Cosmology and Gravitation

The standard model of cosmology has some puzzles/problems such as the cosmological constant problem and the horizon problem which according to many stem from our lack of understanding of the very early universe. This in turn means that almost none of the theories of quantum gravity are at a stage where anything substantial can be said about observational cosmology. In the past few years Causal Set theory has proved itself different in this case where a possible solution to the Cosmological constant problem was proposed. Now some work in progress has also shown that some models of Causal Set dynamics give exponential expansion in the early universe. I hope to discuss both of these exciting prospects but this talk will mainly focus on the first proposal.

We present the first year SDSS-II Supernova Survey results and their implications for cosmology and future supernova surveys. We then discuss challenges that face next-generation surveys, such as LSST, which will deliver of order a million supernovae without spectroscopic confirmation. As a way to address these challenges, we introduce BEAMS, a statistical method to do photometric supernova cosmology, and present a preliminary application to SDSS data. Finally we highlight the importance of future surveys such as LSST, given the surprising result that we may not detect dark energy dynamics for the next decade, if the dark energy scales during matter and radiation domination.

The Baryon Acoustic Oscillations (BAO) are the latest weapon in the quest for precision cosmology and dark energy. Many presentations on BAO are complicated and unclear and I will therefore present BAO with particular emphasis on trying to give the simplest theoretical description, both at the linear and nonlinear level, and will describe some of the observational challenges to measuring BAO.

The standard theorist's model for the dynamics of galaxies is the limit of a Newtonian N-body system at fixed mass as the number of particles goes to infinity - i.e a phase space fluid After going over conventional wisdom, some interesting open issues which remain will be highlighted, and their relation to real galaxies explored.

Strongly warped regions, or throats, are a common feature of string theory compactifications. In the early, hot universe, energy will be transferred between these throats and between throats and the standard model. Using the gauge-gravity duality, we calculate the rate of this energy transfer. Due to the warping, the resulting decay rate of throat-localized Kaluza-Klein states to other throats or the standard model is strongly suppressed. If their lifetime is longer than the current age of the universe, these states are an interesting dark matter candidate. We discuss a scenario along these lines.