An anisotropic bouncing universe in non-local gravity

TL;DR

This paper demonstrates that non-local gravity theories can produce a cosmological bounce in anisotropic universes, with controlled anisotropy growth and a matter sector including radiation, stiff matter, and k-matter, avoiding ghosts.

Contribution

It introduces a novel bouncing solution in anisotropic non-local gravity, showing controlled anisotropy and ghost-free behavior during the bounce and late-time acceleration.

Findings

Anisotropic shear peaks at the bounce and then decreases.

The matter sector includes radiation, stiff matter, and k-matter.

Non-local gravity allows for ghost-free bouncing solutions.

Abstract

We show that it is possible to realize a cosmological bouncing solution in an anisotropic but homogeneous Bianchi-I background in a class of non-local, infinite derivative theories of gravity. We show that the anisotropic shear grows slower than in general relativity during the contraction phase, peaks to a finite value at the bounce point, and then decreases as the universe asymptotes towards isotropy and homogeneity, and ultimately to de Sitter. Along with a cosmological constant, the matter sector required to drive such a bounce is found to consist of three components - radiation, stiff matter and $k$-matter (whose energy density decays like the inverse square of the average scale factor). Generically, $k$-matter exerts anisotropic pressures. We will test the bouncing solution in local and non-local gravity and show that in the latter case it is possible to simultaneously satisfy positivity of energy density and, at least in the late time de Sitter phase, avoid the introduction of propagating ghost/tachyonic modes.