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The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an
interferometric Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
array of 1024 6-
meter The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pr ...
(20 ft) diameter radio telescopes, operating at 400-800 MHz, that will be deployed at the Square Kilometer Array site in the
Karoo The Karoo ( ; from the Afrikaans borrowing of the South Khoekhoe !Orakobab or Khoemana word ''ǃ’Aukarob'' "Hardveld") is a semi- desert natural region of South Africa. No exact definition of what constitutes the Karoo is available, so its ...
region of
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring count ...
. The array is designed to measure red-shifted 21-cm
hydrogen line The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
emission on large angular scales, in order to map out the
baryon acoustic oscillations In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe, caused by acoustic density waves in the primordial plasma of the early universe. In the same way ...
, and constrain models of
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the unive ...
and
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
. The HIRAX collaboration is made up of over a dozen institutions, mainly from South Africa, the United States, and Canada, including the
University of KwaZulu-Natal The University of KwaZulu-Natal (UKZN) is a university with five campuses in the province of KwaZulu-Natal in South Africa. It was formed on 1 January 2004 after the merger between the University of Natal and the University of Durban-Westville ...
, the Durban University of Technology, the
African Institute for Mathematical Sciences The African Institute for Mathematical Sciences (AIMS) is a tertiary education and research institute in Muizenberg, South Africa, established in September 2003, and an associated network of linked institutes in Senegal, Ghana, Cameroon, Tanza ...
, the
Botswana International University of Science and Technology The Botswana International University of Science & Technology, or BIUST, is an international university located in the town of Palapye, Botswana. It is the nation's second university, after the University of Botswana in the country's capital, Gab ...
, the
University of the Western Cape The University of the Western Cape (UWC) is a public research university in Bellville, near Cape Town, South Africa. The university was established in 1959 by the South African government as a university for Coloured people only. Other ...
,
Rhodes University Rhodes University is a public university, public research university located in Makhanda, Eastern Cape, Makhanda (Grahamstown) in the Eastern Cape Province of South Africa. It is one of four universities in the province. Established in 1904, ...
, the
University of Cape Town The University of Cape Town (UCT) ( af, Universiteit van Kaapstad, xh, Yunibesithi ya yaseKapa) is a public research university in Cape Town, South Africa. Established in 1829 as the South African College, it was granted full university statu ...
,
McGill University McGill University (french: link=no, Université McGill) is an English-language public research university located in Montreal, Quebec, Canada. Founded in 1821 by royal charter granted by King George IV,Frost, Stanley Brice. ''McGill Universit ...
, the
University of Toronto The University of Toronto (UToronto or U of T) is a public research university in Toronto, Ontario, Canada, located on the grounds that surround Queen's Park. It was founded by royal charter in 1827 as King's College, the first institu ...
, the
University of British Columbia The University of British Columbia (UBC) is a public university, public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks a ...
, the
Inter-University Centre for Astronomy and Astrophysics The Inter-University Centre for Astronomy and Astrophysics (IUCAA) is an autonomous institution set up by the University Grants Commission of India to promote nucleation and growth of active groups in astronomy and astrophysics in Indian unive ...
,
Yale University Yale University is a Private university, private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the List of Colonial Colleges, third-oldest institution of higher education in the United Sta ...
,
Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
,
Carnegie Mellon Carnegie may refer to: People * Carnegie (surname), including a list of people with the name * Clan Carnegie, a lowland Scottish clan Institutions Named for Andrew Carnegie *Carnegie Building (Troy, New York), on the campus of Rensselaer Polyt ...
, the
University of Wisconsin A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. ''University'' is derived from the Latin phrase ''universitas magistrorum et scholarium'', which ...
, the
West Virginia University West Virginia University (WVU) is a public land-grant research university with its main campus in Morgantown, West Virginia. Its other campuses are those of the West Virginia University Institute of Technology in Beckley, Potomac State Coll ...
,
Oxford University Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...
, the
Astroparticle and Cosmology Laboratory The Astroparticle and Cosmology (APC) laboratory in Paris gathers researchers (experimentalists, theorists and observers) working in different areas including high-energy astrophysics, cosmology, gravitation, and neutrino physics. The institu ...
, the
Nelson Mandela University Nelson Mandela University (formerly known as ''Nelson Mandela Metropolitan University (NMMU)'' ) and before that - the University of Port Elizabeth (UPE), the Port Elizabeth Technikon and Vista University's Port Elizabeth campus. This South Af ...
, EPFL, the
ETH Zurich (colloquially) , former_name = eidgenössische polytechnische Schule , image = ETHZ.JPG , image_size = , established = , type = Public , budget = CHF 1.896 billion (2021) , rector = Günther Dissertori , president = Joël Mesot , a ...
, and the NASA
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA ...
. It is funded by the
National Research Foundation of South Africa South Africa’s National Research Foundation (NRF) is the intermediary agency between the policies and strategies of the Government of South Africa and South Africa's research institutions. It was established on 1 April 1999 as an autonomous ...
, and by the partner institutions. The HIRAX array is named in reference to the
hyrax Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simil ...
, a local mammal, and in parallel to the neighboring
meerKAT MeerKAT, originally the Karoo Array Telescope, is a radio telescope consisting of 64 antennas in the Meerkat National Park, in the Northern Cape of South Africa. In 2003, South Africa submitted an expression of interest to host the Square K ...
radio telescope and its eponymous
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
.


