Cosmology with torsion: An alternative to cosmic inflation (1007.0587v2)

Abstract: We propose a simple scenario which explains why our Universe appears spatially flat, homogeneous and isotropic. We use the Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity which naturally extends general relativity to include the spin of matter. The torsion of spacetime generates gravitational repulsion in the early Universe filled with quarks and leptons, preventing the cosmological singularity: the Universe expands from a state of minimum but finite radius. We show that the dynamics of the closed Universe immediately after this state naturally solves the flatness and horizon problems in cosmology because of an extremely small and negative torsion density parameter, $\Omega_S \approx -10^{-69}$. Thus the ECKS gravity provides a compelling alternative to speculative mechanisms of standard cosmic inflation. This scenario also suggests that the contraction of our Universe preceding the bounce at the minimum radius may correspond to the dynamics of matter inside a collapsing black hole existing in another universe, which could explain the origin of the Big Bang.

• The paper presents the Einstein-Cartan-Kibble-Sciama (ECKS) theory, incorporating spacetime torsion from fermion spin, as a framework that solves flatness and horizon problems without needing cosmic inflation.
• Modelling fermionic matter as a Weyssenhoff spin fluid reveals that torsion's significant effects, like preventing singularities and generating repulsion, manifest at densities exceeding nuclear matter.
• The framework offers a novel perspective, including a scenario where the universe originates within a collapsing black hole, providing insights into cosmic time asymmetry and the arrow of time.

Cosmology with Torsion: An Alternative to Cosmic Inflation

The paper explores the implications of the Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity as an alternative to cosmic inflation in addressing fundamental cosmological problems. This research leverages the ECKS gravity framework, which extends general relativity by incorporating matter's intrinsic angular momentum (spin) to explain the universe's large-scale spatial flatness, homogeneity, and isotropy without the need for the speculative mechanisms inherent in traditional inflationary models.

Key Contributions:

1. Einstein-Cartan-Kibble-Sciama Theory: This paper elucidates the ability of the ECKS theory to prevent singularity formation by introducing spacetime torsion, which emerges from the intrinsic spin of fermions, such as quarks and leptons. By considering torsion as a dynamical variable alongside the metric tensor, ECKS theory provides a robust extension to standard classical gravitational models, offering predictions consistent with quantum field theories focussing on fermions.
2. Torsion and Cosmology: The introduction of spacetime torsion in the early universe produces gravitational repulsion, which solves both the flatness and horizon problems that challenge the Big Bang framework. The torsion of spacetime contributes a negative energy density parameter (S ≈ -10^-69), which adjusts the density parameter dynamics without creating singularities, and avoids the necessity of invoking cosmic inflation.
3. Spin Fluid Dynamics: A critical aspect of the research is the modeling of fermionic matter as a Weyssenhoff spin fluid. This allows for the examination of macroscopic averages of spin and energy-momentum in a cosmological context, with non-zero contributions from torsion becoming significant only at matter densities far exceeding that of nuclear matter.
4. Theoretical Cosmological Implications: By proposing a universe born within a collapsing black hole existing in another universe, the paper presents an innovative scenario of cosmological evolution. Such a model posits our universe's origin as the interior of a black hole, leveraging torsion to avoid singularities while naturally providing for an expanding universe. This introduces a compelling viewpoint on the time asymmetry and the arrow of cosmic time.

Implications and Future Directions:

The paper offers a novel approach to cosmology that bypasses the assumptions and challenges associated with the scalar fields and other elements in standard inflationary cosmology. It opens up a line of inquiry regarding the interaction of intrinsic spin and torsion, which may harbor observable consequences in both astrophysical and experimental settings.

Future developments could probe the empirical detectability of torsion-related effects or explore alternative couplings within the framework to test torsion's viability as a universal phenomenon. Additionally, cosmology emerging from black holes could provide meaningful insights into unresolved questions about dark matter, dark energy, and the initial conditions of the universe.

In conclusion, adopting the ECKS theory within cosmological contexts provides a promising alternative paradigmatic shift. It emphasizes a gravitational framework that seamlessly links early universe evolution with contemporary quantum mechanics, thus potentially offering solutions to some of the most persistent problems in cosmology without reliance on inflation. The application of torsion in cosmological models remains an intriguing avenue for theoretical and observational investigation.