21 cm cosmology and spin temperature reduction via spin-dependent dark
  matter interactions
  The EDGES low-band experiment has measured an absorption feature in the
cosmic microwave background radiation (CMB), corresponding to the 21 cm
hyperfine transition of hydrogen at redshift $z \simeq 17$, before the era of
cosmic reionization. The amplitude of this absorption is connected to the ratio
of singlet and triplet hyperfine states in the hydrogen gas, which can be
parametrized by a spin temperature. The EDGES result suggests that the spin
temperature is lower than the expected temperatures of both the CMB and the
hydrogen gas. A variety of mechanisms have been proposed in order to explain
this signal, for example by lowering the kinetic temperature of the hydrogen
gas via dark matter interactions. We introduce an alternative mechanism, by
which a sub-GeV dark matter particle with spin-dependent coupling to nucleons
or electrons can cause hyperfine transitions and lower the spin temperature
directly, with negligible reduction of the kinetic temperature of the hydrogen
gas. We consider a model with an asymmetric dark matter fermion and a light
pseudo-vector mediator. Significant reduction of the spin temperature by this
simple model is excluded, most strongly by coupling constant bounds coming from
stellar cooling. Perhaps an alternative dark sector model, subject to different
sets of constraints, can lower the spin temperature by the same mechanism.
