Evolution of the Chameleon Scalar Field in the Early Universe

TL;DR

This paper investigates how primordial magnetic fields influence the early universe evolution of the chameleon scalar field, relaxing initial condition constraints and assessing implications for dark energy models.

Contribution

It demonstrates that primordial magnetic fields can relax the initial condition fine-tuning required for the chameleon field to reach its attractor solution.

Findings

Primordial magnetic fields relax initial condition constraints for the chameleon.

A lower bound on magnetic field strength is established.

Chameleon evolution is affected by electromagnetic coupling in the early universe.

Abstract

In order to satisfy limits on the allowed variation of particle masses from big bang nucleosynthesis (BBN) until today, the chameleon scalar field is required to reach its attractor solution early on in its cosmological evolution. Brax et al. (2004) have shown this to be possible for certain specific initial conditions on the chameleon field at the end of inflation. However the extreme fine-tuning necessary to achieve this, poses a problem if the chameleon is to be viewed a natural candidate for dark energy. In this article we revisit the behaviour of the chameleon in the early Universe, including the additional coupling to electromagnetism proposed by Brax et al. (2011). Solving the chameleon evolution equations in the presence of a primordial magnetic field, we find that the strict initial conditions on the chameleon field at the end of inflation can be relaxed, and we determine the associated lower bound on the strength of the primordial magnetic field.