Editing Physical cosmology (section)

== Subject history ==
<!--[[History of physical cosmology]] redirects here.-->
{{Nature timeline|upright=0.6}}
{{see also|Timeline of cosmology|List of cosmologists}}
Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of [[general relativity]], which provided a unified description of gravity as a geometric property of space and time.<ref>{{cite web |title=Albert Einstein – Biographical |url=http://nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html |url-status=live |archive-url=https://web.archive.org/web/20250122000800/https://www.nobelprize.org/prizes/physics/1921/einstein/biographical/ |archive-date=2025-01-22 |access-date=2025-01-23 |publisher=Nobel Prize}}</ref> At the time, Einstein believed in a [[static universe]], but found that his original formulation of the theory did not permit it.<ref name="Liddle, A. 51">{{cite book | author = Liddle, A. | title = An Introduction to Modern Cosmology | url = https://archive.org/details/introductiontomo00lidd_717 | url-access = limited | publisher = Wiley | page=[https://archive.org/details/introductiontomo00lidd_717/page/n67 51] | isbn =978-0-470-84835-7 | date = 2003 }}</ref> This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time.<ref>{{cite book | last = Vilenkin | first = Alex | title = Many worlds in one: the search for other universes | publisher = Hill and Wang, A division of Farrar, Straus and Giroux | location = New York | date = 2007 | isbn = 978-0-8090-6722-0 | page=19}}</ref> However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this ''[[cosmological constant]]'' to his field equations in order to force them to model a static universe.<ref>{{cite book |last1=Jones |first1=Mark |last2=Lambourne |first2=Robert | title = An introduction to galaxies and cosmology | publisher = Open University Cambridge University Press | location = Milton Keynes Cambridge, UK; New York | date = 2004 | isbn = 978-0-521-54623-2 | page= 228}}</ref> The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). However, this so-called Einstein model is unstable to small perturbations—it will eventually start to [[metric expansion of space|expand]] or contract.<ref name="Liddle, A. 51"/> It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the [[cosmological principle]]. The cosmological solutions of general relativity were found by [[Alexander Friedmann]] in the early 1920s.<ref>{{cite book |last1=Jones |first1=Mark |last2=Lambourne |first2=Robert | title = An introduction to galaxies and cosmology | publisher = Open University Cambridge University Press | location = Milton Keynes Cambridge, UK; New York | date = 2004 | isbn = 978-0-521-54623-2 |page=232 }}</ref> His equations describe the [[Friedmann–Lemaître–Robertson–Walker]] universe, which may expand or contract, and whose geometry may be open, flat, or closed.
[[File:History of the Universe.svg|thumb|upright=1.9|History of the [[Universe]] – [[gravitational wave]]s are hypothesized to arise from [[Inflation (cosmology)|cosmic inflation]], a rapidly accelerated expansion just after the [[Big Bang]]<ref name="BICEP2-2014" /><ref name="NASA-20140317" /><ref name="NYT-20140317" />]]
In the 1910s, [[Vesto Melvin Slipher|Vesto Slipher]] (and later [[Carl Wilhelm Wirtz]]) interpreted the [[red shift]] of [[nebula|spiral nebulae]] as a [[Doppler shift]] that indicated they were receding from Earth.<ref>{{cite journal | last1=Slipher | first1=V. M. | title=Further Notes on Spectrographic Observations of Nebulae and Clusters | journal=Publications of the American Astronomical Society| volume=4| date=1922 | pages=284–286 | bibcode=1922PAAS....4..284S }}</ref><ref>{{cite journal | last1=Seitter | first1=Waltraut C. | last2=Duerbeck | first2=Hilmar W. | title=Carl Wilhelm Wirtz – Pioneer in Cosmic Dimensions  | journal=Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era | series=ASP Conference Series | volume=167 | editor1-first=Daniel | editor1-last=Egret | editor2-first=Andre | editor2-last=Heck | isbn=978-1-886733-88-6 | date=1999 | pages=237–242 | bibcode=1999ASPC..167..237S }}</ref> However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its [[angular size]], but a physical size must be assumed in order to do this. Another method is to measure the [[brightness]] of an object and assume an intrinsic [[luminosity]], from which the distance may be determined using the [[inverse-square law]]. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own [[Milky Way]], nor did they speculate about the cosmological implications. In 1927, the [[Belgium|Belgian]] [[Roman Catholic]] [[priest]] [[Georges Lemaître]] independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval [[atom]]"<ref>{{cite journal |last1=Lemaître |first1=Georges |date=1927 |title=Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques |journal=Annales de la Société Scientifique de Bruxelles |language=fr |volume=A47 |pages=49–59 |bibcode=1927ASSB...47...49L}}</ref>—which was later called the Big Bang. In 1929, [[Edwin Hubble]] provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of [[Cepheid variable]] stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance from Earth.<ref>{{cite journal | last1=Hubble | first1=Edwin  | title=A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=15 | issue=3 | pages=168–173 | date=March 1929 | doi=10.1073/pnas.15.3.168  | bibcode=1929PNAS...15..168H | pmid=16577160 | pmc=522427| doi-access=free }}</ref> This fact is now known as [[Hubble's law]], though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.

