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The first [[first observation of gravitational waves|direct observation of gravitational waves]] was made in 2015, when a signal generated by the merger of two black holes was received by the [[LIGO]] [[gravitational wave detector]]s in Livingston, Louisiana, and in Hanford, Washington. The 2017 [[Nobel Prize in Physics]] was subsequently awarded to [[Rainer Weiss]], [[Kip Thorne]] and [[Barry Barish]] for their role in the direct detection of gravitational waves.
In [[gravitational-wave astronomy]], [[List of gravitational wave observations|observations of gravitational waves]] are used to infer data about the sources of gravitational waves. Sources that can be studied this way include [[binary star]] systems composed of [[white dwarf]]s, [[neutron star]]s,<ref name="NYT20210629">{{cite news |last=Chang |first=Kenneth |title=A Black Hole Feasted on a Neutron Star. 10 Days Later, It Happened Again – Astronomers had long suspected that collisions between black holes and dead stars occurred, but they had no evidence until a pair of recent detections. |url=https://www.nytimes.com/2021/06/29/science/black-holes.html |date=29 June 2021 |work=[[The New York Times]] |accessdate=29 June 2021 }}</ref><ref name="AJL-20210629">{{cite journal |author=Abbott, R. |display-authors=et al. |title=Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences
==Introduction==
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In 2017, the [[Nobel Prize in Physics]] was awarded to [[Rainer Weiss]], [[Kip Thorne]] and [[Barry Barish]] for their role in the detection of gravitational waves.<ref name="BBC-20171003">{{cite news |last1=Rincon |first1=Paul |last2=Amos |first2=Jonathan |url=https://www.bbc.co.uk/news/science-environment-41476648|title=Einstein's waves win Nobel Prize |work=[[BBC News]] |date=3 October 2017 |access-date=3 October 2017}}</ref><ref name="NYT-20171003">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers |url=https://www.nytimes.com/2017/10/03/science/nobel-prize-physics.html |date=3 October 2017 |work=[[The New York Times]] |access-date=3 October 2017 }}</ref><ref name="NYT-20171003dk">{{cite news |last=Kaiser |first=David |author-link=David Kaiser |title=Learning from Gravitational Waves |url=https://www.nytimes.com/2017/10/03/opinion/gravitational-waves-ligo-funding.html |date=3 October 2017 |work=[[The New York Times]] |access-date=3 October 2017 }}</ref>
In 2023, NANOGrav, EPTA, PPTA, and IPTA announced that they found evidence of a universal gravitational wave background.<ref name="SA-20230804">{{cite news |last=O'Callaghan |first=Jonathan |title=A Background
Similar results are published by European Pulsar Timing Array, who claimed a <math>3\sigma</math>-significance. They expect that a <math>5\sigma</math>-significance will be achieved by 2025 by combining the measurements of several collaborations.<ref>[https://cloud.mpifr-bonn.mpg.de/index.php/s/5BS4QnZaKWnn3Ti The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals</ref><ref>{{cite web | url=https://www.mpifr-bonn.mpg.de/7919388/news_publication_20524892_transferred | title=Ein neuer Zugang zum Universum }}</ref>
==Effects of passing==
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Although the waves from the Earth–Sun system are minuscule, astronomers can point to other sources for which the radiation should be substantial. One important example is the [[Hulse–Taylor binary]]{{snd}} a pair of stars, one of which is a [[binary pulsar|pulsar]].<ref>{{Cite journal|arxiv=astro-ph/0407149 |title=Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis|journal=Binary Radio Pulsars|volume=328|pages=25|author1=The LIGO Scientific Collaboration|author2=the Virgo Collaboration|year=2004|bibcode=2005ASPC..328...25W}}</ref> The characteristics of their orbit can be deduced from the [[Doppler shift]]ing of radio signals given off by the pulsar. Each of the stars is about {{Solar mass|1.4}} and the size of their orbits is about 1/75 of the [[Earth's orbit|Earth–Sun orbit]], just a few times larger than the diameter of our own Sun. The combination of greater masses and smaller separation means that the energy given off by the Hulse–Taylor binary will be far greater than the energy given off by the Earth–Sun system{{snd}} roughly 10<sup>22</sup> times as much.
