Talks & Events
Braneworld Black Holes
Braneworlds are a fascinating way of hiding extra dimensions by confining ourselves to live on a brane. One particular model (Randall-Sundrum) has a link to string theory via living in anti de Sitter space. I'll describe how the ads/cft correspondence has been used to claim that a braneworld black hole would tell us how Hawking radiation back reacts on spacetime, thus solving one of the outstanding problems of quantum gravity - the ultimate fate of an evaporating black hole. I'll review evidence for this
conjecture, ending with some recent work that shows it may be problematic.
Charged Cosmic Rays and Dark Matter
Recent results from the PAMELA and ATIC experiments have lead us to the conclusion that highly relativistic (10-600 GeV) electrons and positrons are surprisingly ubiquitous in cosmic ray spectrum. Although the source of these unexpected particles is currently not known, an exciting possibility is that they might be the product of dark matter particles annihilating in the local halo of the Milky Way. I will discuss what it would take for dark matter to produce these signals, and the future measurements that will enable us to identify the origin of these particles once and for all.
GLAST, HESS and Beyond, Status and Future of Gamma-ray Astronomy
The field of gamma-ray astronomy has received considerable attention beyond the high-energy astrophysics community in the recent years. This is in part due to the success of Imaging Atmospheric Cherenkov Telescopes such as HESS, MAGIC and VERITAS measuring gamma-rays in the energy regime above 100 GeV. All these new facilities have lifted gamma-ray astronomy in the last few years from an exotic discipline with a handful of detected sources to a solid astronomical discipline. In addition, the recent launch of the Fermi Space Telescope (FST) and its main instrument, the Large Area Telescope (LAT) measuring gamma-rays outside Earth's atmosphere the energy range beyond 100~MeV adds to the attention and excitement. The Fermi-LAT instrument will solidify the field by detecting several thousands of new sources and by bridging the energy spectra of ground-based detected VHE gamma-ray sources to well-studies objects at X-ray energies. In this seminar I will discuss the current status of the field, as well as the potential for future observatories.
Expectations, and Probes of Intergalactic Magnetic Fields
It is likely that intergalactic magnetic fields are naturally seeded by (1) “normal” stellar processes in galaxies, and (2) by central galactic black holes. There may also be (3) a primordial component from the pre-galactic Universe, but this is probably overwhelmed by the first two processes except possibly in cosmic voids. I discuss some methods for probing intergalactic fields, and some recent results. Some are new and tentative, and serve to focus on what better data of the same kinds could be obtained with present instruments. I present a brief overview of what we know about galactic and extragalactic magnetic fields from the local universe from “here” up to z ~ 3. This includes some global a priori calculations of IGM field strengths based on known facts of galactic magnetic energy outflows. I briefly include ideas on of where some distributed UHECR acceleration sites may be found.
Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs
Suggestive evidence has accumulated that intermediate mass black holes (IMBH) exist in dwarf galactic nuclei and some globular clusters. As stars diffuse in the cluster, some will inevitable wander sufficiently close to the hole that they suffer tidal disruption. An attractive feature of the IMBH hypothesis is its potential to disrupt not only solar-type stars but also compact white dwarf stars. Attention is given to the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such extent that explosive nuclear burning may be triggered. Consistent modeling of the gas dynamics together with the nuclear reactions allows for a realistic determination of the explosive energy release. Although the explosion will increase the mass fraction escaping on hyperbolic orbits, a good fraction of the debris remains to be swallowed by the hole, causing a bright soft X-ray flare lasting for about a year. Such transient signatures, if detected, would be a compelling testimony for the presence of a moderately mass black hole.
