Too Hot, Too Cold or Just Right? Implications of a 21-cm Signal for Dark
  Matter Annihilation and Decay
  Measurements of the temperature of the baryons at the end of the cosmic dark
ages can potentially set very precise constraints on energy injection from
exotic sources, such as annihilation or decay of the dark matter. However,
additional effects that lower the gas temperature can substantially weaken the
expected constraints on exotic energy injection, whereas additional radiation
backgrounds can conceal the effect of an increased gas temperature in
measurements of the 21-cm hyperfine transition of neutral hydrogen. Motivated
in part by recent claims of a detection of 21-cm absorption from a redshift of
17 by the EDGES experiment, we derive the constraints on dark matter
annihilation and decay that can be placed in the presence of extra radiation
backgrounds or effects that modify the gas temperature, such as dark
matter-baryon scattering and early baryon-photon decoupling. We find that if
the EDGES observation is confirmed, then constraints on light dark matter
decaying or annihilating to electrons will in most scenarios be stronger than
existing state-of-the-art limits from the cosmic microwave background,
potentially by several orders of magnitude. More generally, our results allow
mapping any future measurement of the global 21-cm signal into constraints on
dark matter annihilation and decay, within the broad range of scenarios we
consider.
