Electromagnetic field in an expanding universe

Abstract An expanding universe filled with a massive electromagnetic field is studied. The electromagnetic field (photon) is found to acquire mass during an inflationary period. The photon mass varies with the Hubble parameter as m γ = 3 ℏ H / c 2 , where H , ℏ and c are the Hubble parameter, Planck constant, and speed of light. A leftover magnetic field since the inflationary era permeates the space between galaxies and interstellar media. The strength of the electromagnetic field changes with the scale factor, E ∝ a −3 . The photon gains mass when interacting with the gravitational field while remaining massless in flat space. The massive electromagnetic field adds energy density to the Universe proportional to m 2 .