Cosmic separation of phases

A first-order QCD phase transition that occurred reversibly in the early universe would lead to a surprisingly rich cosmological scenario. Although observable consequences mould not necessarily survive, it is at least conceivable that the phase transition mould concentrate most of the quark excess ln dcnsc, lnvlslblc qualk nuggcts, prov1d1ng an cxplanat1on for thc dark matter ln terms of QCD effects only. This possibility is viable only if quark matter has energy per baryon 1ess than 938 MeV. Tvvo Ielated issues are considered in appendices: the possibility that neutron stars generate a quark-matter component of cosmic rays, and the possibility that the QCD phase transition may have produced a detectable gravitational signal.