Nonsingular, big-bounce cosmology from spinor-torsion coupling

The Einstein-Cartan-Sciama-Kibble theory of gravity removes the constraint of general relativity that the affine connection be symmetric by regarding its antisymmetric part, the torsion tensor, as a dynamical variable. The minimal coupling between the torsion tensor and Dirac spinors generates a spin-spin interaction which is significant in fermionic matter at extremely high densities. We show that 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. This scenario also explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic.

Figure 1

The normalized temperature $\frac{T}{T_{\mathrm{cr}}}$ as a function of the normalized cosmic time $\frac{t}{t_0}$.

Figure 2

The normalized scale factor $\frac{a}{a_{\mathrm{cr}}}$ as a function of the normalized cosmic time $\frac{t}{t_0}$.