Superhorizon Isocurvature as a Window into Dark Matter Production

TL;DR

This paper explores how primordial isocurvature perturbations in dark sectors influence superhorizon curvature evolution and discusses how future measurements could distinguish dark matter production mechanisms.

Contribution

It provides a systematic, first-principles analysis of superhorizon isocurvature evolution and links it to observable signatures in dark matter models.

Findings

Superhorizon isocurvature can be generated without direct interactions between sectors.

Future measurements can discriminate between freeze-in and freeze-out dark matter mechanisms.

A simple framework to understand superhorizon curvature and isocurvature evolution.

Abstract

In the presence of primordial isocurvature perturbations, for example in a separate dark radiation sector, the superhorizon evolution of curvature perturbations becomes nontrivial. If the dark sector is radiation-like and constitutes a significant fraction of the energy density, its isocurvature can imply isocurvature in the inflaton sector even without direct interactions between the sectors. In this article, we revisit superhorizon curvature and isocurvature evolution in the long-wavelength limit systematically, drawing a simple picture of how to understand the nature of these fluctuations from first principles and without brute-force cosmic perturbation theory. We show how the described setup is able to source isocurvature in simple models of dark matter such as freeze-in and freeze-out and demonstrate that future measurements of matter and neutrino isocurvature can potentially discriminate between these two mechanisms.