Sound velocity bound and neutron stars

TL;DR

This paper discusses a theoretical bound on sound velocity in media, examines its validity in strongly coupled theories, and highlights its tension with astrophysical observations of neutron stars.

Contribution

It challenges the sound velocity bound by connecting it to neutron star observations and the equation of state of hadronic matter.

Findings

The sound velocity bound is supported in certain theories but is challenged by neutron star data.

Neutron stars with around two solar masses are in tension with the bound.

The equation of state at low densities impacts the validity of the bound.

Abstract

It has been conjectured that the velocity of sound in any medium is smaller than the velocity of light in vacuum divided by $\sqrt{3}$. Simple arguments support this bound in non-relativistic and/or weakly coupled theories. The bound has been demonstrated in several classes of strongly coupled theories with gravity duals and is saturated only in conformal theories. We point out that the existence of neutron stars with masses around two solar masses combined with the knowledge of the equation of state of hadronic matter at "low" densities is in strong tension with this bound.