title: Sound velocity bound and neutron stars.
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 nonrelativistic 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.
authors: Department of Physics, University of Maryland, College Park, Maryland 20742, USA.
authors: Paulo
authors: P
authors: Bedaque
authors: Institute for Nuclear Theory, University of Washington, Seattle, Washington 98195, USA; Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA; and Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors: Andrew W
authors: AW
authors: Steiner