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 z \approx 30 to z \approx 6 - 12, 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
damped Lyman-alpha systems, where it is protected from
ionizing UV radiation. These are predominantly hosted in galaxies, so the neutral hydrogen signal is effectively a tracer of the galaxy distribution. 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 can also be measured using
redshift space distortions. 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. This should be compared with the resolution of a redshift survey, which must detect individual galaxies that are typically only tens of kiloparsecs across. Because intensity mapping surveys can be carried out much faster than conventional optical redshift surveys, it is possible to map-out significantly larger volumes of the Universe. As such, intensity mapping has been proposed as a way of measuring phenomena on extremely large scales, including
primordial non-Gaussianity from
inflation and general relativistic corrections to the
matter correlation function.
Molecular and fine structure lines In principle, any emission line can be used to make intensity maps if it can be detected. Other emission lines that have been proposed as cosmological tracers include: • Rotational transitions in molecules, such as carbon monoxide • Fine structure transitions from species such as ionized carbon • Lyman-alpha emission from hydrogen == Experiments ==