Stefan Hilbert, Max Planck Institute for Astrophysics

Gravitational lensing is playing an increasingly important role in astrophysics and cosmology. In collaboration with people from MPA and Bonn, I investigate gravitational lensing by carrying out ray-tracing through the Millennium Simulation, one of the largest simulations of cosmic structure formation.



In this talk, I will present results for the statistics of strong lensing. Furthermore, I will talk about our efforts towards realistic simulations of weak galaxy-galaxy lensing and cosmic shear surveys. Finally, I will discuss the potential capabilities of future radio telescopes for imaging the cosmic matter distribution.

Tony Wong, University of Illinois at Urbana-Champaign

I discuss the role of the molecular gas (or perhaps more accurately, the dense gas) fraction in the evolution of galaxies, with emphasis on recent data from the Large Magellanic Cloud (LMC).

Martin K. Pohl, Iowa State University

The Universe is filled with energetic particles that are collectively referred to as cosmic rays. It appears that efficient acceleration of cosmic rays proceeds in systems with outflow phenomena, in which a fraction of the flow kinetic energy can be transferred to cosmic rays. One class of those systems are shell-type supernova remnants (SNR). The question of cosmic-ray acceleration in SNRs includes aspects of the generation, interaction, and damping of magnetic turbulence in non-equilibrium plasmas. I will report on recent results of kinetic simulations, that may shed light on the strange symbiosis of cosmic rays and magnetic turbulence.

Joe Silk, Oxford

...AND HOW TO DO BETTER

Mark C Wyman, Perimeter Institute for Theoretical Physics

Detecting the parity-odd, or B-mode, polarization pattern in the cosmic microwave background radiation due to primordial gravity waves is considered to be the final observational key to confirming the inflationary paradigm. The search for viable models of inflation from particle physics and string theory has (re) discovered another source for B-modes: cosmic strings.

Strings naturally generate as much vector mode perturbation as they do scalar, producing B-mode polarization with a spectrum distinct from that expected from inflation itself. In a large set of models, B-modes arising from cosmic strings are more prominent than those expected from primordial gravity waves. In light of this, we study the physical underpinnings of string- sourced B-modes and the model dependence of the amplitude and shape of the $C_l^{BB}$ power spectrum. Observational detection of a string-sourced B-mode spectrum would be a direct probe of post-inflationary physics near the GUT scale.
Conversely, non-detection would put an upper limit on a possible cosmic string tension of Gmu < 10^{-7} within the next three years.

Christian Reichardt, UC Berkeley

The Arcminute Cosmology Bolometer Array Receiver (ACBAR) is a multi-frequency 16-element bolometer array which observed from the 2m Viper telescope at the South Pole during the 2001, 2002, 2004 and 2005 Austral winters. ACBAR's small (5') beams allow it to probe the damping tail of the cosmic microwave background (CMB) power spectrum, making it highly complementary to experiments at larger angular scales such as WMAP and Boomerang. I will present recent results from the complete set of ACBAR's CMB temperature anisotropy observations. We include new data from the final 2005 observing season, expanding the number of detector-hours by 210% and the sky coverage by 490% over the data set used in the previous ACBAR release. As a result, the band-power uncertainties have been reduced by more than a factor of two on angular scales encompassing the third to fifth acoustic peaks as well as the damping tail of the CMB power spectrum. The calibration uncertainty has been reduced from 6% to 2.2% in temperature through a direct comparison of the CMB anisotropy measured by ACBAR with that of the dipole-calibrated WMAP3 experiment. The measured power spectrum is consistent with a spatially flat, LambdaCDM cosmological model. We see evidence for weak gravitational lensing of the CMB by comparing the likelihood for the best-fit lensed/unlensed models to the ACBAR+WMAP3 data. On fine angular scales, there is weak evidence (1.7 sigma) for excess power above the level expected from primary anisotropies. The source of this power cannot be constrained by the ACBAR 150 GHz observations alone; however, if it is the same signal seen at 30 GHz by the CBI and BIMA experiments, then it has a spectrum consistent with the Sunyaev-Zel'dovich effect.

Justin Khoury, Perimeter Institute for Theoretical Physics

We present a class of modified theories of gravity in which vacuum energy is screened or degravitated. We argue that any such theory must, at the linear level, reduce to a theory of massive or resonance graviton. The immediate implication is that there are new degrees of freedom, associated with the extra polarization states of the graviton, the most interesting of which is the helicity-0 or longitudinal mode. Phenomenologically, this mode leads to a fifth force which is suppressed in regions of high density but kicks in on large (>Mpc) scales. We discuss the implications for cosmological evolution and structure formation.

