September 25, 2002 | 3:30 PM | RI 480 | Wednesday colloquium Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment Gunter Sigl, GReCO, IAP
We discuss recent results based on simulations of ultra-high energy cosmic ray propagation in the Local Supercluster with magnetic fields reaching fractions of a micro Gauss in galaxy filaments and clusters, as suggested by several arguments. It is shown that scenarios involving a small number of sources around 10 can be consistent with all observations including the newest HiRes data. Since in such scenarios the contribution of sources at cosmological distances to the observed flux is in general negligible, the observation of a GZK cutoff is not necessarily associated with cosmological sources as often claimed.
October 2, 2002 | 3:30 PM | RI 480 | Wednesday colloquium The Pierre Auger Observatories: Why and How Jim Cronin, University of Chicago
I will give a very personal view of how the project came about. As in the Japanese tale "Roshomon" my view may be different from others but it is an honest one. Auger has been nearly eleven years in the making and will require about three more to complete. What I thought was an obvious thing to do took many years to convince others. I will describe briefly the features of the design and show some of the beautiful events from the large scale prototype which is now running. The talk will be a prelude to the mini-symposium that will be held on Friday afternoon.
October 16, 2002 | 3:30 PM | RI 480 | Wednesday colloquium Cosmological constraints from evolution of X-ray clusters at z=0.5 Alexey Vikhlinin, Smithsonian Astrophysics Observatory
Evolution of the mass function of clusters of galaxies is a sensitive measure of the cosmological parameters, especially Omega. I will discuss some recent results on cluster evolution from the CfA-IfA-ROSAT survey (160 deg2) which found more than 40 X-ray bright clusters at z>0.4. I will discuss how to use the cluster baryon masses as a proxy for the total mass function and argue that this is a better approach than using temperatures or X-ray luminosities. An application of this method to the 160 deg2 sample indicates a strong evolution of the cluster mass function since z=0.5. The observed evolution defines a narrow band in the Omega-Lambda plane which intersects with the cosmological constraints from CMB and SNIa near Omega=0.3 and Lambda=0.7.
October 30, 2002 | 3:30 PM | RI 480 | Wednesday colloquium Small-Scale Structure in Lens Galaxies Charles Keeton, University of Chicago
Strong gravitational lensing offers two unique tests of small-scale structure in distant galaxies. First, lens theory predicts that each lens should have a faint image near the center of the lens galaxy. These "core" images provide the only direct probe of the mass profile in the inner 100 pc of distant galaxies. The fact that core images have not been observed, even in radio lenses, places lower limits on the densities on these scales. Second, the flux ratios between the images in 4-image lenses provide a powerful test of the smoothness of galaxy mass distributions, which in turn tests the prediction from CDM that galaxies are lumpy. Dalal & Kochanek (2002) claimed that lensing proves the existence of substructure in amounts consistent with CDM. I will assess the state of the data and theory behind this claim, and discuss future prospects on both fronts.
November 13, 2002 | 3:30 PM | RI 480 | Wednesday colloquium First results from the DEEP2 Redshift Survey Marc Davis, University of California, Berkeley
The DEIMOS spectrograph was delivered and successfully commissioned on the Keck-II telescope in Hawaii last Spring, and the DEEP2 Redshift survey began in July, 2002. This survey will provide high resolution spectra of ~60,000 faint galaxies over 3.5 square degrees at a median redshift z~1. To date we have successful observations from 70 out of a total of 480 masks, approximately 15% of the survey. I will review the design of the survey, the science goals, and initial, very preliminary results. In two year's time, we hope to use the measured abundance of groups and clusters within the survey volume to set a new, improved constraint on the dark energy equation of state. Some details of the survey can be found at http://deep.berkeley.edu/
November 27, 2002 | 12:00 PM | LASR Conference Room | Wednesday colloquium A Sensitive New All Sky Survey in the Thermal Infrared Ned Wright, UCLA
The Next Generation Sky Survey, a MidEx mission proposed for launch in late 2006, will map the the sky in four bands from 3.5 to 23 microns with sensitivities 3 to 6 orders of magnitude better than previous work. This survey should identify the closest stars to the Sun and the most luminous galaxies in the Universe.
