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.
At late times, after the Universe has reionized, most of the remaining neutral hydrogen is stored in dense gas clouds called damped Lyman-alpha systems, where it is protected from ionizing UV radiation.
As with galaxy redshift surveys, intensity mapping observations can be used to measure the geometry and expansion rate of the Universe (and therefore the properties of dark energy[1]) by using the baryon acoustic oscillation feature in the matter power spectrum as a standard ruler.
Both of these features are found at large scales of tens to hundreds of megaparsecs, which is why low angular resolution (unresolved) maps of neutral hydrogen are sufficient to detect them.
As such, intensity mapping has been proposed as a way of measuring phenomena on extremely large scales, including primordial non-Gaussianity from inflation[11] and general relativistic corrections to the matter correlation function.