  The underlying physics of QCD phase transition in the early Universe remains
largely unknown due to its strong-coupling nature during the quark-gluon
plasma/hadron gas transition, yet a holographic model has been proposed to
quantitatively fit the lattice QCD data while with its duration of the
first-order phase transition (FoPT) left undetermined. At specific baryon
chemical potential, the first-order QCD phase transition agrees with the
observational constraint of baryon asymmetry. It, therefore, provides a
scenario for phase transition gravitational waves (GWs) within the Standard
Model of particle physics. If these background GWs could contribute dominantly
to the recently claimed common-spectrum red noise from pulsar timing array
(PTA) observations, the duration of this FoPT can be well constrained, and the
associated primordial black holes are still allowed by current observations.