Precision Cosmological Constraints from Optically-Selected Clusters
I will present new constraints on the matter density and clustering amplitude of matter in the Universe,
based on the largest galaxy cluster catalog available to date, the SDSS maxBCG cluster catalog. When combined with CMB data, these data improve the precision of the parameter constraints by a factor of two, with comparable accuracy to the most recent determinations from X-ray clusters. Our analysis is fully self-calibrated in the sense that we do not rely on a priori determinations of the relation between the number of galaxies in a cluster and a cluster's mass. I will discuss how this approach can be extended to the next generation of photometric surveys, focusing on the Dark Energy Survey. I will discuss which aspects of the future survey are most critical, highlighting the role of uncertainties in the mass-observable relation
and the power of well-selected followup observations to improve the dark energy constraints.
Neutrinos and the dark side of the light fermions
The past decade has been marked by some remarkable discoveries in the neutrino physics: the particles once believed to be massless have turned out to be massive and have shown evidence of lepton family number violation, as well as other interesting phenomena. While this is exciting, the future may hold even more dramatic discoveries, the hints for which begin to appear in astrophysics and cosmology. The observed neutrino masses imply the existence of some yet undiscovered "right-handed" states, which can be very massive and unreachable, but which can also be light enough to constitute the cosmological dark matter and to account for a number of astrophysical phenomena, from supernova asymmetries and the pulsar kicks to the peculiarities in the reionization and formation of the first stars. I will review the recent progress in neutrino physics, as well as the clues that may lead to future discoveries.
Ice Fishing for Cosmic Neutrinos
Cosmic rays at GZK energies produce neutrinos as a consequence of their interactions with the cosmic microwave background radiation. When these neutrinos interact in the Antarctic ice sheet, the resulting showers produce coherent Cherenkov radiation at radio frequencies via the Askaryan mechanism. A balloon instrument, the Antarctic Impulsive transient Antenna (ANITA), has flown over Antarctica twice, most recently during the past austral summer. Results from the first flight and information about the second flight will be presented.
Recent Results from IceCube
After two more construction seasons IceCube, the first cubic-kilometer
neutrino telescope, will be completed as initially planned. The instrumentation of this extremely large volume allows to measure neutrinos in the energy range from about 100 GeV up to energies larger than 10^17 eV. IceCube will reach sensitivites well below expected neutrino fluxes from astrophysical sources accelerating hadrons. A ground-based extensive air-shower, IceTop, measuring showers induced by primaries of energy between 10^15 - 10^17 eV, enriches the physics potential of this observatory at the South Pole operating standalone and in coincidence with the deep ice detector.
The current results of IceCube in incomplete configurations and the physics reach of the full detector will be discussed as well as the low energy extension DeepCore and possible high energy extensions.
A Dustier Universe Than You'd Like
Using galaxies and quasars from the Sloan Digital Sky Survey, we have extended previous work measuring the weak lensing magnification. We can now compare our measurements directly to those made using galaxy-galaxy shear lensing and find that the results agree remarkably well. Our new technique also allows us to make the first measurements of the extended dust halos associated with the galaxies in our sample, detecting a level of intergalactic dust roughly twice that expected from theoretical estimates. At a mean redshift of z ~ 0.35, we find that the dust halos of galaxies extend to Mpc scales, following a power-law density distribution and exhibiting a reddening slope equivalent to that seen in the LMC. From this we infer a smooth halo component to the dust that cannot be accounted for by radially averaging satellite galaxies and find a dust mass density for the universe roughly twice that from previous estimates. Finally, we present some preliminary estimates for the impact of this dustier universe on current and future supernova cosmology measurements.
The Radio Background from ARCADE 2
The ARDCADE 2 instrument has measured the absolute temperature at 3-90 GHz, with an open aperture cryogenic instrument observing at balloon altitudes with no windows with an in situ blackbody reference. The radio background is larger than expected with a temperature of 1.2 K at 1 GHz with an index of -2.6. Then CMB temperaure agrees with FIRAS.
Constraining Dark Energy: First Results from the SDSS-II Supernova Survey
A decade ago, two teams studying distant type Ia supernovae discovered that the expansion of the Universe is speeding up. Since then, supernova surveys from the ground and from space have brought major improvements in the quality and quantity of SN Ia data, confirming the discovery of cosmic acceleration. This talk will focus on early cosmological results from the Sloan Digital Sky Survey-II Supernova Survey, which operated for 9 months in 2005-7, discovering and measuring light curves and spectra for over 500 SNe Ia. I will discuss the observational challenges to determining supernova distances for cosmology and how those challenges are being addressed. These results inform the prospects for using supernovae to obtain improved cosmology measurements from planned and proposed surveys in the future.
