chi-gamma mixing in an expanding universe

We establish the equation of motion of pseudoscalar particles coupled to an electro-
magnetic field in a classical gravitational background through the use of conformal time and flat
geometry. We show that the expansion of the universe can lead to larger mixing than in a station-
ary universe. We also show that for a broad range of parameters, one can obtain resonant mixing,
i.e. a region in which the mixing becomes maximum.