Cosmographic constraints on a class of Palatini f(R) gravity

TL;DR

This paper uses cosmographic data to constrain a specific class of Palatini f(R) gravity models, finding that only models with positive n are viable, thus narrowing the possible modifications to general relativity.

Contribution

It provides new cosmographic constraints on the f(R)=R - α/R^n models in the Palatini approach, especially restricting the parameter n to positive values.

Findings

Models with negative n are excluded by cosmographic constraints.

The evolution of the deceleration parameter q(z) supports positive n.

Broad restrictions on the class of f(R) gravity models are established.

Abstract

Modified gravity, known as $f(R)$ gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. For this kind of gravity the expansion of the Universe may accelerate while containing only baryonic and cold dark matter. The aim of the present investigation is to place cosmographic constraints on the class of theories of the form $f(R)=R - \alpha/R^n$ within the Palatini approach. Although extensively discussed in recent literature and confronted with several observational data sets, cosmological tests are indeed inconclusive about the true signal of $n$ in this class of theories. This is particularly important to define which kind of corrections (infra-red or high-energy) to general relativity this class of theory indeed represent. We shed some light on this question by examining the evolution of the deceleration parameter $q(z)$ for these theories. We find that for a large range of $\alpha$, models based on $f(R) = R - \alpha/R^{n}$ gravity in the Palatini approach can only have positive values for $n$, placing thus a broad restriction on this class of gravity.