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 do not need to be individually detected and measured, making intensity mapping a considerably faster method.
- 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.
- 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.
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 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) by using the baryon acoustic oscillation feature in the matter power spectrum as a standard ruler. The growth rate of structure, useful for testing modifications to general relativity, 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
The following telescopes have either hosted intensity mapping surveys, or plan to carry them out in future.
- BINGO (Brazil/Uruguay/UK)
- CHIME (Canada)
- COMAP (USA) 
- FAST (China)
- Green Bank Telescope (USA)
- HIRAX (South Africa)
- KAT7 (South Africa)
- Parkes radio telescope (Australia)
- PAPER (USA/South Africa/Australia)
- Square Kilometre Array (South Africa/Australia)
- Bull, Philip; Ferreira, Pedro G.; Patel, Prina; Santos, Mario G. (2014-05-06). "Late-time cosmology with 21cm intensity mapping experiments". The Astrophysical Journal. 803: 21. arXiv: . Bibcode:2015ApJ...803...21B. doi:10.1088/0004-637X/803/1/21.
- Madau, Piero; Meiksin, Avery; Rees, Martin J. "21 Centimeter Tomography of the Intergalactic Medium at High Redshift". The Astrophysical Journal. 475 (2): 429–444. arXiv: . Bibcode:1997ApJ...475..429M. doi:10.1086/303549.
- Bharadwaj, Somnath; Sethi, Shiv K. (December 2001). "HI fluctuations at large redshifts: I-visibility correlation". Journal of Astrophysics and Astronomy. 22 (4): 293–307. arXiv: . Bibcode:2001JApA...22..293B. doi:10.1007/BF02702273.
- Battye, Richard A.; Davies, Rod D.; Weller, Jochen. "Neutral hydrogen surveys for high-redshift galaxy clusters and protoclusters". Monthly Notices of the Royal Astronomical Society. 355 (4): 1339–1347. arXiv: . Bibcode:2004MNRAS.355.1339B. doi:10.1111/j.1365-2966.2004.08416.x.
- Chang, Tzu-Ching; Pen, Ue-Li; Bandura, Kevin; Peterson, Jeffrey B. (22 July 2010). "An intensity map of hydrogen 21-cm emission at redshift z ≈ 0.8". Nature. 466 (7305): 463–465. Bibcode:2010Natur.466..463C. doi:10.1038/nature09187. PMID 20651685.
- "Construction begins on Canada's largest radio telescope". Phys.org. 2013-01-24. Retrieved 17 August 2014.
- Loeb, Abraham; Zaldarriaga, Matias (May 2004). "Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations through 21 cm Tomography Prior to the Epoch of Structure Formation". Physical Review Letters. 92 (21): 211301. arXiv: . Bibcode:2004PhRvL..92u1301L. doi:10.1103/PhysRevLett.92.211301. PMID 15245272.
- Hall, Alex; Bonvin, Camille; Challinor, Anthony (19 March 2013). "Testing general relativity with 21-cm intensity mapping". Physical Review D. 87 (6). arXiv: . Bibcode:2013PhRvD..87f4026H. doi:10.1103/PhysRevD.87.064026.
- Camera, Stefano; Santos, Mário G.; Ferreira, Pedro G.; Ferramacho, Luís. "Cosmology on Ultralarge Scales with Intensity Mapping of the Neutral Hydrogen 21 cm Emission: Limits on Primordial Non-Gaussianity". Physical Review Letters. 111 (17): 171302. arXiv: . Bibcode:2013PhRvL.111q1302C. doi:10.1103/PhysRevLett.111.171302. PMID 24206474.
- Maartens, Roy; Zhao, Gong-Bo; Bacon, David; Koyama, Kazuya; Raccanelli, Alvise (26 February 2013). "Relativistic corrections and non-Gaussianity in radio continuum surveys". Journal of Cosmology and Astroparticle Physics. 2013 (02): 044–044. arXiv: . Bibcode:2013JCAP...02..044M. doi:10.1088/1475-7516/2013/02/044.
- Lidz, Adam; Furlanetto, Steven R.; Peng Oh, S.; Aguirre, James; Chang, Tzu-Ching; Doré, Olivier; Pritchard, Jonathan R. (10 November 2011). "INTENSITY MAPPING WITH CARBON MONOXIDE EMISSION LINES AND THE REDSHIFTED 21 cm LINE". The Astrophysical Journal. 741 (2): 70. arXiv: . Bibcode:2011ApJ...741...70L. doi:10.1088/0004-637X/741/2/70.
- Gong, Yan; Cooray, Asantha; Silva, Marta; Santos, Mario G.; Bock, James; Bradford, C. Matt; Zemcov, Michael (January 2012). "Intensity Mapping of the [CII] Fine Structure Line during the Epoch of Reionization". The Astrophysical Journal. 745: 49. arXiv: . Bibcode:2012ApJ...745...49G. doi:10.1088/0004-637X/745/1/49.
- Pullen, Anthony R.; Doré, Olivier; Bock, Jamie (May 2014). "Intensity Mapping across Cosmic Times with the Lyα Line". The Astrophysical Journal. 786: 111. arXiv: . Bibcode:2014ApJ...786..111P. doi:10.1088/0004-637X/786/2/111.
- "Cahill Radio Astronomy Lab - CRAL". www.astro.caltech.edu. Retrieved 2017-11-06.