The Trajectory of the Cosmic Plasma Through the Quark Matter Phase Diagram

Abstract

Experimental studies of the Quark-Gluon Plasma (QGP) focus on two, in practice distinct, regimes: one in which the baryonic chemical potential $μ_B$ is essentially zero, the other in which it is of the same order of magnitude as the temperature. The cosmic QGP which dominates the early Universe after reheating is normally assumed to be of the first kind, but recently it has been suggested that it might well be of the second: this is the case in the theory of "Little Inflation". If that is so, then it becomes a pressing issue to fix the trajectory of the Universe, as it cools, through the quark matter phase diagram: in particular, one wishes to know where in that diagram the plasma epoch ends, so that the initial conditions of the hadronic epoch can be determined. Here we combine various tools from strongly coupled QGP theory (the latest lattice results, together with gauge-gravity duality) in order to constrain that trajectory, assuming that Little Inflation did occur.