This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.
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.
I discuss the backreaction of inhomogeneities on the expansion of the universe. The average behaviour of an inhomogeneous spacetime is not given by the Friedmann-Robertseon-Walker equations. The new terms in the exact equations hold the possibility of explaining the observed acceleration without a cosmological constant or new physics. In particular, the coincidence problem may be solved by a connection with structure formation.
We express the total equation of state parameter of a spatially flat Friedman-Robertson-Walker universe in terms of derivatives of the red-shift dependent spin-weighted angular moments of the two-point correlation function of the three dimensional cosmic shear. In the talk I will explain all the technical terms in the first sentence, I will explain how such an expression is obtained and highlight its relevance for determining the expansion history of the universe.
We express the total equation of state parameter of a spatially flat Friedman-Robertson-Walker universe in terms of derivatives of the red-shift dependent spin-weighted angular moments of the two-point correlation function of the three dimensional cosmic shear. In the talk I will explain all the technical terms in the first sentence, I will explain how such an expression is obtained and highlight its relevance for determining the expansion history of the universe.
The recently released WMAP 3-year data on the anisotropy and polarization of the Cosmic Microwave Background is a milestone in cosmology. For the first time, it is possible to rule out popular models of inflation in the early universe. However, the WMAP3 data contain interesting hints which indicate that it may be too early to declare a "slam dunk" for simple single-field models of inflation.