Slowly-Rotating Neutron Stars in Massive Bigravity

TL;DR

This paper investigates slowly-rotating neutron stars within ghost-free massive bigravity, revealing how modifications to gravity influence their structure, mass distribution, and observable properties, with potential for future astrophysical constraints.

Contribution

It provides the first detailed numerical solutions for rotating neutron stars in bigravity, analyzing mass functions, frame-dragging, and observable relations under realistic conditions.

Findings

Mass function asymptotes to a constant outside the star, differing from GR.

Bigravity modifications are generally small but significant in mass-radius and moment of inertia relations.

The graviton mass and gravitational constant ratio critically affect neutron star properties.

Abstract

We study slowly-rotating neutron stars in ghost-free massive bigravity. This theory modifies General Relativity by introducing a second, auxiliary but dynamical tensor field that couples to matter through the physical metric tensor through non-linear interactions. We expand the field equations to linear order in slow rotation and numerically construct solutions in the interior and exterior of the star with a set of realistic equations of state. We calculate the physical mass function with respect to observer radius and find that, unlike in General Relativity, this function does not remain constant outside the star; rather, it asymptotes to a constant a distance away from the surface, whose magnitude is controlled by the ratio of gravitational constants. The Vainshtein-like radius at which the physical and auxiliary mass functions asymptote to a constant is controlled by the graviton mass scaling parameter, and outside this radius, bigravity modifications are suppressed. We also calculate the frame-dragging metric function and find that bigravity modifications are typically small in the entire range of coupling parameters explored. We finally calculate both the mass-radius and the moment of inertia-mass relations for a wide range of coupling parameters and find that both the graviton mass scaling parameter and the ratio of the gravitational constants introduce large modifications to both. These results could be used to place future constraints on bigravity with electromagnetic and gravitational-wave observations of isolated and binary neutron stars.