DBI Galileons in the Einstein Frame: Local Gravity and Cosmology

TL;DR

This paper demonstrates that disformally coupled theories with Einstein-Hilbert gravity are equivalent to quartic DBI Galileon models, revealing new screening mechanisms and implications for dark energy and structure formation.

Contribution

It establishes the equivalence between disformal couplings and DBI Galileon theories, simplifying dynamics and exploring their cosmological and local gravity effects.

Findings

Disformal screening mechanism exists in dense environments.

A Vainshtein radius characterizes the breakdown of solutions.

Disformally coupled dark matter models face observational challenges.

Abstract

It is shown that a disformally coupled theory in which the gravitational sector has the Einstein-Hilbert form is equivalent to a quartic DBI Galileon Lagrangian, possessing non-linear higher derivative interactions, and hence allowing for the Vainshtein effect. This Einstein Frame description considerably simplifies the dynamical equations and highlights the role of the different terms. The study of highly dense, non-relativistic environments within this description unravels the existence of a disformal screening mechanism, while the study of static vacuum configurations reveals the existence of a Vainshtein radius, at which the asymptotic solution breaks down. Disformal couplings to matter also allow the construction of Dark Energy models, which behave differently than conformally coupled ones and introduce new effects on the growth of Large Scale Structure over cosmological scales, on which the scalar force is not screened. We consider a simple Disformally Coupled Dark Matter model in detail, in which standard model particles follow geodesics of the gravitational metric and only Dark Matter is affected by the disformal scalar field. This particular model is not compatible with observations in the linearly perturbed regime. Nonetheless, disformally coupled theories offer enough freedom to construct realistic cosmological scenarios, which can be distinguished from the standard model through characteristic signatures.