Astrophysics

Cosmology, in particular applying the physics of elementary particles to the extremely hot and violent conditions of the early universe, and exploring deep questions about the big bang, the fate of our universe, and the hope for intelligent life (here or elsewhere)

The growth of matter perturbations in the presence of dark energy with small fluctuations depends on the speed of sound of these fluctuations and the comoving scale. The growth index can differ from the value that it takes in the limit of no dark energy perturbations by an amount comparable to the accuracy of future observations. This may contribute to a better characterization of the dark energy properties.

If primordial black holes are produced at the end of inflation, they should quickly decay via Hawking radiation. For the most part the radiation signature of these black holes will be wiped out, as the universe is still radiation dominated when they disappear. The exception to this would be a stochastic background of gravity waves. I present an algorithm by which the spectrum of radiation can be calculated, and discuss the dependence on the initial energy density and the number of relativistic species.

I will discuss the various sources of non-Gaussianity (NG) in a class of multi-field models of inflation. I will show that there is both an intrinsic and a local contribution to the NG although they both have the same shape. It is also possible in this class of models that the dominant part to the 3-pt function comes from loop diagrams. These models are of the hybrid type and while they occur naturally in string theory, the conditions for the NG to be important are not generic.

There is an ongoing debate in the literature concerning the effects of averaging out inhomogeneities (``backreaction\'\') in cosmology. In particular, it has been suggested that the backreaction can play a significant role at late times, and that the standard perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes nonlinear. After a brief introduction to the problem, I will show using Zalaletdinov\'s covariantaveraging scheme that as long as the metric of the universe can be described by the perturbed FLRW form, the corrections due to averaging remain negligibly small. Further, using a fully relativistic and reasonably generic model of pressureless spherical collapse, I will show that as long as matter velocities remain small (which is true in this model even at late times), the perturbed FLRW form of the metric can be explicitly recovered. Together with the observation that real peculiar velocities are in fact nonrelativistic, these results imply that the backreaction remains small even during nonlinear structure formation.

Non-Gaussianity is a powerful observable that may reveal important properties of the fundamental physics of inflation, with qualitative and quantitative features of higher order correlation functions distinguishing between models. Here I will discuss the structure of correlation functions in the most general single field inflation model and explain why this information is important for making use of observations from the CMB and large scale structure.

WMAP measurements of CMB temperature anisotropies reveal a power asymmetry: the average amplitude of temperature fluctuations in one hemisphere is larger than the average amplitude in the opposite hemisphere at the 99% confidence level. This power asymmetry may be generated during inflation by a large-amplitude superhorizon perturbation that causes the mean energy density to vary across the observable Universe. Such a superhorizon perturbation would also induce large-scale temperature anisotropies in the CMB; measurements of the CMB quadrupole and octupole (but not the dipole!) therefore constrain the perturbation\'s amplitude and wavelength. I will show how a superhorizon perturbation in a multi-field inflationary theory, the curvaton model, can produce the observed power asymmetry without generating unacceptable temperature fluctuations in the CMB. I will also discuss how this mechanism for generating the power asymmetry will be tested by forthcoming CMB experiments.

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 laws 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.

The AdS/CFT correspondence has recently been extended to field theories satisfying the non-relativistic generalization of conformal symmetry, the Schroedinger symmetry. These holographic descriptions offer the potential to do calculations in the strong coupling regime of experimentally-realized condensed matter systems, such as fermions at unitarity. In this talk, we will outline the holographic formulation of such NRCFTs at zero temperature. We will then discuss the embedding of the appropriate geometry into IIB supergravity, and the finite temperature generalization that results. We will conclude with a brief discussion of the holographic description of non-relativistic conformal hydrodynamics and current research projects.

I will show the calculation of the probability distribution for the volume of the Universe after slow-roll inflation both in the eternal and the non-eternal regime. Far from the eternal regime the probability distribution for the number of e-foldings, defined as one third of the logarithm of the volume, is sharply peaked around the number of e-foldings of the classical inflaton trajectory. At the transition to the eternal regime this probability is still peaked (with the width of order one e-foldings) around the average, which however gets twice larger at the transition point. As one enters the eternal regime the probability for the volume to be finite rapidly becomes exponentially small. In addition to developing techniques to study eternal inflation, these results allow us to establish the quantum generalization of the recently proposed bound on the number of e-foldings in non-eternal regime: the probability for slow-roll inflation to produce a finite volume larger than Exp[S_dS/2], where S_dS is the de Sitter entropy at the end of the inflationary stage, is smaller than the uncertainty due to non-perturbative quantum gravity effects. The existence of such a bound provides a consistency check for the idea of de Sitter complementarity.