Editing Black hole (section)

=== Detection of gravitational waves from merging black holes ===
[[File:LIGO measurement of gravitational waves.svg|thumb|LIGO measurement of the gravitational waves at the Livingston (right) and Hanford (left) detectors, compared with the theoretical predicted values]]
On 14 September 2015, the [[LIGO]] gravitational wave observatory made the first-ever successful [[first observation of gravitational waves|direct observation of gravitational waves]].<ref name="PRL-20160211">{{cite journal |author=Abbott, B.P. |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=[[Phys. Rev. Lett.]] |volume=116 |issue=6 |page=061102 |year=2016 |doi=10.1103/PhysRevLett.116.061102 |display-authors=etal |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |pmid=26918975|s2cid=124959784 }}</ref><ref name="NYT-20160211-db">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Physicists Detect Gravitational Waves, Proving Einstein Right |url=https://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html |date=11 February 2016 |work=[[The New York Times]] |access-date=11 February 2016 |url-status=live |archive-url=https://web.archive.org/web/20160211165128/http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html |archive-date=11 February 2016}}</ref> The signal was consistent with theoretical predictions for the gravitational waves produced by the merger of two black holes: one with about 36 solar masses, and the other around 29 solar masses.<ref name="PRL-20160211" /><ref name="Properties">{{cite journal |collaboration=[[LIGO Scientific Collaboration]] & [[Virgo interferometer|Virgo Collaboration]] |last1=Abbott |first1=Benjamin P. |arxiv=1602.03840 |title=Properties of the binary black hole merger GW150914 |journal=Physical Review Letters |volume=116 |issue=24 |page=241102 |date=11 February 2016 |bibcode=2016PhRvL.116x1102A |doi=10.1103/PhysRevLett.116.241102 |pmid=27367378|s2cid=217406416 }}</ref> This observation provides the most concrete evidence for the existence of black holes to date. For instance, the gravitational wave signal suggests that the separation of the two objects before the merger was just 350&nbsp;km, or roughly four times the Schwarzschild radius corresponding to the inferred masses. The objects must therefore have been extremely compact, leaving black holes as the most plausible interpretation.<ref name="PRL-20160211" />

More importantly, the signal observed by LIGO also included the start of the post-merger [[Binary black hole#Ringdown|ringdown]], the signal produced as the newly formed compact object settles down to a stationary state. Arguably, the ringdown is the most direct way of observing a black hole.<ref name="Cardoso2016">{{cite journal |author1=Cardoso, V. |author2=Franzin, E. |author3=Pani, P. |title=Is the gravitational-wave ringdown a probe of the event horizon? |doi=10.1103/PhysRevLett.116.171101 |pmid=27176511 |journal=[[Physical Review Letters]] |arxiv=1602.07309 |year=2016 |volume=116 |issue=17 |page=171101 |bibcode=2016PhRvL.116q1101C|s2cid=206273829 }}</ref> From the LIGO signal, it is possible to extract the frequency and damping time of the dominant mode of the ringdown. From these, it is possible to infer the mass and angular momentum of the final object, which match independent predictions from numerical simulations of the merger.<ref name="tests">{{cite journal |url=https://dcc.ligo.org/P1500213/public |title=Tests of general relativity with GW150914 |collaboration=[[LIGO Scientific Collaboration]] & [[Virgo interferometer|Virgo Collaboration]] |last1=Abbott |first1=Benjamin P. |journal=Physical Review Letters |date=11 February 2016 |volume=116 |issue=22 |page=221101 |doi=10.1103/PhysRevLett.116.221101 |pmid=27314708 |arxiv=1602.03841 |bibcode=2016PhRvL.116v1101A |s2cid=217275338 |access-date=12 February 2016 |archive-url=https://web.archive.org/web/20160215165039/https://dcc.ligo.org/P1500213/public |archive-date=15 February 2016 }}</ref> The frequency and decay time of the dominant mode are determined by the geometry of the photon sphere. Hence, observation of this mode confirms the presence of a photon sphere; however, it cannot exclude possible exotic alternatives to black holes that are compact enough to have a photon sphere.<ref name="Cardoso2016"/><ref name="Murk2023">{{cite journal |last=Murk |first=Sebastian |title=Nomen non est omen: Why it is too soon to identify ultra-compact objects as black holes |journal=International Journal of Modern Physics D |year=2023 |volume=32 |issue=14 |pages=2342012–2342235 |doi=10.1142/S0218271823420129 |arxiv=2210.03750 |bibcode=2023IJMPD..3242012M |s2cid=252781040}}</ref>

The observation also provides the first observational evidence for the existence of stellar-mass black hole binaries. Furthermore, it is the first observational evidence of stellar-mass black holes weighing 25 solar masses or more.<ref name="implications">{{cite journal |collaboration=[[LIGO Scientific Collaboration]] & [[Virgo interferometer|Virgo Collaboration]] |title=Astrophysical Implications of the Binary Black Hole Merger GW150914 |doi=10.3847/2041-8205/818/2/L22 |journal=Astrophys. J. Lett. |volume=818 |number=2 |page=L22 |url=https://dcc.ligo.org/P1500262/public |arxiv=1602.03846 |bibcode=2016ApJ...818L..22A |year=2016 |last1=Abbott |first1=B. P. |hdl=1826/11732 |s2cid=209315965 |url-status=live |archive-url=https://web.archive.org/web/20160316053938/https://dcc.ligo.org/P1500262/public |archive-date=16 March 2016 |doi-access=free }}</ref>

Since then, many more [[List of gravitational wave observations|gravitational wave events]] have been observed.<ref name="ligo list">{{cite web|title=Detection of gravitational waves|url=https://www.ligo.org/detections.php|access-date=9 April 2018|publisher=[[LIGO]]|archive-date=20 May 2020|archive-url=https://web.archive.org/web/20200520134427/https://www.ligo.org/detections.php|url-status=live}}</ref>