5 January 2007	2 February 2007	9 March 2007
12 January 2007	16 February 2007	16 March 2007
19 January 2007	23 February 2007
26 January 2007	2 March 2007

Emil Martinec, The University of Chicago

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.

Cliff Burgess, McMaster University/Perimeter Institute

String/Brane 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.

Jochen Weller, University College London

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.

Brian Humensky, The University of Chicago

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.

Dan Marrone, The University of Chicago

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.

Shri Kulkarni, California Institute of Technology

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.

Richard O'Shaughnessy, Northwestern University

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.

Marusa Bradac, KIPAC

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.

Gajus Miknaitis, Fermilab

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.

Steve Shectman, Carnegie

Astronomy & Astrophysics Colloquium  

10 January 2007	7 February 2007	14 March 2007
31 January 2007	28 February 2007

Maxim Markevitch, Harvard-Smithsonian Center for Astrophysics

Proof of dark matter existence and other results from a collision of galaxy clusters  

Collisions of galaxy clusters let us study the properties of normal and dark matter that are inaccessible by other means. I will review recent results obtained using the merging cluster 1E0657-56. A combination of a long Chandra X-ray observation with accurate weak and strong gravitational lensing maps has provided the first direct, model-independent proof of the dark matter existence (as opposed to the modified gravity paradigm), and a direct constraint on self-interaction cross-section of the dark matter particles. This cluster also exhibits a rare example of a shock front in the intergalactic gas. Its X-ray observations can be used for interesting physical tests, such as determining the electron-ion equilibration timescale in magnetized astrophysical plasmas.

Michael Rauch, Carnegie Observatories

The Structure of the Gaseous Cosmic Web  

Over the past decade, observational and theoretical studies of the general intergalactic medium (IGM) have transformed our understanding of the material universe at high redshift (z>2). The IGM has been recognized as the dominant baryonic component of a large scale cosmic web. It is the matrix from which galaxies are forming, and at the same time provides galaxies with a sink of metal-enriched gas, energy and radiation. The astrophysical simplicity of the IGM and its strong evolution over time have made it an excellent laboratory for studying the baryon contents, initial density fluctuations, temperature, enrichment history, and the ionizing background of the universe. I'll describe some new aspects of the IGM that have recently come to light, using QSO absorption lines seen in the spectra of lensed background QSOs. In particular, we will see how it is possible to study the kinematics of the cosmic web, as a function of density and spatial scale. I shall further discuss the possible impact of galactic winds on the properties of the IGM, and will report briefly on some recent results that may require a revision in our current understanding of the course of reionization.

Frank C. van den Bosch, Max-Planck Institute for Astronomy

The Galaxy-Dark Matter Connection  

Understanding how galaxies are biased with respect to the dark matter is of crucial importance if we are to use observations of large scale structure in order to constrain cosmological parameters. In this talk I present a powerful, statistical method that links galaxies to their dark matter haloes, and which allows us to completely specify their bias. I present applications of this method to both the 2dFGRS and the SDSS, and discuss implications for both cosmology and galaxy formation. I will also present a new method to identify galaxy groups in redshift surveys. Applying this method to the 2dFGRS and SDSS yields the largest galaxy group catalogues constructed to date, which put tight constraints on the galaxy-dark matter connection.

Alexey Vikhlinin, Harvard-Smithsonian Center for Astrophysics

Dark Energy Constraints from Growth of Structure: Results from the 400 deg² X-ray Cluster Survey  

The growth of matter density perturbation is a dark energy probe complementary to the geometrical cosmological tests provided, e.g., by SN Ia or baryonic acoustic oscillations in the galaxy power spectrum. Evolution of the mass function of galaxy clusters is a sensitive measure of growth of structure and hence, dark energy. The cluster sample used in this work is selected in X-ray using ROSAT pointed observation, covering 400 deg^2 of the extragalactic sky. The high-quality X-ray data later obtained with Chandra provide accurate total mass estimates in individual cluster. This sample provides the most precise at present determination of the evolution in the cluster mass function. The dark energy equation of state parameter, w, is constrained to +-0.2 by clusters alone; to +-0.1 by combination of the cluster and CMB data; to +- 0.07 by combining CMB, SN Ia, BAO, and clusters.