Greg Bryan, Columbia University

The new X-ray observatories have unleashed an explosion of data about the hot gas in galaxy clusters, in the process overturning cherished theories and posing new puzzles, particularly about the impact of AGN on the thermal state of the cluster gas. I review what can be understood from observations, simple theory and high-resolution numerical simulations, showing that in some areas the latest observational results are in surprisingly good agreement with theory. On the other hand, the impact of cooling and heating on cluster cores and -- to a lesser extent -- on global scaling relations, is still not well understood. This uncertainty is important when using clusters for constraining cosmological parameters and I will talk about some ways to improve both our understanding of AGN feedback, and our cosmological constraints.

Steven Furlanetto, UCLA

Finding and understanding the earliest generations of galaxies is one of the frontiers of modern cosmology. Although enormous strides have been made in the past decade, the current observational evidence is ambiguous at best. I will describe two routes toward improving the situation. First, searches for high-redshift galaxies through their Lyman-alpha emission lines can teach us not only about the galaxies themselves but also about the intergalactic medium (IGM). While current measurements constrain their abundance, the clustering of these objects promises to reveal even more information. Second, three-dimensional ''tomography'' 21 cm emission (or absorption) by the neutral IGM has the potential to unlock the detailed distribution of baryons between recombination and reionization. I will describe how this cosmic background can teach us about the eras of the first stars, first black holes, and reionization itself. I will also describe some of the challenges facing these measurements.

Yu Qingjuan, University of California at Santa Cruz

Hypervelocity stars, with velocities up to 10^3 km/s, were predicted to exist in the Galaxy as a consequence of dynamical interaction of stars with the central supermassive black hole. This prediction has been confirmed with the discovery of the first hypervelocity star in the Galactic halo in 2005. In this talk, I will review the dynamical mechanisms of ejecting hypervelocity stars and recent observations. I will talk about how hypervelocity stars can reveal the properties of the Galactic center and the binarity of the central supermassive black hole. I will also talk about the kinematics of hypervelocity stars and how they can be used as a diagnostic tool to constrain the triaxial shape of the Galactic halo potential. Future observational tests have potential implications in the cosmological contexts, as triaxial shapes of galactic halos and the existence of binary black holes are the generic predictions of the hierarchical structure formation model in the Lambda-CDM cosmogony.

Mark R Krumholz, Princeton University

The similarity between the mass and spatial distributions of pre-stellar gas cores in star-forming clouds and young stars in clusters provides strong circumstantial evidence that these gas cores are the direct progenitors of individual stars. I describe a physical model for the evolution of massive cores into stars, starting with the intial phases of collapse and fragmentation, through disk formation and fragmentation, the later phases of stellar feedback, and finally interaction of the newly formed stars with their environments. This model shows that a direct mapping from cores to stars is the natural physical outcome of massive core evolution, and thereby allows us to explain many of the properties of young star clusters as direct imprints of their gas-phase progenitors.

Sunil Golwala, Caltech

Blind surveys for galaxy clusters and anisotropy using the Sunyaev-Zeldovich effect are one of the new frontiers in cosmology, promising to tell us about both about global cosmological parameters as well as cluster formation and astrophysics. We present results from such a survey at 150 GHz using Bolocam, a 144-element mm-wave bolometer camera, on the Caltech Submillimeter Observatory, as well as preliminary maps from a program to observe massive clusters in the SZ effect at high signal-to-noise. Finally, we describe our upcoming MKID camera for the CSO and a prospective follow-up camera on the Cornell-Caltech Atacama Telescope and their expected impact on SZ observations.

Michael D Niemack, Princeton University

The Atacama Cosmology Telescope (ACT) is measuring the temperature fluctuations of the Cosmic Microwave Background on arcminute to degree angular scales. Measuring these angular scales will allow us to probe the primary CMB anisotropies as well as explore secondary mechanisms such as the Sunyaev-Zel’dovich effect, the Ostriker-Vishniac effect, and gravitational lensing of the CMB. These measurements combined with follow-up redshift measurements of galaxies will allow us to constrain the dark energy equation of state and the neutrino mass as well as study the growth of structure and the ionization history of the universe. ACT observations began in 2007 with the installation of the Millimeter Bolometer Array Camera (MBAC). During the first season of observations MBAC consisted of a kilopixel detector array of TES bolometers operated at 150 GHz. We are now preparing for the second season of observations by integrating two additiona l detector arrays into MBAC, which will be operated at 220 and 280 GHz.

