Astrophysics

I discuss two instances in which nonlinear perturbations in cosmological models are important. First, in de Sitter space-time, the bare necessity that the perturbations should be part of a consistent Taylor expansion of the field equations leads to the requirement, using the 'linearization stability' arguments of the '70's, that the quantum field theory of a scalar field on de Sitter space-time is manifestly de Sitter invariant (not covariant). Second, the concern that in slow-roll inflation the effect of second order perturbations on the long wavelength (super Hubble) perturbations could be much stronger than that of the first order perturbations, for a wide range of slow-roll conditions, is explored in the context of a linear inflation potential and chaotic inflation.

Modified gravity models seem to have classical instabilities, ghosts degrees of freedom and superluminal modes. Besides these constraints new dynamical bounds have found to be typical of these models. The cosmological nature of all these constraints is discussed.

Existence of dark energy and nonzero nu mass are two most exciting discoveries of recent years. More excitingly, the similarity between the energy scales of these two raise the question: "Are they related?" I will explore how such connection could be there in nature and its cosmological consequences mainly in structure formation.

It will be shown that eternal inflation of the random walk type is generically absent in the brane inflationary scenario. Eternal inflation will be analysed both in the context of KKLMMT and the DBI inflationary models. A Langevin analysis will be employed for a more careful treatment. The DBI action, and the relativistic nature of the brane motion in DBI inflationary model, leads to new subtleties in formulating a Langevin approach.

The initial conditions for structure formation, and hence the dark matter distribution on sub-galactic scales, depend on the microphysics of the dark matter in the early Universe. I will focus on WIMPs and explain how collisional damping and free-streaming erase perturbations on comoving scales k> ~1/pc. Consequently the first structures to form in the Universe are mini-halos with mass of order the Earth. I will then describe the status of calculations of the subsequent dynamical evolution of these mini-halos. Finally, if time permits, I'll briefly overview the microphysics of axions.

If low-scale supersymmetry exists in nature, then it it will be very likely that a number of superpartners will be discovered at the LHC. It is also very likely, however, that much of the supersymmetric spectrum will go unobserved, leaving many important holes in our understanding of the TeV scale. Direct and indirect astrophysical probes of neutralino dark matter can enable for some of these holes to be filled. By studying the interactions of the lightest neutralino, in many models, a much more complete understanding of supersymmetry can be achieved than is possible by using hadron colliders alone.

We discuss motivations, observational constraints and consequences of modifying the fundamental laws of gravity at large distances. Such modifications of gravity can be the reason for the observed late-time acceleration of the Universe, and can be differentiated from conventional dark energy via precision cosmology. The inevitable additional polarizations of graviton lead to observably large perihelion precession of the Lunar and Martian orbits. These theories also have potentially observable consequences at LHC .

Weakly interacting massive particles (WIMPs) are excellent candidates for cold dark matter. After the first millisecond, WIMPs have decoupled from standard model matter, both chemically and kinetically, they enter the free streaming regime and the formation of cosmic structure begins. Another 40 million years pass before the typical first structures enter the nonlinear regime and collapse to the first WIMPy halos. Therefore, it has been assumed that structure formation is insensitive to the WIMP field theory and can be neglected. However, this leads to a monotonically increasing power of structure formation on small scales and some kind of regularization procedure would be required to make the hierarchical picture of structure formation well defined. It will be shown that nonequilibrium processes give rise to a physical regularization of hierarchical structure formation. This has important consequences for indirect and direct dark matter searches which are sensitive to sub-galactic and sub-milli-parsec scales. Furthermore, due the existence of a physical regulator, the problem of structure formation can consistently be solved using N-body simulations.

Realizations of inflation in string theory hold the promise of connecting the theory to observational tests, and at the same time providing new insights for field theory models of inflation. I will report on progress towards realizing inflation on D-branes in type IIB string theory. Moduli stabilization effects generically lead to an eta problem in this scenario, and to analyze the model it is necessary to compute a particular correction to the nonperturbative effects arising on wrapped D-branes. I will explain this calculation, then present results for the full inflaton potential that establish the existence of successful models, albeit with qualitatively new predictions.