Editing Gravity (section)

==Astrophysics==

=== Stars and black holes ===
{{main|Star formation}}
During star formation, gravitational attraction in a cloud of hydrogen gas competes with thermal gas pressure. As the gas density increases, the temperature rises, then the gas radiates energy, allowing additional gravitational condensation. If the mass of gas in the region is low, the process continues until a [[brown dwarf]] or [[gas-giant planet]] is produced. If more mass is available, the additional gravitational energy allows the central region to reach pressures sufficient for [[nuclear fusion]], forming a [[star]]. In a star, again the gravitational attraction competes, with thermal and radiation pressure in [[hydrostatic equilibrium]] until the star's atomic fuel runs out. The next phase depends upon the total mass of the star. Very low mass stars slowly cool as [[white dwarf]] stars with a small core balancing gravitational attraction with [[electron degeneracy pressure]]. Stars with masses similar to the Sun go through a [[red giant]] phase before becoming white dwarf stars. Higher mass stars have complex core structures that burn helium and high atomic number elements ultimately producing an [[iron]] core. As their fuel runs out, these stars become unstable producing a [[supernova]]. The result can be a [[neutron star]] where gravitational attraction balances [[neutron degeneracy pressure]] or, for even higher masses, a [[black hole]] where gravity operates alone with such intensity that even light cannot escape.<ref>{{Cite book |last=Demtröder |first=Wolfgang |title=Astrophysics |date=2024 |chapter=Birth, Lifetime and Death of Stars |series=Undergraduate Lecture Notes in Physics |chapter-url=https://link.springer.com/10.1007/978-3-031-22135-4_5 |access-date=2025-05-04 |publisher=Springer Nature Switzerland |pages=121–175 |language=en |doi=10.1007/978-3-031-22135-4_5|isbn=978-3-031-22133-0 }}</ref>{{rp|121}}

===Gravitational radiation===
{{Main|Gravitational wave}}
[[File:LIGO Hanford aerial 05.jpg|alt=LIGO Hanford Observatory|thumb|upright=1.2|The [[LIGO]] Hanford Observatory located in Washington (state), United States, where gravitational waves were first observed in September 2015]]
General relativity predicts that energy can be transported out of a system through gravitational radiation also known as gravitational waves. The first indirect evidence for gravitational radiation was through measurements of the [[Hulse–Taylor binary]] in 1973. This system consists of a [[pulsar]] and neutron star in orbit around one another. Its orbital period has decreased since its initial discovery due to a loss of energy, which is consistent for the amount of energy loss due to gravitational radiation. This research was awarded the [[Nobel Prize in Physics]] in 1993.<ref name="npp1993">{{cite web |title=The Nobel Prize in Physics 1993 |publisher=[[Nobel Foundation]] |url=https://www.nobelprize.org/prizes/physics/1993/press-release/ |date=13 October 1993 |quote=for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation |access-date=22 December 2023 |archive-date=10 August 2018 |archive-url=https://web.archive.org/web/20180810182047/https://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html |url-status=live }}</ref>

