View source for Hydrogen Intensity and Real-time Analysis eXperiment

{{Infobox telescope
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The '''Hydrogen Intensity and Real-time Analysis eXperiment''' ('''HIRAX''') is an  [[interferometry|interferometric]] array of 1024 6-[[meter]] (20[[Foot (unit)|ft]]) diameter [[radio telescope]]s, operating at 400-800[[megahertz|MHz]], that will be deployed at the [[Square Kilometer Array]] site in the [[Karoo]] region of [[South Africa]]. The array is designed to  measure red-shifted 21-cm [[hydrogen line]] emission on large angular scales, in order to map out the [[baryon acoustic oscillations]], and constrain models of [[dark energy]] and [[dark matter]].<ref name = newburgh15>
{{Cite journal|author = L. Newburgh|title = HIRAX: A Probe of Dark Energy and Radio Transients
|journal = Proceedings of SPIE
|volume = 9906
|issue = 9906
|pages = 99065X
|arxiv = 1607.02059
|display-authors=etal|year = 2016
|doi = 10.1117/12.2234286
|series = Ground-based and Airborne Telescopes VI
|editor1-last = Hall
|editor1-first = Helen J
|editor2-last = Gilmozzi
|editor2-first = Roberto
|editor3-last = Marshall
|editor3-first = Heather K
|bibcode = 2016SPIE.9906E..5XN
|s2cid = 119280190
}}</ref>

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 [[Durban University of Technology]], the [[African Institute for Mathematical Sciences]], the [[Botswana International University of Science and Technology]], the [[University of the Western Cape]], [[Rhodes University]], the [[University of Cape Town]], [[McGill University]], the [[University of Toronto]], the [[University of British Columbia]], the [[Inter-University Centre for Astronomy and Astrophysics]], [[Yale University]], [[Caltech]], [[Carnegie Mellon]], the [[University of Wisconsin]], the [[West Virginia University]], [[Oxford University]], the [[Astroparticle and Cosmology Laboratory]], the [[Nelson Mandela University]], [[EPFL]], the [[ETH Zurich]], and the NASA [[Jet Propulsion Laboratory]]. It is funded by the [[National Research Foundation of South Africa]], and by the partner institutions.

The HIRAX array is named in reference to the [[hyrax]], a local mammal, and in parallel to the neighboring [[meerKAT]] radio telescope and its eponymous [[meerkat|animal]].

