In recent years, observational evidence for an accelerated expansion of the Universe has triggered investigation of non-conventional cosmological models. At the same time, the precision reached in the observation of the Cosmic Microwave Background Radiation (CMBR) offers the possibility of detecting deviations from the predictions of ordinary models of the Early Universe.
In particular, models in which the observed Universe is a four-dimensional surface in a higher dimensional space-time (so called Brane-World models) offer a wide range of possibly interesting new phenomenology. In this context, we investigate scenarios in which Lorentz invariance is violated by the geometry of the higher dimensional space-time. We will study the consequences of this fact on both the early- and late-time cosmological evolution. In particular, we will address the questions whether these type of constructions can accommodate the possibility of a late-time accelerated expansion, whether there can be distinctive signatures of this type of Lorentz-violation in the spectrum of the CMBR, and whether these models provide new string-theoretical realizations of Inflation.
We will also explore the consequences of breaking Lorentz invariance from an effective field theoretical point of view. We will address these questions in two kinds of models with different nature of Lorentz violation: in the first class of models Lorentz invariance is broken by the bulk metric; in the second, the bulk metric is Lorentz-invariant but the boundary conditions are not. Within the latter class of models we will focus our study on a recently found time-dependent string theoretical construction. This model is particularly promising for realising inflationary cosmology in string theory.
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