Intensity mapping is the measurement of cosmic matter density, in particular, the density of matter through time and space, i.e., producing an intensity map. This helps lay out the history of the universe and its parameters. It is referred to as line intensity mapping as generally it involves mapping the intensity of one or more spectral lines. Common are surveys such as 21cm experiments, and [CII] mapping.
Lyman-alpha forest-based surveys, both of which are examples of hydrogen intensity mapping.
Molecular lines are also of interest, such as carbon monoxide lines, e.g., to map out potential star-forming regions, both within the Milky Way and star formation over cosmological time and space. The 21cm line, [C II], Hα, and Lyα are also signs of starformation that can be viewed at a distance, e.g., to detect the presence of distant star-forming galaxies.
The intent of intensity mapping and some other surveys is coverage of all or a significant portion of the sky. Issues include the field of view, scan speed and the scan pattern for efficient coverage and for scheduling time. In some cases, the Earth's rotation is used, e.g., for surveys using cylindrical telescopes. For space-born surveys, rotation of the satellite might be used.
Intensity maps may be of emissions made at a range of redshifts (e.g., a tracer indicating star formation) or may be from an early emission that is influenced during its travel history through redshifts, such as studies of the CMB or 21 cm surveys. In the latter case, anisotropy is analyzed with interest to see whether it was in the original source (e.g., quantum fluctuations in the CMB) and if not, whether it is an indication of large scale structure at substantial redshifts, or is the result of some phenomena within the Milky Way, and if it is distant, whether it is due to some particular object (e.g., a known galaxy cluster) or can be ascribed to unresolved distant objects, e.g., faint galaxies. Thus such studies contribute to all these areas. The influences on the signal that are not of interest may be termed interlopers. Cross-correlations between intensity maps and other types of surveys, e.g., those covering the large scale structures and/or distant objects such as quasars are means of performing such analysis.