Editing Cyclic model

{{Short description|Cosmological models involving indefinite, self-sustaining cycles}}
{{Primary sources|article|date=February 2013}}
{{cosmology}}
A '''cyclic model''' (or '''oscillating model''') is any of several [[cosmological model]]s in which the [[universe]] follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by [[Albert Einstein]] in 1930 theorized a universe following an [[eternity|eternal]] series of oscillations, each beginning with a [[Big Bang]] and ending with a [[Big Crunch]]; in the interim, the universe would [[metric expansion of space|expand]] for a period of time before the gravitational attraction of matter causes it to collapse back in and undergo a [[Big Bounce|bounce]].

== 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]].

== The Steinhardt&ndash;Turok model ==
{{main|Ekpyrotic universe}}
In this cyclic model, two parallel [[orbifold]] planes or [[Membrane (M-Theory)|M-branes]] collide periodically in a higher-dimensional space.<ref name="Paul J. Steinhardt, Neil Turok 2004">
{{cite journal |author=Steinhardt |first1=P. J. |last2=Turok |first2=N. |year=2005 |title=The Cyclic Model Simplified |journal=New Astronomy Reviews |volume=49 |issue=2–6 |pages=43–57 |arxiv=astro-ph/0404480 |bibcode=2005NewAR..49...43S |doi=10.1016/j.newar.2005.01.003 |s2cid=16034194}}</ref> The visible four-dimensional universe lies on one of these [[branes]]. The collisions correspond to a reversal from contraction to expansion, or a [[Big Crunch]] followed immediately by a [[Big Bang]]. The matter and radiation we see today were generated during the most recent collision in a pattern dictated by [[quantum fluctuation]]s created before the branes. After billions of years the universe reached the state we observe today; after additional billions of years it will ultimately begin to contract again. [[Dark energy]] corresponds to a force between the branes, and serves the crucial role of solving the [[monopole problem|monopole]], [[horizon problem|horizon]], and [[flatness problem|flatness]] problems. Moreover, the cycles can continue indefinitely into the past and the future, and the solution is an [[attractor]], so it can provide a complete history of the universe.

As [[Richard C. Tolman]] showed, the earlier cyclic model failed because the universe would undergo inevitable [[thermodynamics|thermodynamic]] [[heat death of the universe|heat death]].<ref name="Tolman 1934" /> However, the newer cyclic model evades this by having a net expansion each cycle, preventing [[entropy]] from building up. However, there remain major open issues in the model. Foremost among them is that colliding [[Membrane (M-Theory)|branes]] are not understood by string theorists, and nobody knows if the [[scale invariance|scale invariant]] spectrum will be destroyed by the big crunch. Moreover, as with [[cosmic inflation]], while the general character of the forces (in the [[ekpyrotic]] scenario, a force between branes) required to create the [[quantum fluctuation|vacuum fluctuation]]s is known, there is no candidate from [[particle physics]].<ref>{{cite book |author=Woit |first=Peter |title=Not Even Wrong |date=2006 |publisher=[[Random House]] |isbn=978-0-09-948864-4 |location=London |author-link=Peter Woit}}</ref>

== The Baum–Frampton model ==
This more recent cyclic model of 2007 assumes an exotic form of dark energy called [[phantom energy]],<ref name="Baum, Frampton 2007" /><ref name="Baum, Frampton 2006">
{{cite journal |author=Baum |first1=L. |last2=Frampton |first2=P. H. |date=2007 |title=Turnaround in Cyclic Cosmology |journal=Physical Review Letters |volume=98 |issue=7 |page=071301 |arxiv=hep-th/0610213 |bibcode=2007PhRvL..98g1301B |doi=10.1103/PhysRevLett.98.071301 |pmid=17359014 |s2cid=17698158}}</ref> which possesses negative kinetic energy and would usually cause the universe to end in a [[Big Rip]]. This condition is achieved if the universe is dominated by dark energy with a cosmological [[Equation of state (cosmology)|equation of state]] parameter <math>w</math> satisfying the condition <math>w\equiv \frac{p}{\rho} <-1</math>, for [[energy density]] <math>{\rho}</math> and [[pressure]] p. By contrast, Steinhardt–Turok assume <math>w {\geq}-1</math>. In the Baum–Frampton model, a septillionth (or less) <!-- Use 10<sup>−X</sup> notation please-->of a second (i.e. 10<sup>−24</sup> seconds or less) before the would-be Big Rip, a turnaround occurs and only one causal patch is retained as our universe. The generic patch contains no [[quark]], [[lepton]] or [[force carrier]]; only [[dark energy]] – and its entropy thereby vanishes. The [[adiabatic process]] of contraction of this much smaller universe takes place with constant vanishing entropy and with no matter including no [[black holes]] which disintegrated before turnaround.

