Editing LIGO (section)

==Observations==
[[File:LIGO on Hanford Reservation.jpg|thumb|300px|Western leg of LIGO [[interferometer]] on [[Hanford Reservation]]]]
{{see also|First observation of gravitational waves|List of gravitational wave observations}}
Based on current models of astronomical events, and the predictions of the [[general theory of relativity]],{{r|Pretorius2005|CampanelliLousto2006|BakerCentrella2006}} gravitational waves that originate tens of millions of light years from Earth are expected to distort the {{convert|4|km|adj=on}} mirror spacing by about {{val|e=−18|u=m}}, less than one-thousandth the [[charge radius|charge diameter]] of a [[proton]].  Equivalently, this is a relative change in distance of approximately one part in {{10^|21}}.  A typical event which might cause a detection event would be the late stage inspiral and merger of two 10-[[solar-mass]] black holes, not necessarily located in the Milky Way galaxy, which is expected to result in a very specific sequence of signals often summarized by the slogan ''chirp,'' ''burst,'' ''quasi-normal mode ringing,'' ''exponential decay.''

In their fourth Science Run at the end of 2004, the LIGO detectors demonstrated sensitivities in measuring these displacements to within a factor of two of their design.

During LIGO's fifth Science Run in November 2005, sensitivity reached the primary design specification of a detectable strain of one part in {{10^|21}} over a {{val|100|u=Hz}} bandwidth. The baseline inspiral of two roughly solar-mass neutron stars is typically expected to be observable if it occurs within about {{convert|8|e6pc|e6ly|lk=on}}, or the vicinity of the [[Local Group]], averaged over all directions and polarizations. Also at this time, LIGO and [[GEO 600]] (the German-UK interferometric detector) began a joint science run, during which they collected data for several months. [[Virgo interferometer|Virgo]] (the French-Italian interferometric detector) joined in May 2007. The fifth science run ended in 2007, after extensive analysis of data from this run did not uncover any unambiguous detection events.

In February 2007, GRB&nbsp;070201, a short [[gamma-ray burst]] arrived at Earth from the direction of the [[Andromeda Galaxy]].  The prevailing explanation of most short gamma-ray bursts is the merger of a neutron star with either a neutron star or a black hole. LIGO reported a non-detection for GRB&nbsp;070201, ruling out a merger at the distance of Andromeda with high confidence. Such a constraint was predicated on LIGO eventually demonstrating a direct detection of gravitational waves.<ref>{{cite press release |title=LIGO Sheds Light on Cosmic Event |first=Kathy |last=Svitil |url=http://www.caltech.edu/news/ligo-sheds-light-cosmic-event-1367 |date=2 January 2008 |access-date=14 February 2016 |publisher=[[California Institute of Technology]]}}</ref>

===Enhanced LIGO===
[[File:Northern leg of LIGO interferometer on Hanford Reservation.JPG|thumb|300px|Northern leg (x-arm) of LIGO [[interferometer]] on [[Hanford Reservation]]]]
After the completion of Science Run 5, initial LIGO was upgraded with certain technologies, planned for Advanced LIGO but available and able to be retrofitted to initial LIGO, which resulted in an improved-performance configuration dubbed Enhanced LIGO.<ref>{{cite tech report |first1=Rana |last1=Adhikari |first2=Peter |last2=Fritschel |first3=Sam |last3=Waldman |title=Enhanced LIGO |url=https://dcc.ligo.org/public/0007/T060156/001/T060156-01.pdf |id=[https://dcc.ligo.org/LIGO-T060156-x0/public LIGO-T060156-01-I] |date=17 July 2006}}</ref> Some of the improvements in Enhanced LIGO included:
* Increased laser power
* [[Homodyne detection]]
* Output mode cleaner
* In-vacuum readout hardware

Science Run 6 (S6) began in July 2009 with the enhanced configurations on the 4&nbsp;km detectors.<ref>{{cite web |url=http://ligonews.blogspot.com/2009/06/firm-date-set-for-start-of-s6.html |title=Firm Date Set for Start of S6 |first=Dave |last=Beckett |date=15 June 2009 |website=LIGO Laboratory News}}</ref> It concluded in October 2010, and the disassembly of the original detectors began.