December 11, 2002 | 3:30 PM | RI 480 | Wednesday colloquium Gravitational 3-D mass Tomography Tony Tyson, Bell Labs, Lucent Technologies
There is evidence for vacuum energy accelerating the recent expansion of the universe. Does this "dark energy" exist? What is it's equation of state? A new and direct probe of dark energy is provided by 3-D mass tomography. Dark matter may be "seen" directly via its coherent gravitational lens warping of thousands of high redshift galaxy images. Tomographic inversion of these cosmic mirages in deep wide-field imaging surveys enables a unique 3-D mass view of our universe. Images of dark matter structures and their development over cosmic time will sharply constrain dark energy and its physical properties. These multiple weak lens probes complement those from cosmic background radiation, SZ probes, and supernovae, and thus test the consistency of our cosmology. The Deep Lens Survey, and plans for the Large Synoptic Survey Telescope will be reviewed.
September 27, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar Studying the Reionization of the Universe with Recent and Future Observations Rennan Barkana,
The first stars and quasars reionized the hydrogen in the universe by redshift six. Observations are beginning to probe this phase transition in the universe, and the near future should produce a wealth of data on the first sources of light and the reionization era. We model and interpret the spectra of current sources at the highest known redshifts, and study what can be learned in the future from Lyman alpha absorption.
October 11, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar Probing the State and Composition of the Intergalactic Medium Steven Furlanetto, CfA
Radiative and mechanical feedback from galaxies and quasars play a crucial role in determining the characteristics of the intergalactic medium (IGM), including its temperature, ionization state, and heavy element content. However, the extent and power of feedback remain uncertain, and observational probes of the IGM at a variety of redshifts are needed. I will describe several techniques to constrain feedback scenarios in the coming years. For example, observations of the 21 cm hyperfine transition of hydrogen in the neutral IGM at high redshifts allow us to probe its thermal state and the early radiation background. Studies of metal absorption lines at high redshift can also constrain the extent of heavy element pollution in the IGM. Finally, I will describe how observations of the environments of radio jets can probe the physics of relativistic outflows.
October 25, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar CBR Anisotropy as a Probe of New Physics: Holographic discreteness of inflationary fluctuations Craig Hogan,
Arguments based on black hole thermodynamics and quantum unitaritary suggest that the Hilbert space of any physical system is discrete and finite, with a maximum information content equivalent to n=A/4 binary spins, where A is the area of the two-dimensional bounding surface in Planck units. Thus the effect of quantizing gravity considerably reduces the number of degrees of freedom below that of quantum field theory. This "holographic entropy bound" is described and used to estimate the quantum-gravitational discreteness of inflationary perturbations. In the context of scalar inflation perturbations produced during standard slow-roll inflation, and assuming that horizon-scale perturbations ``freeze-out'' in discrete steps separated by one bit of total observable entropy, it is shown that the Hilbert space of a typical horizon-scale inflation perturbation is equivalent to that of about 10^5 binary spins-- approximately the inverse of the final scalar metric perturbation amplitude, independent of other parameters. Holography thus suggests that in a broad class of fundamental theories, inflationary perturbations carry a limited amount of information (about 10^5 bits per mode) and should therefore display discreteness not predicted by the standard field theory. Some manifestations of this discreteness may be observable in cosmic background anisotropy.
November 1, 2002 | 12:00 PM | Astronomy Conference Room AAC 123 | Friday noon seminar Mapping the Dark Matter: Mass Selected Galaxy Clusters Joseph F. Hennawi, Princeton University
The distortion of images of faint high-redshift background galaxies can be used to probe the intervening mass distribution. This weak gravitational lensing effect can be used to detect dark matter in clusters of galaxies, allowing one to effectively "image" and "weigh" these dark objects. In addition, if photometric redshifts of background source galaxies are available, mass tomography enables one to ascertain the cluster redshift. This opens up the possibility of mapping the 3-d locations of a mass selected sample of galaxy clusters from weak gravitational lensing alone. The efficacy and reliability of these techniques is investigated using a large ensemble of fast cosmological N-body simulations specifically tailored to investigate the statistics of lensing by clusters. Recent purported detections of baryon poor "dark clusters" are reviewed and interpreted. An Adaptive Matched Filtering scheme which combines tomography and matched filtering is introduced and proves superior to filtering techniques used in previous studies. The possibility of using mass selected cluster samples to probe cosmological parameters is discussed. Specifically, the redshift distribution of clusters is a sensitive probe of the equation of state parameter of the dark energy w, and is robust against the uncertain state of baryons in clusters.
