Effect of Torsion on Neutron Star Structure in Einstein-Cartan Gravity

TL;DR

This paper investigates how spacetime torsion, in Einstein-Cartan gravity, affects neutron star structure, revealing that rotation-induced torsion can significantly alter star radii and densities, with potential for observational testing.

Contribution

It introduces a model for torsion effects in neutron stars, especially focusing on rotation-induced torsion, and quantifies their impact on star properties, which was previously under-explored.

Findings

Torsion leads to smaller neutron star radii and higher central densities.

Microphysical spin torsion effects are negligible for star structure.

Rotation-induced torsion can change star radii by up to 900 meters.

Abstract

Einstein-Cartan gravity is a close historical sibling of general relativity that allows for spacetime torsion. As a result, angular momentum couples to spacetime geometry in a similar way to energy. While consequences of this are well studied on cosmological scales, their role in neutron star physics is largely under-explored. We study the effects that torsion, sourced by either microphysical spin or macroscopic angular momentum, has on neutron stars. For this, we use a simplified polytropic model to quantify the microphysical coupling to torsion. We also derive expressions to model rotation-induced torsion effects and estimate the consequences for rotating neutron stars with different rotation rates. We find that the presence of torsion in general leads to neutron stars with smaller radii and masses, but higher central densities. Realistic models for microphysical spin lead to torsion effects that have no relevant influence on the neutron star structure. Rotation-induced torsion effects however, can decrease the radius by up to $900\,m$, which is comparable to the increase due to centrifugal forces. Depending on which effect dominates, this leads to a torsion-induced spin-up or spin-down of the neutron star. We conclude that torsion effects due to rotation can not be neglected and are large enough to be tested using current or near-future technology.