Dimitrios Psaltis, University of Arizona

New Tests of Strong-Field General Relativity with Black Holes and Neutron Stars  

In contrast to gravity in the weak-field regime, which has been subjected to numerous experimental tests, gravity in the strong-field regime is largely unconstrained by experiments. Indeed, a large class of gravity theories can be constructed that obey the Einstein equivalence principle and cannot be rejected by solar system tests, but that diverge from general relativity in the strong-field regime. I show that such theories predict black holes and neutron stars with significantly different properties than their general relativistic counterparts. I then discuss how recent observations with current telescopes have provided interesting new constraints on scalar-tensor and braneworld gravity models that are comparable to solar-system and table-top experiments.

3 January 2007	24 January 2007	21 February 2007
17 January 2007	14 February 2007	7 March 2007

Fausto Cattaneo, The University of Chicago

Some Recent Developments in the Theory of MHD Turbulence  

There are many situations of astrophysical interest, like the solar wind or the interstellar medium, in which the turbulent motions of an electrically conducting fluid are affected by a background magnetic field. The resulting coupled system, which goes under the name of magneto-hydrodynamic (MHD) turbulence,has been the subject of study for over forty years. There are two competing theories of MHD turbulence, following Kolmogorov, both describe an energy cascade process from large to small scales, but their phenomenological underpinning and their predictions are drastically different. The first, developed independently by Iroshnikov and Kreichnan in the sixties, is based on the assumption that the energy cascade is isotropic at all scales. It predicts that the nonlinear interactions weaken at small scales and that the slope of the energy spectrum in the inertial range is -3/2. By contrast, the second theory, developed jointly by Goldreich and Shreidar in the nineties, assumes that the energy cascade is anisotropic, and that the degree of anisotropy increases at small scales. It assumes further that the nonlinear interactions remain strong at all scales and that the slope of the energy spectrum in the inertial range is -5/3 in the directions transverse to the local magnetic field. Interestingly, numerical simulations over the last decade, rather than clarifying the issues, have produced a picture in which the energy cascade is clearly anisotropic, in agreement with the Goldreich-Shridar phenomenology, but the slope of the energy spectrum in the inertial range is close to -3/2 as predicted by the Iroshnikov-Kreichnan theory. In my talk I will show how the numerical results and the Goldreich-Shridar theory can be reconciled by the introduction of the idea of dynamical alignment. According to this picture, the polarization vectors of the velocity and magnetic field fluctuations become increasingly more aligned at small scales. I will show that this tendency is associated with the conservation of cross-helicity in the ideal problem, and that one of the consequences is that the nonlinear interactions are depleted so that the slope of the energy spectrum agrees with the observed results.

Mitchell Begelman, JILA, University of Colorado

The First Supermassive Black Holes  

The existence of a supermassive black hole in nearly every galactic nucleus is no longer in doubt, but the question of how these black holes formed is wide open. I will argue that they could have formed via the direct infall and collapse of gas in pregalactic haloes at redshifts ~10-20, without the intermediate stage of Pop III star formation. Global gravitational instabilities get rid of excess angular momentum, and the infalling gas forms a self-gravitating, optically thick structure - a "quasistar". As matter piles on, the core of the quasistar heats up until it undergoes runaway neutrino cooling and collapses to form a 10 solar mass black hole. The black hole then grows by accreting from the quasistar at an extremely super-Eddington rate, reaching thousands of solar masses in less than a million years. Concurrently, the quasistar expands to form a radiation pressure-dominated, convective envelope reminiscent of a red giant. I will discuss the structure and evolution of quasistars and their detectability with the James Webb Space Telescope.

Edwin A. Bergin, University of Michigan

The Fossil History of the Solar System: Links to Interstellar Chemistry  

In a few key instances the chemical composition of meteorites and comets provides a fossil record of the physical conditions present at their creation. In this talk I will explore lines of evidence that link the chemistry of the solar nebula to that seen in the interstellar medium today. †In particular, I will present results that address two long-standing questions in solar system chemistry: (1) the overall nitrogen deficiency in comets (when compared to carbon and oxygen) and† (2) the origin of oxygen isotopic anomalies seen in primitive meteorites. I will show how an answer to these questions has implications for the origin of other anomalies seen in comets and meteorites, but also implies the birth of the Sun in a large star cluster.

