Cosmology and Gravitation

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.

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.

I argue that all necessary ingredients for successful inflation are present in the minimal supersymmetric standard model (MSSM). The potential for the supersymmetric flat directions (which can be viewed as moduli near points of enhanced symmetry) generically has metastable minima at large field values. An adjustment of soft supersymemtry breaking parameters results in a point where the first two derivatives of the potential vanish. One can then have more than 10^3 e-foldings of slow roll inflation with a Hubble rate ~ O(1-10) GeV in the vicinity of this point. The model has a robust prediction for the scalar spectral index: n_s \leq 0.93, which agrees with the current WMAP and LSS data within 2\sigma. It can be implemented purely within MSSM, or MSSM plus Dirac neutrinos. It has the unique feature of having a concrete and real connection to physics that can be observed in earthbound laboratories.

I will demonstrate how one can realize Cascade inflation in M-theory. Cascade inflation is a realization of assisted inflation which is driven by non-perturbative interactions of N M5-branes. Its power spectrum possesses three distinctive signatures: a decisive power suppression at small scales, oscillations around the scales that cross the horizon when the inflaton potential jumps and stepwise decrease in the scalar spectral index. All three properties result from features in the inflaton potential. The features in the inflaton potential are generated whenever two M5-branes collide with the boundaries. The derived small-scale power suppression serves as a possible explanation for the dearth of observed dwarf galaxies in the Milky Way halo. The oscillations, furthermore, allow to directly probe M-theory by measurements of the spectral index and to distinguish cascade inflation observationally from other string inflation models.

In this talk I will discuss some aspects of graviton production by moving branes. After a brief introduction to braneworld cosmology I will focus on braneworlds in a five-dimensional bulk, where cosmological expansion is mimicked by motion through AdS_5. The moving brane acts naturally as a time-dependent boundary for the five-dimensional graviton (five-dimensional tensor perturbations) leading to graviton production out of quantum vacuum fluctuations. This effect is related to the so-called dynamical Casimir effect, i.e. the generation of real photons out of vacuum fluctuations of the quantized electromagnetic field in dynamical cavities. By applying the formalism used to study the dynamical Casimir effect I will show explicitly that the five-dimensional graviton reduces to the four-dimensional one in the late time approximation of such braneworlds. In the last part of the talk I will study a (toy) model where two branes approach each other in a radiation dominated phase, bounce off and move apart from each other afterwards. Thereby generation of massive gravitons takes place caused by the coupling of the Kaluza-Klein modes to the gravitational zero mode which exhibits a blue spectrum. At the end I will discuss possible applications of the formalism to more interesting scenarios (braneworld inflation etc).

The best studied class of dark matter candidates in Supersymmetric theories is the WIMP, Weakly Interacting Massive Particles, which makes cold dark matter. There is a well-motivated alternative to the WIMP -- dark matter populated by decays of WIMPs. This dark matter from decays is closer in spirit to warm dark matter. They can be distinguished from cold dark matter by observations of structure on scales smaller than about a megaparsec, where cold dark matter models seem to face difficulty. Big Bang Nucleosynthesis predictions are also modified in interesting ways.