Effective dark energy through spin-gravity coupling

TL;DR

This paper explores a cosmological model where spin-gravity coupling modifies Einstein's equations, leading to an effective dark energy component that can explain late-time cosmic acceleration and align with observational data.

Contribution

It introduces a novel spin-gravity coupling framework that modifies Friedmann equations, providing an alternative explanation for dark energy and cosmic acceleration.

Findings

The model recovers standard $\\Lambda$CDM when spin coupling is zero.

It predicts a stable late-time accelerated expansion.

Deviations from $\\Lambda$CDM depend on the spin coupling parameter.

Abstract

We investigate cosmological scenarios with spin-gravity coupling. In particular, due to the spin of the baryonic and dark matter particles and its coupling to gravity, they probe an effective spin-dependent metric, which can be calculated semi-classically in the Mathisson-Papapetrou-Tulczyjew-Dixon formalism. Hence, the usual field equations give rise to modified Friedmann equations, in which the extra terms can be identified as an effective dark-energy sector. Additionally, we obtain an effective interaction between the matter and dark-energy sectors. In the case where the spin-gravity coupling switches off, we recover standard $\Lambda$CDM cosmology. We perform a dynamical system analysis and we find a matter-dominated point that can describe the matter era, and a stable late-time solution corresponding to acceleration and dark-energy domination. For small values of the spin coupling parameter, deviations from $\Lambda$CDM concordance scenario are small, however for larger values they can be brought to the desired amount, leading to different dark-energy equation-of-state parameter behavior, as well as to different transition redshift from acceleration to deceleration. Finally, we confront the model predictions with Hubble function data.