Dark Stars, or the Effect of Dark Matter on the First Stars
The first stars in the universe may have been powered by dark matter heating rather than nuclear fusion. They were dark matter-powered stars, or for short Dark Stars. The annihilation of weakly interacting massive particles would provide the heating. This talk presents the story of Dark Stars: how they form, how long they might live, and what they might become at the end of their lives.
Empirical Constraints on the Formation and Evolution of Low-Mass Stars and Brown Dwarfs: A Data-Intensive Approach
Recent and ongoing large surveys, both from the ground and from space, are enabling new data-intensive approaches to a variety of problems in stellar astrophysics. This talk describes three such projects, each serving as a vignette of a different but complementary mode of data-intensive research into low-mass star formation and evolution. The X10000 Project takes a panchromatic, time-domain approach to study the structures of young stellar coronae in order to understand the role of extreme coronal mass ejections in the angular momentum evolution of young stars. SLoWPoKES takes an ensemble, data-mining approach to extract from the Sloan Digital Sky Survey the largest sample of wide low-mass binaries ever assembled, which can be used to constrain binary formation theory and for refining the fundamental mass-age-activity-rotation-metallicity relations for low-mass stars. The EB Factory project takes a time-domain, data-mining approach to identify rare, but astrophysically very interesting, case studies from among the large numbers of eclipsing binaries being harvested by surveys for transiting exoplanets. We will highlight recent discoveries from this work, and will draw these results together to elucidate the physical interrelationships between stellar rotation, magnetic field generation, and stellar structure during the star-formation process.
Early Results from the Fermi Large Area Telescope Gamma-ray Sky Survey
The Fermi Gamma-ray Space Telescope was launched in June 2008. The Large Area Telescope (LAT) instrument on Fermi is designed to study the gamma-ray sky in the energy range from 20 MeV to 300 GeV. The first year of the Fermi mission is devoted to a full sky survey with the LAT, started in August 2008. I will summarize the Fermi mission capabilities and report on the survey progress with details of some interim science results.
Astronomy from Dome A, Antarctica
The Antarctic plateau provides many exciting possibilities for astronomical observations. The next decade may see a significant astronomical buildup on the Antarctic Plateau. Dome A and Dome C are currently the two most promising sites. These high points on the plateau have unique properties for astronomical observations. Two of these arise from the extreme cold: the column density of water vapor is lower than at any other site, thus opening unique windows at infrared and submillimeter wavelengths; and the ambient temperature, and thus the thermal background emission of telescope mirrors, is lower than at any other site. Two more advantages arise from the unique character of the atmospheric turbulence: the atmospheric boundary layer is extremely thin, only tens of meters, which opens the possibility of wide field, high resolution imaging by either adaptive correction of the thin ground layer or by raising the telescope above the boundary layer; the wind speeds at all levels of the atmosphere are low, which is highly favorable for adaptive correction. It will likely be possible to form diFFraction limited images over a good fraction of the sky down to visible light wavelengths three times HST resolution for an 8 m telescope. Dome A, being the highest and coldest point in Antarctica, is especially promising based on the results of recent theoretical models and site surveys. More comprehensive site monitoring should be planned for the next decade. Based on existing data, the site has certain areas of astronomical observations can already be planned with little risks. Wide field near-IR imaging, for example, relies critically on the thermal background and the low temperature at Dome A makes it an ideal site. Another key area is likely to be exoplanet imaging and spectroscopy in the L-band, where the combination of super-diFFraction limited AO correction and the very low thermal background will enable very high contrast imaging at very close inner working angle, for example 0.15 arcsec for an 8 m telescope.