November 8, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar TeV Gamma-Rays, Active Galaxies and Cosmology John Quinn, University of College Dublin
A new observational window of the extreme cosmos has been opened with the advent of second generation ground based Gamma-ray detectors. The majority of these detectors utilise the Imaging Atmospheric Cerenkov Technique (IACT) which has been pioneered by the Whipple Collabortion. One of the main successes of IACT has been the detection of objectsbelonging to the blazar class of Active Galactic Nuclei, at energies above 300 GeV. These TeV observations are beginning to provide an important tool in constraining the models of particle acceleration in AGN. As TeV gamma-rays interact with the Cosmic Infrared Background, they provide us with a novel way of probing this background. By accurately measuring the energy spectra at TeV energies from extragalactic objects at different distances, preliminary estimates of the CIB flux have been derived. TeV gamma-ray observations may also be used to contrain the energy scale of quantum gravity, and to search for signatures of dark-matter particles. An overview of the IACT, and results of TeV observations on AGN to date, will be presented.
November 15, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar Kaluza-Kein Dark Matter Geraldine Servant, Univ. of Chicago, Argonne National Lab
In a certain class of models with extra dimensions, the Lightest Kaluza-Klein Particle (LKP) is stable and turns out to be a viable dark matter candidate and a typical WIMP. In this talk, I will start by reviewing the particle physics framework for this class of models with TeV^-1 compactification radius and their phenomenological motivations. I will next present the relic density calculation of the LKP (which is likely to be a Kaluza-Klein photon) and discuss (direct and indirect) detection issues.
November 22, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar Is the hunt for the B-mode a waste of time? William H. Kinney, Columbia University
Recent results appear to indicate that the detection of primordial gravity waves from inflation may be a hopeless task. First, foregrounds from lensing put a strict lower limit on the detectability of the B-mode polarization signal in the Cosmic Microwave Background, the "smoking gun" for tensor (gravity wave) fluctuations. Meanwhile, widely accepted theoretical arguments indicate that the amplitude of gravity waves produced in inflation will be below this limit. I will argue that failure is not inevitable, and that the effort to detect the primordial signal in the B-mode, whether it succeeds or fails, will yield crucial information about the nature of inflation.
December 6, 2002 | 12:00 PM | LASR Conference Room | Friday noon seminar Signatures of primordial helicity in the CMBR Levon Pogosian,
I will discuss ways in which helical primordial magnetic fields could be constrained by measurements of the CMBR. If there were helical flows in the primordial plasma at the time of recombination, they would produce parity violating temperature-polarization correlations. However, the magnitude of helical flows induced by helical magnetic fields is unobservably small. I will describe an alternate scheme for extracting the helicity of a stochastically homogeneous and isotropic primordial magnetic field using Faraday rotation measure maps of the CMBR and the power spectrum of B-type polarization.
October 17, 2002 | 12:00 PM | LASR Conference Room | Special seminar Applications of Secondary CMB Anisotropy Llloyd Knox, University of California, Davis, Department of Physics
Secondary effects in CMB maps are induced by weak gravitational lensing, by cold electrons in the inter--galactic medium, and by hot electrons in clusters of galaxies. Through these effects the CMB can be used to teach us about physical processes that are only important well after the epoch of last-scattering. Examples include the reionization of the inter-galactic medium and the equation-of-state of the dark energy.
October 31, 2002 | 4:15 PM | Physics Colloquium-KPTC 106 | Special seminar The Highest Energy Particles in Nature Alan Watson, University of Leeds, CFCS
December 10, 2002 | 12:00 PM | LASR Conference Room | Special seminar The Measurement of Radiance and the van Cittert-Zernike Roland Winston,
These topics have a fundamental connection. Nonimaging optics is the optics of extended objects, hence of phase-space transport. Obviously, measurement of radiance (the density in phase space) depends on the statistical properties of the wave field and of the instrument. Playing the movie backwards gives a van Cittert-Zernike-like theorem for the instrument. The observable effects, important for measurements near the diffraction limit, have been tested experimentally with high resolution long-wave IR radiometers. The qualitative result is that object are closer (or warmer) than you think.
December 16, 2002 | 12:00 PM | LASR Conference Room | Special seminar CDM Substructure in a Cosmological Context Andrew Zentner, The Ohio State University
The LCDM cosmological model has emerged as the standard framework for the growth of cosmic structure. This "standard model" matches large-scale observations extremely well however, the LCDM paradigm faces several challenges on galactic and sub-galactic scales. I will briefly discuss how dropping the assumption of a scale-invariant power spectrum and allowing for tilts (n sim 0.9) or otherwise modified power spectra can alleviate some of the problems with the central densities of low surface brightness galaxies. Moreover, along with the COBE normalization, the favored tilts imply a value of $sigma_8 sim 0.75$ that is provocatively close to many recent estimates of "low" values of $sigma_8$. It seems worthwhile then, to extend studies of the power spectrum dependence of small-scale structure to CDM substructure and to address the "dwarf satellite problem" that has plagued LCDM as well as recent lensing measurements of substructure.