Talks & Events
KICP Colloquia: 2007
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.
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.
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.
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.
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.
Three-Year Results from WMAP (with Commentary)
The data from the first three years of operation of the Wilkinson Microwave Anisotropy Probe (WMAP) satellite provide detailed full-sky maps of the cosmic microwave background temperature anisotropy and new full-sky maps of the polarization. Together, the data provide a wealth of cosmological information, including the age of the universe, the epoch when the first stars formed, and the overall composition of baryonic matter, dark matter, and dark energy. The results also provide constraints on the period of inflationary expansion in the very first moments of time. These and other aspects of the mission results will be discussed and commented on.
WMAP, part of NASA's Explorers program, was launched on June 30, 2001. The WMAP satellite was produced in a partnership between the Goddard Space Flight Center and Princeton University. The WMAP team also includes researchers at the Johns Hopkins University; the Canadian Institute of Theoretical Astrophysics; University of Texas; University of Chicago; Brown University; University of British Columbia; and University of California, Los Angeles.
Wide-Field Surveys of the Optical Sky: The Large Synoptic Survey Telescope (LSST)
Recent technological advances have now made it possible to carry out deep optical surveys of major fractions of the visible sky. Such surveys enable a diverse array of astronomical investigations, ranging from the search for small moving objects in the solar system to studies of the assembly history of the Milky Way. In terms of cosmology, wide-field surveys can yield tight constraints on models of dark energy using a variety of independent techniques.
The Large Synoptic Survey Telescope (LSST) is the most ambitious project of this kind that has yet been proposed. With an 8.4 m primary mirror, and its 3.2 Gigapixel, 10 square degree camera, LSST will provide a nearly an order of magnitude improvement in survey speed over all existing surveys, or those which are currently in development. Over its ten years of operation, LSST will survey 20,000 square degrees of sky in six optical colors down to 27th magnitude. At least a thousand distinct images will be acquired of every field, enabling a plethora of statistical investigations for intrinsic variability and for control of systematics in deep imaging studies.
I will describe some of the science that will be made possible by the construction of LSST and give a brief overview of the technical design.
Star Formation at High z: Evidence for two types of DLAs?
Damped Ly_ absorption systems (DLAs) are a population of objects that act as neutral-gas reservoirs for star formation at high redshift. If the star formation efficiency in DLAs is the same as in :current galaxies, a significant fraction of the sky would be covered by emission from low surface-brightness objects. I describe results of a recent survey for such emission using deep images from the Hubble Ultra Deep Field. The low rate of detection implies a low efficiency for in situ star formation throughout the neutral gas. But evidence that the gas emits cooling radiation suggests it is being heated. I discuss why in DLAs with cooling rates below a critical value the gas is heated by FUV background radiation, and in DLAs with cooling rates above the critical value local sources of FUV radiation are required. The local sources are likely to be compact Lyman Break galaxies (LBGs) embedded in the DLA gas. I discuss evidence that the critical cooling rate divides the DLA sample into two distinct populations with different physical properties.
Three Mysteries of Matter
Particle physicists are on the verge of a direct experimental look at the mysterious Higgs field, which allegedly gives mass to all the elementary particles. Beyond the Higgs, other mysteries of matter face us, such as the identity of dark matter and the origin of the baryon asymmetry of the universe, both of which require new laws of physics beyond the Standard Model. I will summarize our present theoretical understanding of how these mysteries may be related within the context of TeV-scale Supersymmetry, and discuss experimental probes of this scenario.
GammeV - a gamma to milli-eV particle search
The GammeV experiment uses a high-power laser and an accelerator magnet for photons to potentially oscillate into new milli-eV mass astroparticles. I will describe how I spent my Summer vacation to set interesting limits for these new class of particles.