We will review methods for constraining the dark energy equation of state using ACT observations combined with photometric redshift measurements. Then, we will describe the ACT and MBAC design, development, and status as well as preliminary results from the first season of observations. Finally, we will present the results of a new approach for improving photometric redshift measurements by combining ground-based optical observations with ultraviolet observations from the GALEX satellite telescope.

Bruce Winstein, The University of Chicago

The CAPMAP experiment recently submitted final results on the Polarization of the Cosmic Microwave Background Radiation at fine angular scales. CAPMAP's methodology will be discussed. The experiment used coherent detectors which are also the basis for QUIET which aims to study the possible Inflationary Gravity Wave Background (GWB). This alternative (to TES Bolometers) and still viable approach will be discussed. The challenges in detecting the GWB are formidable; we'll take a little time to consider if the current approach is the ideal one.

Ellen G Zweibel, U Wisconsin, Madison

Magnetic fields are ubiquitous in astrophysical systems, but despite many years of progress in cosmology, we know very little about how and when they originated, or how they evolve. I will review traditional and nontraditional evidence for magnetic fields and discuss key processes in their maintenance and evolution.

Joanna Dunkley, Princeton/Oxford

I will present new results from five years of WMAP observations. The WMAP satellite measures the Cosmic Microwave Background temperature and polarization anisotropy over the whole sky. With five years of data, we detect no significant deviations from the simple LCDM cosmological model:
a flat universe filled with baryons, photons, cold dark matter, neutrinos, and a cosmological constant. I will describe the observations, and discuss the cosmological implications for cosmic inflation, for the reionization of the universe by the first stars, and for the contents of the universe including neutrinos and dark energy.

Jenny E Greene, Princeton

The evolution of supermassive black holes and galaxies are apparently inextricably linked, and we might hope to gain unique insight into the nature of their connection by studying galaxies with currently accreting black holes. The study of active host galaxies has a long history; I focus here on three innovative experiments that exploit both the recent availability of large spectroscopic surveys such as the SDSS and improved techniques to estimate black hole masses in active galactic nuclei (AGNs). The first uses HI observations of local AGNs to measure their neutral gas fractions, dynamical masses, and disturbance levels. The second studies the host galaxies of local obscured AGNs. Because the accretion disk is hidden, it is possible to look for direct evidence of AGN-induced disturbance in the hosts of these intrinsically very luminous systems. Finally, I examine the host galaxies of AGNs selected to have the lowest black hole masses known, a regime in which black hole-bulge relations appear to break down. I discuss the implications for AGN fueling and feedback, and the possibility of observing continuing evolution in the relations between black hole mass and bulge properties at the present epoch.

Greg Bryan, Columbia University

The new X-ray observatories have unleashed an explosion of data about the hot gas in galaxy clusters, in the process overturning cherished theories and posing new puzzles, particularly about the impact of AGN on the thermal state of the cluster gas. I review what can be understood from observations, simple theory and high-resolution numerical simulations, showing that in some areas the latest observational results are in surprisingly good agreement with theory. On the other hand, the impact of cooling and heating on cluster cores and -- to a lesser extent -- on global scaling relations, is still not well understood. This uncertainty is important when using clusters for constraining cosmological parameters and I will talk about some ways to improve both our understanding of AGN feedback, and our cosmological constraints.

Richard Kessler, The University of Chicago

I will present an overview of the three-year SDSS Survey that resulted in approximately 500 spectroscopically confirmed SN Ia. Using ~100 SNe Ia from the first season of the SDSS survey, along with another 140 SNe~Ia from other surveys, preliminary results are presented for the cosmological parameters w and Omega_M.
Results are compared for both the SALT2 and MLCS methods.

Andisheh Mahdavi, University of Victoria

Clusters of galaxies are dominated by dark matter. We can see the gravitational effect of this dark material on the orbits of cluster members, the thermodynamics of the hot gas, and the lensed shapes of galaxies behind the cluster. I will show that combining X-ray, lensing, and SZ data for a single relaxed cluster can yield powerful constraints on its dark matter distribution and on the equation of state of the intracluster plasma. At the same time, multiwavelength observations of merging clusters can yield significant and perhaps even more interesting constraints on dark matter properties. Both relaxed and merging clusters are well-represented in the Canadian Cluster Comparison Project, an X-ray, optical, and radio survey of fifty nearby systems. I will conclude by discussing an unusual, massive, X-ray bright core nearly devoid of galaxies at the heart of Abell 520, and discuss whether it can advance our understanding of the fundamental nature of dark matter.

