Weakly Broken Galileon Symmetry in Cosmology
Abstract
In the present thesis, using an effective field theory point of view, we explore theories of singlefield inflation where higher derivative operators become relevant, affecting in a novel way the dynamics and therefore the observations. For instance, concerning the scalar spectrum, they allow for measurable equilateral nonGaussianity, whose amplitude can differ significantly from the predictions of other existing models. Moreover, we show that the stability and the consistency of such theories are ensured by an approximate Galileon symmetry. Indeed, being generically possible to build an invariant theory under Galileon transformations in flat spacetime, it is instead well known that such a symmetry is unavoidably broken by gravity. In principle, this might ruin the nice and interesting properties of the Galileons in flat backgrounds, such as the nonrenormalization theorem. However, we find that this does not happen if the Galileon invariance is broken only weakly, in a well defined sense, by a suitable coupling to gravity, providing therefore an extension of the quantum nonrenormalization properties in curved spacetimes. Hence, besides discussing the phenomenological consequences and the observational predictions for inflation, we apply such Galileon theories to the context of the latetime acceleration of the Universe. In the last part, in order to probe nonstandard primordial scenarios, they are also employed in a cosmology where the Big Bang singularity is smoothed down and the Universe emerges from a Minkowski spacetime, in a well defined extension at all times of the Galilean Genesis scenario.
 Publication:

arXiv eprints
 Pub Date:
 December 2016
 arXiv:
 arXiv:1612.01154
 Bibcode:
 2016arXiv161201154S
 Keywords:

 High Energy Physics  Theory
 EPrint:
 PhD thesis, Scuola Normale Superiore, Oct 2016