  Heavy-ion colliders have revealed the process of "fast thermalization". This
experimental breakthrough has led to new theoretical tools to study the
thermalization process at both weak and strong coupling. We apply this to the
reheating epoch of inflationary cosmology, and the formation of a cosmological
quark gluon plasma (QGP). We compute the thermalization time of the QGP at
reheating, and find it is determined by the energy scale of inflation and the
shear viscosity to entropy ratio $\eta/s$; or equivalently, the
tensor-to-scalar ratio and the strong coupling constant at the epoch of
thermalization. Thermalization is achieved near-instantaneously in low-scale
inflation and in strongly coupled systems, and takes of order or less than a
single e-fold of expansion for weakly-coupled systems or after high-scale
inflation. We demonstrate that the predictions of inflation are robust to the
physics of thermalization, and find a stochastic background of gravitational
waves at frequencies accessible by interferometers, albeit with a small
amplitude.
