  We propose a method of projecting the quantum states from a state space of a
given geometry into another state space generated by a different geometry,
taking care on the correct normalization which is crucial in interpreting the
quantum theory. Thanks to this method we can define on any spatially flat FLRW
spacetime states in which genuine Minkowskian parameters are measured. We use
these Minkowskian states for separating the frequencies in the rest frames of
the massive scalar particles defining thus the scalar rest frame vacuum. We
show that this vacuum is stable on the de Sitter expanding universe where the
energy is conserved. In contrast, on a spatially flat FLRW spacetime with a
Milne-type scale factor this vacuum results to be dynamic, corresponding to a
time-dependent rest energy interpreted as an effective mass. This dynamic
vacuum give rise to a cosmological particle creation which is significant only
in the early Milne-type universe considered here. Some interesting features of
this new effect are pointed out in a brief analysis.
