Talks & Events
KICP Friday Noon Seminars: 2007
The Cosmological State of String Theory
After a brief introduction to the ingredients of string theory and in particular duality between gauge theory and gravity, I will describe some of the challenges faced in attempts to apply string theory to cosmology.
String theory is our best candidate for a theory of the physics at very short distances, but is very much a theory in search of an observable application. Conversely, scalar field theories can successfully describe the phenomenology of inflation or dark energy, but have proven difficult to embed into a realistic theory of short distances. However, the branes arising in string theory can have strong implications for the naturalness problems which obstruct this embedding. This is interesting in itself, since short distance physics (like string theory) normally decouples from long-distance phenomena (like cosmology). But can cosmology and string theory help answer each other's problems? This talk describes recent progress in bringing these ideas together.
Constraining Inverse Curvature Gravity with Supernovae
We show that the current accelerated expansion of the Universe can be explained without resorting to dark energy. Models of generalized modified gravity, with inverse powers of the curvature can have late time accelerating attractors without conflicting with solar system experiments. We have solved the Friedman equations for the full dynamical range of the evolution of the Universe. This allows us to perform a detailed analysis of Supernovae data in the context of such models that results in an excellent fit. Hence, inverse curvature gravity models represent an example of phenomenologically viable models in which the current acceleration of the Universe is driven by curvature instead of dark energy. If we further include constraints on the current expansion rate of the Universe from the Hubble Space Telescope and on the age of the Universe from globular clusters, we obtain that the matter content of the Universe is 0.07 <= omega_m <= 0.21 (95% Confidence). Hence the inverse curvature gravity models considered can not explain the dynamics of the Universe just with a baryonic matter component.
VERITAS: Status and Plans
VERITAS, the Very Energetic Radiation Imaging Telescope Array System, is a major new ground-based observatory for studying nonthermal astrophysics in the gamma-ray band above 100 GeV.
Stereo observations with the first two of four telescopes began in January, 2006 at the Fred Lawrence Whipple Observatory in southern Arizona, USA. Construction and commissioning of the remainder of the array has taken place during the Summer and Fall of 2006, in parallel with engineering and science observations by the first two telescopes. We discuss the performance of the VERITAS telescopes during this commissioning period and discuss the key science program planned for VERITAS during the first two years of routine array operation, beginning in January 2007.
This program includes a sky survey in the galactic plane, observations of SNRs and PWNe, studies of TeV-emitting AGN, and a search for dark matter.
Accretion Onto the Nearest AGN, Sagittarius A*
Sagittarius A* is the source associated with the 3.5x10^6 solar mass black hole at the center of our galaxy; its extremely low luminosity
(10^-9 L_Edd) is a challenge for models accretion physics. The linear polarization of Sgr A*, observable above ~100 GHz, originates just outside the event horizon and provides unique clues about the conditions in the inner accretion flow. Using the Submillimeter Array (SMA) and a multi-frequency polarimeter built for these observations, we have greatly improved the frequency coverage and sensitivity available for polarimetric studies of this source. We have made the first measurement of Faraday rotation in this source, which restricts the accretion rate to the range 2x10^-7 to 2x10^-9 Msun/yr. We are also able to detect polarization variability on timescales as short as minutes, indicating rapid changes in the inner accretion flow. In particular, one interesting type of variation may enable future measurements of the black hole spin. Finally, we have observed a flaring event in Sgr A* across seven decades in frequency through coordinated X- ray/IR/submillimeter observations. The surprising spectral and temporal properties of the flare provide strong constraints on the mechanism responsible for these events.
The Space Interferometer Mission: Parallaxes, Planets and More
The Space Interferometry Mission PlanetQuest (SIM) is a 9-m Michelson interferometer operating in the visible band with an anticipated accuracy of 4 microarcseconds (all sky, wide angle) and precision of 1 microarcseconds (narrow angle). These two modes will enable making fundamental advances in Galactic astronomy, determine the matter makeup of our Galaxy and the Local Cluster, study dark matter halos, decisively define the cosmic distance scale, and undertake comprehensive search for planets down to the mass of Earth.
