# Theoretical cosmology

Modern cosmology poses some of the deepest challenges of modern physics, which are closely related to the former core area. Perhaps more than any other field, cosmology has recently undergone rapid progress due to the profitable interplay between theory and observations. Three sets of observations in particular have shaped our thinking about the universe: the cosmic microwave background, the discovery of dark energy and the detailed observations of the effects of dark matter. Cosmic microwave background observations are compatible with certain classes of inflationary and ekpyrotic models which, in many ways, provide very simple effective dynamics for the early universe.

However, there remain many fundamental questions, such as: how were the initial conditions for an inflationary or ekpyrotic phase set? Why is it justified to assume that the early universe already behaved so classically, with small quantum fluctuations superimposed on a classical background? Is there one universe, or a multiverse populated for instance by the proposed mechanism of eternal inflation? Can the big bang be understood? Dark energy and dark matter provide similar challenges of our understanding of the universe. Current particle physics models cannot account for either of these effects. In fact, it seems clear that these issues can only be resolved by a better understanding of quantum gravity and "beyond the standard model" particle physics.

The above arguments imply that there are many research opportunities at the interface of quantum gravity and cosmology, and the partners in the IMPRS are particularly well positioned to offer projects in these areas. Recent research at the AEI has touched on many of these issues and includes the discovery of an ekpyrotic model fitting all current CMB data, the elucidation of supergravity effects on inflationary models with string theory motivated higher-derivative kinetic terms, an analysis of the construction of de Sitter space in string theory, an exploration of the manner in which space-time can become classical due to an inflationary or ekpyrotic phase, and the construction of non-singular bounces as effective resolutions of the big bang.