Physics Colloquium

In the standard cosmological model, the universe has a tiny baryon-to- photon ratio as determined from big-bang nucleosynthesis and the cosmic microwave background radiation. At a timescale of 10 microseconds, the early universe passes through the QCD phase transition, the earliest phase transition one can probe in the laboratory by the study of relativistic heavy-ion collisions.
The QCD phase diagram might exhibit a first order phase transition, but according to lattice data only for high baryon densities. It is possible to dilute an initial high baryon density to the observed value by a little inflationary epoch where the universe is trapped in a false state of QCD. Bubble nucleation to the true minimum can lead to primordial production of the seeds of extragalactic magnetic fields, primordial black holes and gravitational waves. In addition the power spectrum of cold dark matter can be affected up to mass scales of a billion solar masses. Interestingly, the imprints of the cosmological QCD phase transition on the gravitational wave background can be explored with the future gravitational wave detectors and with pulsar timing.