A first-order deconfinement phase transition in the early universe and gravitational waves

TL;DR

This paper investigates the conditions under which the early universe's QCD phase transition could be first order, derives a general potential formula, and predicts gravitational wave signals detectable by future space interferometers.

Contribution

It distinguishes between chiral and deconfinement phase transitions, extends the deconfinement transition to dark QCD, and provides a quantitative prediction of gravitational wave signals from the transition.

Findings

Chiral transition unlikely to be first order at zero chemical potential.

Deconfinement transition can be first order with an extra scalar.

Predicted gravitational wave signals are within the reach of future detectors.

Abstract

We clarify the conditions of the cosmic quantum chromodynamics (QCD) first-order phase transition in the early universe by carefully distinguishing the chiral and deconfinement phase transitions. While the chiral one with light quarks at zero chemical potential is unlikely to be first order based on the recent lattice QCD calculations, the latter one can be naturally extended with one extra rolling scalar to be first order. The argument is also valid for the dark QCD theory with arbitrary $N_c$ with a wide range of phase transition temperatures, which can be from hundreds of MeV up to beyond TeV. Notably, here we derive the general formula for the deconfinement phase transition potential of SU($N_c$) gauge theory characterized by the Polyakov loop. With the effective potential in hand, the gravitational wave spectrum is then determined via the sound shell model, which then enables us to give for the first time the quantitative analysis of the gravitational wave signals coming from the QCD deconfinement phase transition and awaits the check from future space interferometers.