Thermal Perturbations from Cosmological Constant Relaxation

TL;DR

This paper explores a cosmological model where thermal perturbations, sourced by a relaxion field interacting with a thermal bath, can produce the observed CMB anisotropies despite low inflationary Hubble scales.

Contribution

It introduces a model where thermal fluctuations from a relaxion field explain CMB anisotropies, addressing the cosmological constant problem and inflationary perturbation sourcing.

Findings

Thermal perturbations can seed CMB anisotropies at low inflationary scales.

A derivatively coupled relaxion model explains both dark energy and CMB fluctuations.

The model naturally produces the small cosmological constant.

Abstract

We probe the cosmological consequences of a recently proposed class of solutions to the cosmological constant problem. In these models, the universe undergoes a long period of inflation followed by a contraction and a bounce that sets the stage for the hot big bang era. A requirement of any successful early universe model is that it must reproduce the observed scale-invariant density perturbations at CMB scales. While these class of models involve a long period of inflation, the inflationary Hubble scale during their observationally relevant stages is at or below the current Hubble scale, rendering the de Sitter fluctuations too weak to seed the CMB anisotropies. We show that sufficiently strong perturbations can still be sourced thermally if the relaxion field serving as the inflaton interacts with a thermal bath, which can be generated and maintained by the same interaction. We present a simple model where the relaxion field is derivatively (i.e. technically naturally) coupled to a non-abelian gauge sector, which gets excited tachyonically and subsequently thermalizes due to its nonlinear self-interactions. This model explains both the smallness of the cosmological constant and the amplitude of CMB anisotropies.