The BIG ghost

University dissertation from JCAP

Abstract: In this thesis we present work done in an analysis of models of brane induced gravity. These are higher dimensional generalizations of Einstein's General relativity where our universe is considered as a brane in a higher dimensional bulk and where the gravitational dynamics on the brane is enhanced. This provides a modification of gravity on the brane as compared to ordinary general relativity, primarily at very large distances. These models are therefore very interesting for adressing the cosmological constant problem. Even though the basic setup is natural to consider from the perspective of effective field theory and also follow from certain string theoretical considerations, the models have been plagued by inconsistencies in the form of unstable modes. In particular, a ghostlike and tachyonic scalar mode appears already at the linear level in a perturbative treatment. In order to gain a deeper insight into the nature of these consistency problems we have revisited the models, performing a more extensive analysis of the generic models than has previously been done. We have worked entirely in a gauge invariant formalism in order not to be obscured by gauge issues. We have also incorporated an effective thickness of the brane in our analysis and performed an explicit analysis of the effect of contributions from the extrinsic geometry. Although our analysis has been carried out at the linear level in a perturbative treatment we are able to get a deeper understanding of the unstable mode and illuminate some of the difficulties of these models that would likely persist even in a full nonlinear analysis.