Given the cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by [[George Gamow]]. The other explanation was [[Fred Hoyle]]'s [[steady state model]] in which new matter is created as the galaxies move away from each other. In this model, the universe is roughly the same at any point in time.<ref>{{cite journal |last1=Hoyle |first1=Fred |date=1948 |title=A New Model for the Expanding Universe |journal=Monthly Notices of the Royal Astronomical Society |volume=108 |issue=5 |pages=372–382 |bibcode=1948MNRAS.108..372H |doi=10.1093/mnras/108.5.372 |doi-access=free}}</ref><ref name="aip">{{cite web | title=Big Bang or Steady State? | work=Ideas of Cosmology | publisher=American Institute of Physics | url=https://www.aip.org/history/cosmology/ideas/bigbang.htm | access-date=2015-07-29 | archive-date=12 June 2015 | archive-url=https://web.archive.org/web/20150612115523/http://www.aip.org/history/cosmology/ideas/bigbang.htm | url-status=dead }}</ref>

For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model,<ref name="aip"/> and since the precise measurements of the cosmic microwave background by the [[Cosmic Background Explorer]] in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the universe began with a [[Gravitational singularity|singularity]], as demonstrated by [[Roger Penrose]] and [[Stephen Hawking]] in the 1960s.<ref name=Earman1999>{{cite book | last1=Earman | first1=John | title=The Penrose-Hawking Singularity Theorems: History and Implications – The expanding worlds of general relativity | journal=The Expanding Worlds of General Relativity | pages=235–267 | editor1-first=Hubert | editor1-last=Goenner | editor2-last=Jürgen | editor3-first=Jim | editor3-last=Ritter | editor4-first=Tilman | editor4-last=Sauer | series=Birk presentations of the fourth conference on the and gravitation | year=1999 | bibcode=1999ewgr.book..235E | doi=10.1007/978-1-4612-0639-2_7 | doi-broken-date=1 November 2024 | isbn=978-1-4612-6850-5 }}</ref>

An alternative view to extend the Big Bang model, suggesting the universe had no beginning or singularity and the age of the universe is infinite, has been presented.<ref name="LS-20150226">{{cite web |last=Ghose |first=Tia |title=Big Bang, Deflated? Universe May Have Had No Beginning |url=http://www.livescience.com/49958-theory-no-big-bang.html |date=26 February 2015 |website=[[Live Science]] |access-date=28 February 2015 }}</ref><ref name="PL-20150204">{{cite journal |last=Ali |first=Ahmed Faraq |title=Cosmology from quantum potential |journal=[[Physics Letters B]] |volume=741 |issue=2015 |pages=276–279 |doi=10.1016/j.physletb.2014.12.057 |date=4 February 2015 |arxiv=1404.3093 |bibcode=2015PhLB..741..276F |s2cid=55463396 }}</ref><ref name="AR-20141118">{{cite journal|last1=Das|first1=Saurya|last2=Bhaduri|first2=Rajat K|title=Dark matter and dark energy from a Bose–Einstein condensate|journal=Classical and Quantum Gravity|date=21 May 2015|volume=32|issue=10|pages=105003|doi=10.1088/0264-9381/32/10/105003|arxiv = 1411.0753 |bibcode = 2015CQGra..32j5003D |s2cid=119247745}}</ref>

In September 2023, astrophysicists questioned the overall current view of the [[universe]], in the form of the [[Standard Model of Cosmology]], based on the latest [[James Webb Space Telescope]] studies.<ref name="NYT-20230902">{{cite news |last1=Frank |first1=Adam |last2=Gleiser |first2=Marcelo |title=The Story of Our Universe May Be Starting to Unravel |url=https://www.nytimes.com/2023/09/02/opinion/cosmology-crisis-webb-telescope.html |date=2 September 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20230902161629/https://www.nytimes.com/2023/09/02/opinion/cosmology-crisis-webb-telescope.html |archivedate=2 September 2023 |accessdate=3 September 2023 }}</ref>