I-Love-Q Relations for Neutron Stars in dynamical Chern Simons Gravity

TL;DR

This paper investigates the I-Love-Q universal relations for neutron stars within dynamical Chern-Simons gravity, demonstrating their robustness and potential for constraining modified gravity theories through future observations.

Contribution

It extends the study of I-Love-Q relations to dynamical Chern-Simons gravity, analyzing their universality, observational constraints, and internal structure effects in this modified theory.

Findings

I-Love-Q relations remain approximately universal in dynamical Chern-Simons gravity.

Future observations could constrain the theory's coupling constants six orders of magnitude better than current bounds.

Eccentricity variation inside neutron stars is smaller in this theory, supporting the universality of isodensity contours.

Abstract

Neutron stars are ideal to probe, not only nuclear physics, but also strong-field gravity. Approximate universal relations insensitive to the star's internal structure exist among certain observables and are useful in testing General Relativity, as they project out the uncertainties in the equation of state. One such set of universal relations between the moment of inertia $(I)$, the tidal Love number and the quadrupole moment $(Q)$ has been studied both in General Relativity and in modified theories. In this paper, we study the relations in dynamical Chern-Simons gravity, a well-motivated, parity-violating effective field theory, extending previous work in various ways. First, we study how projected constraints on the theory using the I-Love relation depend on the measurement accuracy of $I$ with radio observations and that of the Love number with gravitational-wave observations. Provided these quantities can be measured with future observations, we find that the latter could place bounds on dynamical Chern-Simons gravity that are six orders of magnitude stronger than current bounds. Second, we study the I-Q and Q-Love relations in this theory by constructing slowly-rotating neutron star solutions to quadratic order in spin. We find that the approximate universality continues to hold in dynamical Chern-Simons gravity, and in fact, it becomes stronger than in General Relativity, although its existence depends on the normalization of the dimensional coupling constant of the theory. Finally, we study the variation of the eccentricity of isodensity contours inside a star and its relation to the degree of universality. We find that, in most cases, the eccentricity variation is smaller in dynamical Chern-Simons gravity than in General Relativity, providing further support to the idea that the approximate self-similarity of isodensity contours is responsible for universality.