In cosmology, intensity mapping is an observational technique for surveying the
large-scale structure of the universe
The observable universe is a spherical region of the universe consisting of all matter that can be observed from Earth; the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of th ...
by using the integrated
radio emission from unresolved gas clouds.
In its most common variant, 21 cm intensity mapping, the
21 cm 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
redshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...
ed 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 need to be individually detected and measured, making intensity mapping a considerably faster method.
History
* Aug 1977: Varshalovich and Khersonskii calculate the effect of 21cm line absorption at high redshift on the spectrum of the CMB.
* 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
In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure format ...
.
* Jun 2006: Peterson, Bandura, and Pen propose the Hubble Sphere Hydrogen Survey
* Mar 2009: Cosmological HI signal observed for the first time out to redshift 1.12 by the
Green Bank Telescope
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Rad ...
.
* Jan 2013: Construction begins on the
CHIME experiment in
British Columbia
British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...
, Canada.
Scientific applications
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
In the fields of Big Bang theory and physical cosmology, cosmology, reionization is the process that caused electrically neutral atoms in the primordial universe to reionize after the lapse of the "Timeline of the Big Bang#Dark Ages, dark ages". ...
, the
baryon
In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...
ic 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
In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure format ...
) 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
megaparsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s, 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
In economics, inflation is an increase in the average price of goods and services in terms of money. This increase is measured using a price index, typically a consumer price index (CPI). When the general price level rises, each unit of curre ...
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
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Rad ...
(USA)
HIRAX(South Africa)
*
KAT7 (South Africa)
*
MeerKAT
The meerkat (''Suricata suricatta'') or suricate is a small mongoose found in southern Africa. It is characterised by a broad head, large eyes, a pointed snout, long legs, a thin tapering tail, and a brindled coat pattern. The head-and-body ...
(South Africa)
*
Parkes radio telescope
Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...
(Australia)
*
PAPER
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses, Feces#Other uses, herbivore dung, or other vegetable sources in water. Once the water is dra ...
(USA/South Africa/Australia)
*
Square Kilometre Array
The Square Kilometre Array (SKA) is an intergovernmental organisation, intergovernmental international radio telescope project being built in Australia (low-frequency) and South Africa (mid-frequency). The combining infrastructure, the Square ...
(South Africa/Australia)
The
Goddard Space Flight Center
The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory located approximately northeast of Washington, D.C., in Greenbelt, Maryland, United States. Established on May 1, 1959, as NASA's first space flight center, GSFC ...
also host a list of intensity mappin
experiments
References
{{reflist
External links
Oxford Martin workshop on intensity mappingRAS discussion meeting on intensity mappingCHIME experiment
Physical cosmology
Observational astronomy
Large-scale structure of the cosmos