Cosmology and Gravitation

Two possible explanations for the type SNe Ia supernovae observations are a nonlinear, underdense void embedded in a matter dominated Einstein-de Sitter spacetime or dark energy in the ?CDM model. Both of these alternatives are faced with Copernican fine-tuning problems. A case is made for the void scenario that avoids introducing undetected dark energy.

Underlying the standard cosmological model is the assumption that it is possible to coarse-grain the energy density of the Universe, and that the dynamical and optical properies of space-time should be well modelled by the result. However, even if the average coarse-grained geometry does have the same dynamical properties as the fine-grained system it is intended to imitate, there are good reasons to suspect that the optical properties may be different. To investigate this we consider a simple model of the Universe in which the matter content is in the form of uniformly distributed discrete islands, rather than a continuous fluid. It is found that in the appropriate limits the resulting large-scale dynamics of the model approach those of an FRW universe, while the optical properties do not. We find the angular diameter distance, luminosity distance and redshifts that would be measured by observers inside such a space-time, and use preliminary results to show that the effect on estimates of the cosmological constant can be of the order of 10%.

Weak gravitational lensing is a powerful probe of modifications of General Relativity on cosmological scales, since such modifications can affect both how matter produces gravitational potential wells and how photons move within these wells. I will discuss alternative theories of gravitation and how we may constrain such theories using weak lensing observables, including those that could be obtained with the balloon-borne High Altitude Lensing Observatory (HALO). I will also discuss the "parametrized-post-Friedmannian" approach for obtaining model-independent constraints, in which new parameters are introduced to characterize the departure from General Relativity on large scales.

Near the Planckian scales, quantum gravity is expected to drastically change the structure of spacetime, one feature of which may be noncomutativity of the coordinates. Based on the recent advances in quantum field theories on such noncommutative spaces, I will consider the fluctuations of inflaton and look for possible noncommutative corrections in the CMB. Anisotropy and non-gaussianity are the result. The resultant distribution is then compared with ACBAR, CBI and WMAP data to constrain the scale of noncommutativity parameter.

I describe a number of techniques that allow for the generation of (near) scale-invariant fluctuations in the early Universe without inflation or ekpyrosis. The basic ingredient is a decaying maximal speed of propagation, for which a Universal law is found. Connections are made with k-essence, the cuscaton, and the DBI action. However the simplest realizations result from bimetric theories and deformed dispersion relations and DSR. A number of implications to theories of quantum gravity are discussed.

In addition to its ability to probe Inflation, CMB polarization offers the intriguing possibility to detect CP-symmetry violation. In some sense these predictions, if true, would be more surprising than confirmation of the inflationary paradigm -- for which ample, albeit circumstantial, evidence already exists. Moreover, recent theoretical predictions imply that, not only are parity violating CMB polarization effects possible, but that they have already been detected at 3\sigma confidence levels in existing polarization data. I will present a worked example showing the impact of experimental systematic effects on such measurements, and present a robust test to help determine the veracity of the theoretical predictions. I will show that the CP-symmetry violating observables are more susceptible to certain systematic effects, and discuss the future prospects for such CMB polarization probes of fundamental physics.

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.