Talks & Events
Astronomy Colloquia: 2012
Why I compute....
Looking for Dark Matter here, there and everywhere
The hunt for physics beyond the standard model at the LHC is in full swing. We already know of the existence of (at least) one new particle that is not in the standard model, dark matter. The existence of dark matter was first inferred from astrophysical observations and later confirmed by cosmological measurements. There is considerable ongoing effort to see the effects of dark matter, which makes up the majority of the matter in our galaxy, in a more terrestrial setting. I will outline what is, and what is not, known about dark matter, and explain how we may soon learn a lot more, as well as explaining how the conventional search methods can be complemented by searches at the LHC, and elsewhere.
The DARKSIDE of Dark Matter
The search for Dark Matter represents one of the most intriguing open frontiers in modern cosmology and astroparticle physics. The science case is extremely strong: observations of the cosmic microwave background fluctuation, large-scale galaxy surveys, studies of large scale structure formation and of the dynamics of galaxy clusters, all point to the existence of cold dark matter. Weakly Interacting Massive Particles (WIMPs) are an excellent candidate for cold dark matter. These particles, predicted in many new theories extending beyond the standard model, may collide with ordinary nuclei via ultra-weak interactions, and could be detected by means of special, low-background detectors, capable of selectively identifying nuclear recoils - the likely signature of WIMP interactions.
I will present and discuss the DarkSide Project for direct dark matter detection. The use of depleted argon as a target in a two-phase time projection chamber, coupled with a powerful neutron veto based on the Borexino technology, results in a unique detector, capable of achieving background-free conditions. DarkSide-50 is the first detector in the DarkSide program, featuring an active mass of 50 kg of depleted argon. It is designed to reach a sensitivity to the WIMP interaction cross section of 10^-45 cm^2 and will be deployed at Laboratori Nazionali del Gran Sasso in Italy in 2012.
Constraining Cosmology using the Growth of Structure and the Cosmic Microwave Background
While dark energy explains the apparent acceleration of the universe, its properties and nature remains a complete mystery. Measurements of the growth of structure are affected by dark energy in a fundamentally different way than distance-redshift based tests, such as from type Ia supernovae and baryon acoustic oscillations. This makes them useful to break cosmological parameter degeneracies, reduce systematic uncertainty, and are an important systematic test of the standard dark energy paradigm. I will present the most recent cosmological constraints from the South Pole Telescope (SPT), and discuss the role of multi-wavelength cluster observations, primarily through Chandra X-ray and optical weak lensing observations, in improving the dark energy constraints for the full SPT cluster survey. I will also discuss the role of these observations in helping to understand the formation and evolution of massive clusters, and their relevance to future cluster surveys, such as the Dark Energy Survey. Finally, I will discuss the next frontier for CMB experiments of using the lensing of the CMB to measure the growth of structure, the technological challenges for the next generation of experiments, and their projected constraints on dark energy, neutrino mass, and the energy scale of Inflation.
What Stars are Useful For - A Particle Physicist's Point of View
Stars realize a variety of physical conditions inaccessible in the lab. Various stages of stellar evolution are influenced by microphysical processes that are sensitive to the fundamental properties of elementary particles. This makes it possible to use stars to search for new particle physics beyond the Standard Model. I will show some examples of resulting bounds, including scenarios with neutrino magnetic moment, extra space-time dimensions, and axion particles. As a particularly amusing example of conditions that cannot be reproduced in the lab, I will discuss the phenomenon of collective oscillations of neutrinos streaming out of the supernova core.
Observational constraints on the kinetic luminosity of quasar outflows: A key to assessing real AGN feedback mechanisms
The potential importance of quasar outflows on the growth of super-massive black holes, enrichment of the intergalactic medium, evolution of the host galaxy, cluster cooling flows and the luminosity function of quasars has been widely recognized. I will briefly review these theoretical developments and describe the efforts of our research group, and others, to determine the most relevant parameter for these models: kinetic luminosity of observed quasar outflows.
New Observational Handles on the Primordial Universe
Early universe cosmology is entering a new phase thanks to more precise measurements constraining the primordial density inhomogeneities. The Planck satellite and current and near future Large Scale Structure surveys are pursuing statistics of the inhomogeneities beyond the well-measured power spectrum. The potential of these new statistics has changed the way we think about theories of the primordial universe, including inflation. I will present the current understanding of how new data may decode the particle physics of inflation and provide more compelling tests of the theoretical framework for the primordial universe.
The Galaxy-Halo Connection Across Mass and Time
Dark matter halos are the fundamental building blocks in the growth of structure and they provide the framework for our modern understanding of galaxy formation. I will discuss the current state of the art in our understanding of the connection between galaxy properties and their dark matter hosts over a range of masses and redshifts. In the context of a given cosmological model, I will show how the galaxy-halo relation can be tightly constrained at low redshift, and how it can be used to infer the full star formation histories of galaxies. This model for the co-evolution of galaxies and LCDM halos is in excellent agreement with a wide range of data, including the evolution of the stellar mass function, galaxy clustering statistics, and the statistics of satellites around Milky Way mass hosts. I will discuss applications to several issues in cosmology, including modeling of the faintest dwarfs and extracting cosmological constraints from the largest galaxy surveys.
Massive Gravity and Cosmology
Could the late-time acceleration of the Universe and the apparent discrepancy with standard particle physics be the first sign of the breakdown of gravity at very large distances? This is the question I will explore within the context of massive gravity. Whilst massive gravity is one of the earliest and most natural generalization of General Relativity, its fully consistent formulation was only unraveled recently. After discussing its realization I will explore its phenomenological implications, both on cosmological scales than within the solar system.