== Science goals ==

The nature of [[dark energy]] and [[dark matter]] are among the greatest unsolved mysteries in modern cosmology.<ref name="albrecht/etal">{{cite arXiv|title=Report of the Dark Energy Task Force |author=Andreas Albrecht |display-authors=etal |eprint=astro-ph/0609591 |date=2006}}</ref> It has been known since the late 1920s, with the discovery of [[Hubble's law]], that the universe is expanding,<ref name=Lemaitre1>
{{cite journal
 |last=Lemaître |first=G. |author-link=Georges Lemaître
 |title=Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques
 |date=1927
 |journal=Annales de la Société Scientifique de Bruxelles A
 |volume= 47|issue=47 |pages=49–59
 |bibcode=1927ASSB...47...49L
}}</ref><ref name=Lemaitre2>
{{Cite journal
 |last=Lemaître |first=G.
 |title=Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae
 |date=1931
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=91 |issue= 5|pages=483–490
 |bibcode=1931MNRAS..91..483L
 |doi=10.1093/mnras/91.5.483
|doi-access=free
 }}</ref><ref name=hubble>
{{cite journal
 |last=Hubble |first=E.
 |date=1929
 |title=A relation between distance and radial velocity among extra-galactic nebulae
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=15 |issue=3 |pages=168–73
 |bibcode=1929PNAS...15..168H
 |doi=10.1073/pnas.15.3.168
 |pmid=16577160
 |pmc=522427
|doi-access=free
 }}</ref>
but for most of the 20th century it was assumed that this was a decelerating expansion, following a hot [[Big Bang]]. However, in the late 1990s it was discovered that the [[expansion of the universe]] is in fact accelerating.<ref name="riess">{{cite journal|author=Riess, Adam G.|author-link=Adam Riess|year=1998|title=Observational evidence from supernovae for an accelerating universe and a cosmological constant|journal=Astronomical Journal|volume=116|issue=3|pages=1009–38|arxiv=astro-ph/9805201 |doi=10.1086/300499|bibcode=1998AJ....116.1009R|last2=Filippenko|last3=Challis|last4=Clocchiatti|last5=Diercks|last6=Garnavich|last7=Gilliland|last8=Hogan|last9=Jha|last10=Kirshner|last11=Leibundgut|last12=Phillips|last13=Reiss|last14=Schmidt|last15=Schommer|last16=Smith|last17=Spyromilio|last18=Stubbs|last19=Suntzeff|last20=Tonry|s2cid=15640044}}</ref><ref name="perlmutter">{{cite journal|author=Perlmutter, S.|author-link=Saul Perlmutter|journal=Astrophysical Journal|volume=517|issue=2|pages=565–86|year=1999|title=Measurements of Omega and Lambda from 42 high redshift supernovae|arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P|last2=Aldering|last3=Goldhaber|last4=Knop|last5=Nugent|last6=Castro|last7=Deustua|last8=Fabbro|last9=Goobar|last10=Groom|last11=Hook|last12=Kim|last13=Kim|last14=Lee|last15=Nunes|last16=Pain|last17=Pennypacker|last18=Quimby|last19=Lidman|last20=Ellis|last21=Irwin|last22=McMahon|last23=Ruiz‐Lapuente|last24=Walton|last25=Schaefer|last26=Boyle|last27=Filippenko|last28=Matheson|last29=Fruchter|last30=Panagia|s2cid=118910636|display-authors=29}}</ref>
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]], but not the [[electromagnetic force]], 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 [[baryon|baryonic matter]] which we can see; the [[star]]s, [[interstellar medium|gas]] and [[cosmic dust|dust]] that makes up [[galaxy|galaxies]] and [[galaxy cluster]]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 [[hydrogen line|21-cm line emission]] produced by hot diffuse neutral hydrogen from distant [[galaxy cluster]]s and from the [[intracluster medium]].<ref name = newburgh15 /> This neutral hydrogen traces out the [[Observable universe#Large-scale structure|large scale structures]] in the universe, and so can be used to map out the large scale [[Baryon Acoustic Oscillations|Baryon Acoustic Oscillation]] (BAO) structure of the universe. The BAO are a fixed [[comoving and proper distances|comoving]] size, and so they act as a [[standard ruler]], 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]], as the standard [[Lambda-CDM model|ΛCDM]] 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&nbsp;MHz operating band of the HIRAX instrument corresponds to [[redshift]]ed 21-cm emission from <math>0.8 < z < 2.5</math> (7-11[[Bya]], or when the universe was between 2.5 and 6.5 billion years old).<ref name = newburgh15 /><ref name = phys-org>{{cite web|url=https://phys.org/news/2018-08-telescope-mysteries-radio-dark-energy.html|title=New telescope chases the mysteries of radio flashes and dark energy|publisher=}}</ref> This range encompasses the period when the standard [[Lambda-CDM model|ΛCDM]] 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]] 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 burst|Fast Radio Bursts]] (FRBs) and [[Pulsars]]. FRBs are short (~1 [[millisecond|ms]]) bright (~1 [[Jansky|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.<ref name = newburgh15 /> Pulsars are rapidly rotating [[neutron stars]], 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]], 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 [[Observational error|instrumental systematics]]. The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a next-generation radio telescope, proposed for construction to start immediately. CHORD is a pan-Canadian project, designed to work with and build on the success of the CHIME. It will act as a sister experiment to HIRAX.<ref>{{Cite report |url=https://zenodo.org/records/3765414 |title=The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) |last=Vanderlinde |first=Keith |last2=Liu |first2=Adrian |date=2019-10-21 |publisher=[object Object] |doi=10.5281/zenodo.3765414 |language=en |last3=Gaensler |first3=Bryan |last4=Bond |first4=Dick |last5=Hinshaw |first5=Gary |last6=Ng |first6=Cherry |last7=Chiang |first7=Cynthia |last8=Stairs |first8=Ingrid |last9=Brown |first9=Jo-Anne}}{{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> CHORD will incorporate CHIME’s best innovations alongside new Canadian technology. Small cylinders derived from the CHIME design and operating from 400-800MHz will be deployed at remote outrigger sites and provide milli-arcsecond-level localization of radio transients. These will be complemented by focused arrays of 6m composite dishes at each site, instrumented with novel ultra-wideband (UWB) feeds, covering a 5:1 radio band from 300–1500MHz.

