  The action of the discrete symmetries on the scalar mode functions of the de Sitter spacetime is studied. The invariance with respect to a combination of discrete symmetries is put forward as a criterion to select a certain vacuum out of a family of vacua. The implications of the choices for eigenfunctions of various common sets of commuting operators are explored, and the results are compared to the different original choices from the literature that have been utilized in order to exhibit thermal effects. 
Discrete symmetries determining scalar quantum modes on the de Sitter   spacetime