This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.
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 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
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.
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 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.
Experiments have ruled out unit-strength scalar-mediated fifth forces on scales ranging from 0.1 mm to 10,000 AU. However, allowing the scalar to have a quartic self-interaction weakens these constraints considerably. This weakening is due to the "chameleon mechanism", which gives the scalar field an effective mass that depends on the local matter density. I will describe the chameleon mechanism and discuss experimental constraints on self-interacting scalar fields.
It has recently been proposed by Nayeri, Brandenberger and Vafa, that the thermodynamics of strings in the early universe can provide us with a causal mechanism to generate a scale invariant spectrum of primordial density fluctuations, without requiring an intervening epoch of inflation. We will review this mechanism, and report on more recent work which has uncovered several observational consequences of the NBV mechanism, some of which in principle, will be distinguishable from the generic predictions of inflation.
We propose a new brane world scenario. In our model, the Universe starts as a small bulk filled with a dense gas of branes. The bulk is bounded by two orbifold fixed planes. An initial stage of isotropic expansion ends once a weak potential between the orbifold fixed planes begins to dominate, leading to contraction of the extra spatial dimensions. Depending on the form of the potential, one may obtain either a non-inflationary scenario which solves the entropy and horizon problem, or an improved brane-antibrane inflation model.
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X S_5 bulk is introduced. Although there is no special points in the bulk, the brane Universe has a center and is isotropic around it. The model has an accelerating expansion and its effective cosmological constant is inversely proportional to the distance from the center, giving a possible geometrical origin for the smallness of a present-day cosmological constant.