Astrophysical Constraints on BH-NS and NS-NS Mergers and the Short GRB Redshift Distribution
Gravitational-wave detectors are expected to observe binary mergers in the near future, not merely providing opportunities to test general relativity itself but also, through measurements of merger rates, providing substantial constraints on our understanding of compact binary formation and evolution. These observations may agree with existing observational constraints, allowing us to refine our models for binary evolution, or they may disagree, forcing us to extend those models. In this talk we lay the groundwork for comparison: we describe the range of merger rates expected from state-of-the-art population synthesis models for the Milky Way; we summarize existing observational constraints in the Milky Way; and we describe how constraints improve our understanding of binary evolution, using existing (electromagnetic) and expected future (gravitational-wave)observations. However, because long delays can occur between binary birth and merger and because most star formation occurred long ago, binaries born long ago in old elliptical galaxies can also contribute significantly to the present-day merger rate. Using recent results on the cosmological census and star formation history, we summarize the presently plausible range of LIGO detection rates. Though these additional uncertainties complicate astrophysical interpretations of LIGO detections, we suggest that additional observations of short GRBs and of merging binary parameter distributions can reduce the associated ambiguity and allow gravitational-wave observations to significantly constrain binary evolution.
As an example, we describe the range of short GRB redshift distributions, detection rates, and host galaxy associations expected from star formation in a heterogeneous universe, as predicted from concrete population synthesis calculations, the star formation rate and cosmological census, and a good fit to the GRB luminosity function. We demonstrate that several plausible models can fit observations, and we interpret the models which best reproduce the observed short GRB detection rate and redshift distribution.
Shedding Light on Dark Matter: Seeing the Invisible with Gravitational Lensing
The cluster of galaxies 1E0657-56 has been the subject of intense ongoing research in the last few years. This system is remarkably well-suited to addressing outstanding issues in both cosmology and fundamental physics. It is one of the hottest and most luminous X-ray clusters known and is unique in being a major supersonic cluster merger occurring nearly in the plane of the sky, earning it the nickname, "the Bullet Cluster". In this talk I will present our measurements of the composition of this system, show the evidence for existence of dark matter, and describe limits that can be placed on the intrinsic properties of dark matter particles. In addition, I will explain how this cluster offers a serious challenge to MOdified Newtonian Dynamics (MOND) theories.
Dark Energy Constraints from the ESSENCE Supernova Survey
The ESSENCE survey is an ongoing six year effort to use high-redshift (0.2 < z < 0.8) supernovae to probe the expansion rate of the universe and constrain w to 10%. From analysis of 60 supernovae from the first four years of ESSENCE data, in conjunction with constraints from baryonic acoustic oscillations and geometric flatness, we obtain a value for a constant equation of state parameter of w=-1.05 +- 0.13 (stat) +- 0.11 (sys). I will also present preliminary constraints on the evolution of the equation of state parameter, as well as on other more exotic dark energy models. While the current constraints are dominated by statistical error, as the sample grows, our ability to constrain dark energy will depend on mastering systematic uncertainties, such as the treatment of extinction in supernova host galaxies. I will explain some of the challenges in understanding sources of systematic error for current and future supernova surveys.
Astronomy & Astrophysics Colloquium
What's the Trouble with the Anthropic Principle?
Anthropic arguments based on selection effects for observers have been claimed to successfully explain the measured value of the cosmological constant.
In this talk I review the foundation of such claims in the context of probability theory and show that different (and equally legitimate) ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, I discuss a weighting scheme based on the total number of possible observations that observers can carry out over the entire lifetime of the Universe. I show that this leads to an extremely small probability for observing a value of the cosmological constant equal to or greater than what we now measure, in marked contrast with the usual result.
I also discuss principles of consistent probabilistic reasoning, showing that the anthropic principle as applied in most of the literature is logically inconsistent. I conclude that current implementations of the anthropic principle display a worrisome lack of predictivity, and cannot be used to explain the value of the cosmological constant, nor, likely, any other physical parameters.
Ultrahigh Energy Cosmic Neutrinos
Ultrahigh energy (UHE) astrophysical neutrinos that originate in interactions of cosmic rays with the microwave background radiation, or from astrophysical sources such as Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRBs), provide a unique way of studying astrophysics as well as particle physics. I will discuss what we can learn from UHE cosmic neutrinos. I will show that the effect of neutrino oscillations is production of tau neutrinos which can provide an enhanced signal for the detection of cosmic neutrinos. Furthermore, interactions of UHE cosmic neutrinos could potentially lead to the production of microscopic black holes predicted in theories of extra dimensions, or they may produce supersymmetric charged particles that travel large distances, such as sleptons. I will discuss these processes and their signals in neutrino detectors such as Auger, Anita, IceCube, EUSO and OWL.
