Constraining a possible time-variation of the gravitational constant through "gravitochemical heating" of neutron stars

TL;DR

This paper uses neutron star temperature observations and theoretical models of their thermal evolution to set upper limits on the possible rate of change of the gravitational constant over time.

Contribution

It introduces a novel method to constrain the variation of G by analyzing the thermal state of neutron stars affected by gravitochemical heating.

Findings

Upper limit of |Ġ/G| < 2×10⁻¹⁰ yr⁻¹ with direct Urca reactions

Stricter upper limit of |Ġ/G| < 4×10⁻¹² yr⁻¹ considering only modified Urca reactions

Among the most restrictive limits from astrophysical observations

Abstract

A hypothetical time-variation of the gravitational constant $G$ would make neutron stars expand or contract, so the matter in their interiors would depart from beta equilibrium. This induces non-equilibrium weak reactions, which release energy that is invested partly in neutrino emission and partly in internal heating. Eventually, the star arrives at a stationary state in which the temperature remains nearly constant, as the forcing through the change of $G$ is balanced by the ongoing reactions. Using the surface temperature of the nearest millisecond pulsar (PSR J0437$-$4715) inferred from ultraviolet observations and results from theoretical modelling of the thermal evolution, we estimate two upper limits for this variation: (1) $|\dot G/G| < 2 \times 10^{-10}\mathrm{yr}^{-1},$ if the fast, "direct Urca" reactions are allowed, and (2) $|\dot G/G|<4\times 10^{-12}\mathrm{yr}^{-1},$ considering only the slower, "modified Urca" reactions. The latter is among the most restrictive upper limits obtained by other methods.