Massimo Ricotti, University of Maryland at College Park

In this talk I show results of cosmological simulations suggesting a possible identification of at least some dwarf spheroidal galaxies in the Local Group as the fossils of the first galaxies ("pre-reionization fossils"). I also revisit the problem of gas accretion onto minihalos after reionization. I show that primordial minihalos with $v_{cir}<20$~km~s$^{-1}$ stop accreting gas after reionization, as it is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases, they may have a late phase (at redshift $z<2$) of gas accretion and possibly star formation. As a result we expect that pre-reionization fossils have a more complex star formation history than previously envisioned. The dwarf spheroidal galaxy Leo~T fits with this scenario. Another prediction of the model is the existence of a population of gas rich minihalos that never formed stars. A subset of compact high-velocity clouds may be identified as such objects but the bulk of them may still be undiscovered.

Janna J Levin, Barnard College/Columbia University

We are on the verge of a truly remarkable observational possibility: the direct detection of black holes through gravitational radiation. Knowledge of the dynamics around rotating black holes is imperative to the success of gravitational wave observatories. While they have been studied extensively, a general understanding of the elaborate orbits has been elusive. We demonstrate that the entire dynamics around black holes can be understood through a beautiful, geometric taxonomy of perfectly periodic orbits. Through a correspondence to the rational numbers, we build a periodic table of black hole orbits in analogy with the chemical periodic table. A remarkable implication of this taxonomy is that the simple precessing ellipse familiar from planetary orbits is not allowed in the strong-field regime. Instead, eccentric orbits trace out precessions of multi-leaf clovers in the final stages of inspiral.

Fulai Guo, University of California, Santa Barbara

AGN feedback plays a key role in suppressing cooling flows in galaxy clusters, and thus in shaping the high-luminosity end of the galaxy luminosity function. Recent Chandra observations have detected X-ray deficient bubbles in many galaxy clusters. In this talk, I will use hydrodynamic simulations to show that cosmic rays leaked from these bubbles may heat the intracluster medium, and thus efficiently suppress the cooling catastrophe. No fine-tuning of various parameters is required in our model. In the second part of my talk, I will perform the first global linear stability analysis of (AGN) feedback models. I will show that thermal instability in cool core clusters can be suppressed by the AGN feedback mechanism, provided that the feedback efficiency exceeds a critical lower limit. Furthermore, our analysis naturally shows that the clusters can exist in two distinct forms. Globally stable clusters are expected to have either: 1) cool cores stabilized by both AGN feedback and conduction, or 2) non-cool cores stabilized primarily by conduction. Intermediate central temperatures typically lead to globally unstable solutions.

Matthias Beilicke, Washington University of St.Louis

The giant radio galaxy M87 is located at a distance of ~16 Mpc and harbours a supermassive black hole in its center. The structure of the relativistic plasma jet of M87 is resolved at radio, optical and X-ray wavelengths. M87 belongs to the class of active galactic nuclei (AGN) and is the only extragalactic source not belonging to the class of blazars (plasma jet pointing towards the observer) which is detected at very high energies (E>100 GeV, i.e. VERITAS). This makes it a unique laboratory to study jet physics and the corresponding emission processes at very high energies. The current status as well as the prospects of future simultaneous multi-wavelength observations are discussed.

Douglas Spolyar, ucsc

One of the outstanding problems in astrophysics is to determine the mass and properties of the first stars, as these determine the astrophysical environment of the early universe. These stars form when the universe is 200 million years old and are thought to begin the process of element enrichment necessary for the beginnings of life. They form at the very centers of million solar mass concentrations of dark matter (DM), where the DM density is exceptionally high. We propose that the first phase of stellar evolution in the history of the Universe may be Dark Stars (DS), powered by dark matter heating rather than by nuclear fusion, and in this paper we examine the early history of these DS. The power source is annihilation of Weakly Interacting Massive Particles (WIMPs) which are their own antiparticles. These WIMPs are the best motivated DM candidates and may be discovered by GLAST or at the Large Hadron Collider at CERN, which have just started taking data. We build up the dark stars from the time DM heating becomes the dominant power source, accreting more and more matter onto them. We have included many new effects in the current study, including a variety of particle masses, accretion rates, energy transport mechanisms, nuclear burning, feedback mechanisms, and possible repopulation of DM density due to capture. Remarkably, we find, that in all these cases, we obtain the same result: the first stars are very large, 500-1000 times as massive as the Sun, and cool (Tsurf < 10, 000K) during the accretion. These result differs markedly from the standard picture in the absence of DM heating, in which the maximum mass is about 140 M⊙ (Tan and McKee) and the temperatures are much hotter (Tsurf > 50, 000K). DS lead to element enrichment that provides a better match to data, and eventually collapse to form massive black holes that may provide seeds for the otherwise unexplained supermassive black holes observed at early times as well as intermediate black holes.