Exploring the High Energy Universe: GLAST Mission and Science
The Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch in late 2007, is a satellite based observatory to study the high energy gamma-ray sky. There are two instruments on GLAST: the Large Area Telescope (LAT) which provides coverage from 20 MeV to over 300 GeV, and the GLAST Burst Monitor (GBM) which provides observations of transients from 8 keV to 30 MeV. GLAST will provide well beyond those achieved by the highly successful EGRET instrument on the Compton Gamma-Ray Observatory, with dramatic improvements in sensitivity, angular resolution and energy range. The very large field of view will make it possible to observe ~20% of the sky at any instant, and the entire sky on timescales of a few hours. In addition to the science opportunities, this talk describes the design and expected performance of the instruments, the opportunities for guest investigators, and the mission status.
Mega-masers, Hubble Constant and Dark Energy
In contrast to laboratory environments, cosmic conditions allow water maser emission to arise naturally. Powerful water maser emission (water mega-masers) can be found in accretion disks in the nuclei of some galaxies. Besides providing a measure of the mass at the nucleus, such mega-masers can be used to determine the distance to the host galaxy, based on a kinematic model. Such mega=masers are useful for determining the Hubble Constant, independent of the traditional approach based on Cepheid variables. We will explain the importance of determining the Hubble Constant to high accuracy for constraining the equation of state of Dark Energy and describe the Mega-maser Cosmology Project that has the goal of determining the Hubble Constant to better than 3%. Time permitting, we will also present the scientific capabilities of the current and future NRAO facilities for addressing key astrophysical problems.
Imprints of Tachyonic Preheating on the CMB
We have recently proven that the fluctuations of a tachyonic field, which arise at the end of hybrid or brane-antibrane inflation, can act as a source of density perturbations at second order in cosmological perturbation theory. This typically results in a very blue (n=4) contamination of the power spectrum at small scales, as well as nongaussianities. The effect gives rise to powerful new constraints on the parameter space of hybrid-like inflation models, as well as the possibility of new features in the power spectrum.
Kinematics of the Ultra-Faint Milky Way Dwarf Galaxy Satellites
In the past year alone, the number of satellite galaxies around the Milky Way has doubled. These newly discovered dwarf galaxies have surface brightnesses and luminosities that are an order of magnitude lower than any previously known galaxy. I present Keck/DEIMOS spectroscopy for eight of these new objects. All are highly dark matter-dominated with mass-to-light ratios of several hundred. The measured velocity dispersions of these galaxies are inversely correlated with their luminosity, indicating that a minimum mass for luminous galactic systems has not yet been reached. I will discuss the importance of these dwarf galaxies in a cosmological context.
Is the low l CMB cosmic?
The Cosmic Microwave Background Radiation is our most important source of information about the early universe. Many of its features are in good agreement with the predictions of the so-called standard model of cosmology -- the Lambda Cold Dark Matter Inflationary Big Bang. However, the large-angle correlations in the microwave background exhibit several statistically significant anomalies compared to the predictions of the standard model. Not only is there a lack of large angle correlations, but the lowest multipoles seem to be correlated with each other, rather than statistically independent. Indeed, they also seem to be correlated with the geometry of the solar system, suggesting that what little power there is on large scales is locally not cosmologically produced.
Determining the Nature of Dark Matter with Astrometry
The dwarf spheroidal satellite population of the Milky Way provides an ideal laboratory for studying structure formation on small scales and testing the nature of dark matter. Observationally, their proximity allows for kinematic studies of individual stars. Theoretically, their small masses make them ideal candidates for prominent warm dark matter cores. I will discuss how the present data is able to shed light on the 'missing satellites problem' in cold dark matter, and how future astrometric data will reveal the presence of dark matter cores or cusps in these systems.
Searching for Stochastic Gravitational Wave Background with LIGO: Results and Implications
The Laser Interferometer Gravitational-wave Observatory (LIGO) has built three multi-km scale interferometers, designed to search for gravitational waves (GW). One of the targets for these searches is the stochastic GW background, whose existence is expected both due to cosmological and due to astrophysical sources. We discuss the status of LIGO, the most recent results of the search for stochastic GW radiation with LIGO interferometers, and the implications of these results for some of the theoretical models of stochastic GW background.
ALPACA: A Uniquely Powerful & Inexpensive Deep Imaging Survey
ALPACA (Advanced Liquid-mirror Probe of Astrophysics, Cosmology and Asteroids) is an 8m-class telescope planned for Cerro Tololo, Chile which will survey a roughly 1000 square degree patch of sky to superlative depth, half of the field imaged each night. This can be done at extremely low cost yet provide uniquely sensitive data on supernovae, active galactic nuclei, variable stars, asteroids and many other classes of objects. ALPACA takes advantage of recent advances in liquid mirror technology, which we will also discuss.