S. R. ("Shri") Kulkarni, Caltech

An Explosion of Cosmic Explosions  

Only about a hundred years ago astronomers came to recognize cosmic explosive events. What was once termed as Stella Nova are now divided into two major families, novae and supernovae (with real distinct classes in each). The speaker will first summarize new developments in the field of cosmic explosions (namely gamma-ray bursts) and proceed to speculate on (and announce) new classes of cosmic explosions. Such a discussion is timely given the imminent commissioning of wide-angle optical surveys for transient sources (e.g. SkyMapper, PS1, LSST).

Anatoly Spitkovsky, Princeton University

How Collisionless Shocks Work (And How They Don't)  

Collisionless shocks are ubiquitous in astrophysics. Besides their primary role of terminating supersonic flows, collisionless shocks are commonly inferred to accelerate nonthermal particles and cosmic rays,and to generate significant magnetic fields. How and if this actually happens remains largely a realm of speculations. I will present the results of a systematic study of relativistic collisionless shocks through ab-initio particle-in-cell simulations, focusing on the basic physics that mediates a shock. I will show how shock properties depend on magnetization and composition of the flow and describe which parameter regimes are conducive to particle acceleration. In particular, I will show the first evidence for self-consistent Fermi particle acceleration in simulations, and address the issue of the electron-ion temperature ratio in relativistic shocks. These simulations begin to place constraints on the composition and magnetization of relativistic outflows in astrophysics.

Lynne Hillenbrand, Caltech

From Protostars to Planets to Debris Around Young Suns  

This talk will summarize our basic understanding of the evolutionary time scales associated with Circumstellar material. I will cover both the dissipation of primordial dust and gas, which is associated with the formation of planets, and the generation and evolution of secondary or debris dust, which is the most readily apparent astronomical signature of our own mature planetary system.

16 January 2007

17 January 2007

22 January 2007

Amol Upadhye, Princeton University

Fifth Forces from a Self-interacting Scalar Field  

Experiments have ruled out unit-strength scalar mediated fifth forces on scales ranging from 0.1 mm to 100 AU. However, allowing the scalar to have a quartic self-interaction weakens these constraints considerably. This weakening is due to the "chameleon mechanism", which gives the scalar field an effective mass that depends on the matter density. I will describe the chameleon mechanism and discus experimental constraints on self-interacting scalar fields. In particular, I will compare the chameleon-mediated self-interaction to constraints from the Eot-Wash experiment, which is now able to exclude the most likely regions of parameter space. Predictions for the next-generation Eot-Wash experiment will also be discussed.

Michael Turner, The University of Chicago

Trends in Basic Research Funding: From NSF to the American Competitiveness Initiative  

Dr. Michael Turner, former Assistant Director of the National Science Foundation's Mathematical and Physical Sciences Directorate, will share his perspectives on trends in federal research funding in the sciences. The discussion will explore how federal initiatives and priorities, such as the American Competitiveness Initiative, may impact grant seekers at various stages of their academic careers, shape the types of funding opportunities available, and inform strategies for securing support for basic research in a competitive environment. Junior Faculty, Research Associates and Research Administrators are welcome to attend. Co-sponsored by University Research Administration and the Physical Sciences Division Mentoring Committee.

Fabio Gastaldello, University of California Irvine

X-ray Bright Galaxy Groups as Cosmological Tools  

We present radial mass profiles for 16 relaxed galaxy groups (kT 1-3 keV) selected for optimal mass constraints from the Chandra and XMM data archives. The resulting mass profiles are described well by a two component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. For the first time we find that the NFW concentration parameter (c) for groups decreases with increasing virial mass (M) as expected in standard Lambda-CDM models. When combined with our own results for 7 elliptical galaxies and clusters from the literature, the X-ray c-M relation agrees with the relation produced by the flat, concordance Lambda CDM model provided the sample is comprised of the most relaxed, early forming systems, which is consistent with our selection criteria. The tilted, low sigma_8 model suggested by the 3-yr WMAP analysis is rejected at > 99.99% confidence, but it can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w = -0.8.