Science goals

The nature of
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the unive ...
and
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
are among the greatest unsolved mysteries in modern cosmology. It has been known since the late 1920s, with the discovery of
Hubble's law Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
, that the universe is expanding, but for most of the 20th century it was assumed that this was a decelerating expansion, following a hot
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from t ...
. However, in the late 1990s it was discovered that the
expansion of the universe The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not exp ...
is in fact accelerating. Dark energy is the hypothesized form of energy which causes this acceleration, however little is known about it beyond the fact that it must currently comprise approximately 70% of the energy density of the universe. Dark matter also plays a significant role in the growth of structures within the universe. It is believed to be a form of matter that interacts with the
gravitational force In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
, but not the
electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
, and it is known to make up approximately 25% of the energy density of the universe, but the exact nature of it is not understood. The remaining 5% of the energy density of the universe is the
baryonic matter In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifi ...
which we can see; the
star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
s, gas and
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
that makes up
galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
and
galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-la ...
s. HIRAX is designed to measure the effects of dark energy and dark matter on the dynamics of the universe over a long period of time (~4 billion years) to learn more about their nature. This is accomplished by looking at the 21-cm line emission produced by hot diffuse neutral hydrogen from distant
galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-la ...
s and from the intracluster medium. This neutral hydrogen traces out the large scale structures in the universe, and so can be used to map out the large scale Baryon Acoustic Oscillation (BAO) structure of the universe. The BAO are a fixed comoving size, and so they act as a
standard ruler A standard ruler is an astronomical object for which the actual physical size is known. By measuring its angular size in the sky, one can use simple trigonometry to determine its distance from Earth. In simple terms, this is because objects of a fi ...
, marking the expansion of the universe over time, and therefore giving information about dark energy and dark matter. For example, if dark energy is not a
cosmological constant In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: ), alternatively called Einstein's cosmological constant, is the constant coefficient of a term that Albert Einstein temporarily added to his field eq ...
, as the standard
ΛCDM The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda (Greek Λ) associated wit ...
theory of cosmology predicts, then the rate of acceleration of the universe may not be constant over time. Due to the expansion of the universe, the 400-800 MHz operating band of the HIRAX instrument corresponds to
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 simultaneous increase in f ...
ed 21-cm emission from 0.8 < z < 2.5 (7-11 Bya, or when the universe was between 2.5 and 6.5 billion years old). This range encompasses the period when the standard
ΛCDM The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda (Greek Λ) associated wit ...
cosmological model predicts that dark energy is beginning to affect the dynamics of the universe, causing it to transition from decelerating expansion to accelerating expansion. The HIRAX array will survey most of the southern sky to map out BAO, and its large
field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans a ...
and large survey area will additionally make it a very powerful tool for detecting radio transient events. In particular, HIRAX will be extremely efficient at detecting
Fast Radio Bursts In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to 3 seconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB rel ...
(FRBs) and
Pulsars A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
. FRBs are short (~1 ms) bright (~1 Jy) radio bursts, whose origins are completely unknown. Only approximately 612 have been detected as of 2021, but the HIRAX array expects to detect tens of FRBs per day. Pulsars are rapidly rotating
neutron stars A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. wh ...
, whose rotation causes them to appear to emit radio frequency pulses at very regular rates. Precise measurements of the rates of their pulses could be used to detect
gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
, because the gravitational waves would distort the size of the space the pulses travel through, and thus their arrival times at Earth. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a sister experiment to HIRAX. It has similar science objectives, but observes in the northern hemisphere, and has different instrumental systematics.


