The Paths of Gravity in Galileon Cosmology

TL;DR

This paper investigates the evolution of gravitational strength in various Galileon cosmological models, analyzing their stability, and constraining their parameters based on early universe instabilities.

Contribution

It provides a comprehensive analysis of three classes of Galileon models, exploring their expansion history, perturbations, and stability constraints, highlighting the severe limitations of coupled models.

Findings

Linear and derivative coupled models are constrained by early universe instabilities.

The gravitational strength varies significantly over cosmic time in these models.

Parameter space is restricted by ghost-free and stability criteria.

Abstract

Galileon gravity offers a robust gravitational theory for explaining cosmic acceleration, having a rich phenomenology of testable behaviors. We explore three classes of Galileon models -- standard uncoupled, and linearly or derivatively coupled to matter -- investigating the expansion history with particular attention to early time and late time attractors, as well as the linear perturbations. From the relativistic and nonrelativistic Poisson equations we calculate the generalizations of the gravitational strength (Newton's constant), deriving its early and late time behavior. By scanning through the parameters we derive distributions of the gravitational strength at various epochs and trace the paths of gravity in its evolution. Using ghost-free and stability criteria we restrict the allowed parameter space, finding in particular that the linear and derivative coupled models are severely constrained by classical instabilities in the early universe.