CMB and 21-cm signals for dark matter with a long-lived excited state

Motivated by the eXciting dark matter model of Finkbeiner and Weiner, hypothesized to explain the 511 keV signal in the center of the Milky Way, we consider the cosmic microwave background 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 10^13 s, we discuss the signatures of such relic excitation on cosmic microwave background and high-z 21-cm observations. Such models have potentially richer astrophysical signals than the traditional weakly interacting massive particle annihilations and decays, and may have observable consequences for future generations of experiments.