Editing Cosmic inflation (section)

=== 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.