Editing Gravitational wave (section)

==Introduction==
{{More citations needed|section|date=August 2024}}
In [[Albert Einstein]]'s [[general theory of relativity]], gravity is treated as a phenomenon resulting from the [[curvature of spacetime]]. This curvature is caused by the presence of mass. {{crossreference|(See: [[Stress–energy tensor]])}} If the masses move, the curvature of spacetime changes. If the motion is not spherically symmetric, the motion can cause gravitational waves which propagate away at the [[speed of light]].<ref name=Penrose-1965/>

As a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of [[Strain (materials science)|strain]]. Distances between objects increase and decrease rhythmically as the wave passes, at a frequency equal to that of the wave. The magnitude of this effect is [[inversely proportional]] to the distance (not distance squared) from the source.<ref name="Schutz2009">{{Cite book |last=Schutz |first=Bernard F. |url=https://archive.org/details/firstcourseingen00bern_0 |title=A first course in general relativity |date=2009 |publisher=[[Cambridge University Press]] |isbn=978-0-521-88705-2 |edition=2nd |location=Cambridge; New York |author-link=Bernard F. Schutz}}</ref>{{rp|227}}

Inspiraling [[Neutron star#Binary neutron star systems|binary neutron stars]] are predicted to be a powerful source of gravitational waves as they [[Neutron star merger|coalesce]], due to the very large acceleration of their masses as they [[orbit]] close to one another. However, due to the astronomical distances to these sources, the effects when measured on Earth are predicted to be very small, having strains of less than 1 part in 10<sup>20</sup>.

Scientists demonstrate the existence of these waves with highly-sensitive detectors at multiple observation sites. {{As of|2012}}, the [[LIGO]] and [[Virgo interferometer|Virgo]] observatories were the most sensitive detectors, operating at resolutions of about one part in {{val|5|e=22}}.<ref>{{Cite journal |last=Abadie |first=J. |display-authors=etal |date=2012-04-19 |title=Search for gravitational waves from low mass compact binary coalescence in LIGO's sixth science run and Virgo's science runs 2 and 3 |url=https://link.aps.org/doi/10.1103/PhysRevD.85.082002 |journal=Physical Review D |volume=85 |issue=8 |page=082002 |arxiv=1111.7314 |bibcode=2012PhRvD..85h2002A |doi=10.1103/PhysRevD.85.082002 |hdl=2440/74812 |issn=1550-7998 |s2cid=6842810|collaboration=[[LIGO Scientific Collaboration]]; [[Virgo Collaboration]]}}</ref> The Japanese detector [[KAGRA]] was completed in 2019; its first joint detection with LIGO and VIRGO was reported in 2021.<ref name="MPG">{{cite web |title=LIGO, Virgo, and KAGRA raise their signal score to 90 |url=https://www.aei.mpg.de/816408/ligo-virgo-and-kagra-raise-their-signal-score-to-90 |website=aei.mpg.de |publisher=Max Planck Institute for Gravitational Physics |access-date=13 November 2021 |language=en}}</ref> Another European ground-based detector, the [[Einstein Telescope]], is under development. A space-based observatory, the [[Laser Interferometer Space Antenna]] (LISA), is also being developed by the [[European Space Agency]].
[[File:Quadrupol Wave.gif|thumb|right|Linearly polarized gravitational wave]]

Gravitational waves do not strongly interact with matter in the way that electromagnetic radiation does.<ref name="Flanagan2005"/>{{rp|33–34}} This allows for the observation of events involving exotic objects in the distant universe that cannot be observed with more traditional means such as [[optical telescope]]s or [[radio telescope]]s; accordingly, [[gravitational wave astronomy]] gives new insights into the workings of the universe.<ref name="Flanagan2005"/>{{rp|36–40}}

In particular, gravitational waves could be of interest to cosmologists as they offer a possible way of observing the very early universe. This is not possible with conventional astronomy, since before [[recombination (cosmology)|recombination]] the universe was opaque to electromagnetic radiation.<ref>{{cite journal |last1=Krauss |last2=Dodelson |last3=Meyer |first3=S |title=Primordial Gravitational Waves and Cosmology |journal=Science |volume=328 |issue=5981 |pages=989–92 |doi=10.1126/science.1179541 |pmid=20489015 |first1=LM |first2=S |bibcode=2010Sci...328..989K |arxiv=1004.2504 |year=2010 |s2cid=11804455 }}</ref> Precise measurements of gravitational waves will also allow scientists to test more thoroughly the general theory of relativity.

In principle, gravitational waves can exist at any frequency. Very low frequency waves can be detected using pulsar timing arrays. In this technique, the timing of approximately 100 pulsars spread widely across our galaxy is monitored over the course of years. Detectable changes in the arrival time of their signals can result from passing gravitational waves generated by merging [[supermassive black hole]]s (SMBH) with wavelengths measured in lightyears. These timing changes can be used to locate the source of the waves.<ref>{{Cite journal |last1=Agazie |first1=Gabriella |last2=Anumarlapudi |first2=Akash |last3=Archibald |first3=Anne M. |last4=Arzoumanian |first4=Zaven |last5=Baker |first5=Paul T. |last6=Bécsy |first6=Bence |last7=Blecha |first7=Laura |last8=Brazier |first8=Adam |last9=Brook |first9=Paul R. |last10=Burke-Spolaor |first10=Sarah |last11=Burnette |first11=Rand |last12=Case |first12=Robin |last13=Charisi |first13=Maria |last14=Chatterjee |first14=Shami |last15=Chatziioannou |first15=Katerina |date=2023-07-01 |title=The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background |journal=The Astrophysical Journal Letters |volume=951 |issue=1 |pages=L8 |doi=10.3847/2041-8213/acdac6 |doi-access=free |arxiv=2306.16213 |bibcode=2023ApJ...951L...8A |issn=2041-8205}}</ref>

Using this technique, astronomers have discovered the 'hum' of various SMBH mergers occurring in the universe. [[Stephen Hawking]] and [[Werner Israel]] list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10<sup>−7</sup>&nbsp;Hz up to 10<sup>11</sup>&nbsp;Hz.<ref name="HI">{{cite book |last1=Hawking |first1=S. W. |last2=Israel |first2=W. |title=General Relativity: An Einstein Centenary Survey |publisher=Cambridge University Press |location=Cambridge |year=1979 |isbn=978-0-521-22285-3 |page=98 |url=https://books.google.com/books?id=pxA4AAAAIAAJ&pg=PA98 }}</ref>