Gravity seminar - Andy O'Bannon Seminar

Abstract: I will discuss the bold conjecture of Cherman, Cohen, and Nellore in 0905.0903 that the value of the speed of sound in a conformal fluid provides an upper bound on the speed of sound in any fluid. This "sound bound" has important implications. For example, the bound places an upper limit on neutron star masses of roughly two solar masses, consistent with observations. The sound bound conjecture was motivated by examples from holography involving scalar fields in asymptotically Anti-de Sitter black brane spacetimes. However, counter-examples have also been discovered in holography, and as a result the proposed bound has been dismissed. I will present work in progress showing that all of the known counter-examples in holography in fact have unstable quasi-normal modes, so in a sense a bound does exist after all: violation of the bound always implies a dynamical instability.