Editing Cosmic inflation (section)

==Alternatives and adjuncts==
Other models have been advanced that are claimed to explain some or all of the observations addressed by inflation.

=== Big bounce ===
The big bounce hypothesis attempts to replace the cosmic singularity with a cosmic contraction and bounce, thereby explaining the initial conditions that led to the big bang. The flatness and horizon problems are naturally solved in the [[Einstein–Cartan theory|Einstein–Cartan]]–Sciama–Kibble theory of gravity, without needing an exotic form of matter or free parameters.<ref>
{{cite journal |author=Poplawski |first=N. J. |year=2010 |title=Cosmology with torsion: An alternative to cosmic inflation |journal=[[Physics Letters B]] |volume=694 |issue=3 |pages=181–185 |arxiv=1007.0587 |bibcode=2010PhLB..694..181P |doi=10.1016/j.physletb.2010.09.056}}
</ref><ref>
{{cite journal |author=Poplawski |first=N. J. |year=2012 |title=Nonsingular, big-bounce cosmology from spinor-torsion coupling |journal=[[Physical Review D]] |volume=85 |issue=10 |pages=107502 |arxiv=1111.4595 |bibcode=2012PhRvD..85j7502P |doi=10.1103/PhysRevD.85.107502 |s2cid=118434253}}
</ref> This theory extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the [[torsion tensor]], as a dynamical variable. The minimal coupling between torsion and [[Dirac spinor]]s generates a spin-spin interaction that is significant in fermionic matter at extremely high densities. Such an interaction averts the unphysical Big Bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting. The rapid expansion immediately after the [[Big Bounce]] explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic. As the density of the Universe decreases, the effects of torsion weaken and the Universe smoothly enters the radiation-dominated era.

=== Ekpyrotic and cyclic models ===
The [[Ekpyrotic universe|ekpyrotic]] and [[cyclic model]]s are also considered adjuncts to inflation. These models solve the [[horizon problem]] through an expanding epoch well ''before'' the Big Bang, and then generate the required spectrum of primordial density perturbations during a contracting phase leading to a [[Big Crunch]]. The Universe passes through the Big Crunch and emerges in a hot [[Big Bang]] phase. In this sense they are reminiscent of [[Richard Chace Tolman]]'s [[oscillatory universe]]; in Tolman's model, however, the total age of the Universe is necessarily finite, while in these models this is not necessarily so. Whether the correct spectrum of density fluctuations can be produced, and whether the Universe can successfully navigate the Big Bang/Big Crunch transition, remains a topic of controversy and current research. Ekpyrotic models avoid the [[magnetic monopole]] problem as long as the temperature at the Big Crunch/Big Bang transition remains below the Grand Unified Scale, as this is the temperature required to produce magnetic monopoles in the first place. As things stand, there is no evidence of any 'slowing down' of the expansion, but this is not surprising as each cycle is expected to last on the order of a trillion years.<ref>{{Cite journal |last=Lehners |first=Jean-Luc |date=2 June 2009 |title=Ekpyrotic and cyclic cosmology |journal=Physics Reports |volume=465 |issue=6 |pages=223–263 |doi=10.1016/j.physrep.2008.06.001 |arxiv=0806.1245 |s2cid=17534907 }}</ref>

=== String gas cosmology ===
[[String theory]] requires that, in addition to the three observable spatial dimensions, additional dimensions exist that are curled up or [[compactification (physics)|compactified]] (see also [[Kaluza–Klein theory]]). Extra dimensions appear as a frequent component of [[supergravity]] models and other approaches to [[quantum gravity]]. This raised the contingent question of why four space-time dimensions became large and the rest became unobservably small. An attempt to address this question, called ''string gas cosmology'', was proposed by [[Robert Brandenberger]] and [[Cumrun Vafa]].<ref>
{{cite journal
 |last1=Brandenberger |first1=R.
 |last2=Vafa |first2=C.
 |year=1989
 |title=Superstrings in the early universe
 |journal=[[Nuclear Physics B]]
 |volume=316 |issue=2 |pages=391–410
 |bibcode=1989NuPhB.316..391B |citeseerx=10.1.1.56.2356
 |doi=10.1016/0550-3213(89)90037-0
}}
</ref> This model focuses on the dynamics of the early universe considered as a hot gas of strings. Brandenberger and Vafa show that a dimension of [[spacetime]] can only expand if the strings that wind around it can efficiently annihilate each other, which became known as [[Brandenberger–Vafa mechanism]]. Each string is a one-dimensional object, and the largest number of dimensions in which two strings will [[Transversality (mathematics)|generically intersect]] (and, presumably, annihilate) is three. Therefore, the most likely number of non-compact (large) spatial dimensions is three. Current work on this model centers on whether it can succeed in stabilizing the size of the compactified dimensions and produce the correct spectrum of primordial density perturbations.<ref>{{cite journal |last1=Battefeld|first1=Thorsten |last2=Watson |first2=Scott |year=2006 |title=String Gas Cosmology |journal=[[Reviews of Modern Physics]] |volume=78|issue=2|pages=435–454 |arxiv=hep-th/0510022 |bibcode=2006RvMP...78..435B |doi=10.1103/RevModPhys.78.435 |s2cid=2246186}}</ref> The original model did not "solve the entropy and flatness problems of standard cosmology",<ref>{{cite journal|last1=Brandenberger|first1=Robert H.|last2=Nayeri|first2=ALI|last3=Patil|first3=Subodh P.|last4=Vafa|first4=Cumrun|date=2007|title=String Gas Cosmology and Structure Formation|url=https://cds.cern.ch/record/978863|journal=[[International Journal of Modern Physics A]]|volume=22|issue=21|pages=3621–3642|arxiv=hep-th/0608121|bibcode=2007IJMPA..22.3621B|doi=10.1142/S0217751X07037159|s2cid=5899352}}</ref> although Brandenburger and coauthors later argued that these problems can be eliminated by implementing string gas cosmology in the context of a bouncing-universe scenario.<ref>{{Cite journal|last1=Lashkari|first1=Nima|last2=Brandenberger|first2=Robert H|date=2008-09-17|title=Speed of sound in string gas cosmology|journal=[[Journal of High Energy Physics]]|volume=2008|issue=9|pages=082|doi=10.1088/1126-6708/2008/09/082|issn=1029-8479|arxiv=0806.4358|bibcode=2008JHEP...09..082L|s2cid=119184258}}</ref><ref>{{Cite journal|last1=Kamali|first1=Vahid|last2=Brandenberger|first2=Robert|date=2020-05-11|title=Creating spatial flatness by combining string gas cosmology and power law inflation|journal=[[Physical Review D]]|language=en|volume=101|issue=10|pages=103512|doi=10.1103/PhysRevD.101.103512|arxiv=2002.09771|bibcode=2020PhRvD.101j3512K|issn=2470-0010|doi-access=free}}</ref>

=== Varying ''c'' ===
{{Further|Variable speed of light}}
Cosmological models employing a [[variable speed of light]] have been proposed to resolve the horizon problem of and provide an alternative to cosmic inflation. In the VSL models, the fundamental constant ''c'', denoting the [[speed of light]] in vacuum, is greater in the [[early universe]] than its present value, effectively increasing the [[particle horizon]] at the time of decoupling sufficiently to account for the observed isotropy of the CMB.