Editing Flatness problem (section)

===Einstein–Cartan theory===
{{Main|Einstein–Cartan theory}}
The flatness problem is naturally solved by the [[Einstein–Cartan theory|Einstein–Cartan–Sciama–Kibble theory of gravity]], without an exotic form of matter required in inflationary theory.<ref>{{cite journal |author=Poplawski, N. J. |date=2010 |title=Cosmology with torsion: An alternative to cosmic inflation| journal=Phys. Lett. B |volume=694 |issue=3 |pages=181–185 |doi=10.1016/j.physletb.2010.09.056|arxiv = 1007.0587 |bibcode = 2010PhLB..694..181P }}</ref><ref>{{cite journal |author=Poplawski, N. |date=2012 |title=Nonsingular, big-bounce cosmology from spinor-torsion coupling |journal=Phys. Rev. D |volume=85 |issue=10 |pages=107502 |doi=10.1103/PhysRevD.85.107502|arxiv = 1111.4595 |bibcode = 2012PhRvD..85j7502P |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. It has no free parameters. Including torsion gives the correct conservation law for the total (orbital plus intrinsic) [[angular momentum]] of matter in the presence of gravity. The minimal coupling between torsion and Dirac spinors obeying the [[nonlinear Dirac equation]] generates a spin-spin interaction which is significant in [[fermion]]ic matter at extremely high densities. Such an interaction averts the unphysical big bang singularity, replacing it with a 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.