Search Talks

While understanding the evolution of galaxies is one of the major themes of contemporary astronomy, most empirical studies focus only on the evolution of distribution functions (e.g., the luminosity function), effectively treating galaxies in isolation. The new generation of large imaging and spectroscopic surveys make it possible to measure the clustering of galaxies with different physical properties as a function of redshift, providing complementary information to traditional distribution function studies. This approach is especially powerful, because most theoretical models of galaxy evolution are based on the underlying distribution of dark matter halos and they make strong predictions for clustering evolution. To link galaxies to dark matter halos from the observed galaxy clustering, I will introduce the halo occupation distribution (HOD), which characterizes the relation between galaxies and dark matter halos by the probability distribution that a halo of virial mass M contains N galaxies of a given type, together with the spatial and velocity distributions of galaxies within halos. I will present HOD modeling results for galaxy clustering measured in several surveys, including the SDSS (z~0), the DEEP2 (z~1), and the NOAO Deep Wide-Field Surveys (0<z<1). I will demonstrate that, by linking galaxies to dark matter halos, HOD modeling of galaxy clustering opens a new direction in studying galaxy evolution (and cosmology).

Cosmic strings, generic in brane inflationary models, may be detected by the current generation of gravitational wave detectors. An important source of gravitational wave emission is from isolated events on the string called cusps and kinks. I first review cosmic strings, discussing their effective action and motion, and showing how cusps and kinks arise dynamically. I then show how allowing for the motion of the strings in extra dimensions gives a potentially significant reduction in signal strength, and comment on current LIGO bounds.

I will discuss a powerful way to examine the nature of dark energy using a measurement of the growth of galaxy clusters over cosmic time. A novel technique that uses the Cosmic Microwave Background as a backlight allows the detection of galaxy clusters out to the time of their first formation. Using this technique, I will present the first constraints on cosmological parameters obtained with the Atacama Cosmology Telescope, as well as exciting prospects for the future.

Black holes are associated with a variety of the most extreme and counter-intuitive phenomena in astronomy and physics. However, despite the passage of nearly 40 years since the discovery of the first strong black hole candidate, we have scant evidence that general relativity provides an accurate description of gravity in the immediate vicinity of astrophysical black holes. Over the next few years this will change dramatically.

In the picture of eternal inflation, our observable universe resides inside a single bubble nucleated from an inflating false vacuum. Some of the theories giving rise to eternal inflation predict that we have causal access to collisions with other bubble universes, providing an opportunity to confront these theories with observation. In this talk, I will outline progress on the theoretical description of eternal inflation and bubble collisions, and present results from the first search for the effects of bubble collisions in the WMAP 7-year data.

Strong lensing galaxy clusters provide promising probes of cosmological structure formation. Strong lensing halos can be identified in cosmological simulations through ray tracing techniques and their properties measured. Previous studies have found some evidence that strong lensing clusters are more concentrated than expected, with possible explanations including baryonic effects or more exotic physics such as early dark energy. Using Sunyaev-Zeldovich (SZ) observations to measure the mass on large scales and strong lensing mass modeling for small scales, we find that the clusters are more concentrated than expected at the 97% confidence level. We also investigate the placement of lensed images with respect to the SZ gas and Brightest Cluster Galaxy orientations. I will also discuss the redshift evolution of radio Active Galactic Nuclei in clusters, which is relevant for determining cosmological parameters from SZ surveys as well as for understanding the energy budgets in cluster cores.

Inflation is one of the foundational paradigms of our picture of the Universe. Yet distinguishing between the multitude of different inflationary models presents major observational challenges. In this talk, I will discuss a number of inflationary observables, specifically the tilt and running of the primordial power spectrum, compensated isocurvature modes, and non-Gaussianity, and the extent to which they might be constrained by future galaxy surveys and 21 cm experiments.

Theories with extra dimensions naturally give rise to a large landscape of vacua stabilized by flux. I will show that the fastest decay is a giant leap to a wildly distant minimum, in which many different fluxes discharge at once. Indeed, the fastest decay is frequently the giantest leap of all, where all the fluxes discharge at once, which destabilizes the extra dimensions and begets a bubble of nothing. Finally, I will discuss how these giant leaps are mediated by the nucleation of "monkey branes" that wrap the extra dimensions.

Five decades ago, Aharonov and Bohm illustrated the indispensable role of the vector potential in quantum dynamics by showing (theoretically) that scattering electrons around a solenoid, no matter how thin, would give rise to a non-trivial cross section that had a periodic dependence on the product of charge and total magnetic flux. (This periodic dependence is due to the topological nature of the interaction.) We extend the Aharonov-Bohm analysis to the field theoretic domain: starting with the quantum vacuum (with zero particles) we compute explicitly the rate of production of electrically charged particle-antiparticle pairs induced by shaking a solenoid at some fixed frequency. (This body of work can be found in arXiv: 0911.0682 and 1003.0674.) Part II: The N-Body Problem in General Relativity from Perturbative QFT In the second portion of the talk, I will describe how one may use methods usually associated with perturbative quantum field theory to develop what is commonly known as the post-Newtonian program in General Relativity -- the weak field, non-relativistic, gravitational dynamics of compact astrophysical objects. The 2 body aspect of the problem is a large industry by now, driven by the need to model the gravitational waves expected from compact astrophysical binaries. I will discuss my efforts to generalize these calculations to the N-body case. (This work can be found in arXiv: 0812.0012.)

This talk will describe the best current understanding of the interior structure of astronomically realistic black holes. A common misconception is that matter falling into a black hole simply falls to a central singularity, and that's that. Reality is much more interesting. Rotating black holes have not only outer horizons, but also inner horizons. Penrose (1968) first pointed out that an infaller falling through the inner horizon would see the outside Universe infinitely blueshifted, and he speculated that this would destabilize the inner horizon. The expectation was supported by linear perturbation theory, but it was not until 1990 that Poisson & Israel were able to clarify the nonlinear evolution of the instability at the inner horizon, which they called mass inflation. Inflation accelerates ingoing (positive energy) and outgoing (negative energy) streams to exponentially huge energies. The black hole thus behaves like a particle accelerator of extraordinary power, accelerating ingoing and outgoing particles to collide with each other at super-Planckian energies. The talk raises the fundamental question: What does Nature do with this remarkable accelerator?

I will discuss the emergence of large, localized, pseudo-stable configurations (oscillons) from inflaton fragmentation at the end of inflation. Remarkably, the emergent oscillons take up >50 per cent of the energy density of the inflaton. First, I will give an overview of oscillons, provide some analytic solutions and discuss their stability. Then, I will discuss the conditions necessary for their emergence and provide estimates for their cosmological number density. I will show results from detailed 3+1-dimensional numerical simulations and compare them to the analytic estimates. Finally, I discuss possible observational consequences of oscillons in the early universe.

The gravitational observatory LISA will detect radiation from massive black hole sources at cosmological distances, accurately measure their luminosity distance and help identify the electromagnetic counterparts that such sources may generate. I will describe various astrophysical scenarios for the generation of electromagnetic counterparts and discuss observational strategies aimed at identifying them. Successful identifications will enable novel studies of black hole astrophysics and cosmological physics.