encounter solar radiation.

Early answers may lie in the 22 gigabytes of data Parker Solar Probe collected during its first two encounters with the sun, but as Parker said: "I wouldn't be surprised if the most surprising things have yet to come."

Related:
Department members: Eugene N. Parker

New measure of Hubble constant adds to mystery about universe's expansion rate
July 16, 2019
Click on the image to enlarge
UChicago News, by Louise Lerner
Prof. Wendy Freedman leads study of red giant stars for new measurement of disputed constant
University of Chicago scientists have made a new measurement of how fast the universe is expanding - using an entirely different kind of star than previous endeavors. That value falls in the center of a hotly debated question in astrophysics that may call for an entirely new model of the universe.

Scientists have known for almost a century that the universe is expanding, but the exact number for how fast it's going has remained stubbornly elusive. In 2001, Prof. Wendy Freedman led a team that used distant stars to make a landmark measurement of this number, called the Hubble constant - but it disagrees with another major measurement, and the tension between the two numbers has persisted even as each side makes more and more accurate readings.

In a new paper to be published shortly in the Astrophysical Journal, Freedman and her team announced a new measurement of the Hubble constant using a kind of star known as a red giant. Their observations, made with NASA's Hubble Space Telescope, indicate that the expansion rate for our corner of the universe is just under 70 kilometers per second per megaparsec - slightly smaller than their previous measurement, but not alleviating the tension.

"The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves," said Freedman, the John and Marion Sullivan University Professor in Astronomy and Astrophysics and a world-renowned astronomer. "The discrepancy that we saw before has not gone away, but this new evidence suggests that the jury is still out on whether there is an immediate and compelling reason to believe that there is something fundamentally flawed in our current model of the universe."

A number behind the theory of the universe
The Hubble constant, named after pioneering astronomer and UChicago alum Edwin Hubble, underpins everything in the universe - from our estimate of when the Big Bang happened to how much dark matter exists. It helps scientists sketch out a theory of the history and structure of the universe; and conversely, if there are fault lines in that theory, an accurate measurement of the Hubble constant might lead them to it.

"The Hubble constant...is one of the most direct ways we have of quantifying how the universe evolves."
- Prof. Wendy Freedman

Twenty years ago, the Hubble Space Telescope Key Project team, which Freedman led, announced it had measured the value using distant stars called Cepheids, which pulse at regular intervals. Their program concluded that the value of the Hubble constant for our universe was 72. As astronomers have refined their analyses and gathered new data, this number has remained fairly stable, at about 73.

But more recently, scientists took a very different approach: building a model based on the rippling structure of light left over from the earliest moments of the Big Bang, which is called the Cosmic Microwave Background. If they ran a model forward in time, extrapolating from the first few moments of the universe, they reached a value of 67. That disagreement is significant - nearly 10 percent - and it has continued to solidify over time.

Both camps have looked for anything that might be causing the mismatch. "Naturally, questions arise as to whether the discrepancy is coming from some aspect that astronomers don't yet understand about the stars we're measuring, or whether our cosmological model of the universe is still incomplete," Freedman said. "Or maybe both need to be improved upon."

Mapping the stars
A central part of the challenge in measuring the universe is that it is very difficult to accurately calculate distances to distant objects. Freedman's team originally looked at two types of stars that have reliable characteristics which allow astronomers to use them in combination as cosmological measuring sticks: Type Ia supernovae, which explode at a uniform brightness; and Cepheid variables, stars which pulse at regular intervals that can be matched to their peak brightnesses. But it's still possible that there is something about Cepheids that scientists don't yet fully understand, which could be introducing errors.

Freedman's team sought to check their results by establishing a new and entirely independent path to the Hubble constant using an entirely different kind of star.

Certain stars end their lives as a very luminous kind of star called a red giant. At a certain point, the star undergoes a catastrophic event called a helium flash, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity. (This will one day happen to our own sun, which will also become a red giant). Astronomers can see the point where all the luminosities of the stars drop off, and they can use this as a way to tell distance.

"Our initial thought was that if there's a problem to be resolved between the Cepheids and the Cosmic Microwave Background, then the red giant method can be the tie-breaker," said Freedman.