The first direct evidence for gravitational radiation was measured on 14 September 2015 by the [[LIGO]] detectors. The gravitational waves emitted during the collision of two black holes 1.3 billion light years from Earth were measured.<ref name='Clark 2016'>{{Cite web|title = Gravitational waves: scientists announce 'we did it!'{{snd}}live|url = https://www.theguardian.com/science/across-the-universe/live/2016/feb/11/gravitational-wave-announcement-latest-physics-einstein-ligo-black-holes-live|website = the Guardian|date = 11 February 2016|access-date = 11 February 2016|first = Stuart|last = Clark|archive-date = 22 June 2018|archive-url = https://web.archive.org/web/20180622055957/https://www.theguardian.com/science/across-the-universe/live/2016/feb/11/gravitational-wave-announcement-latest-physics-einstein-ligo-black-holes-live|url-status = live}}</ref><ref name="Discovery 2016">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News |url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |date=11 February 2016 |last1=Castelvecchi |first1=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361 |s2cid=182916902 |access-date=11 February 2016 |archive-date=12 February 2016 |archive-url=https://web.archive.org/web/20160212082216/http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |url-status=live }}</ref> This observation confirms the theoretical predictions of Einstein and others that such waves exist. It also opens the way for practical observation and understanding of the nature of gravity and events in the Universe including the Big Bang.<ref>{{cite web|title=WHAT ARE GRAVITATIONAL WAVES AND WHY DO THEY MATTER?|date=13 January 2016 |url=http://www.popsci.com/whats-so-important-about-gravitational-waves|publisher=popsci.com|access-date=12 February 2016|archive-date=3 February 2016|archive-url=https://web.archive.org/web/20160203130600/http://www.popsci.com/whats-so-important-about-gravitational-waves|url-status=live}}</ref> [[Neutron star]] and [[black hole]] formation also create detectable amounts of gravitational radiation.<ref name="PhysRev2017">{{cite journal |last1=Abbott |first1=B. P. |display-authors=etal. |collaboration=[[LIGO Scientific Collaboration]] & [[Virgo interferometer|Virgo Collaboration]] |title=GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral |journal=[[Physical Review Letters]] |date=October 2017 |volume=119 |issue=16 |pages=161101 |doi=10.1103/PhysRevLett.119.161101 |pmid=29099225 |doi-access=free |arxiv=1710.05832 |url=http://www.ligo.org/detections/GW170817/paper/GW170817-PRLpublished.pdf |bibcode=2017PhRvL.119p1101A |access-date=28 September 2019 |archive-date=8 August 2018 |archive-url=https://web.archive.org/web/20180808012441/https://www.ligo.org/detections/GW170817/paper/GW170817-PRLpublished.pdf |url-status=live }}</ref> This research was awarded the Nobel Prize in Physics in 2017.<ref>{{cite web|title=Nobel prize in physics awarded for discovery of gravitational waves|url=https://www.theguardian.com/science/2017/oct/03/nobel-prize-physics-discovery-gravitational-waves-ligo|website=the Guardian|date=3 October 2017|access-date=3 October 2017|last1=Devlin|first1=Hanna|archive-date=3 October 2017|archive-url=https://web.archive.org/web/20171003102211/https://www.theguardian.com/science/2017/oct/03/nobel-prize-physics-discovery-gravitational-waves-ligo|url-status=live}}</ref>

=== Dark matter ===
{{main|Dark matter}}
At the cosmological scale, gravity is a dominant player. About 5/6 of the total mass in the universe consists of dark matter which interacts through gravity but not through electromagnetic interactions. The gravitation of clumps of dark matter known as [[dark matter halo]]s attract hydrogen gas leading to stars and galaxies.<ref>{{Cite journal |last1=Wechsler |first1=Risa H. |last2=Tinker |first2=Jeremy L. |date=2018-09-14 |title=The Connection Between Galaxies and Their Dark Matter Halos |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-081817-051756 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=56 |issue=1 |pages=435–487 |doi=10.1146/annurev-astro-081817-051756 |issn=0066-4146|arxiv=1804.03097 }}</ref>

