  Quantum electrodynamics predicts that photons undergo one-loop scattering.
The combined effect of this on the behaviour of a photon gas for temperatures
above 10^{10} K results in a softening of the equation of state. We calculate
the effect this has on the effective equation of state in the early universe,
taking into account all the species of the Standard Model. The change to the
dynamics of the early universe is discussed.
