21 cm cosmology and spin temperature reduction via spin-dependent dark matter interactions

Understand

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.