Þyngdarbylgja was directly detected for the first time in 2015 after a century of its prediction by Einstein’s General Theory of Relativity in 1916. But, the continuous, low frequency Þyngdarafl-wave Background (GWB) that is thought to be present throughout the alheimurinn has not been detected directly so far. The researchers at North American Nanohertz Observatory for Þyngdarbylgjur (NANOGrav) have recently reported detection of a low-frequency signal that could be ‘Gravitational-wave Background (GWB)’.
Almenn afstæðiskenning sem Einstein setti fram árið 1916 spáir því að stórir geimviðburðir eins og sprengistjarna eða samruni svarthol ætti að framleiða þyngdarbylgjur that propagate through the Universe. Earth should be awash with þyngdarbylgjur from all directions all the time but these are undetected because they become extremely weak by the time they reach earth. It took about a century to make a direct detection of gravitational ripples when in 2015 LIGO-Virgo team was successful in detecting þyngdarbylgjur produced due to merger of two svarthol situated at a distance of 1.3 billion light-years from the Earth (1). Þetta þýddi einnig að gárurnar sem greindust voru handhafar upplýsinga um kosmískan atburð sem átti sér stað fyrir um 1.3 milljörðum ára.
Frá fyrstu uppgötvun árið 2015, góður fjöldi þyngdarafl gára have been recorded till date. Most of them were due to merger of two svarthol, few were due to collision of two neutron stars (2). All detected þyngdarbylgjur so far were episodic, caused due to binary pair of svarthol or neutron stars spiralling and merging or colliding with each other (3) og voru með hátíðni, stuttri bylgjulengd (í millisekúndna bili).
However, since there is possibility of large number of sources of þyngdarbylgjur í alheimurinn hence many þyngdarbylgjur together from all over the alheimurinn may be continuously passing through the earth all the time forming a background or noise. This should be continuous, random and of low frequency small wave. It is estimated that some part of it may even have originated from the Big Bang. Called Þyngdarafl-wave Background (GWB), this has not been detected so far (3).
But we may be on the verge of a breakthrough – the researchers at the North American Nanohertz Observatory for Þyngdarbylgjur (NANOGrav) have reported detection of a low-frequency signal that could be ‘Gravitational-wave Background (GWB) (4,5,6).
Unlike LIGO-virgo team who detected gravitational wave from individual pairs of svarthol, NANOGrav team have looked for persistent, noise like, ‘combined’ gravitational wave created over very long period of time by countless blackholes í alheimurinn. The focus was on ‘very long wavelength’ gravitational wave at the other end of ‘gravitational wave spectrum’.
Ólíkt ljósi og annarri rafsegulgeislun er ekki hægt að fylgjast beint með þyngdarbylgjunum með sjónauka.
NANOGrav liðið valdi millisekúndu pulsars (MSPs) that rotate very rapidly with long term stability. There is steady pattern of light coming from these pulsers which should be altered by the gravitational wave. The idea was to observe and monitor an ensemble of ultra-stable millisecond pulsars (MSP) for correlated changes in the timing of the arrival of the signals at the Earth thus creating a “galaxy-sized” gravitational-wave detector within our own Galaxy. The team created a pulsar timing array by studying 47 of such pulsars. The Arecibo Observatory and the Green Bank Telescope were the útvarp telescopes used for the measurements.
Gagnasettið sem aflað hefur verið hingað til inniheldur 47 MSP og yfir 12.5 ára athuganir. Byggt á þessu er ekki hægt að sanna með óyggjandi hætti beina uppgötvun GWB þó að lágtíðnimerkin sem greindust bendi mjög til þess. Kannski væri næsta skref að setja fleiri tjaldstjörnur í fylkið og rannsaka þær í lengri tíma til að auka næmni.
To study the alheimurinn, scientists were exclusively dependant on electromagnetic radiations like light, X-ray, útvarp wave etc. Being completely unrelated to electromagnetic radiation, detection of gravitational in 2015 opened a new window of opportunity to scientists to study celestial bodies and understanding the alheimurinn especially those celestial events which are invisible to electromagnetic astronomers. Further, unlike electromagnetic radiation, gravitational waves do not interact with matter hence travel virtually unimpeded carrying information about their origin and source free of any distortion.(3)
Detection of Gravitational-wave Background (GWB) would broaden the opportunity further. It may even become possible to detect the waves generated from Big Bang which may help us understand origin of alheimurinn in a better way.
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Tilvísanir:
- Castelvecchi D. og Witze A., 2016. Þyngdarbylgjur Einsteins fundust loksins. Náttúrufréttir 11. febrúar 2016. DOI: https://doi.org/10.1038/nature.2016.19361
- Castelvecchi D., 2020. Það sem 50 þyngdarbylgjuviðburðir sýna um alheiminn. Náttúrufréttir birtar 30. október 2020. DOI: https://doi.org/10.1038/d41586-020-03047-0
- LIGO 2021. Heimildir og tegundir þyngdarbylgna. Fæst á netinu á https://www.ligo.caltech.edu/page/gw-sources Skoðað 12. janúar 2021.
- NANOGrav Collaboration, 2021. NANOGrav finnur mögulegar „fyrstu vísbendingar“ um lágtíðni þyngdarbylgjubakgrunn. Fæst á netinu á http://nanograv.org/press/2021/01/11/12-Year-GW-Background.html Skoðað þann 12. janúar 2021
- Samstarf NANOGrav 2021. Fréttatilkynning – Leitað að þyngdarbylgjubakgrunni á 12.5 árum af NANOGrav gögnum. 11. janúar 2021. Fæst á netinu á http://nanograv.org/assets/files/slides/AAS_PressBriefing_Jan’21.pdf
- Arzoumanian Z., o.fl. 2020. NANOGrav 12.5 ára gagnasafnið: Leitaðu að ísótrópískum stochastic þyngdarbylgjubakgrunni. The Astrophysical Journal Letters, Volume 905, Number 2. DOI: https://doi.org/10.3847/2041-8213/abd401
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