Editing Quasar (section)

=== Development of physical understanding (1960s) ===
{{main|Redshift|Universe|Expansion of the universe}}

An extreme redshift could imply great distance and velocity but could also be due to extreme mass or perhaps some other unknown laws of nature. Extreme velocity and distance would also imply immense power output, which lacked explanation. The small sizes were confirmed by [[interferometry]] and by observing the speed with which the quasar as a whole varied in output, and by their inability to be seen in even the most powerful visible-light telescopes as anything more than faint starlike points of light. But if they were small and far away in space, their power output would have to be immense and difficult to explain. Equally, if they were very small and much closer to this galaxy, it would be easy to explain their apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe.

Schmidt noted that redshift is also associated with the expansion of the universe, as codified in [[Hubble's law]]. If the measured redshift was due to expansion, then this would support an interpretation of very distant objects with extraordinarily high [[luminosity]] and power output, far beyond any object seen to date. This extreme luminosity would also explain the large radio signal. Schmidt concluded that 3C 273 could either be an individual star around 10&nbsp;km wide within (or near to) this galaxy, or a distant active galactic nucleus. He stated that a distant and extremely powerful object seemed more likely to be correct.<ref name="schmidt1963">{{Cite journal |last=Schmidt |first=M. |author-link=Maarten Schmidt |date=March 1963 |title=3C 273 : A Star-Like Object with Large Red-Shift |journal=Nature |language=en |volume=197 |issue=4872 |pages=1040 |bibcode=1963Natur.197.1040S |doi=10.1038/1971040a0 |issn=0028-0836 |s2cid=4186361 |doi-access=free}}</ref>

Schmidt's explanation for the high redshift was not widely accepted at the time. A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. In the 1960s no commonly accepted mechanism could account for this. The currently accepted explanation, that it is due to [[matter]] in an [[accretion disc]] falling into a [[supermassive black hole]], was only suggested in 1964 by [[Edwin E. Salpeter]] and [[Yakov Zeldovich]],<ref>{{cite journal |last1=Shields |first1=G. A. |title=A Brief History of Active Galactic Nuclei |journal=Publications of the Astronomical Society of the Pacific |date=1999 |volume=111 |issue=760 |page=661 |doi=10.1086/316378 |bibcode=1999PASP..111..661S |arxiv = astro-ph/9903401|s2cid=18953602 }}</ref> and even then it was rejected by many astronomers, as at this time the existence of [[black holes]] at all was widely seen as theoretical.

Various explanations were proposed during the 1960s and 1970s, each with their own problems. It was suggested that quasars were nearby objects, and that their redshift was not due to the [[Redshift#Expansion of space|expansion of space]] but rather to [[gravitational redshift|light escaping a deep gravitational well]]. This would require a massive object, which would also explain the high luminosities. However, a star of sufficient mass to produce the measured redshift would be unstable and in excess of the [[Hayashi limit]].<ref>{{Cite journal |last=Chandrasekhar |first=S. |author-link=Subrahmanyan Chandrasekhar |date=August 1964 |title=The Dynamical Instability of Gaseous Masses Approaching the Schwarzschild Limit in General Relativity. |journal=The Astrophysical Journal |language=en |volume=140 |issue=2 |pages=417 |bibcode=1964ApJ...140..417C |doi=10.1086/147938 |issn=0004-637X |s2cid=120526651 |doi-access=free}}</ref> Quasars also show [[forbidden lines|forbidden]] spectral emission lines, previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well.<ref>{{Cite journal |last1=Greenstein |first1=Jesse L. |author-link=Jesse L. Greenstein |last2=Schmidt |first2=Maarten |date=July 1964 |title=The Quasi-Stellar Radio Sources 3c 48 and 3c 273. |journal=The Astrophysical Journal |language=en |volume=140 |issue=1 |pages=1 |bibcode=1964ApJ...140....1G |doi=10.1086/147889 |issn=0004-637X |s2cid=123147304 |doi-access=free}}</ref> There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including [[nuclear fusion]]. There were suggestions that quasars were made of some hitherto unknown stable form of [[antimatter]] in similarly unknown types of region of space, and that this might account for their brightness.<ref>{{Cite journal |last=Gray |first=G. K. |year=1965 |title=Quasars and Antimatter |journal=Nature |language=en |volume=206 |issue=4980 |page=175 |bibcode=1965Natur.206..175G |doi=10.1038/206175a0 |issn=0028-0836 |s2cid=4171869 |doi-access=free}}</ref> Others speculated that quasars were a [[white hole]] end of a [[wormhole]],<ref>{{cite book |last1=Haven |first1=Kendall F. |date=2001 |title=That's weird!: awesome science mysteries |others=Illustrated by Jason Lynch |publisher=Fulcrum Resources |location=Golden, Colo. |isbn=9781555919993|pages=39–41 |url=https://books.google.com/books?id=2zVo1lirhi4C&pg=PA39}}</ref><ref>{{cite book |last1=Santilli |first1=Ruggero Maria |title=Isodual theory of antimatter: with applications to antigravity, grand unification and cosmology |date=2006 |publisher=Springer |location=Dordrecht |isbn=978-1-4020-4517-2 |page=304 |url=https://books.google.com/books?id=xJwUB--qflEC&pg=PA304 |bibcode=2006itaa.book.....S }}</ref> or a [[chain reaction]] of numerous [[supernova]]e.<ref name="Caltech_ipac_4">{{Cite web |last=Shields |first=Gregory A. |date=1999 |title=A Brief History of AGN. 4.2.&nbsp;Energy Source |url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields4_2.html |website=[[California Institute of Technology]]}}</ref>

