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. History Aug 1977: Varshalovich and Khersonskii calculate the effect of 21cm line absorption at high redshift on the spectrum of the CMB. Aug 1996: Madau, Meiksin & Rees propose intensity mapping as a way of probing the Epoch of Reionization. Dec 2001: Bharadwaj & Sethi propose using intensity maps of neutral hydrogen to observe the matter distribution in the post-reionisation epoch. Jan 2004: Battye, Davies & Weller propose using 21 cm intensity maps to measure dark energy. Jun 2006: Peterson, Bandura, and Pen propose the Hubble Sphere Hydrogen Survey Mar 2009: Cosmological HI signal observed for the first time out to redshift 1.12 by the Green Bank Telescope. Jan 2013: Construction begins on the CHIME experiment in British Columbia, Canada. Scientific applications Intensity mapping has been proposed as a way of measuring the cosmic matter density field in several different regimes. Epoch of Reionization Between the times of recombination and reionization, the baryonic content of the Universe – mostly hydrogen – existed in a neutral phase. Detecting the 21 cm emission from this time, all the way through to the end of reionization, has been proposed as a powerful way of studying early structure formation. This period of the Universe's history corresponds to redshifts of to , implying a frequency range for intensity mapping experiments of 50 – 200 MHz. Large-scale structure and dark energy At late times, after the Universe has reionized, most of the remaining neutral hydrogen is stored in dense gas clouds called