Quartic Chameleons: Safely Scale-Free in the Early Universe

TL;DR

This paper explores a quartic chameleon scalar field model that avoids early universe problems of runaway potentials, remaining well-behaved during nucleosynthesis and preventing catastrophic perturbations.

Contribution

It introduces a scale-free quartic potential for chameleon fields, demonstrating improved stability and calculability in the early universe compared to traditional runaway potentials.

Findings

Quartic potential prevents early universe breakdowns.

Chameleon field remains well-behaved at nucleosynthesis.

High-energy perturbations only occur at very high temperatures.

Abstract

In chameleon gravity, there exists a light scalar field that couples to the trace of the stress-energy tensor in such a way that its mass depends on the ambient matter density, and the field is screened in local, high-density environments. Recently it was shown that, for the runaway potentials commonly considered in chameleon theories, the field's coupling to matter and the hierarchy of scales between Standard Model particles and the energy scale of such potentials result in catastrophic effects in the early Universe when these particles become nonrelativistic. Perturbations with trans-Planckian energies are excited, and the theory suffers a breakdown in calculability at the relatively low temperatures of Big Bang Nucleosynthesis. We consider a chameleon field in a quartic potential and show that the scale-free nature of this potential allows the chameleon to avoid many of the problems encountered by runaway potentials. Following inflation, the chameleon field oscillates around the minimum of its effective potential, and rapid changes in its effective mass excite perturbations via quantum particle production. The quartic model, however, only generates high-energy perturbations at comparably high temperatures and is able remain a well-behaved effective field theory at nucleosynthesis.