Catastrophic Consequences of Kicking the Chameleon

TL;DR

This paper reveals that the chameleon mechanism, proposed to hide scalar fields in modified gravity theories, leads to catastrophic quantum particle production during the early universe, challenging previous classical models.

Contribution

It demonstrates that quantum effects during cosmic 'kicks' invalidate classical chameleon cosmology and introduces a new framework for analyzing particle production with strong dissipation.

Findings

Quantum particle production is significant during early universe kicks.

Dissipative effects alter the chameleon field's evolution.

High-energy fluctuations question the validity of the effective theory.

Abstract

The physics of the "dark energy" that drives the current cosmological acceleration remains mysterious, and the dark sector may involve new light dynamical fields. If these light scalars couple to matter, a screening mechanism must prevent them from mediating an unacceptably strong fifth force locally. Here we consider a concrete example: the chameleon mechanism. We show that the same coupling between the chameleon field and matter employed by the screening mechanism also has catastrophic consequences for the chameleon during the Universe's first minutes. The chameleon couples to the trace of the stress-energy tensor, which is temporarily non-zero in a radiation-dominated universe whenever a particle species becomes non-relativistic. These "kicks" impart a significant velocity to the chameleon field, causing its effective mass to vary non-adiabatically and resulting in the copious production of quantum fluctuations. Dissipative effects strongly modify the background evolution of the chameleon field, invalidating all previous classical treatments of chameleon cosmology. Moreover, the resulting fluctuations have extremely high characteristic energies, which casts serious doubt on the validity of the effective theory. Our results demonstrate that quantum particle production can profoundly affect scalar-tensor gravity, a possibility not previously considered. Working in this new context, we also develop the theory and numerics of particle production in the regime of strong dissipation.