Requirements for Cosmological 21-cm Masers
  We perform Monte-Carlo calculations of the radiative transfer of Lyman Alpha (hereafter Lya) photons emitted by a source embedded in a neutral collapsing gas cloud. This represents a young galaxy or quasar during the early stages of the epoch of reionisation (EoR). After computing the Lya spectrum as function of radius and time, we find that the Lya color temperature may be negative in large volumes surrounding the central source. Motivated by this result, we explore the prospects for a population inversion in the hyperfine levels of atomic hydrogen via the Wouthuysen-Field (WF) effect. The reason for this exploration is clear: if 21-cm masers exist during the EoR, they could greatly boost the expected 21-cm flux from this epoch. We find that population inversion is unlikely to occur for several reasons: (1) the required Lya luminosities of the central source exceed ~10e45 erg/s. The radiation pressure exerted by such a large Lya flux likely halts the collapse of the cloud; (2) When quantum corrections to the WF-coupling strength are applied, the required Lya luminosities are (even) larger by orders of magnitude; (3) A relatively low flux of Lya photons that is produced via other channels (x-ray heating, collisional excitation of hydrogen) prevents the Lya color temperature from becoming negative. 
Mark Dijkstra, Abraham Loeb