"The principle is simple," Freedman said. "Imagine that you are standing near a street light that you know is 10 feet away. At regular intervals down the street you can seen more street lights, which get progressively dimmer the further away that they are. Knowing how far away and how bright the lamp is beside you, and then measuring how much fainter the more distant lamps appear to be, allows you to estimate the distances to each of the other lamps all down the road."

Freedman's team put this into action using sensitive cameras on the Hubble Space Telescope, searching for their new cosmic lampposts. By comparing the apparent luminosities of the distant red giants with nearby ones that we've measured with other methods, and pairing these readings with those from Type Ia supernovae, Freedman and her team were able to determine how far away each of the host galaxies were.

The next step is straightforward: How fast that galaxy is moving away from us is the product of its distance times the Hubble constant. Luckily, a galaxy's velocity is simple to measure - the light coming from galaxies shifts depending on how fast the galaxy is moving away from us.

Their calculations gave a Hubble constant of 69.8 - straddling the two previously determined numbers.

"The red giant method is independent of the Cepheids and is incredibly precise. The stars used are of lower mass, have different evolutionary histories and are located in different regions of distant galaxies," said Taylor Hoyt, a University of Chicago graduate student and co-author on the paper.

But the results do not appear to strongly favor one answer over the other.

"We are working at the frontier of what is currently known about cosmology," Freedman concluded. "These results suggest that we do not have the final answer yet. The burden of proof is high when claims of new physics hang in the balance, but that's what makes it exciting," she said. "Either way the conflict resolves, it is important. We either confirm our standard model of cosmology, or we learn something new about the universe."

The other University of Chicago co-author was Dylan Hatt, PhD'18. Carnegie scientist Barry Madore also has a visiting appointment at UChicago. Other co-authors included scientists with the Observatories of the Carnegie Institution for Science, Princeton University, Seoul National University, Penn State, Florida Atlantic University and the Leibniz Institute for Astrophysics-Potsdam.

Related:
Department members: Wendy L. Freedman, Barry Madore
Department students: Taylor Hoyt

New Hubble Constant Measurement Adds to Mystery of Universe's Expansion Rate
July 16, 2019
Click on the image to enlarge
hubblesite.org
Red Giant Stars Used as Milepost Markers
In 1924, American astronomer Edwin Hubble announced that he discovered galaxies outside of our Milky Way by using the powerful new Hooker telescope perched above Los Angeles. By measuring the distances to these galaxies, he realized the farther away a galaxy is, the faster it appears to be receding from us. This was incontrovertible evidence the universe is uniformly expanding in all directions. This was a big surprise, even to Albert Einstein, who predicted a well-balanced, static universe. The expansion rate is the basis of the Hubble constant. It is a sought-after value because it yields clues to the origin, age, evolution, and future fate of our universe.

For nearly the past century astronomers have worked meticulously to precisely measure the Hubble constant. Before the Hubble Space Telescope was launched in 1990, the universe's age was thought to lie between 10 and 20 billion years, based on different estimates of the Hubble constant. Improving this value was one of the biggest justifications for building the Hubble telescope. This paid off in the early 1990s when a team led by Wendy Freedman of the University of Chicago greatly refined the Hubble constant value to a precision of 10%. This was possible because the Hubble telescope is so sharp at finding and measuring Cepheid variable stars as milepost markers - just as Edwin Hubble did 70 years earlier.

But astronomers strive for ever greater precision, and this requires further refining yardsticks for measuring vast intergalactic distances of billions of light-years. Freedman's latest research looks at aging red giant stars in nearby galaxies. They are also milepost markers because they all reach the same peak brightness at a critical stage of their late evolution. This can be used to calculate distances.

Freedman's research is one of several recent studies that point to a nagging discrepancy between the universe's modern expansion rate and predictions based on the universe as it was more than 13 billion years ago, as measured by the European Space Agency's Planck satellite. This latest measurement offers new evidence suggesting that there may be something fundamentally flawed in the current model of the universe.

Astronomers have made a new measurement of how fast the universe is expanding, using an entirely different kind of star than previous endeavors. The revised measurement, which comes from NASA's Hubble Space Telescope, falls in the center of a hotly debated question in astrophysics that may lead to a new interpretation of the universe's fundamental properties.

