Big bounce from spin and torsion (1105.6127v2)

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.

• The paper shows that in Einstein-Cartan-Sciama-Kibble gravity, the interaction of fermion spin and spacetime torsion generates a gravitational repulsion.
• This spin-torsion interaction provides a mechanism for a cosmological 'big bounce' that naturally avoids the initial singularity predicted by standard general relativity.
• The theory suggests significant implications like the potential for black holes to form 'daughter universes' and classical behavior potentially existing below the Planck scale.

Overview of "Big Bounce from Spin and Torsion"

Nikodem J. Poplawski's paper explores an extension to the general theory of relativity known as the Einstein-Cartan-Sciama-Kibble (ECSK) theory of gravity. This extension incorporates the intrinsic quantum-mechanical properties of fermions, particularly focusing on their intrinsic angular momentum or spin. ECSK gravity modifies general relativity to account for spacetime torsion, which is not present in the standard cosmological models.

Key Concepts and Theoretical Framework

The ECSK theory introduces non-zero torsion in the affine connection of spacetime, a departure from Einstein's general relativity (GR) that leads to notable cosmological implications. Importantly, Poplawski suggests that this spin-torsion interplay induces gravitational repulsion, significantly impacting high-density fermionic matter events, such as those believed to occur in the early universe or within black holes. This gravitational repulsion could prevent the formation of singularities typically predicted by GR, replacing the traditional big bang with a "big bounce."

The Big Bounce Model

In this alternative cosmological scenario, the Universe undergoes a contraction phase before reaching a minimum size and rebounding into an expansion phase. Poplawski calculates that for known standard-model particles, the energy density at this bounce is several orders greater than the Planck energy density. The theoretical implications are significant: according to the ECSK theory, the Universe's minimum scale factor is about 50 μm, suggesting the possibility of classical behaviors even below the Planck scale.

Implications and Numerical Results

The paper posits strong implications for the nature of spacetime and cosmic evolution:

• Absence of a Singular Big Bang: ECSK theory provides a mechanism that naturally avoids singularities without additional assumptions required by models like cosmic inflation, thus offering an alternative view on the initial conditions of the Universe.
• Potential for Derived Universes: The coupling effect in ECSK suggests a scenario where black holes may gravitationally interact to create daughter universes. This idea aligns with observations of inexplicable large-scale galaxy cluster flows, providing a framework for explaining potential pre-existing conditions before the big bang.
• Energy Density at the Big Bounce: Poplawski refines previous results by considering all ultrarelativistic standard-model particles, concluding that the energy density during the bounce could either affirm the ECSK classical bounce without requiring a quantum gravity framework or necessitate it if conditions or additional fermionic species are discovered.

Speculations on Future Research Directions

The results postulated in this paper point towards several prospective research trajectories:

1. Integration with Quantum Gravity Models: While the ECSK provides classical conditions for the big bounce, future explorations into loop quantum gravity (LQG) can assess whether quantum effects become prevalent under conditions near the Planck density.
2. Exploration of Sub-Planckian Phenomena: Should additional fermionic species be verified, further paper into ECSK scenarios where bounces occur below the Planck scale would yield insights into how classical physics might smoothly transition into quantum regimes.
3. Astrophysical Observations: Revisiting high-energy astrophysical observations to determine whether existing black hole formations could hint at inter-universal interactions, thus empirically supporting Poplawski's proposals.

Conclusion

Poplawski's paper delineates a profound modification to classical cosmology through the unification of fermion spin effects and torsion in the ECSK framework. While bridging theoretical predictions with observational data remains a challenge, such extensions form an exciting frontier in gravitational physics. These models, if substantiated, could redefine foundational concepts of universe genesis, black hole cosmology, and early universe physics. Future advancements in both theoretical models and observational techniques will be paramount in either validating or refuting the ECSK theory's predictions.