Cosmology with explicit and spontaneous background fields

TL;DR

This paper investigates how explicit and spontaneous background fields that break diffeomorphism invariance influence cosmological models, deriving modified Friedmann equations and identifying conditions for accelerated expansion.

Contribution

It provides a comparative analysis of explicit and spontaneous background fields in cosmology, focusing on their dynamics and effects on universe expansion.

Findings

Explicit $t$-sector allows accelerated expansion with matter

Background fields can preserve isotropy and homogeneity

Modified Friedmann equations derived for both cases

Abstract

We study a general class of effective backgrounds that break diffeomorphism invariance and their potential roles in cosmology. Specifically, we examine both explicit and spontaneous background fields which display distinct transformation properties and are characterized with different dynamics. For explicit breaking, we focus on the $t$-sector of the minimal gravitational Standard-Model Extension (SME) and for spontaneous breaking on a vector field called the bumblebee model. In both cases, we derive the modified Friedmann equations and find a configuration of the background fields that preserve isotropy and homogeneity. We show that the explicit $t$-sector admits phases of accelerated expansion of the universe with standard matter.