Astrophysics

Recent progress in massive gravity has made it possible to construct consistent theories of interacting spin-2 fields.  In this talk I'll describe these developments, focusing on the resolution of the Boulware-Deser ghost problem and the promotion of massive gravity to a bimetric theory of gravity with two dynamical, interacting spin-2 fields.  I'll then discuss the generalization of these bimetric theories to theories of multiple interacting spin-2 fields.

The standard cosmological model posits that the universe is homogeneous and statistically isotropic on its largest scales. However, there is no fundamental reason why these properties have to hold, and in fact they can be broken due to interesting new physics. Moreover, there is some evidence from recent WMAP observations for 'anomalies' - including departures from statistical isotropy - on the largest observable scales. Large-scale structure (LSS) - including the distribution of galaxies in the universe - presents a new frontier in testing statistical isotropy and homogeneity, and we are entering an epoch with orders-of-magnitude improvement in the statistics of LSS. In this talk I first review general tests of statistical isotropy using LSS. I then describe results from research done in collaboration with my student Cameron Gibelyou on testing aspects of the statistical isotropy - in particular, dipolar modulations of the galaxy counts - using existing LSS surveys.

Inflation, a postulated epoch of accelerated expansion in the early universe, has become a principal component of the standard model of cosmology. From a wide variety of initial conditions, inflation produces a nearly homogeneous universe populated by density fluctuations that seed large-scale structure. However, inflation is such a good homogenizer that, once unleashed, in many cases it becomes eternal, ending only within spontaneously nucleated bubbles. In this scenario, our observable universe resides inside one such bubble. Surprisingly, it is possible to perform direct observational tests of eternal inflation. The most dramatic and detectable signatures of eternal inflation arise from the collision between bubbles in the very early universe, which leave an imprint on the cosmic microwave background (CMB) radiation. In this talk, I will motivate and describe the eternal inflation scenario and present the results of a search for the signatures of eternal inflation in CMB data from the Wilkinson Microwave Anisotropy Probe.

First part: The research group in Yaounde (Cameroon), working on Mathematical Modelling and Applications is introduced. Second part: Global existence of solutions to the spatially homogeneous Einstein-Maxwell-Boltzmann system on a Bianchi type 1 space-time is proved.