Big bounce from spin and torsion

TL;DR

This paper discusses how the Einstein-Cartan theory predicts a big bounce replacing the big bang, caused by spacetime torsion from fermion spin, preventing singularities at extremely high densities.

Contribution

It introduces a classical spin-torsion mechanism in Einstein-Cartan gravity that replaces the big bang with a bounce, avoiding quantum singularities.

Findings

The big bounce occurred at energy densities about 15 times the Planck energy.

The minimum scale factor at the bounce was approximately 50 micrometers.

Observations support classical behavior of spacetime at sub-Planckian scales.

Abstract

The Einstein-Cartan-Sciama-Kibble theory of gravity naturally extends general relativity to account for the intrinsic spin of matter. Spacetime torsion, generated by spin of Dirac fields, induces gravitational repulsion in fermionic matter at extremely high densities and prevents the formation of singularities. Accordingly, the big bang is replaced by a bounce that occurred when the energy density $\epsilon\propto gT^4$ was on the order of $n^2/m_\textrm{Pl}^2$ (in natural units), where $n\propto gT^3$ is the fermion number density and $g$ is the number of thermal degrees of freedom. If the early Universe contained only the known standard-model particles ($g\approx 100$), then the energy density at the big bounce was about 15 times larger than the Planck energy. The minimum scale factor of the Universe (at the bounce) was about $10^{32}$ times smaller than its present value, giving $\approx 50 \mum$. If more fermions existed in the early Universe, then the spin-torsion coupling causes a bounce at a lower energy and larger scale factor. Recent observations of high-energy photons from gamma-ray bursts indicate that spacetime may behave classically even at scales below the Planck length, supporting the classical spin-torsion mechanism of the big bounce. Such a classical bounce prevents the matter in the contracting Universe from reaching the conditions at which a quantum bounce could possibly occur.