  A crossover QCD phase transition in the early Universe, involving a formation
scenario of stable strangeon nuggets is studied. The
Polyakov-Nambu-Jona-Lasinio model is applied to calculate the thermodynamics of
the QCD phase with u, d, s quarks, and the relativistic mean-field model
describes the hadronic matter. The crossover phase transition from quarks to
hadrons occurred at cosmic temperature of T~170 MeV, and those two phases are
connected in a three-window model. Due to quark's non-perturbative coupling,
quark clusters with net strangeness (i.e., strangeons) and then strangeon
nuggets could form during the transition process. A distribution function of
the nugget baryon number, A, is introduced to describe the nuggets' number
density. All the strangeon nuggets with A>A_c are considered to be stable,
where the critical number, A_c, is determined by both the weak and strong
interactions. A non-relativistic equation of state is applied to calculate the
thermodynamics of stable nuggets. The calculation shows that the
thermodynamical contributions (pressure, entropy, etc.) of the stable strangeon
nuggets are negligible. The resultant mass density of the strangeon nuggets
survival from the early Universe is comparable to the dark matter, that
indicates a possible explanation of the cold dark matter without introducing
any exotic particles beyond the standard model.