Cosmology and Astrophysics with Galaxy Clusters
Clusters of galaxies are unique probes of cosmology and astrophysics, promising to provide new insights into both the nature of dark energy and dark matter and the physics of galaxy formation. One of the key challenges facing this approach lies in our understanding of cluster physics and their impact on cluster structure and evolution. In this talk, I will review recent development of theoretical and computational modeling of galaxy cluster formation, with focus on thermodynamic of intracluster medium. Numerical simulations including gas cooling and star formation reproduce global properties of the intracluster medium (ICM) and observable-mass relations with the accuracy of ~10%. I will show that non-thermal processes, such as turbulence, cosmic-rays, and ICM plasma physics, are the dominant sources of systematic uncertainties in the current cluster mass estimate. I will discuss the future prospect for improving our understanding of cluster astrophysics and cosmological constraints with upcoming large-scale cluster surveys.
Lessons from the Nearby Universe about Star Formation in Galaxies
The study of the relation between gas and star formation in galaxies is a matter of great current interest, and a crucial piece of information in our understanding of galaxy evolution. In order to determine how primordial density fluctuations become observable structures in the present day universe, it is crucial to characterize the processes that drive star formation on galactic scales. In turn, I will argue, this requires considering the creation of the self-gravitating cold molecular phase of the interstellar medium: Giant Molecular Clouds (GMCs).
In this talk I will present multi-scale measurements of the relation between gas and star formation in galaxies (the "star formation law"), and discuss our current knowledge of this relation. I will also discuss our new results on dust formation by supernovae, frequently assumed to be the major dust creation pathway in the early universe. The properties of GMCs, the major reservoirs of star-forming gas, play a key role in setting the initial conditions for the formation of stars. I will discuss the results from a comprehensive study of the resolved GMC properties in a number of extragalactic systems, including both normal and dwarf galaxies, and I will contrast the results of the virial and high-resolution far-infrared studies and what they tell use about molecular clouds in primitive galaxies and the relation between gas and the formation of stars.
Radio detection of extensive air showers
The origin of cosmic rays, among them being the highest energy particles in the Universe is one of the main open questions in Astroparticle Physics. Air showers are induced by interactions of high-energy cosmic rays in the atmosphere. Secondary electrons (and positrons) are deflected in the Earth's magnetic field and emit synchrotron radiation. This radio emission is detected by dipole antennas. The technique has been pioneered by the LOPES experiment, operated in coincidence with the KASCADE-Grande air shower experiment.
Radio emission of ultra high-energy cosmic particles offers a number of interesting advantages. Since radio waves suffer no attenuation, radio measurements allow the detection of very distant or highly inclined showers, can be used day and night, and provide a calorimetric measure of the electromagnetic shower component.
The LOPES experiment has detected the radio emission from cosmic rays, confirmed the geosynchrotron effect for extensive air showers, and provided a good calibration formula to convert the radio signal into primary particle energy. Future steps will be the installation of a 20 km^2 radio antenna field at the Pierre Auger Observatory to measure the composition of cosmic rays in the energy region between 10^17 and 10^18 eV. In this region a transition from galactic to extra-galactic origin of the particles is expected. Prototype studies are presently conducted on site in Argentina. Future activities also include the use of the LOFAR radio telescope as cosmic-ray detector. The LOFAR telescope is presently constructed in the Netherlands and in Europe as a digital radio interferometer.
The present status of the activities is reviewed and the perspectives will be discussed. It is expected that the radio technique will develop into a mature and independent method to detect ultra high-energy cosmic rays within the next decade.
Strong Lensing by Optically-Selected Galaxy Clusters
Gravitational lensing by galaxy clusters was predicted in the 1930s, and finally discovered in 1986. Since these initial discoveries, several dozen significant cluster lenses have been discovered in a variety of ways.
Lensing clusters probe the distribution of massive haloes in the universe; the expected arc production frequency can be predicted from simulations and compared to existing data. Massive lensing clusters act as 'natural telescopes', providing highly magnified images of background sources which cannot otherwise be studied using the current generation of telescopes.