Cosmological perturbations have been traditionally assumed to be initially Gaussian. The main reason for this assumption is a simplicity criterion which ascribes the coherent structures in the universe today to the non-linear action of gravitational instability. A theoretical basis for this simplicity assumption comes from single-field slow-roll models of inflation, which predict that curvature perturbations were generated by quantum vacuum fluctuations of a scalar field. These perturbations are almost Gaussian as a consequence of the flatness of the inflaton potential, non-linearities (non-Gaussianities) being suppressed by the smallness of the inflaton self-coupling. However, a large variety of alternative models has been proposed during the last ten years, which generally predict that larger deviations from Gaussianity are possible.
One of the most important goals of modern cosmology has then become that of trying to either detect or constrain primordial Non-Gaussianity (NG). I will describe how theoretical predictions of NG and the search for primordial NG signatures in cosmological datasets has become a new and very promising way to discriminate among different models for the origin of primordial perturbations.
Millimeter Wavelength Exploration of Submillimeter Galaxies
Submillimeter galaxies, first detected by the SCUBA instrument in 1998, are thought to be host to some of the most prolific bursts of star formation in the early universe. Notoriously difficult detect at mm-wavelengths and even more difficult to follow-up with other facilities, a comprehensive understanding of this population has remained elusive over the past decade. In this talk I will describe an ongoing program to create and exploit new catalogs of SMGs to provide a new accounting of the star formation history of the early universe. Our work is centered on imaging made with the AzTEC mm-wavelength camera on three telescopes: the JCMT, the ASTE telescope, and the future Large Millimeter Telescope (LMT). I will describe a collection of survey results from the JCMT and ASTE telescopes and conclude with the exciting prospects of first light with AzTEC on the LMT in 2009.
What can we do with CMB as a backlight?
The Cosmic Microwave Background has been providing us with a wealth of information for cosmology. Can we learn more from the CMB apart from its primary anisotropies?
Here, we are going to present the next frontiers of the CMB: the Integrated Sachs-Wolfe Effect, Weak Lensing of the CMB and Kinetic Sunyaev-Zeldovich effect.
In order to understand the gravitational potential of the universe, we make use of the ISW effect and weak lensing of CMB via the following
datasets: WMAP, 2MASS, SDSS photometric LRGs and Quasars and NVSS. We perform a joint analysis of all samples, allowing a reliable covariance matrix to be constructed including all the cross-correlations of different samples, which is necessary for joint cosmological parameter fitting.
We will present our methodologies, analysis and cosmological results.
To understand the evolution of electron density of the universe, we utilize the kinetic SZ signature of the CMB. This method is very promising with the upcoming surveys such as SPT. ACT and APEX, as long as we have a galaxy survey such as DES, ADEPT or Panstarrs available.
We will present our prediction for ACT cross ADEPT correlation and results of the application of this method to the WMAP and SDSS main spectroscopic galaxy sample as a trial example.
Testing Dark Matter Forces with Tidal Streams
Satellite galaxies can be tidally disrupted as they orbit a more massive host galaxy. If dark matter (DM) experiences a stronger self-attraction than baryons, stars will preferentially gain rather than lose energy during tidal disruption leading to an enhancement in the trailing compared to the leading tidal stream. The Sgr dwarf galaxy, a satellite of our own Milky Way, is seen to have roughly equal streams, challenging models in which DM and baryons accelerate differently by more than 10%. Future observations and a better understanding of DM distribution should allow detection of DM forces only a few percent the strength of gravity.
BONUS: Binary Black Hole Merger: Symmetry and the Spin Expansion
We regard binary black hole (BBH) merger as a map from a simple initial state (two well separated Kerr black holes) to a simple final state (a single Kerr black hole with a recoil). By Taylor expanding this map around the spinless case and systematically applying symmetry constraints, we obtain a formalism that is simple, yet remarkably successful at explaining existing BBH simulations.
A New Picture of Star Formation in Field Galaxies since z~1
The "All Wavelength Extended Groth Strip Survey" (AEGIS) has recently provided a first comprehensive picture of star formation in field galaxies out to z>1.
(1) Our data showed that star formation histories are a strong function of galaxy mass.
(2) Star formation evolved predominantly gradually declining, not through an evoloving role of starbursts, placing a tight constraint on the role of major mergers in the evolution of star formation rates.
(3) The observed mass dependencies of star formation histories imply not only a slower decline, but also a systematically later onset, of star formation in less massive galaxies. These mass dependencies jointly generate the observed "downsizing" phenomena.
