Breaking the sound barrier in AdS/CFT

TL;DR

This paper presents counterexamples in holographic models showing that the speed of sound can exceed previously conjectured bounds, impacting the understanding of dense matter in neutron stars.

Contribution

It introduces specific strongly coupled theories with finite density that violate the conjectured speed of sound limit in holography.

Findings

Speed of sound approaches conformal value from above at high chemical potentials.

Counterexamples include ${\

Theories involve ${\cal N}=4$ super Yang-Mills with finite R-charge and gaugino masses.

Abstract

It has been conjectured that the speed of sound in holographic models with UV fixed points has an upper bound set by the value of the quantity in conformal field theory. If true, this would set stringent constraints for the presence of strongly coupled quark matter in the cores of physical neutron stars, as the existence of two-solar-mass stars appears to demand a very stiff Equation of State. In this article, we present a family of counterexamples to the speed of sound conjecture, consisting of strongly coupled theories at finite density. The theories we consider include ${\cal N}=4$ super Yang-Mills at finite R-charge density and non-zero gaugino masses, while the holographic duals are Einstein-Maxwell theories with a minimally coupled scalar in a charged black hole geometry. We show that for a small breaking of conformal invariance, the speed of sound approaches the conformal value from above at large chemical potentials.