===Advanced LIGO===
[[File:Simplified diagram of an Advanced LIGO detector.png|thumb|300px|Simplified diagram of an Advanced LIGO detector (not to scale).]]
[[File:AdvLIGO noise curve.webp|thumb|300px|Design sensitivity of Advanced LIGO interferometer with major noise sources, maximum sensitivity is around 500 Hz<ref>{{Cite journal|last1=Danilishin|first1=Stefan L.|last2=Khalili|first2=Farid Ya.|last3=Miao|first3=Haixing|date=29 April 2019|title=Advanced quantum techniques for future gravitational-wave detectors|url=http://link.springer.com/10.1007/s41114-019-0018-y|journal=Living Reviews in Relativity|language=en|volume=22|issue=1|pages=2|doi=10.1007/s41114-019-0018-y|arxiv=1903.05223|bibcode=2019LRR....22....2D|s2cid=119238143|issn=2367-3613}}</ref>]]
After 2010, LIGO went offline for several years for a major upgrade, installing the new Advanced LIGO detectors in the LIGO Observatory infrastructures.

The project continued to attract new members, with the [[Australian National University]] and [[University of Adelaide]] contributing to Advanced LIGO, and by the time the LIGO Laboratory started the first observing run 'O1' with the Advanced LIGO detectors in September 2015, the LIGO Scientific Collaboration included more than 900 scientists worldwide.<ref name="Nature_2015_Sept_15" />

The first observing run operated at a sensitivity roughly three times greater than Initial LIGO,<ref name="LIGO_sep_2015">{{cite web |title=The Newest Search for Gravitational Waves has Begun |url=https://www.ligo.caltech.edu/news/ligo20150918 |publisher=LIGO Scientific Collaboration |access-date=9 September 2017 |last=Burtnyk |first=Kimberly |date=18 September 2015 |archive-url=https://web.archive.org/web/20170704121637/https://www.ligo.caltech.edu/news/ligo20150918 |archive-date=4 July 2017 |quote=LIGO’s advanced detectors are already three times more sensitive than Initial LIGO was by the end of its observational lifetime}}</ref> and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies.<ref>{{cite journal |last1=Aasi |first1=J |title=Advanced LIGO |journal=Classical and Quantum Gravity |date=9 April 2015 |volume=32 |issue=7 |page=074001 |doi=10.1088/0264-9381/32/7/074001 |arxiv=1411.4547 |bibcode=2015CQGra..32g4001L |s2cid=118570458 }}</ref>

On 11 February 2016, the LIGO and [[Virgo interferometer|Virgo]] collaborations announced the [[first observation of gravitational waves]].{{r|Nature_11Feb16|PRL-20160211}} The signal, named [[GW150914]],<ref name="PRL-20160211" /><ref name="Naeye">{{cite news |last=Naeye |first=Robert |url=http://www.skyandtelescope.com/astronomy-news/gravitational-wave-detection-heralds-new-era-of-science-0211201644/ |title=Gravitational Wave Detection Heralds New Era of Science |work=Sky and Telescope |date=11 February 2016 |access-date=11 February 2016 }}</ref> was recorded on 14 September 2015, just two days after Advanced LIGO started collecting data following the upgrade.<ref name="Nature_11Feb16" /><ref name="cho2016">{{cite journal |last1=Cho |first1=Adrian |title=Here's the first person to spot those gravitational waves |url=https://www.science.org/content/article/here-s-first-person-spot-those-gravitational-waves |journal=Science |date=2016-02-11 |doi=10.1126/science.aaf4039}}</ref><ref name="BBC_11Feb16">{{cite news|title=Gravitational waves from black holes detected|url=https://www.bbc.co.uk/news/science-environment-35524440|work=BBC News|date=11 February 2016}}</ref> It matched the [[Tests of general relativity|predictions of general relativity]]<ref name="Pretorius2005">{{cite journal|last1=Pretorius|first1=Frans|title=Evolution of Binary Black-Hole Spacetimes|journal=Physical Review Letters|volume=95|issue=12|page=121101|year=2005|issn=0031-9007|doi=10.1103/PhysRevLett.95.121101|pmid=16197061|arxiv = gr-qc/0507014 |bibcode = 2005PhRvL..95l1101P |s2cid=24225193}}</ref><ref name="CampanelliLousto2006">{{cite journal |last1=Campanelli |first1=M. |author-link=Manuela Campanelli (scientist) |last2=Lousto |first2=C.O. |author-link2=Carlos Lousto |last3=Marronetti |first3=P. |last4=Zlochower |first4=Y. |year=2006 |title=Accurate Evolutions of Orbiting Black-Hole Binaries without Excision |journal=Physical Review Letters |volume=96 |issue=11 |page=111101 |arxiv=gr-qc/0511048 |bibcode=2006PhRvL..96k1101C |doi=10.1103/PhysRevLett.96.111101 |issn=0031-9007 |pmid=16605808 |s2cid=5954627}}</ref><ref name="BakerCentrella2006">{{cite journal|last1=Baker|first1=John G.|last2=Centrella|first2=Joan|author2-link= Joan Centrella |last3=Choi|first3=Dae-Il|last4=Koppitz|first4=Michael|last5=van Meter|first5=James|title=Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes|journal=Physical Review Letters|volume=96|issue=11|page=111102|year=2006|issn=0031-9007|doi=10.1103/PhysRevLett.96.111102|pmid=16605809|arxiv = gr-qc/0511103 |bibcode = 2006PhRvL..96k1102B |s2cid=23409406}}</ref> for the inward spiral and [[Stellar collision|merger]] of a [[Binary black hole|pair]] of [[black hole]]s and subsequent ringdown of the resulting single black hole. The observations demonstrated the existence of binary stellar-mass black hole systems and the first observation of a binary black hole merger.