===Gravitational lensing===
{{main|Gravitational lensing}}
[[File:Einstein cross.jpg|thumb|upright=1|[[Einstein's Cross]], four images of the same distant [[quasar]] around a foreground galaxy due to gravitational lensing – a single quasar is actually hidden behind a massive foreground object (a galaxy in this case)]]
Gravity acts on light and matter equally, meaning that a sufficiently massive object could warp light around it and create a gravitational lens. This phenomenon was first confirmed by observation in 1979 using the 2.1 meter telescope at [[Kitt Peak National Observatory]] in Arizona, which saw two mirror images of the same quasar whose light had been bent around the galaxy [[YGKOW G1]].<ref>{{cite book |author1=Kar |first=Subal |url=https://books.google.com/books?id=IWFkEAAAQBAJ |title=Physics and Astrophysics: Glimpses of the Progress |publisher=CRC Press |year=2022 |isbn=978-1-000-55926-2 |edition=illustrated |page=106}} [https://books.google.com/books?id=IWFkEAAAQBAJ&pg=PT106 Extract of page 106].</ref><ref>{{Cite web |title=Hubble, Hubble, Seeing Double! |url=https://www.nasa.gov/content/goddard/hubble-hubble-seeing-double/#.YpZyvYOZrRl |access-date=31 May 2022 |website=NASA |date=24 January 2014 |archive-date=25 May 2022 |archive-url=https://web.archive.org/web/20220525041837/https://www.nasa.gov/content/goddard/hubble-hubble-seeing-double/#.YpZyvYOZrRl |url-status=live }}</ref>
Many subsequent observations of gravitational lensing provide additional evidence for substantial amounts of dark matter around galaxies. Gravitational lenses do not focus like [[eyeglass]] lenses, but rather lead to annular shapes called [[Einstein rings]].<ref name="Zee-2013"/>{{rp|370}}

===Speed of gravity===
{{Main|Speed of gravity}}
In December 2012, a research team in China announced that it had produced measurements of the phase lag of [[Earth tide]]s during full and new moons which seem to prove that the speed of gravity is equal to the speed of light.<ref>[http://www.astrowatch.net/2012/12/chinese-scientists-find-evidence-for.html Chinese scientists find evidence for speed of gravity] {{Webarchive|url=https://web.archive.org/web/20130108083729/http://www.astrowatch.net/2012/12/chinese-scientists-find-evidence-for.html |date=8 January 2013 }}, astrowatch.com, 12/28/12.</ref> This means that if the Sun suddenly disappeared, the Earth would keep orbiting the vacant point normally for 8 minutes, which is the time light takes to travel that distance. The team's findings were released in ''[[Science Bulletin]]'' in February 2013.<ref>{{cite journal|last=TANG|first=Ke Yun|author2=HUA ChangCai |author3=WEN Wu |author4=CHI ShunLiang |author5=YOU QingYu |author6=YU Dan |title=Observational evidences for the speed of the gravity based on the Earth tide|journal=Chinese Science Bulletin|date=February 2013|volume=58|issue=4–5|pages=474–477|doi=10.1007/s11434-012-5603-3|bibcode=2013ChSBu..58..474T|doi-access=free}}</ref>

In October 2017, the [[LIGO]] and [[Virgo interferometer]] detectors received gravitational wave signals within 2 seconds of [[gamma ray]] satellites and optical telescopes seeing signals from the same direction. This confirmed that the speed of gravitational waves was the same as the speed of light.<ref>{{cite web|url=https://www.ligo.caltech.edu/page/press-release-gw170817|title=GW170817 Press Release|website=LIGO Lab – Caltech|access-date=24 October 2017|archive-date=17 October 2017|archive-url=https://web.archive.org/web/20171017010137/https://www.ligo.caltech.edu/page/press-release-gw170817|url-status=live}}</ref>

