Observational Bounds on Modified Gravity Models

TL;DR

This paper investigates broad classes of modified gravity models as explanations for cosmic acceleration, using a general Taylor expansion approach and current observational data to constrain model parameters.

Contribution

It introduces a model-independent Taylor expansion method for modified gravity functions and constrains their parameters using current cosmological observations.

Findings

Current data constrain the Taylor expansion coefficients.

Next-generation surveys will significantly tighten these constraints.

The approach assesses theoretical viability related to ghosts and instabilities.

Abstract

Modified gravity provides a possible explanation for the currently observed cosmic accelaration. In this paper, we study general classes of modified gravity models. The Einstein-Hilbert action is modified by using general functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in the Palatini formalisms. We do not use an explicit form for the functions, but a general form with a valid Taylor expansion up to second order about redshift zero in the Riemann-scalars. The coefficients of this expansion are then reconstructed via the cosmic expansion history measured using current cosmological observations. These are the quantities of interest for theoretical considerations relating to ghosts and instabilities. We find that current data provide interesting constraints on the coefficients. The next-generation dark energy surveys should shrink the allowed parameter space for modifed gravity models quite dramatically.