String Cosmology

Group Leader:  Jean-Luc Lehners

The aim of the String Cosmology research group is to enhance our understanding of the very early universe and its most mysterious aspect, the big bang. It seems likely that a great many features of our universe were determined by the physical processes taking place around the time of the big bang, as the initial conditions for the future evolution of the universe were set at that time. Moreover, we already know that these initial conditions must have been rather special: extrapolating back from current observations, it is clear that the universe must have been exceptionally flat, homogeneous and isotropic at these early times, but at the same time small density fluctuations (with a nearly scale-invariant spectrum) were needed in order to provide seeds for the formation of the large-scale structures via subsequent gravitational collapse.

There currently exists no complete theory that satisfactorily explains these “initial” conditions. However, there are promising candidate theories, such as inflationary cosmology and the theory of the cyclic universe, among others. We study and develop these cosmological theories within the context of string theory, which is currently the most promising framework for a unified description of nature. Our research encompasses both conceptual issues and the all-important predictions for cosmological observations.

The relatively new discipline of string cosmology is entering an exciting time allowing it to mature and to re-establish a close link between fundamental theory and experiments/observations. What were so far loose connections between the disciplines of string theory and cosmology are currently developing into a field of their own, which, if successful, will have the power to address some of the most basic questions we have about the universe: what was its origin? What happened at the big bang, and was the big bang the beginning of time? Is our universe unique? How much of the universe is predetermined and fixed by mathematical consistency requirements, and how much is historical accident? What is the fate of the universe in the far future? Is our universe a one-off event, or does it regenerate itself by having cycles of evolution? It is clear that progress in this field will have a strong impact, both on science and on the perception that we humans have of our place in the universe.