  We demonstrate that the static ground state atom, which interacts with a
conformally coupled massless scalar field in the de Sitter invariant vacuum,
can obtain a position-dependent energy-level shift and this shift could cause a
Casimir-Polder-like force on it. Interestingly no such force arises on the
inertial atom bathed in a thermal radiation in the Minkowski universe. Thus,
although the energy-level shifts of the static atom for these two cases are
structurally the same, whether the energy-level shift causes the
Casimir-Polder-like force, in principle, could be as an indicator to
distinguish de Sitter universe from the thermal Minkowski spacetime.
