Editing Cosmic inflation (section)

=== 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>