Cosmology

Inflation is generically a never ending process, with new \'pocket universes\' constantly being formed. All possible events will happen an infinite number of times in such an eternally inflating universe. Unless we learn how to compare these infinities, we will not be able to make any predictions at all. I will discuss some proposed approaches to this \'measure problem\'.

With LHC commissioned in just a few month ahead, all sorts of ideas about physics beyond the standard model are being explored intensively. A strong-coupling chiral theory appearing at TeV scale remains a possibility but also a very hard scenario to study. When it comes to strongly coupled theories, lattice regularization is by far the most reliable method. But defining exact chiral gauge theory on the lattice remains a difficult problem on its own. We show that the idea to use additional non-gauge, high-scale mirror-sector dynamics to decouple the mirror fermions without breaking the gauge symmetry might lead to a practically manageable solution. We demonstrate, using the exact lattice chirality, that partition functions of lattice gauge theories with vector like fermion representations can be split into \'light\' and \'mirror\' parts, and each contains a chiral representation. Such a splitting is only well defined when both sectors are separately anomaly free. We also prove that, the generating function and therefore the spectrum of an arbitrary chiral gauge theory is a smooth function of the background gauge field, if and only if the anomaly free condition is satisfied. We reached this conclusion by proving some very general properties of an arbitrary chiral gauge theory on lattice, and the results should be of importance for further studies in this field.

Within the context of F-theory compactified on Calabi-Yau fourfolds, we describe a class of string theory vacua which contain several features necessary in supersymmetric grand unified models of particle physics. Focussing on a simple class of local Calabi-Yau fourfolds, we explain how the matter content and superpotential in four dimensions are determined by a topological gauge theory. Along these lines, we present some minimal examples of GUT models.

The process of reheating in warped brane world models was initially thought to be quite efficient. However, the identification of long-lived Kaluza-Klein (KK) relics associated with isometries along the internal directions suggests that reheating may not be efficient, and may conflict with BBN and baryogenesis constraints. This talk discusses processes which may accommodate their decay and quantifies their expectant lifetimes, resulting in strong constraints on the parameters of the underlying theory. We also point out several shortcomings of other, recent investigations into the decay mechanisms of the KK relics.