In cosmology, intensity mapping is an observational technique for surveying the large-scale structure of the universe by using the integrated radio emission from unresolved gas clouds.
In its most common variant, 21 cm intensity mapping, the 21cm emission line of neutral hydrogen is used to trace the gas. The hydrogen follows fluctuations in the underlying cosmic density field, with regions of higher density giving rise to a higher intensity of emission. Intensity fluctuations can therefore be used to reconstruct the power spectrum of matter fluctuations. The frequency of the emission line is redshifted by the expansion of the Universe, so by using radio receivers that cover a wide frequency band, one can detect this signal as a function of redshift, and thus cosmic time. This is similar in principle to a galaxy redshift survey, with the important distinction that galaxies need to be individually detected and measured, making intensity mapping a considerably faster method.[1]