I-Love-Q to the extreme

TL;DR

This paper tests the robustness of the I-Love-Q relations in neutron stars using extreme equations of state, including superluminal sound speeds, and finds they hold to 1% accuracy, supporting their universality.

Contribution

It demonstrates that the I-Love-Q relations remain valid even under extreme and physically questionable equations of state, confirming their robustness.

Findings

I-Love-Q relations hold to 1% accuracy with extreme equations of state.

Relations are independent of transition density and superluminal sound speeds.

Supports the idea of an emergent symmetry due to constant eccentricity inside stars.

Abstract

Certain bulk properties of neutron stars, in particular their moment of inertia, rotational quadrupole moment and tidal Love number, when properly normalized, are related to one another in a nearly equation of state independent way. The goal of this paper is to test these relations with extreme equations of state at supranuclear densities constrained to satisfy only a handful of generic, physically sensible conditions. By requiring that the equation of state be (i) barotropic and (ii) its associated speed of sound be real, we construct a piecewise function that matches a tabulated equation of state at low densities, while matching a stiff equation of state parametrized by its sound speed in the high-density region. We show that the I-Love-Q relations hold to 1 percent with this class of equations of state, even in the extreme case where the speed of sound becomes superluminal and independently of the transition density. We also find further support for the interpretation of the I-Love-Q relations as an emergent symmetry due to the nearly constant eccentricity of isodensity contours inside the star. These results reinforce the robustness of the I-Love-Q relations against our current incomplete picture of physics at supranuclear densities, while strengthening our confidence in the applicability of these relations in neutron star astrophysics.