Universal Behaviors of Speed of Sound from Holography

We investigate the speed of sound in $(d+1)$-dimensional field theory by studying its dual $(d+2)$-dimensional gravity theory from gauge/gravity correspondence. Instead of the well known conformal limit $c_s^2 \rightarrow 1/d$ at high temperature, we reveal two more universal quantities in various limits: $c_s^2\to (d-1)\mathrm{/}16\pi$ at low temperature and $c_s^2 \rightarrow (d-1)/16\pi d$ at large chemical potential.