Eventually, starting from about the 1970s, many lines of evidence (including [[Uhuru (satellite)|the first]] [[X-ray]] [[space observatory|space observatories]], knowledge of [[black hole]]s and modern models of [[cosmology]]) gradually demonstrated that the quasar redshifts are genuine and due to the [[expansion of space]], that quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested, and that their energy source is matter from an accretion disc falling onto a supermassive black hole.<ref name="keel2009">{{cite web |first=William C. |last=Keel |date=October 2009 |title=Alternate Approaches and the Redshift Controversy |publisher=The University of Alabama |url=http://www.astr.ua.edu/keel/galaxies/arp.html |access-date=2010-09-27}}</ref> This included crucial evidence from optical and X-ray viewing of quasar host galaxies, finding of "intervening" absorption lines, which explained various spectral anomalies, observations from [[gravitational lensing]], [[James Gunn (astronomer)|Gunn]]'s 1971 finding that galaxies containing quasars showed the same redshift as the quasars,<ref>{{cite journal | last1 = Gunn | first1 = James E. | title = On the Distances of the Quasi-Stellar Objects | journal = The Astrophysical Journal | date = March 1971 | volume = 164 | page = L113 | doi = 10.1086/180702 | bibcode = 1971ApJ...164L.113G | doi-access = free }}</ref> and [[Jerome Kristian|Kristian]]'s 1973 finding that the "fuzzy" surrounding of many quasars was consistent with a less luminous host galaxy.<ref>{{cite journal | last1 = Kristian | first1 = Jerome | title = Quasars as Events in the Nuclei of Galaxies: the Evidence from Direct Photographs | journal = The Astrophysical Journal | date = January 1973 | volume = 179 | page = L61 | doi = 10.1086/181117| bibcode = 1973ApJ...179L..61K }}</ref>

This model also fits well with other observations suggesting that many or even most galaxies have a massive central black hole. It would also explain why quasars are more common in the early universe: as a quasar draws matter from its accretion disc, there comes a point when there is less matter nearby, and energy production falls off or ceases, as the quasar becomes a more ordinary type of galaxy.

The accretion-disc energy-production mechanism was finally modeled in the 1970s, and black holes were also directly detected (including evidence showing that supermassive black holes could be found at the centers of this and many other galaxies), which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature. By 1987 it was "well accepted" that this was the correct explanation for quasars,<ref name="thomsen_1987" /> and the cosmological distance and energy output of quasars was accepted by almost all researchers.