Instrument

The HIRAX array will consist of 1024 6-meter diameter parabolic dish reflectors with a field of view of 5–10°. The dishes will ''not'' be steered, but fixed in position and sweep the sky as the Earth rotates. Every few months, they will be manually re-pointed in elevation to survey a new strip of the sky. The dishes are extremely deep, with an
f-number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill ...
of 0.23, to shield the feeds from ground pickup, and
crosstalk In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, in ...
from neighboring dishes in the array. The antennas have been optimized to have low loss and high
reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electroni ...
across the 400–800 MHz observing band of the telescope. Each dish is coupled to a single dual-
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
clover-leaf
dipole antenna In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole w ...
. The signal is amplified by a pair of low noise amplifiers (LNAs), and transmitted to a centralized computation structure (the "back end") by means of fibre-optic links. At the back end the signal is amplified further by
analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analo ...
amplifier chains, then digitized and correlated with the signals from all other dishes to produce a single coherent image from the whole array. The digitization and frequency channelization operations will be performed by custom
field programmable gate array A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturinghence the term '' field-programmable''. The FPGA configuration is generally specified using a hardware ...
(FPGA) boards, and the correlation will be run on a custom
graphics processing unit A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mo ...
(GPU) based
high performance computing High-performance computing (HPC) uses supercomputers and computer clusters to solve advanced computation problems. Overview HPC integrates systems administration (including network and security knowledge) and parallel programming into a multi ...
cluster. This correlation operation is extremely computationally expensive, and is the primary reason why such large interferometric arrays have not previously been fielded. In full array operation, HIRAX will be required to process 6.5 Tb of data per second, which is comparable to the total international internet bandwidth for the continent of Africa. This problem is made feasible by recent advances in GPU based computing, and by the regular spacing between the array elements, which lowers the computational difficulty from O(n^2) to O(n \log n), where ''n'' is the number of elements in the array.


Status

The HIRAX collaboration fielded an 8-element prototype array at the
Hartebeesthoek Radio Astronomy Observatory The Hartebeesthoek Radio Astronomy Observatory (HartRAO) is a radio astronomy observatory, located in a natural bowl of hills at Hartebeesthoek just south of the Magaliesberg mountain range, Gauteng, South Africa, about 50 km west of Johannes ...
(HartRAO) in 2017, which is used as a test bed for hardware and software development leading up to the construction of the full array at the South African Radio Astronomy Observatory (SARAO) site in the Karoo. Construction of a 128-element pathfinder array is slated to begin in 2024. The pathfinder array will then be expanded out to the full 1024-element array over the course of the following three years. The HartRAO 8-element array will be incorporated into the full array as an "outrigger" array, along with several others throughout southern Africa. These outriggers will dramatically improve the angular resolution of the HIRAX array, allowing it to localize FRB detections with sub-arcsecond precision. The
University of KwaZulu-Natal The University of KwaZulu-Natal (UKZN) is a university with five campuses in the province of KwaZulu-Natal in South Africa. It was formed on 1 January 2004 after the merger between the University of Natal and the University of Durban-Westville ...
, and the South African
Department of Science and Technology Department of Science and Technology may refer to: *Department of Science and Technology (Australia), an Australian Government department between November 1980 and December 1984 * Department of Science and Technology (India) * Department of Science ...
and National Research Foundation announced the official launch of the HIRAX experiment in August 2018.


See also

*
Baryon acoustic oscillations In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe, caused by acoustic density waves in the primordial plasma of the early universe. In the same way ...
* Fast radio burst *
Intensity mapping 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. In its most common variant, 21 cm intensity mapping, the ...
*
List of radio telescopes This is a list of radio telescopes – over one hundred – that are or have been used for radio astronomy. The list includes both single dishes and interferometric arrays. The list is sorted by region, then by name; unnamed telescopes are in r ...


References


External links


HIRAX Official Site
{{Radio-astronomy, state=uncollapsed Radio telescopes Science and technology in South Africa Interferometric telescopes Astronomical observatories in South Africa