Restricting Fourth Order Gravity via Cosmology

TL;DR

This paper investigates how fourth order gravity modifications affect cosmological evolution, showing conditions under which they can be consistent with observations and tightening constraints on such theories.

Contribution

It provides explicit conditions for fourth order gravity to match standard cosmology and refines constraints on these theories during different cosmological epochs.

Findings

Constraints on fourth order gravity are improved by up to 30 orders during inflation.

Cosmological history can be approximately reproduced with small deviations from GR.

Tensor mode evolution remains unchanged under certain conditions.

Abstract

The cosmology of general fourth order corrections to Einstein gravity is considered, both for a homogeneous and isotropic background and for general tensor perturbations. It is explicitly shown how the standard cosmological history can be (approximately) reproduced and under what condition the evolution of the tensor modes remain (approximately) unchanged. Requiring that the deviations from General Relativity are small during inflation sharpens the current constraints on such corrections terms by some thirty orders of magnitude. Taking a more conservative approach and requiring only that cosmology be approximately that of GR during Big Bang Nucleosynthesis, the constraints are improved by 4 - 6 orders of magnitude.