Spectator dark matter in non-standard cosmologies

TL;DR

This paper examines how non-standard early Universe expansion rates affect the production and characteristics of spectator dark matter generated during inflation, highlighting implications for observational constraints and testability.

Contribution

It analyzes the robustness of spectator dark matter production under non-standard cosmologies, including effects on density evolution and isocurvature spectra, extending previous models.

Findings

Dark matter density evolution differs in non-standard cosmologies.

Isocurvature perturbation spectra are significantly altered.

Scenario remains testable via primordial isocurvature and non-Gaussianity.

Abstract

It has been shown that the observed dark matter (DM) abundance can be produced by amplification of quantum fluctuations of an energetically subdominant scalar field during inflation. In this paper, we study the robustness of this "spectator dark matter" scenario to changes in the expansion rate of the early Universe. Compared to the standard radiation-dominated (RD) scenario, two aspects will change: the DM energy density evolves differently as a function of time, and also the DM isocurvature perturbation spectrum will be different from the result in the RD case. These can impose sizeable changes to the values of model parameters which allow the field to constitute all DM while simultaneously satisfying all observational constraints. We study both free and self-interacting DM in scenarios with non-standard expansion and quantify the changes to the cases with a standard cosmological history. We also discuss testability of the scenario through primordial DM isocurvature and non-Gaussianity.