I present a quantitative model that parametrizes the evolution of star formation as a function of mass and redshift. This picture of "normal" star formation provides a testbed for current theoretical concepts of star formation and baryon physics and, for the first time, a baseline against which we can measure the effect of additional processes, quenching or mergers, as a function of galaxy mass and z.
Probing dark energy with X-ray galaxy clusters studies
Most of the energy density of the Universe is in the form of dark matter and dark energy, and yet these two components are the most intriguing mysteries in current cosmology. Using two complementary X-ray galaxy clusters studies we present new constraints on the mean matter density of the Universe, dark energy density, normalization of the density fluctuation power spectrum, and dark energy equation of state parameter. First, using Chandra measurements of the X-ray gas mass fraction in 42 hot, X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05
The Least Luminous Galaxies in the Universe - A Nearby Window to Cosmology
Since 2005, nearly 20 dwarf galaxies have been discovered around the Milky Way and M31 that are 100 times less luminous than any galaxy previously known. These discoveries are changing our understanding of galaxy formation at the lowest luminosities, and are also currently our most direct tracers of the properties of dark matter on small scales. I will present the properties of these new discoveries as a population, such as the spectroscopic observations that appear to confirm their residence within dark matter halos and the imaging that will ultimately reveal their star formation histories. I will also examine the properties of the Willman 1 object in detail to critically assess whether we can functionally distinguish globular clusters from dwarf galaxies at the extremely low luminosities at which future surveys may reveal an abundance of objects. I will discuss the implications of all these results in a cosmological plus galaxy formation context, including resolutions to the 'substructure problem' and predictions for the possible gamma-ray fluxes from annihilating dark matter particles in these tiny galaxies.
High-redshift Galaxy Groups and Clusters in DEEP2 and Beyond
Groups and clusters of galaxies, as the largest, most recently formed objects in the universe, carry a great deal of information about the recent history of the cosmos. By studying these systems at a variety of epochs, it is possible to place strong constraints on theories of galaxy formation and on cosmological parameters. With the recent completion of the DEEP2 Galaxy Redshift Survey at z~1, we can now perform detailed studies of galaxy groups and clusters over a wider redshift range than ever before. In this talk I will present evidence that DEEP2 galaxy groups had only recently become suitable environments for shutting off star formation in galaxies. In addition, I will show results suggesting that DEEP2 groups are underluminous in the X-ray band, compared to local systems. I will also discuss an ongoing effort to constrain cosmological parameters by comparing the abundance of groups in DEEP2 to the abundance in the local universe. In particular, I will describe the primary challenges to performing this test reliably, and I will present a realistic plan for overcoming them. Finally, I will briefly mention some important implications of the DEEP2 results for future attempts to constrain cosmology with optical cluster surveys.
Population III stars and the formation of the first protogalaxies
I use the cosmological adaptive mesh refinement code Enzo to do a suite of high-resolution numerical simulations of Population III protostellar clouds in a cosmological context. These calculations examine the formation of primordial protostellar clouds at a range of redshifts and in differing cosmic "neighborhoods." I find that these cores have a wide variety of accretion rates - varying by over two orders of magnitude - which may have significant implications for the IMF of Population III stars. I then simulate supernovae from the inferred stellar mass range and follow the evolution of the ensuing supernova remnant until the deposition of metal-enriched gas in the next generation of halos, which generally occurs ~50 million years after the original supernova. The dense gas in the core of these "child" halos is typically enriched to metallicities of ~10^-3 solar, which is above the "critical metallicity" at which metal line cooling dominates over molecular cooling, and suggests that the stars in these halos will have a significantly lower mass range than their Population III parents. This metal enrichment is a local phenomenon, and the transition of the universe from primordial to metal-enriched gas will be quite extended.
Constraining early universe relics with radio astronomy
I will discuss recent work on using future radio astronomy observations to constrain populations of early universe relics. In the first half, I will present a method for constraining the existence of cosmic strings with gravitational lensing surveys. Past radio surveys have already ruled out a portion of the cosmic string parameter space, and future radio telescope arrays such as LOFAR and SKA have the potential to outperform current constraints from pulsar timing and the CMB by up to two orders of magnitude. In the second half, I will discuss the effect of evaporating primordial black holes on the ionization history of the universe, with an emphasis on the limits derivable from future 21-cm observations of high-redshift neutral hydrogen. In addition to giving us insight into the history of structure formation in the universe, 21cm surveys will also allow us to search for exotic physics in the Dark Ages.