Breaking the sound barrier in AdS/CFT

Understand

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.