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 Experiments The following telescopes have either hosted intensity mapping surveys, or plan to carry them out in future. TIANLAI (China) BINGO (Brazil/Uruguay/UK) CHIME (Canada) COMAP (USA) FAST (China) Green Bank Telescope (USA) HIRAX (South Africa) KAT7 (South Africa) MeerKAT (South Africa) Parkes radio telescope (Australia) PAPER (USA/South Africa/Australia) Square Kilometre Array (South Africa/Australia) References External links Oxford Martin workshop on intensity mapping RAS discussion meeting on intensity mapping CHIME experiment Physical cosmology Observational astronomy Large-scale structure of the cosmos