The details of the observed lensing in clusters also probes the internal properties of these massive haloes. Most cluster strong lens studies to date have been rather limited by the small number and heterogeneous nature of the sample of known lenses (most of which are one-off discoveries). I will report on efforts to take the study of strong lensing clusters to a new statistical regime, by identifying and studying two new samples of strong lenses within large catalogs of optically selected galaxy clusters from the RCS-2 and SDSS surveys. In total we expect to find hundreds of new giant arcs. These efforts are now approximately half-complete; in this mid-course report I will describe some of the successes of these studies, and the remaining challenges. Time permitting, I will also discuss a recently commissioned instrument at the Magellan telescopes which was designed specifically for studying these new lens samples.
Strong Lensing by Optically-Selected Galaxy Clusters
Gravitational lensing by galaxy clusters was predicted in the 1930s, and finally discovered in 1980s. In the two decades following the initial discovery, several dozen significant cluster lenses were found, though only a handful of these have been studied extensively. Lensing clusters probe the distribution of massive halos in the universe; the expected arc production frequency can be predicted from simulations and compared to existing data. Massive lensing clusters act as 'natural telescopes', providing highly magnified images of background sources which cannot otherwise be studied using the current generation of telescopes. The details of the observed lensing in clusters also probes the internal properties of these massive halos. Most cluster strong lens studies to date have been limited by the small number and heterogeneous nature of the sample of known lenses (most of which are one-off discoveries). I will report on efforts to take the study of strong lensing clusters to a new statistical regime, by identifying and studying two new samples of strong lenses within large catalogs of optically selected galaxy clusters from the RCS-2 and SDSS surveys; in total we have found hundreds of new giant arcs. These efforts are now approximately three-quarters-complete; in this progress report I will describe some of the spectacular successes of these studies, and the remaining challenges.
X-ray Cluster Cosmology
X-ray observations of galaxy clusters provide powerful cosmological constraints via two independent methods. The first uses measurements of the baryonic mass fraction in the largest, dynamically relaxed clusters. This method, like type Ia supernovae studies, measures distance as a function of redshift and traces the acceleration of the Universe directly. It also provides a tight constraint on the mean matter density. The second method uses the observed evolution of the cluster mass function. It leads to tight constraints on the amplitude of mass fluctuations and powerful, complementary constraints on dark energy. I will present the latest results from our team's work using both methods, employing a rigorous, self-consistent approach that accounts for survey biases, captures fully the important degeneracies between parameters and includes conservative allowances for systematic uncertainties. I will place the results in context with other current experiments and highlight the prospects for improvements in the near-to-mid term with the incorporation of new SZ, optical and X-ray data and improved hydrodynamical simulations.
The latest results from the Pierre Auger Observatory
Since the first detection of a cosmic ray event with energy above 1020 eV in 1962, their nature and origin remain unknown. Due to the extreme rarity of these ultra high energy cosmic rays, they must be observed indirectly through the observation of extensive air showers, and the lack of knowledge of hadronic interactions at these energies leads to inherent difficulties in characterizing the properties of the primary particle.
A new generation cosmic ray detector, the Pierre Auger Observatory, has been designed to study cosmic rays with energy above 1018 eV and answer the crucial questions of ultra high energy cosmic ray physics. The Southern Observatory in Argentina has been collecting data since January 2004, and its exposure is larger than that of any other cosmic ray experiment. Among the first results from the Pierre Auger Collaboration are the most precise measurement of the suppression of the cosmic ray flux at the highest energies, the first anisotropy result above 6 x 1019 eV, the first photon limit with a fluorescence detector, and the best neutrino limit at EeV energies. Then, after five years of operation, a good question is, "What is left to be done?" In this colloquium, I will describe the Pierre Auger Observatory in its astrophysical context, our most recent results, and the exciting prospects for the near future.