The idea that the universe "comes back empty" is a central new idea of this cyclic model, and avoids many difficulties confronting matter in a contracting phase such as excessive [[structure formation]], proliferation and expansion of [[black holes]], as well as going through [[phase transition]]s such as those of QCD and electroweak symmetry restoration. Any of these would tend strongly to produce an unwanted premature bounce, simply to avoid violation of the [[second law of thermodynamics]]. The condition of <math>w <-1</math> may be logically inevitable in a truly infinitely cyclic cosmology because of the entropy problem. Nevertheless, many technical back up calculations are necessary to confirm consistency of the approach. Although the model borrows ideas from [[string theory]], it is not necessarily committed to strings, or to [[higher dimensions]], yet such speculative devices may provide the most expeditious methods to investigate the [[internal consistency]]. The value of <math>w</math> in the Baum–Frampton model can be made arbitrarily close to, but must be less than, −1.

== Other cyclic models ==
* [[Conformal cyclic cosmology]]—a general relativity based theory by [[Roger Penrose]] in which the universe expands until all the matter decays and is turned to light—so there is nothing in the universe that has any time or distance scale associated with it. This permits it to become identical with the Big Bang, so starting the next cycle.
* [[Loop quantum cosmology]] which predicts a "quantum bridge" between contracting and expanding cosmological branches.

== See also ==
'''Physical cosmologies:'''
* [[Big Bounce]]
* [[Conformal cyclic cosmology]]

'''Religion:'''
* [[Bhavacakra]]
* [[Hindu units of time|Cycles of time in Hinduism]]
* [[Eternal return]]
* [[Historic recurrence]]
* [[Jainism and non-creationism]]
* [[Kalachakra]]
* [[Wheel of time]]

== References ==
{{Reflist}}

== Further reading ==
*{{cite book |author=Steinhardt |title=Endless Universe |last2=Turok |first2=N. |first1=P. J. |url=https://archive.org/details/endlessuniverseb0000stei |url-access=registration |publisher=Doubleday |location=New York, New York |language=en-us |date=2007 |isbn=978-0-385-50964-0}}
*{{cite book |author=Tolman |first=R. C. |title=Relativity, Thermodynamics, and Cosmology |publisher=Dover |location=New York |orig-year=1934 |date=1987 |isbn=978-0-486-65383-9 |lccn=34032023}}
*{{cite journal |author=Baum |first1=L. |last2=Frampton |first2=P. H. |title=Turnaround in Cyclic Cosmology |journal=Physical Review Letters |volume=98 |page=071301 |date=2007 |doi=10.1103/PhysRevLett.98.071301 |pmid=17359014 |issue=7 |arxiv=hep-th/0610213 |bibcode=2007PhRvL..98g1301B |s2cid=17698158}}
* {{cite journal |last1=Dicke |first1=R. H. |last2=Peebles |first2=P. J. E.| last3=Roll |first3=P. G. |last4=Wilkinson |first4=D. T. |title=Cosmic Black-Body Radiation. |journal=The Astrophysical Journal |volume=142 |year=1965 |pages=414 |issn=0004-637X |doi=10.1086/148306 |bibcode=1965ApJ...142..414D |language=en}}
*S. W. Hawking and G. F. R. Ellis, ''The large-scale structure of space-time'' (Cambridge, 1973).
*{{cite book |author=Penrose |first=Roger |title=Cycles of Time: an extraordinary new view of the universe |publisher=[[The Bodley Head]] |location=London |language=en-us |date=2010 |isbn=978-0-224-08036-1 |title-link=Cycles of Time (book)}}

== External links ==
* [https://web.archive.org/web/20040110024422/http://feynman.princeton.edu/~steinh/ Paul J. Steinhardt], Department of Physics, Princeton University
* [https://web.archive.org/web/20070218172218/http://www.physics.unc.edu/~frampton/ Paul H. Frampton], Department of Physics and Astronomy, The University of North Carolina at Chapel Hill
*[https://web.archive.org/web/20070930204426/http://www.edge.org/video/dsl/turok07.html  "The Cyclic Universe": A Talk with Neil Turok]
*[https://www.youtube.com/watch?v=pEIj9zcLzp0 Roger Penrose—Cyclical Universe Model]
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