### Annihilation of the scalar pair into a photon on de Sitter spacetime

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