Non-Riemannian Cosmology: The role of Shear Hypermomentum

TL;DR

This paper explores how shear hypermomentum in a Metric-Affine gravity framework influences cosmological evolution, potentially preventing initial singularities and affecting universe expansion.

Contribution

It introduces a generalized Friedmann equation with shear hypermomentum and analyzes its effects on cosmological dynamics in a FLRW universe.

Findings

Shear hypermomentum can prevent initial singularity formation.

Shear influences accelerating or decelerating expansion.

Exact solutions are provided for shear-dominated eras.

Abstract

We consider the usual Einstein-Hilbert action in a Metric-Affine setup and in the presence of a Perfect Hyperfluid. In order to decode the role of shear hypermomentum, we impose vanishing spin and dilation parts on the sources and allow only for non-vanishing shear. We then consider an FLRW background and derive the generalized Friedmann equations in the presence of shear hypermomentum. By providing one equation of state among the shear variables we study the cases for which shear has an accelerating/decelerating effect on Universe's expansion. In particular, we see that shear offers a possibility to prevent the initial singularity formation. We also provide some exact solutions in the shear dominated era and discuss the physical significance of the shear current.