CMB and 21-cm Signals for Dark Matter with a Long-Lived Excited State

TL;DR

This paper explores how dark matter models with long-lived excited states could produce detectable signals in the CMB and 21-cm observations, offering new avenues for understanding dark matter properties.

Contribution

It calculates relic excitation fractions for dark matter with long-lived excited states and discusses their potential signatures in CMB and 21-cm signals.

Findings

Relic excitation fraction can reach up to 1% in certain parameter regions.

Long-lived excited states can produce observable signatures in cosmological signals.

Models may yield richer signals than traditional WIMP annihilation or decay scenarios.

Abstract

Motivated by the eXciting Dark Matter (XDM) model of Finkbeiner & Weiner, hypothesized to explain the 511 keV signal in the center of the Milky Way, we consider the CMB and 21-cm signatures of models of dark matter with collisional long-lived excited states. We compute the relic excitation fraction from the early universe for a variety of assumptions about the collisional de-excitation cross-section and thermal decoupling. The relic excitation fraction can be as high as 1% for natural regions of parameter space, but could be orders of magnitude smaller. Since the lifetime of the excited state is naturally greater than 10^13s, we discuss the signatures of such relic excitation on cosmic microwave background (CMB) and high-z 21-cm observations. Such models have potentially richer astrophysical signals than the traditional WIMP annihilations and decays, and may have observable consequences for future generations of experiments.