Cosmology of modified (but second order) gravity

TL;DR

This paper reviews modified second-order gravity theories in cosmology, discussing methods to avoid instabilities, and explores their applications to dark energy, constrained by cosmological and Solar System data, with new insights into scalar-tensor and nonlocal models.

Contribution

It provides a comprehensive review of stable, second-order modified gravity models and introduces new perspectives on algebraic scalar-tensor theories and nonlocal actions in cosmology.

Findings

Viable parameter spaces constrained by cosmological and Solar System data

New remarks on algebraic scalar-tensor theories

Insights into nonlocal actions involving inverse d'Alembertian

Abstract

This is a brief review of modified gravity cosmologies. Generically extensions of gravity action involve higher derivative terms, which can result in ghosts and instabilities. There are three ways to circumvent this: Chern-Simons terms, first order variational principle and nonlocality. We consider recent cosmological applications of these three classes of modified gravity models, in particular to the dark energy problem. The viable parameter spaces can be very efficiently constrained by taking into account cosmological data from all epochs in addition to Solar system tests and stability considerations. We make some new remarks concerning so called algebraic scalar-tensor theories, biscalar reformulation of nonlocal actions involving the inverse d'Alembertian, and a possible covariant formulation holographic cosmology with nonperturbative gravity.