Scientists have known for almost a century that the universe is expanding, meaning the distance between galaxies across the universe is becoming ever more vast every second. But exactly how fast space is stretching, a value known as the Hubble constant, has remained stubbornly elusive.

Now, University of Chicago professor Wendy Freedman and colleagues have a new measurement for the rate of expansion in the modern universe, suggesting the space between galaxies is stretching faster than scientists would expect. Freedman's is one of several recent studies that point to a nagging discrepancy between modern expansion measurements and predictions based on the universe as it was more than 13 billion years ago, as measured by the European Space Agency's Planck satellite.

As more research points to a discrepancy between predictions and observations, scientists are considering whether they may need to come up with a new model for the underlying physics of the universe in order to explain it.

"The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves," said Freedman. "The discrepancy that we saw before has not gone away, but this new evidence suggests that the jury is still out on whether there is an immediate and compelling reason to believe that there is something fundamentally flawed in our current model of the universe."

In a new paper accepted for publication in The Astrophysical Journal, Freedman and her team announced a new measurement of the Hubble constant using a kind of star known as a red giant. Their new observations, made using Hubble, indicate that the expansion rate for the nearby universe is just under 70 kilometers per second per megaparsec (km/sec/Mpc). One parsec is equivalent to 3.26 light-years distance.

This measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported by the Hubble SH0ES (Supernovae H0 for the Equation of State) team using Cepheid variables, which are stars that pulse at regular intervals that correspond to their peak brightness. This team, led by Adam Riess of the Johns Hopkins University and Space Telescope Science Institute, Baltimore, Maryland, recently reported refining their observations to the highest precision to date for their Cepheid distance measurement technique.

How to Measure Expansion
A central challenge in measuring the universe's expansion rate is that it is very difficult to accurately calculate distances to distant objects.

In 2001, Freedman led a team that used distant stars to make a landmark measurement of the Hubble constant. The Hubble Space Telescope Key Project team measured the value using Cepheid variables as distance markers. Their program concluded that the value of the Hubble constant for our universe was 72 km/sec/Mpc.

But more recently, scientists took a very different approach: building a model based on the rippling structure of light left over from the big bang, which is called the Cosmic Microwave Background. The Planck measurements allow scientists to predict how the early universe would likely have evolved into the expansion rate astronomers can measure today. Scientists calculated a value of 67.4 km/sec/Mpc, in significant disagreement with the rate of 74.0 km/sec/Mpc measured with Cepheid stars.

Astronomers have looked for anything that might be causing the mismatch. "Naturally, questions arise as to whether the discrepancy is coming from some aspect that astronomers don't yet understand about the stars we're measuring, or whether our cosmological model of the universe is still incomplete," Freedman said. "Or maybe both need to be improved upon."

Freedman's team sought to check their results by establishing a new and entirely independent path to the Hubble constant using an entirely different kind of star.

Certain stars end their lives as a very luminous kind of star called a red giant, a stage of evolution that our own Sun will experience billions of years from now. At a certain point, the star undergoes a catastrophic event called a helium flash, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity. Astronomers can measure the apparent brightness of the red giant stars at this stage in different galaxies, and they can use this as a way to tell their distance.

The Hubble constant is calculated by comparing distance values to the apparent recessional velocity of the target galaxies - that is, how fast galaxies seem to be moving away. The team's calculations give a Hubble constant of 69.8 km/sec/Mpc - straddling the values derived by the Planck and Riess teams.

"Our initial thought was that if there's a problem to be resolved between the Cepheids and the Cosmic Microwave Background, then the red giant method can be the tie-breaker," said Freedman.

But the results do not appear to strongly favor one answer over the other say the researchers, although they align more closely with the Planck results.

NASA's upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid-2020s, will enable astronomers to better explore the value of the Hubble constant across cosmic time. WFIRST, with its Hubble-like resolution and 100 times greater view of the sky, will provide a wealth of new Type Ia supernovae, Cepheid variables, and red giant stars to fundamentally improve distance measurements to galaxies near and far.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Related:
Department members: Wendy L. Freedman, Barry Madore
Department students: Taylor Hoyt