Early Results from the Local Cluster Substructure Survey (LoCuSS)
Gravitational lensing has long been recognized as a powerful tool for mapping the distribution of dark matter in clusters, and yet until recently, it has only been applied routinely to individual clusters. I will show that recent results are changing this picture; both strong and weak lensing are developing rapidly into tools that can routinely be applied to samples rivaling, for example, those considered by X-ray-only scaling relation studies. I will introduce a new survey, the Local Cluster Substructure Survey (LoCuSS) that is assembling a sample of ~100 clusters with HST, Subaru, Chandra, XMM, SZE and Spitzer data. The overall goal is to study how the recent assembly history of the clusters (as revealed by lensing-based mass maps) influences the baryons trapped in the clusters. After outlining the results that motivated the survey (e.g.structural segregation in the mass-temperature plane), I will present new LoCuSS results on the mass-observable scaling relations and the relationship between cluster merger history and the propoerties and thus evolution of brightest cluster galaxies.
Noble Travails: Noble Liquid Detectors Searching for Particle Dark Matter
Particle dark matter is thought to be the overwhelming majority of the matter in the Universe, dwarfing the contribution from conventional material that we, the earth and the stars, are composed of. However, we still have no direct evidence for the existence of particle dark matter. This may soon change...
I will report on the latest results from the XENON10 liquid xenon- based detector which began searching for particle dark matter at Gran Sasso in late 2006 (http://xenon.brown.edu). The experiment has demonstrated (http://arxiv.org/abs/0706.0039) a world class direct search sensitivity which is a factor 4 better than its current nearest rival (CDMS II). I will discuss some of the details of this experiment.
I will also discuss some of the other noble liquid target experiments that are also providing competitive sensitivities in the race for the direct detection of particle dark matter. Theoretical estimates, based on supersymmetric models predict dark matter interaction rates from the best sensitivity of existing direct detection experiments of
~1 evts/kg/month, down to rates of ~1 evts/100 kg/yr, and below this.
Current and future noble liquid experiments for dark matter searches, range in scale from 10's kg to tonnes, and are designed to rise to this challenge. The new liquid xenon detector, LUX, which has begun construction, will be 100 times more sensitive than current best search experiments.
The Enriched Xenon Observatory for Double Beta Decay
The Enriched Xenon Observatory (EXO) is a project aiming at detecting neutrinoless double beta decays of Xe-136. A xenon-filled time projection chamber (TPC) supplemented with scintillation light readout detects ionising particle interactions within its volume. When candidate events are recorded, the Ba-136 ion daughters will be identified, event by event, by means of optical spectroscopy. This coincidence technique would allow for a measurement of double beta decays virtually immune to external radioactive contaminations. The EXO collaboration is planning on combining these experimental techniques in a ton-scale Xe detector using a phased approach.
A smaller detector, EXO-200, employing 200 kg of enriched xenon (80% Xe-136) in liquid form within a TPC with scintillation readout and with no Ba identification, is in advanced stage of assembly. Its cryogenic and xenon handling systems are being re-assembled at the Waste Isolation Pilot Plant (WIPP) underground site in New Mexico. The central detector is planned to be installed by early 2008. As a parallel effort to EXO-200, strategies for Ba tagging are being developed in the laboratory.
I will present the EXO experiment in the context of neutrinoless double beta decay searches and describe the EXO-200 detector in detail, discussing its physics goals, experimental challenges, and schedule. I will also illustrate some of the most promising approaches for tagging single Ba ions produced in a ton-scale Xe detector, show milestone results achieved in laboratory setups, and discuss the EXO timeline for the near future.
Hot on the trail of particle dark matter
Over seventy years, a body of evidence has steadily grown indicating that much of the Universe's mass is non-luminous. Still today, however, we have not identified what makes up this mysteriously dark substance. Many experimental programs that hope to change this are underway, including deep underground detectors, gamma-ray telescopes, neutrino and anti-matter detectors, as well as particle colliders. Each of these efforts are searching for clues of dark matter's identity. With the new technologies needed to observe these particles rapidly developing, the hunt to discover dark matter's identity is well underway.