Quark–gluon plasma as the possible source of cosmological dark radiation

The effective number of neutrinos, NeffNeff, obtained from CMB fluctuations accounts for all effectively massless degrees of freedom present in the Universe, including but not limited to the three known neutrinos. Using a lattice-QCD derived QGP equation of state, we constrain the observed range of NeffNeff in terms of the freeze-out of unknown degrees of freedom near to quark–gluon hadronization. We explore limits on the coupling of these particles, applying methods of kinetic theory, and discuss the implications of a connection between NeffNeff and the QGP transformation for laboratory studies of QGP.