FRW Universe Models in Conformally Flat Spacetime Coordinates. I:   General Formalism
  The 3-space of a universe model is defined at a certain simultaneity. Hence space depends on which time is used. We find a general formula generating all known and also some new transformations to conformally flat spacetime coordinates. A general formula for the recession velocity is deduced. 
Oyvind Gron and Steinar Johannesen
gr-qc