===Anomalies and discrepancies===
{{distinguish|Gravity anomaly}}

There are some observations that are not adequately accounted for, which may point to the need for better theories of gravity or perhaps be explained in other ways.
[[File:GalacticRotation2.svg|thumb|Rotation curve of a typical spiral galaxy: predicted ('''A''') and observed ('''B'''). The discrepancy between the curves is attributed to [[dark matter]].]]
* '''[[Galaxy rotation curve|Galaxy rotation curves]]''': Stars in galaxies follow a distribution of velocities where stars on the outskirts are moving faster than they should according to the observed distributions of luminous matter. Galaxies within [[Galaxy groups and clusters|galaxy clusters]] show a similar pattern. The pattern is considered strong evidence for [[dark matter]], which would interact through gravitation but not electromagnetically; various [[Modified Newtonian dynamics|modifications to Newtonian dynamics]] have also been proposed.<ref>{{Cite journal |last1=Sofue |first1=Yoshiaki |last2=Rubin |first2=Vera |date=2001-09-01 |title=Rotation Curves of Spiral Galaxies |url=https://www.annualreviews.org/content/journals/10.1146/annurev.astro.39.1.137 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=39 |pages=137–174 |doi=10.1146/annurev.astro.39.1.137 |issn=0066-4146|arxiv=astro-ph/0010594 |bibcode=2001ARA&A..39..137S }}</ref>
* '''[[Accelerated expansion]]''': The [[expansion of the universe]] seems to be accelerating.<ref>{{Cite web |title=The Nobel Prize in Physics 2011 : Adam G. Riess Facts |url=https://www.nobelprize.org/prizes/physics/2011/riess/facts/ |access-date=19 March 2024 |website=NobelPrize.org |language=en-US |archive-date=28 May 2020 |archive-url=https://web.archive.org/web/20200528014511/https://www.nobelprize.org/prizes/physics/2011/riess/facts/ |url-status=live }}</ref> [[Dark energy]] has been proposed to explain this.<ref>{{Cite web |title=What is Dark Energy? Inside our accelerating, expanding Universe |url=https://science.nasa.gov/universe/the-universe-is-expanding-faster-these-days-and-dark-energy-is-responsible-so-what-is-dark-energy/ |access-date=19 March 2024 |website=science.nasa.gov |date=5 February 2024 |language=en |archive-date=19 March 2024 |archive-url=https://web.archive.org/web/20240319153930/https://science.nasa.gov/universe/the-universe-is-expanding-faster-these-days-and-dark-energy-is-responsible-so-what-is-dark-energy/ |url-status=live }}</ref>
* '''[[Flyby anomaly]]''': Various spacecraft have experienced greater acceleration than expected during [[gravity assist]] maneuvers.<ref>{{Cite journal |last1=Anderson |first1=John D. |last2=Campbell |first2=James K. |last3=Ekelund |first3=John E. |last4=Ellis |first4=Jordan |last5=Jordan |first5=James F. |date=3 March 2008 |title=Anomalous Orbital-Energy Changes Observed during Spacecraft Flybys of Earth |url=https://link.aps.org/doi/10.1103/PhysRevLett.100.091102 |journal=Physical Review Letters |language=en |volume=100 |issue=9 |page=091102 |doi=10.1103/PhysRevLett.100.091102 |pmid=18352689 |bibcode=2008PhRvL.100i1102A |issn=0031-9007}}</ref> The [[Pioneer anomaly]] has been shown to be explained by thermal recoil due to the distant sun radiation on one side of the space craft.<ref>{{Cite journal |last1=Turyshev |first1=Slava G. |last2=Toth |first2=Viktor T. |last3=Kinsella |first3=Gary |last4=Lee |first4=Siu-Chun |last5=Lok |first5=Shing M. |last6=Ellis |first6=Jordan |date=12 June 2012 |title=Support for the Thermal Origin of the Pioneer Anomaly |url=https://link.aps.org/doi/10.1103/PhysRevLett.108.241101 |journal=Physical Review Letters |volume=108 |issue=24 |pages=241101 |doi=10.1103/PhysRevLett.108.241101|pmid=23004253 |arxiv=1204.2507 |bibcode=2012PhRvL.108x1101T }}</ref><ref>{{Cite journal |last=Iorio |first=Lorenzo |date=May 2015 |title=Gravitational anomalies in the solar system? |url=https://www.worldscientific.com/doi/abs/10.1142/S0218271815300153 |journal=International Journal of Modern Physics D |language=en |volume=24 |issue=6 |pages=1530015–1530343 |doi=10.1142/S0218271815300153 |issn=0218-2718|arxiv=1412.7673 |bibcode=2015IJMPD..2430015I }}</ref>