Inflation in the context of Planck 2013

Abstract

On the frontiers of modern cosmology is quantum cosmology, which brings together two speculative ideas: inflation and quantum gravity. While, there are theories such as the ekpyrotic model, which do not make use of inflation, a vast majority of the theories support inflation. The real motivation for these speculative ideas lie in the fact that they can be observed as inhomogeneities in the Cosmic Microwave Background (CMB), and this has given us a window to the working of the early universe. This was one of the key motivations underlying the Planck collaboration, which measured the CMB data at exceedingly high sensitivity (×3 compared to WMAP data). This has ushered in a new era of what one may call “precision cosmology”. With this increased sensitivity, Planck data has already disfavoured some of the previous inflationary models, and as new theories are being put forth- emphasis has shifted from just phenomenological models to theoretically-motivated models of inflation. The Lyth bound, and the actual bound on tensor-to-scalar ratio have not been very helpful in deciding the energy scale of inflation, but a theory incorporating a grand unified theory or a theory of quantum gravity is well justified. It is in this context that string theory has contributed to some of the models. A form of heterotic string action has been studied in this thesis, at lower energy scales. The popular theories of inflation such as brane-inflation, trace anomaly driven inflation and Starobinsky inflation have been briefly discussed. The research in this topic is a very active one, and in the next year, with the release of Planck data 2014 containing B-mode data, the interest in this field will only increase further. The recent announcement of BICEP2 dataset, supporting inflation at an energy scale higher than previously believed, is clearly going to increase interest in the new field of “String cosmology”.