  We investigate the influence of massive photons on the evolution of the
expanding universe. Two particular models for generalized electrodynamics are
considered, namely de Broglie-Proca and Bopp-Podolsky electrodynamics. We
obtain the equation of state (EOS) $P=P(\varepsilon)$ for each case using
dispersion relations derived from both theories. The EOS are inputted into the
Friedmann equations of a homogeneous and isotropic space-time to determine the
cosmic scale factor $a(t)$. It is shown that the photon non-null mass does not
significantly alter the result $a\propto t^{1/2}$ valid for a massless photon
gas; this is true either in de Broglie-Proca's case (where the photon mass $m$
is extremely small) or in Bopp-Podolsky theory (for which $m$ is extremely
large).
