Editing Cyclic model (section)

== Overview ==
In the 1920s, theoretical physicists, most notably [[Albert Einstein]], noted the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an [[metric expansion of space|expanding universe]]{{Citation needed|reason=Looking for Einstein's Article on the topic, Little doubt of the validity of statement|date=November 2024}}. In 1922, [[Alexander Friedmann]] introduced the Oscillating Universe Theory.<ref>{{Cite web |title=Friedmann universe {{!}} Expanding Universe, Big Bang & Dark Energy {{!}} Britannica |url=https://www.britannica.com/science/Friedmann-universe |access-date=2023-09-24 |website=www.britannica.com |language=en}}</ref> However, work by [[Richard C. Tolman]] in 1934 showed that these early attempts failed because of the cyclic problem: according to the [[second law of thermodynamics]], entropy can only increase.<ref name="Tolman 1934">
{{cite book |author=Tolman |first=R. C. |title=Relativity, Thermodynamics, and Cosmology |date=1987 |publisher=Dover |isbn=978-0-486-65383-9 |location=New York |lccn=34032023 |orig-year=1934}}</ref> This implies that successive cycles grow longer and larger. Extrapolating back in time, cycles before the present one become shorter and smaller culminating again in a Big Bang and thus not replacing it. This puzzling situation remained for many decades until the early 21st century when the recently discovered [[dark energy]] component provided new hope for a consistent cyclic cosmology.<ref name="Frampton 2006">
{{cite arXiv |eprint=astro-ph/0612243 |first=P. H. |last=Frampton |title=On Cyclic Universes |date=2006}}</ref> In 2011, a five-year survey of 200,000 galaxies and spanning 7 billion years of cosmic time confirmed that "dark energy is driving our universe apart at accelerating speeds."<ref>{{Cite web |title=Dark energy is driving universe apart: NASA's Galaxy Evolution Explorer finds dark energy repulsive |url=https://www.sciencedaily.com/releases/2011/05/110519113152.htm |url-status=live |archive-url=https://web.archive.org/web/20241203111057/https://www.sciencedaily.com/releases/2011/05/110519113152.htm |archive-date=2024-12-03 |access-date=2025-01-23 |website=ScienceDaily |language=en}}</ref><ref>{{Cite journal |last1=Mandelbaum |first1=Rachel |last2=Blake |first2=Chris |last3=Bridle |first3=Sarah |last4=Abdalla |first4=Filipe B. |last5=Brough |first5=Sarah |last6=Colless |first6=Matthew |last7=Couch |first7=Warrick |last8=Croom |first8=Scott |last9=Davis |first9=Tamara |author-link9=Tamara Davis |last10=Drinkwater |first10=Michael J. |last11=Forster |first11=Karl |last12=Glazebrook |first12=Karl |last13=Jelliffe |first13=Ben |last14=Jurek |first14=Russell J. |last15=Li |first15=I-hui |year=2011 |title=The WiggleZ Dark Energy Survey: Direct constraints on blue galaxy intrinsic alignments at intermediate redshifts |journal=Monthly Notices of the Royal Astronomical Society |volume=410 |issue=2 |pages=844–859 |arxiv=0911.5347 |bibcode=2011MNRAS.410..844M |doi=10.1111/j.1365-2966.2010.17485.x |s2cid=36510728 |last16=Madore |first16=Barry |last17=Martin |first17=Chris |last18=Pimbblet |first18=Kevin |last19=Poole |first19=Gregory B. |last20=Pracy |first20=Michael |last21=Sharp |first21=Rob |last22=Wisnioski |first22=Emily |last23=Woods |first23=David |last24=Wyder |first24=Ted|doi-access=free }}</ref>

One new cyclic model is the [[brane cosmology]] model of the [[Big Bang|creation of the universe]], derived from the earlier [[Ekpyrotic universe|ekpyrotic]] model. It was proposed in 2001 by [[Paul Steinhardt]] of [[Princeton University]] and [[Neil Turok]] of [[Cambridge University]]. The theory describes a universe exploding into existence not just once, but repeatedly over time.<ref name="Steinhardt, Turok 2001a">
{{cite journal |author=Steinhardt |first1=P. J. |last2=Turok |first2=N. |year=2002 |title=Cosmic Evolution in a Cyclic Universe |journal=Physical Review D |volume=65 |issue=12 |pages=126003 |arxiv=hep-th/0111098 |bibcode=2002PhRvD..65l6003S |doi=10.1103/PhysRevD.65.126003 |s2cid=1342094}}</ref><ref name="Steinhardt, Turok 2001b">
{{cite journal |author=Steinhardt |first1=P. J. |last2=Turok |first2=N. |date=2001 |title=A Cyclic Model of the Universe |journal=Science |volume=296 |issue=5572 |pages=1436–1439 |arxiv=hep-th/0111030 |bibcode=2002Sci...296.1436S |doi=10.1126/science.1070462 |pmid=11976408 |s2cid=1346107}}</ref> The theory could potentially explain why a repulsive form of energy known as the [[cosmological constant]], which is accelerating the expansion of the universe, is several orders of magnitude smaller than predicted by the standard [[Big Bang]] model.

A different cyclic model relying on the notion of [[phantom energy]] was proposed in 2007 by Lauris Baum and [[Paul Frampton]] of the [[University of North Carolina at Chapel Hill]].<ref name="Baum, Frampton 2007">
{{cite journal |author=Baum |first1=Lewis |last2=Frampton |first2=P. H. |year=2008 |title=Entropy of Contracting Universe in Cyclic Cosmology |journal=Modern Physics Letters A |volume=23 |issue=1 |pages=33–36 |arxiv=hep-th/0703162 |bibcode=2008MPLA...23...33B |doi=10.1142/S0217732308026170 |s2cid=719844}}</ref>

Other cyclic models include [[conformal cyclic cosmology]] and [[loop quantum cosmology]].