Moment of inertia of neutron star crust in alternative and modified theories of gravity

TL;DR

This study investigates how alternative and modified theories of gravity affect the moment of inertia of neutron star crusts, which is crucial for understanding pulsar glitches, revealing potential for using pulsar data to test gravity models.

Contribution

It calculates the crustal to total moment of inertia ratio in scalar-tensor and $f(R)$ gravity theories, highlighting differences from general relativity especially in high-mass neutron stars.

Findings

Crust to core moment of inertia ratio remains similar to GR in scalar-tensor theory.

In $f(R)$ gravity, the ratio increases significantly for high-mass neutron stars.

Pulsar glitch activity could serve as a probe for gravity models.

Abstract

The glitch activity of young pulsars arises from the exchange of angular momentum between the crust and the interior of the star. Recently, it was inferred that the moment of inertia of the crust of a neutron star is not sufficient to explain the observed glitches. Such estimates are presumed in the Einstein's general relativity in describing the hydrostatic equilibrium of neutron stars. The crust of the neutron star has a space-time curvature of 14 orders of magnitude larger than that probed in solar system tests. This makes gravity the weakest constrained physics input in the crust related processes. We calculate the ratio of the crustal to the total moment of inertia of neutron stars in the scalar-tensor theory of gravity and the non-perturbative $f({\cal R})={\cal R}+ a {\cal R}^2$ gravity. We find for the former that the crust to core ratio of the moment of inertia does not change significantly from what is inferred in general relativity. For the latter we find that the ratio increases significantly from what is inferred in general relativity in the case of high mass objects. Our results suggest that the glitch activity of pulsars may be used to probe gravity models, although the gravity models explored in this work are not appropriate candidates.