The Cosmology of Dark Energy Radiation

TL;DR

This paper explores the cosmological implications of dark energy radiation, proposing a model where dark energy includes a thermal bath of relativistic particles, and assesses detection prospects and current observational constraints.

Contribution

It introduces a novel model of dark energy involving a thermal bath of relativistic particles and evaluates its observational signatures and detection prospects.

Findings

Dark energy radiation can have an abundance up to $oldsymbol{ ext{Ω}}_{ ext{DER}}=0.03$

Current data cannot distinguish dark energy radiation models from Quintessence

Detection of dark energy radiation requires sensitivities around 6 meV for neutrinos

Abstract

In this work, we quantify the cosmological signatures of dark energy radiation -- a novel description of dark energy, which proposes that the dynamical component of dark energy is comprised of a thermal bath of relativistic particles sourced by thermal friction from a slowly rolling scalar field. For a minimal model with particle production emerging from first principles, we find that the abundance of radiation sourced by dark energy can be as large as $\Omega_{\text{DER}} = 0.03$, exceeding the bounds on relic dark radiation by three orders of magnitude. Although the background and perturbative evolution of dark energy radiation is distinct from Quintessence, we find that current and near-future cosmic microwave background and supernova data will not distinguish these models of dark energy. We also find that our constraints on all models are dominated by their impact on the expansion rate of the Universe. Considering extensions that allow the dark radiation to populate neutrinos, axions, and dark photons, we evaluate the direct detection prospects of a thermal background comprised of these candidates consistent with cosmological constraints on dark energy radiation. Our study indicates that a resolution of $\sim 6 \, \text{meV}$ is required to achieve sensitivity to relativistic neutrinos compatible with dark energy radiation in a neutrino capture experiment on tritium. We also find that dark matter axion experiments lack sensitivity to a relativistic thermal axion background, even if enhanced by dark energy radiation, and dedicated search strategies are required to probe new parameter space. We derive constraints arising from a dark photon background from oscillations into visible photons, and find that viable parameter space can be explored with the LAte Dark Energy RAdiation (LADERA) experiment.