Paul Romatschke

University of Colorado Boulder, March 8, 12:00 pm

An Upper Bound on the Speed of Sound in Neutron Stars

Resummed Second Order Relativistic Fluid Dynamics (aka Israel-Stewart Theory) is the workhorse for simulating viscous relativistic fluids for high energy nuclear collisions. It has long been known that resummed fluid dynamics obeys causality if the ratio of first and second order transport coefficients (e.g. shear viscosity over shear relaxation time) are bounded from above. In this talk, I point out that inverting this logic leads to an upper bound on the speed of sound from transport and causality, specifically c_s^2 < 0.781. All currently known first-principles calculations for cold nuclear matter obey this bound. Using this bound as input for the ultradense matter EoS implies that the maximum mass of neutron stars cannot exceed 2.7 solar masses.