visible in emission. For $T_S \gg T_\gamma$, the magnitude of the emission saturates to a maximum (redshift-dependent) brightness temperature that is about 25 to 35 mK for a mean-density, fully neutral IGM between redshifts 6 and 15, assuming a $\Lambda$CDM cosmology with $\Omega_m=0.3$, $\Omega_\Lambda=0.7$, $\Omega_b=0.04$, and $h=0.7$. At the other extreme, when the spin temperature is very small and $T_S \ll T_\gamma$, the line is visible in absorption against the CMB with a potentially very large (and negative) relative brightness temperature.

A number of factors are involved in predicting the typical differential brightness temperature of the redshifted 21 cm line as a function of redshift. In particular, the spin temperature must be treated in detail, including collisional coupling between the spin and kinetic temperatures of the gas, absorption of CMB photons, and heating by ultra-violet radiation from the first luminous sources. We direct the reader to @2006PhR...433..181F for a good introduction to the topic. The results of several efforts to predict the evolution of the differential brightness temperature of the redshifted 21 cm line have yielded predictions that are generally consistent in overall behavior, but vary highly in specific details . These models tend to agree that, for a finite 