Editing Cyclic model (section)

== 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>