The information about the orbit can be used to predict how much energy (and angular momentum) would be radiated in the form of gravitational waves. As the binary system loses energy, the stars gradually draw closer to each other, and the orbital period decreases. The resulting trajectory of each star is an inspiral, a spiral with decreasing radius. General relativity precisely describes these trajectories; in particular, the energy radiated in gravitational waves determines the rate of decrease in the period, defined as the time interval between successive periastrons (points of closest approach of the two stars). For the Hulse–Taylor pulsar, the predicted current change in radius is about 3 mm per orbit, and the change in the 7.75 hr period is about 2 seconds per year. Following a preliminary observation showing an orbital energy loss consistent with gravitational waves,<ref name="auto"/> careful timing observations by Taylor and Joel Weisberg dramatically confirmed the predicted period decrease to within 10%.<ref>{{cite journal|last=Taylor|first=J. H.|author2=Weisberg, J. M. |title= A New Test of General Relativity: Gravitational Radiation and the Binary Pulsar PSR 1913+16 |journal= Astrophysical Journal |volume=253 |issue=5696|pages= 908–920 |doi= 10.1038/277437a0 |bibcode = 1979Natur.277..437T |date=1979|s2cid=22984747}}</ref> With the improved statistics of more than 30 years of timing data since the pulsar's discovery, the observed change in the orbital period currently matches the prediction from gravitational radiation assumed by general relativity to within 0.2 percent.<ref>{{cite journal|last=Huang|first=Y.|author2=Weisberg, J. M.|title=Relativistic Measurements from Timing the Binary Pulsar PSR B1913+16|journal=Astrophysical Journal|volume=829|issue=1|pages=55|doi=10.3847/0004-637X/829/1/55|bibcode = 2016ApJ...829...55W|date=2016|arxiv=1606.02744|s2cid=119283147 |doi-access=free }}</ref> In 1993, spurred in part by this indirect detection of gravitational waves, the Nobel Committee awarded the Nobel Prize in Physics to Hulse and Taylor for "the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."<ref>{{cite web|url=http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/|title=Nobel Prizes and Laureates – NobelPrize.org|website=NobelPrize.org}}</ref> The lifetime of this binary system, from the present to merger is estimated to be a few hundred million years.<ref>{{cite journal|arxiv=1411.3930 |doi=10.1088/0264-9381/32/12/124009 |title=1974: the discovery of the first binary pulsar|volume=32|issue=12 |journal=Classical and Quantum Gravity|page=124009|bibcode = 2015CQGra..32l4009D |year=2015 |last1=Damour |first1=Thibault |s2cid=118307286 }}</ref>
Inspirals are very important sources of gravitational waves. Any time two compact objects (white dwarfs, neutron stars, or [[binary black hole|black holes]]) are in close orbits, they send out intense gravitational waves. As they spiral closer to each other, these waves become more intense. At some point they should become so intense that direct detection by their effect on objects on Earth or in space is possible. This direct detection is the goal of several large-scale experiments.<ref>[http://calteches.library.caltech.edu/4298/1/BlackHoles.pdf Crashing Black Holes]</ref>
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===Using pulsar timing arrays===
{{main|Pulsar timing array}}
[[File:correlation_vs_angular_separation_between_pulsars.svg|thumb|upright=1.5|Plot of correlation between pulsars observed by NANOGrav vs angular separation between pulsars, compared with a theoretical ''Hellings-Downs'' model (dashed purple) and if there were no gravitational wave background (solid green)<ref>http://iopscience.iop.org/collections/apjl-230623-245-Focus-on-NANOGrav-15-year</ref><ref>{{cite web | url=http://news.berkeley.edu/2023/06/28/after-15-years-pulsar-timing-yields-evidence-of-cosmic-gravitational-wave-background | title=After 15 years, pulsar timing yields evidence of cosmic gravitational wave background | date=2022 }}</ref>]]
[[Pulsar]]s are rapidly rotating stars. A pulsar emits beams of radio waves that, like lighthouse beams, sweep through the sky as the pulsar rotates. The signal from a pulsar can be detected by radio telescopes as a series of regularly spaced pulses, essentially like the ticks of a clock. GWs affect the time it takes the pulses to travel from the pulsar to a telescope on Earth. A [[pulsar timing array]] uses [[millisecond pulsar]]s to seek out perturbations due to GWs in measurements of the time of arrival of pulses to a telescope, in other words, to look for deviations in the clock ticks. To detect GWs, pulsar timing arrays search for a distinct quadrupolar pattern of correlation and anti-correlation between the time of arrival of pulses from different pulsar pairs as a function of their angular separation in the sky.<ref>
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|bibcode=2018ApJ...859...47A
|s2cid=89615050
|doi-access=free
}}</ref> In addition to individual binary systems, pulsar timing arrays are sensitive to a stochastic background of GWs made from the sum of GWs from many galaxy mergers. Other potential signal sources include [[cosmic strings]] and the primordial background of GWs from [[Inflation (cosmology)|cosmic inflation]].
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In June 2023, NANOGrav published the 15-year data release, which contained the first evidence for a stochastic gravitational wave background. In particular, it included the first measurement of the Hellings-Downs curve, the tell-tale sign of the gravitational wave origin of the observed background.<ref>{{Cite journal |
===Primordial gravitational wave===
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In 1964 [https://www.worldscientific.com/doi/10.1142/9789812834300_0041 L. Halpern] and B. Laurent theoretically proved that gravitational spin-2 electron transitions are possible in atoms. Compared to electric and magnetic transitions the emission probability is extremely low. Stimulated emission was discussed for increasing the efficiency of the process. Due to the lack of mirrors or resonators for gravitational waves, they determined that a single pass GASER (a kind of laser emitting gravitational waves) is practically unfeasible.<ref>{{Cite journal |last1=Halpern |first1=L. |last2=Laurent |first2=B. |date=1964-08-01 |title=On the gravitational radiation of microscopic systems |url=https://doi.org/10.1007/BF02749891 |journal=Il Nuovo Cimento |language=en |volume=33 |issue=3 |pages=728–751 |doi=10.1007/BF02749891 |bibcode=1964NCim...33..728H |s2cid=121980464 |issn=1827-6121}}</ref>
In 1998 the possibility of a different implementation of the above theoretical analysis was proposed by Giorgio Fontana. The required coherence for a practical GASER could be obtained by [[cooper pair]]s in [[Superconductivity|superconductors]] that are characterized by a macroscopic collective wave-function. Cuprate [[High-temperature superconductivity|high temperature superconductors]] are characterized by the presence of s-wave and d-wave<ref>{{Cite
In 2009 a paper discussing the possible application of [[Neutronium#In the periodic table|dineutrons]] as sources of gravitational waves at X and gamma-ray frequencies was published by G. Fontana and B. Binder.<ref>{{Cite journal |last1=Fontana |first1=Giorgio |last2=Binder |first2=Bernd |date=2009-03-16 |title=Electromagnetic to Gravitational wave Conversion via Nuclear Holonomy |url=https://aip.scitation.org/doi/abs/10.1063/1.3115561 |journal=AIP Conference Proceedings |volume=1103 |issue=1 |pages=524–531 |doi=10.1063/1.3115561 |bibcode=2009AIPC.1103..524F |issn=0094-243X}}</ref>
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