Hint of an unexpectedly cool early universe suggests interacting dark matter

Just before the first stars turned on, three-quarters of the conventional matter of the universe consisted of neutral hydrogen atoms. As in the case in the modern universe, the spin of a hydrogen atom’s electron was usually antiparallel to the spin of its proton. But some of the hydrogen was in a higher-energy state with parallel spins; the energy difference between the two states corresponds to a photon with a rest-frame wavelength of 21 cm. Because the gas of hydrogen atoms had adiabatically expanded since shortly after the Big Bang, its bulk kinetic temperature was less than that of the ambient cosmic radiation. The two-state spin system, however, was in thermal equilibrium with the cosmic radiation, so in the time just before star formation, neither net 21 cm emission nor absorption occurred.