Photon Propagation in Dense Media
  Using thermal field theory, we derive simple analytic expressions for the spectral density of photons in degenerate QED plasmas, without assuming the usual non or ultra-relativistic limit. We recover the standard results in both cases. Although very similar in ultra-relativistic plasmas, transverse and longitudinal excitations behave very differently as the electron Fermi momentum decreases. 
T. Altherr, E. Petitgirard, T. del Rio Gaztelurrutia