author={Marcelo A. Alvarez and Ue-li Pen and Tzu-Ching Chang},
  journal={The Astrophysical Journal Letters},
  pages={L17 - L21}
Before the universe was reionized, it was likely that the spin temperature of intergalactic hydrogen was decoupled from the cosmic microwave background (CMB) by UV radiation from the first stars through the Wouthuysen–Field effect. If the intergalactic medium (IGM) had not yet been heated above the CMB temperature by that time, then the gas would appear in absorption relative to the CMB. Large, rare sources of X-rays could inject sufficient heat into the neutral IGM, so that δTb>0 at comoving… 
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