Weakly Broken Galileon Symmetry in Cosmology

TL;DR

This paper investigates single-field inflation models with weakly broken Galileon symmetry, highlighting their unique non-Gaussian signatures, stability features, and applications to late-time acceleration and non-singular cosmologies.

Contribution

It introduces a framework where Galileon symmetry is weakly broken by gravity, extending non-renormalization properties to curved spacetime and exploring diverse cosmological implications.

Findings

Enhanced equilateral non-Gaussianity in scalar spectrum

Stable theories with weakly broken Galileon symmetry

Application to non-singular cosmological scenarios

Abstract

In the present thesis, using an effective field theory point of view, we explore theories of single-field 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 non-Gaussianity, 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 space-time, 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 non-renormalization 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 non-renormalization properties in curved space-times. Hence, besides discussing the phenomenological consequences and the observational predictions for inflation, we apply such Galileon theories to the context of the late-time acceleration of the Universe. In the last part, in order to probe non-standard primordial scenarios, they are also employed in a cosmology where the Big Bang singularity is smoothed down and the Universe emerges from a Minkowski space-time, in a well defined extension at all times of the Galilean Genesis scenario.