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We consider, in de Sitter spacetime, both freely falling and static two-levelatoms in interaction with a conformally coupled massless scalar field in the deSitter-invariant vacuum, and separately calculate the contributions of vacuumfluctuations and radiation reaction to the atom's spontaneous excitation rate.We find that spontaneous excitations occur even for the freely falling atom asif there is a thermal bath of radiation at the Gibbons-Hawking temperature andwe thus recover, in a different physical context, the results of Gibbons andHawking that reveals the thermal nature of de Sitter spacetime. Similarly, forthe case of the static atom, our results show that the atom also perceives athermal bath which now arises as a result of the intrinsic thermal nature of deSitter spacetime and the Unruh effect associated with the inherent accelerationof the atom.

Thermal nature of de Sitter spacetime and spontaneous excitation of atoms