Cosmology and Gravitation

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. Besides, if our model is considered as an alternative of early time acceleration, it is shown that the early stage accelerating phase ends in a dust dominated FRW homogeneous Universe. Mirage-driven acceleration thus provides a dark matter component for the brane Universe final state. We finally show that the model fulfills the current constraints on inhomogeneities.

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.

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. I will comment on the successes and the limitations of the new data in the context of inflationary model-building, and discuss the next generation of theoretical tools which will be necessary to make sense of future high-precision data.

I will discuss phenomenological aspects on N=1, four-dimensional Type IIB string theory compactifications with all moduli stabilised. In particular, I will review a class of compactifications with exponentially large volumes of the Calabi-Yau manifold and derive explicit formulae for bulk and D3/D7 moduli masses. Then I will show what patterns of soft supersymmetry breaking terms can arise after renormalisation group running to the weak scale.

The discovery of cosmic acceleration has generated tremendous excitement among researchers in fundamental physics and cosmology. Most experts agree that nothing short of a revolution will be required to fully integrate the observed cosmic acceleration (which many attribute to a mysterious "dark energy") with established physics. Currently this discovery is driving very exciting research in both the theoretical and observational domain. I will present two of these topics that particularly interest me: 1) Dark Energy and Cosmic Equilibrium: How a cosmological constant could make the universe look like a box of gas (and what this could mean for cosmology). 2) Probes of Dark Energy: A host of new probes promise to tell us more about dark energy, but what do we really want to know?