  The annihilation of massive scalar particles in one photon on de Sitter
expanding universe is studied, using perturbation theory. The amplitude and
probability corresponding to this process is computed using the exact solutions
of the Klein-Gordon and Maxwell equations on de Sitter geometry. Our results
show that the expression of the total probability of photon emission is a
function dependent on the ratio mass/expansion\, factor. We perform a graphical
study of the total probability in terms of the parameter mass/expansion factor,
showing that this effect is significant only in strong gravitational fields. We
also obtain that the total probability for this process vanishes in the
Minkowski limit.