Gravitational Waves: A New Window onto the Universe
Over the next decade or so, the gravitational-wave window onto the Universe will be opened in four frequency bands that span 22 orders of magnitude: The high-frequency band, 10 to 10,000 Hz (ground-based interferometers such as LIGO), the low-frequency band, 10^-5 to 0.1 Hz (the space-based interferometer LISA), the very-low-frequency band, 10^-9 to 10^-7 Hz (pulsar timing arrays), and the extremely-low-frequency band, 10^-18 to 10^-16 Hz (polarization of the cosmic microwave background). This lecture will describe these four bands, the detectors that are being developed to explore them, and what we are likely to learn about black holes, neutron stars, white dwarfs and early-universe exotica from these detectors' observations.
[I will focus largely on LIGO and LISA but, unless you advise otherwise, I think it useful to include PTAs and CMB polarization as well.]
"Living on the Edges: Modeling Formation of the Smallest and Largest Luminous Systems in the Universe"
The current Cold Dark Matter paradigm of structure formation in the universe has proven its mettle in numerous stringent tests against observations over the last three decades. Nevertheless, many aspects of the theory related to the physics of baryonic component of galaxies and galaxy clusters remain relatively poorly understood and are therefore subject to continuous rigorous testing.
In this talk, I will focus on formation of the smallest luminous galaxies (some of which contain only a few hundred stars) and the largest virialized systems - galaxy clusters. These systems occupy extremes of the mass range of collapsed objects and are interesting from both astrophysical and cosmological standpoints. The faintest dwarf galaxies give us a window into the process of star formation and stellar feedback in extremely low density and low metallicity environments and, at the same time, provide constraints on the properties of dark matter particles. Clusters of galaxies are excellent laboratories for studying details of galaxy formation and interaction of galaxies with surrounding gas and, at the same time, can be used as sensitive probes of cosmological parameters. I will review some of the recent research developments in modeling these systems and will discuss implications both for our understanding of galaxy formation and for cosmology.
Exoplanets and their Odd Orbital Orientations
In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees. What about planets around other stars? Recently we have measured the orbital orientations (relative to their parent stars' equators) of more than a dozen different exoplanets, using a technique first theorized in the 19th century. Many systems have good alignment, as in the Solar system -- but there are a few surprises. I will discuss these results and their implications for theories of planet formation and migration.
Ettore Majorana meets his shadow
Seventy years have elapsed since Majorana's bold hypothesis about the neutrinos, and almost the same time since he dissappeared in misterious circunstances, a mistery yet as unresolved as the true nature of Neutrino. Probably we will never know what happened to the physicist but neutrinoless double deta decay experiments may unravel if the neutrino is its own antiparticle. I will offer a brief review of the satus of the field and the most promising techniques for the NEXT generation of experiments.
Before the Big Bang
This talk will explore the idea that our universe existed before the big bang, as an alternative to the standard big bang/inflationary model. I will show how a phase of slow contraction before the big bang can explain the observed degree of flatness and homogeneity of our universe, as well as generate a nearly scale-invariant spectrum of primordial density perturbations. I will contrast the predictions of this model with those of inflationary cosmology, and discuss observational prospects for distinguishing between the two scenarios.
Reionization and Cosmology with 21 cm Fluctuations
Searching for 'Missing Baryons' in the Warm-Hot Intergalactic Medium with High Resolution X-ray Spectroscopy
Baryons make up 4.6% of matter and energy content of the universe, but roughly half the baryons in the current epoch have not yet been observed. Simulations suggest that the missing matter is primarily in a "cosmic web" of hot, low-density plasma that traces the large-scale structure of the universe. At temperatures above 106 K, the most promising signature of this so-called WHIM is the resonance line of He-like oxygen, O VII He α, around 20 Å. This line can be observed in absorption against background X-ray point sources with the high resolution spectrometers on the Chandra and XMM-Newton observatories (an X-ray analog to the Ly α forrest). The observations, however, are challenging and results have been controversial. This talk will review evidence for (and against) the detection of the WHIM, including a recently confirmed observation of an X-ray absorption line associated with the large-scale galaxy enhancement in the direction of Sculptor.