  We develop a technique relating scalar fields with different masses in
different conformally flat space-times. We apply this technique to the case of
FRW space-times, with $k=\pm 1,0$, and discuss several examples. We also study
various energy conditions and discuss how they constrain the space-times
related by this technique. In addition, we use this technique to study
correlators for the related space-times in an example.
