de Broglie-Proca and Bopp-Podolsky massive photon gases in cosmology
  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).
