  The vacuum polarization in an external gravitational field due to one loop
electron-positron pair and one loop millicharged fermion-antifermion pair is
studied. Considering the propagation of electromagnetic (EM) radiation and
gravitational waves (GWs) in an expanding universe, it is shown that by taking
into account QED effects in curved spacetime, the propagation velocity of
photons is superluminal and can exceed that of gravitons. We apply these
results to the case of the GW170817 event detected by LIGO. If the EM radiation
and GWs are emitted either simultaneously or with a time difference from the
same source, it is shown that the EM radiation while propagating with
superluminal velocity, would be detected either in advance or in delay with
respect to GW depending on the ratio of millicharged fermion relative charge to
mass $\epsilon/m_\epsilon$.