== Instrument ==
The HIRAX array will consist of 1024 6-meter diameter [[parabolic antenna|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]] of 0.23, to shield the feeds from ground pickup, and [[crosstalk]] from neighboring dishes in the array. The antennas have been optimized to have low [[dielectric loss|loss]] and high [[reflectance|reflectivity]] across the 400–800&nbsp;MHz observing band of the telescope.  Each dish is coupled to a single dual-[[polarization (waves)|polarization]] clover-leaf [[dipole antenna]]. The signal is amplified by a pair of [[low-noise amplifier]]s (LNAs), and transmitted to a centralized computation structure (the "back end") by means of [[Radio over fiber|fibre-optic links]].<ref name = newburgh15 />

At the back end the signal is amplified further by [[analog electronics|analog]] 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]] (FPGA) boards, and the correlation will be run on a custom [[graphics processing unit]] (GPU) based [[high performance computing]] cluster.<ref name = newburgh15 /> 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 [[terabit|Tb]] of data per second, which is comparable to the total international internet bandwidth for the continent of Africa.<ref name = phys-org /><ref name = africabandwidth>{{cite web|url=http://www.africabandwidthmaps.com/|title=Africa Bandwidth Maps|website=www.africabandwidthmaps.com}}</ref> 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 <math>O(n^2)</math> to <math>O(n \log n)</math>, where ''n'' is the number of elements in the array.<ref name = newburgh15 />

== Status ==

[[File:HIRAX prototype array 2017-03-17.jpg|thumb|The HIRAX prototype telescope array at HartRAO in 2017.]]

The HIRAX collaboration fielded an 8-element prototype array at the [[Hartebeesthoek Radio Astronomy Observatory]] (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.<ref name = newburgh15 /><ref name = nature>{{cite journal|title=Dark-energy telescope, asteroid hunters and gene-therapy rules|date=1 August 2018|journal=Nature|volume=560|issue=7719|pages=414–415|doi=10.1038/d41586-018-05983-4|pmid=30135538|bibcode=2018Natur.560..414.|doi-access=free}}</ref>
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.<ref name = ukzn-launch>{{cite web|url=https://acru.ukzn.ac.za/hirax-telescope-project-is-officially-launched/|title=HIRAX telescope project is officially launched. – Astrophysics and Cosmology Research Unit|website=acru.ukzn.ac.za|date=17 August 2018 }}</ref>

The [[University of KwaZulu-Natal]], and the South African  [[Department of Science and Technology (South Africa)|Department of Science and Technology]] and  [[National Research Foundation of South Africa|National Research Foundation]] announced the official launch of the HIRAX experiment in August 2018.<ref name = nature /><ref name = ukzn-launch /><ref name = southafrican-launch>{{cite web|url=https://www.thesouthafrican.com/hirax-radio-telescope-launched-in-south-africa-in-order-to-solve-dark-energy/|title=New radio telescope launched in South Africa in order to solve the mystery that is 'dark energy'|date=20 August 2018|publisher=}}</ref>

== See also ==
* [[Baryon acoustic oscillations]]
* [[Fast radio burst]]
* [[Intensity mapping]]
* [[List of radio telescopes]]

== References ==
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{{reflist}}

== External links ==
* [https://www.acru.ukzn.ac.za/~hirax/ HIRAX Official Site]
{{Radio-astronomy|state=uncollapsed}}

[[Category:Radio telescopes]]
[[Category:Science and technology in South Africa]]
[[Category:Interferometric telescopes]]
[[Category:Astronomical observatories in South Africa]]