On 15 June 2016, LIGO announced the detection of a second gravitational wave event, recorded on 26 December 2015, at 3:38 UTC. Analysis of the observed signal indicated that the event was caused by the merger of two black holes with masses of 14.2 and 7.5 solar masses, at a distance of 1.4 billion light years.<ref name="chu">{{cite news|last1=Chu|first1=Jennifer|title=For second time, LIGO detects gravitational waves|url=https://news.mit.edu/2016/second-time-ligo-detects-gravitational-waves-0615|access-date=15 June 2016|work=MIT News|publisher=MIT|date=15 June 2016}}</ref> The signal was named [[GW151226]].<ref>{{cite journal |url=http://www.ligo.org/science/Publication-GW151226/index.php |title=GW151226: Observation of Gravitational Waves from a 22 Solar-mass Binary Black Hole Coalescence |journal=Physical Review Letters |volume=116 |issue=24 |page=241103 |date=15 June 2016|bibcode=2016PhRvL.116x1103A |last1=Abbott |first1=B.P. |last2=Abbott |first2=R. |last3=Abbott |first3=T.D. |last4=Abernathy |first4=M.R. |last5=Acernese |first5=F. |last6=Ackley |first6=K. |last7=Adams |first7=C. |last8=Adams |first8=T. |last9=Addesso |first9=P. |last10=Adhikari |first10=R.X. |last11=Adya |first11=V.B. |last12=Affeldt |first12=C. |last13=Agathos |first13=M. |last14=Agatsuma |first14=K. |last15=Aggarwal |first15=N. |last16=Aguiar |first16=O.D. |last17=Aiello |first17=L. |last18=Ain |first18=A. |last19=Ajith |first19=P. |last20=Allen |first20=B. |last21=Allocca |first21=A. |last22=Altin |first22=P.A. |last23=Anderson |first23=S.B. |last24=Anderson |first24=W.G. |last25=Arai |first25=K. |last26=Araya |first26=M.C. |last27=Arceneaux |first27=C.C. |last28=Areeda |first28=J.S. |last29=Arnaud |first29=N. |last30=Arun |first30=K.G. |display-authors=3 |arxiv=1606.04855 |doi=10.1103/PhysRevLett.116.241103 |pmid=27367379 |s2cid=118651851 }}</ref>

The second observing run (O2) ran from 30 November 2016<ref>{{cite web |title=VIRGO joins LIGO for the "Observation Run 2" (O2) data-taking period |date=1 August 2017 |url=http://www.virgo-gw.eu/docs/AdV_joins_O2_en.pdf |publisher=LIGO Scientific Collaboration & VIRGO collaboration |access-date=20 October 2017 |archive-date=10 October 2017 |archive-url=https://web.archive.org/web/20171010204215/http://www.virgo-gw.eu/docs/AdV_joins_O2_en.pdf |url-status=dead }}</ref> to 25 August 2017,<ref name=O3_update>{{cite web |title=Update on the start of LIGO's 3rd observing run |date=24 April 2018 |url=https://www.ligo.org/news/index.php#O3updateApr18 |access-date=31 August 2018 |quote=the start of O3 is currently projected to begin in early 2019. Updates will be provided once the installation phase is complete and the commissioning phase has begun. An update on the engineering run prior to O3 will be provided by late summer 2018.}}</ref> with Livingston achieving 15–25% sensitivity improvement over O1, and with Hanford's sensitivity similar to O1.<ref>{{cite news |title=Advanced LIGO ramps up, with slight improvements |last1=Grant |first1=Andrew |date=12 December 2016 |journal=[[Physics Today]] |issue=11 |quote=The bottom line is that [the sensitivity] is better than it was at the beginning of O1; we expect to get more detections.|doi=10.1063/PT.5.9074 }}</ref> In this period, LIGO saw several further gravitational wave events: [[GW170104]] in January; [[GW170608]] in June; and [[List of gravitational wave observations|five others]] between July and August 2017. Several of these were also detected by the Virgo Collaboration.<ref>[https://dcc.ligo.org/LIGO-P1800307/public GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs]</ref>{{r|Chu17}}<ref>{{Cite web | url=https://www.ligo.caltech.edu/news/ligo20170927 | title=Gravitational waves from a binary black hole merger observed by LIGO and Virgo}}</ref> Unlike the black hole mergers which are only detectable gravitationally, [[GW170817]] came from the [[neutron star collision|collision of two neutron stars]] and was also detected electromagnetically by gamma ray satellites and optical telescopes.<ref name=Chu17>{{cite press release |title=LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars |url=https://www.ligo.caltech.edu/page/press-release-gw170817 |date=16 October 2017 |first=Jennifer |last=Chu |publisher=LIGO}}</ref>