Sudeep Das, Princeton University

Large scale structure in the universe deflects cosmic microwave background (CMB) photons by roughly 3 arcminutes. This deflection field reflects the history of cosmological expansion and growth of structure. With high resolution ground based CMB experiments like the South Pole Telescope and the Atacama Cosmology Telescope already underway, and the space based Planck satellite nearing its launch, time is ripe for perfecting a new set of tools and ideas to reap the scientific benefits of this exciting effect. I will describe high resolution simulations, new analysis techniques and theoretical expectations for an array of possible projects involving CMB lensing and large scale structure surveys which will shed new light on dark energy, dark matter and galaxy formation.

Joel R Primack, University of California - Santa Cruz

Mergers of gas-rich spiral galaxies can make rotating elliptical galaxies that have kinematics much like those from the SAURON observations, and recent work led by my just-finished PhD student Matt Covington shows that their properties look very much like those seen at redshifts up to z~3. However, large elliptical galaxies are often round on the sky and non-rotating, and these are not formed by binary mergers. However, at high redshifts z>2 the merger timescale is comparable to the Hubble time so mergers typically overlap. High-resolution simulations by my just-finished PhD student Greg Novak appear to produce massive elliptical galaxies like those observed.

Matthew McQuinn, Harvard

Observations of the intergalactic medium (IGM) suggest that quasars reionize the HeII at z ~ 3. HeII reionization heats the IGM by tens of thousands of Kelvin, and it affects the statistics of the HI and HeII Lyman-alpha forests. I will present a set of simulations of HeII being reionized by quasars. These simulations lead to a different picture for this process than in previous studies. If quasars have a mean spectral index of 1.5, I find that HeII reionization heats regions in the IGM by as much as 30,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~10,000 K and with large temperature fluctuations on ~50 comoving Mpc scales. However, the amount of heating can be much larger if the spectrum is harder. I discuss how temperature fluctuations from HeII reionization bias measurements from the HI Lyman-alpha forest of the IGM temperature and of cosmological parameters, and I quantify the detectability of these fluctuations with wavelet statistics. I conclude by contrasting the morphology of HeII reionization by quasars with that of hydrogen reionization by stars.

Laurie D Shaw, McGill University

Measuring the redshift evolution of the cluster mass function provides us with a sensitive means of constraining cosmological parameters. Sunyaev-Zel'dovich Effect experiments aim to survey a large fraction of the sky searching for clusters via their imprint on the CMB. However, to probe the growth of structure it is vitally important that we can accurately infer the mass of clusters via measurements of their integrated SZ flux. In this talk I will present a detailed analysis of the slope, normalisation and intrinsic scatter in the SZE flux - mass scaling relation via simulated cluster samples. Using synthetic sky-maps constructed from a high-resolution cosmological 'lightcone' simulation, I will evaluate the accuracy with which cluster size and flux can be measured in practice, and discuss the characteristics of the detected cluster catalogues. Finally I will discuss the potential for improved cluster catalogues by cross-matching with optical data.

Fabian Schmidt, The University of Chicago

Instead of adding another dark component to the energy budget of the Universe in trying to explain the accelerated expansion, one can ask whether the cause is in fact the behavior of gravity itself on the largest scales. In this talk, I will consider a sub-class of so-called f(R) gravity theories which closely follow the LambdaCDM expansion history, while at the same time evading tight Solar System constraints on gravity. I will present new results from cosmological N-body simulations which consistently solve for the modified gravitational force. In particular, I will discuss the effects of modified gravity on structure formation, dark matter halo properties, and cosmological observables.

Wenjuan Fang, Physics Department, Columbia University

The accelerated expansion of the universe at the present time may be caused by a different law of gravity from General Relativity on cosmological scales instead of by a missing dark energy component. A leading example is the Dvali-Gabadadze-Porrati (DGP) self-accelerating braneworld model where cosmic acceleration arises from graviational

leakage into an extra dimention. I will show the results of a Markov Chain Monte Carlo study of this model given the current observations of the cosmic microwave background anisotropies from the 5-year WMAP, supernovae from the SNLS and

Hubble constant from the HST key project, and discuss how it is compared to the successful Lamda-CDM model that is based on General Relativity but with a cosmological constant.