Gravitational Lensing of the Cosmic Microwave Background
Gravitational lensing of the temperature and polarization anisotropies of the CMB is rapidly emerging as a new cosmological tool. Current experiments (as an example, I will focus on the South Pole Telescope) are using this effect to make mass maps over large areas. The power spectra of these mass maps allow new constraints on cosmological parameters, while the maps themselves can be cross-correlated with other tracers of the mass distribution to better understand the connections between mass and light.
Through a Glass Darkly: Cluster Lensing Update 2012
When I last spoke to the department in the fall of 2009, I gave an overview of several "work in progress" projects on strong lensing by galaxy cluster- and group-scale halos. In this 2012 update I will highlight the outcomes of these projects, focusing on three major themes: arc counts, the properties of lensing halos, and the lensed Universe. In the past few years, we have shown that, contrary to prior work, arc counts are in approximate agreement with theoretical expectations; the critical measurement to resolving this tension has been our measurement of the lensed source redshift distribution. In several related papers we have also measured the halo concentration distribution for large samples of lensing clusters and have shown that the previously dramatized 'over-concentration' problem in lensing clusters is at best about a 2-sigma effect. Parallel studies targeting the lensed background sources have revealed a wealth of new data on distant galaxies, probing new regimes of galaxy mass and star formation rates untouched by deep blank-field surveys, and revealing the detailed properties of individual galaxies with exquisite detail.
How to Find the Nearest Habitable Exoplanet
The Giant Magellan Telescope and the James Webb Space Telescope may grant us the opportunity to study the atmospheres of potentially habitable worlds orbiting nearby stars. There is only one problem: We don't yet know at which stars we should point these observatories. Using data from the NASA Kepler Mission, we can deduce the distance within which we can expect a few such worlds, and hence the observational effort that will be required to characterize them. It appears extremely likely that the nearest systems will orbit low-mass M-dwarf stars, which is fortunate as the small physical sizes and low-luminosities of the stars permit novel ground-based detection schemes that are not feasible for Sun-like stars. The MEarth Project is an array of automated telescopes that survey several thousand of the closest and least massive stars to search for transiting planets. In pursuit of this quest, MEarth has informed our understanding of the physics governing the structure, evolution, and rotation of the most common stars in the galaxy.
Bridging the Gap: Transients in the Local Universe
With the advent of advanced gravitational wave interferometers and high energy neutrino facilities within this decade, the prospect of searching for their electromagnetic counterparts is promising. I discuss the challenge of poor sky localization, and present a simple solution that leverages the sensitivity limit to the local universe. Next, I discuss the rapidly growing inventory of transients in the local universe. Until recently, the framework of cosmic explosions was plagued with a glaring six-magnitude luminosity "gap" between the brightest novae and faintest supernovae. Systematic synoptic surveys, serendipitous discoveries and archival searches have started uncovering transients fainter, faster and rarer than supernovae only in the past few years. There is now evidence of multiple, distinct populations of rare transients in this gap. Each new class can illuminate a missing piece in our understanding of stellar evolution. Here I present discoveries and unique physics of transients that bridge this gap between novae and supernovae.
Infrared Spectroscopy of Extrasolar Planetary Atmospheres
The geometry of transiting extrasolar planets provides rich scientific opportunities to probe their atmospheres using photometric and spectroscopic techniques. These techniques include detection of thermal emission at secondary eclipse using Spitzer and ground-based infrared detectors, as well as transmission spectroscopy using both the NICMOS and WFC3 instruments on Hubble, and ground-based telescopes as well. I will give my version of the current "scorecard" for exoplanetary spectroscopy, describing those results that I believe to be robust, versus those that are questionable, and I will show new results from Hubble WFC3 that promise to resolve some outstanding issues. In the future, we look forward to exoplanetary spectroscopy using JWST, and to a major role for the new generation of extremely large ground-based telescopes.
Probing Gas Halos of Galaxies with SDSS
The distribution of gas around galaxies - the circumgalactic and intergalactic media - contains signatures of key processes of galaxy formation such as outflows and accretion, and may account for the majority of baryons in the Universe. Absorption-line spectroscopy is a powerful tool to probe gas properties. SDSS has obtained spectroscopy for ~200,000 quasars, providing a perfect dataset for statistical analyses of absorption lines. I will introduce the new techniques we have developed to model quasar spectra and detect absorption lines. With these new tools, we have compiled a metal absorber catalog of ~50,000 systems. I will present a statistical analysis of these absorber systems and discuss implications for gas flow processes. In addition, our results allow us to measure the Galaxy-Gas correlation out to Mpc scales, linking the gas properties of individual galaxies to their large-scale environment.
Heavy-Element Enrichment in the Early Universe Observed with FIRE
Many Chicago astronomers have used the FIRE spectrometer, an infrared echelle instrument on the Magellan/Baade telescope which has now been in operation for 2.5 years. One of my group's main scientific motivations for building FIRE was to observe heavy elements in quasar absorption spectra at the highest possible redshifts. In this colloquium I will report on new science and discoveries about the z ~ 6 universe that have resulted from FIRE's early operations. These include a new survey of circum-galactic metal pollution seen in MgII - previously only characterized at z < 2 - out to z=5.5. Also, we have substantially improved constraints on the CIV mass density and intergalactic carbon enrichment at z = 5.0-6.5. Finally, I will describe our first measurements of chemical abundances in the z > 7 universe, and discuss their significance for reionization and the formation of the first stars.