The third run (O3) began on 1 April 2019<ref>{{cite web|url=https://www.ligo.caltech.edu/news/ligo20190502 |title=LIGO and Virgo Detect Neutron Star Smash-Ups}}</ref> and was planned to last until 30 April 2020; in fact it was suspended in March 2020 due to [[Coronavirus disease 2019|COVID-19]].<ref name="O3suspended"/><ref>{{cite web |title=Observatory Status |url=https://www.ligo.caltech.edu/page/observatory-status |archive-url=https://web.archive.org/web/20200409072730/https://www.ligo.caltech.edu/page/observatory-status |archive-date=2020-04-09 |url-status=live |work=LIGO |date=2020-03-23 |access-date=2020-06-23}}</ref><ref>Diego Bersanetti: [https://indico.cern.ch/event/577856/contributions/3422625/ Status of the Virgo gravitational-wave detector and the O3 Observing Run], EPS-HEP2019</ref> On 6 January 2020, LIGO announced the detection of what appeared to be gravitational ripples from a collision of two neutron stars, recorded on 25 April 2019, by the LIGO Livingston detector. Unlike GW170817, this event did not result in any light being detected. Furthermore, this is the first published event for a single-observatory detection, given that the LIGO Hanford detector was temporarily offline at the time and the event was too faint to be visible in Virgo's data.<ref>{{cite web|url=https://www.ligo.caltech.edu/news/ligo20200106 |title=LIGO-Virgo network catches another neutron star collision}}</ref>

The fourth observing run (O4) was planned to start in December 2022,<ref>{{Cite web |title=LIGO Laboratory statement on long term future observing plans |url=https://www.ligo.caltech.edu/page/ligo-lab-statement-long-term-future-observing-plans |access-date=2022-03-22 |website=LIGO Lab}}</ref> but was postponed until 24 May 2023. O4 is projected to continue until February 2025.<ref name=":1" /> As of O4, the interferometers are operating at a sensitivity of 155-175 Mpc,<ref name=":1" /> within the design sensitivity range of 160-190 Mpc for binary neutron star events.<ref>{{cite journal |title=Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO, Advanced Virgo and KAGRA |journal=Living Reviews in Relativity |date=24 Nov 2020 |doi=10.1007/s41114-020-00026-9 |arxiv=1304.0670 |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Abraham |first4=S. |last5=Acernese |first5=F. |last6=Ackley |first6=K. |last7=Adams |first7=C. |last8=Adya |first8=V. B. |last9=Affeldt |first9=C. |last10=Agathos |first10=M. |last11=Agatsuma |first11=K. |last12=Aggarwal |first12=N. |last13=Aguiar |first13=O. D. |last14=Aiello |first14=L. |last15=Ain |first15=A. |last16=Ajith |first16=P. |last17=Akutsu |first17=T. |last18=Allen |first18=G. |last19=Allocca |first19=A. |last20=Aloy |first20=M. A. |last21=Altin |first21=P. A. |last22=Amato |first22=A. |last23=Ananyeva |first23=A. |last24=Anderson |first24=S. B. |last25=Anderson |first25=W. G. |last26=Ando |first26=M. |last27=Angelova |first27=S. V. |last28=Antier |first28=S. |last29=Appert |first29=S. |last30=Arai |first30=K. |volume=23 |issue=1 |page=3 |pmid=33015351 |pmc=7520625 |bibcode=2020LRR....23....3A |display-authors=1 }}</ref> 

The fifth observing run (O5) is projected to begin in late 2025 or in 2026.<ref name=":1" />