Unsuccessful cosmology with Modified Gravity Models

TL;DR

This study evaluates a class of Modified Gravity Models with inverse curvature invariants, finding they only produce viable, singularity-free cosmologies in limited parameters but are generally unstable, making them unviable.

Contribution

The paper thoroughly analyzes the full parameter space of a specific Modified Gravity Model class and assesses their cosmological viability and stability.

Findings

Singularity-free solutions exist only in restricted parameters.

All models exhibit an unstable scalar mode.

Models are deemed not viable due to instabilities.

Abstract

A class of Modified Gravity Models, consisting of inverse powers of linear combination of quadratic curvature invariants, is studied in the full parameter space. We find that singularity-free cosmological solutions, interpolating between an almost-Friedmann universe at Big Bang Nucleosynthesis and an accelerating universe today, exist only in a restricted parameter space. Furthermore, for all parameters of the models, there is an unstable scalar mode of the gravitational field. Therefore we conclude that this class of Modified Gravity Models is not viable.