Aspects of Infrared Modifications of Gravity

TL;DR

This thesis explores modifications to gravity through Lorentz-invariant models of massive and massless spin-2 and spin-3/2 particles, and investigates non-linear extensions like unimodular gravity and bigravity, analyzing their solutions and causal structures.

Contribution

It provides a comprehensive analysis of consistent Lagrangians for higher-spin particles and develops non-linear gravity models that modify General Relativity at large scales.

Findings

Exact solutions for modified gravity models

Causal structures of non-linear solutions

Perturbation analysis for Lorentz-breaking cases

Abstract

In the first part of the thesis, and after an introduction to certain models of modified gravity, we study consistent Lagrangians for Lorentz invariant (massive and massless) spin-2 and spin-3/2 particles in flat space. The second part of the dissertation is devoted to non-linear extensions for the spin-2 case, focusing on unimodular gravity and bigravity. Both theories lead to modifications of General Relativity at large distances and we will study exact solutions, causal structure of those solutions, and perturbation theory (specially for the Lorentz breaking case). Some comments on quantization of these theories can be found in an appendix.