Symmetron Cosmology

TL;DR

This paper investigates the cosmological evolution of the symmetron scalar field, showing it reaches its symmetry-breaking state by today, but requires a cosmological constant for late-time acceleration, with potential modifications influencing cosmic dynamics.

Contribution

It provides a detailed analysis of symmetron field evolution across cosmic epochs, linking local gravity constraints with cosmological behavior, and explores potential modifications for dark energy effects.

Findings

Symmetron reaches symmetry-breaking vacuum by today.

Simple potentials are too small for dark energy role.

Adding a cosmological constant is necessary for late-time acceleration.

Abstract

The symmetron is a scalar field associated with the dark sector whose coupling to matter depends on the ambient matter density. The symmetron is decoupled and screened in regions of high density, thereby satisfying local constraints from tests of gravity, but couples with gravitational strength in regions of low density, such as the cosmos. In this paper we derive the cosmological expansion history in the presence of a symmetron field, tracking the evolution through the inflationary, radiation- and matter-dominated epochs, using a combination of analytical approximations and numerical integration. For a broad range of initial conditions at the onset of inflation, the scalar field reaches its symmetry-breaking vacuum by the present epoch, as assumed in the local analysis of spherically-symmetric solutions and tests of gravity. For the simplest form of the potential, the energy scale is too small for the symmetron to act as dark energy, hence we must add a cosmological constant to drive late-time cosmic acceleration. We briefly discuss a class of generalized, non-renormalizable potentials that can have a greater impact on the late-time cosmology, though cosmic acceleration requires a delicate tuning of parameters in this case.