FRW Universe Models in Conformally Flat Spacetime Coordinates. II:   Universe models with negative and vanishing spatial curvature
  We deduce general expressions for the line element of universe models with negative and vanishing spatial curvature described by conformally flat spacetime coordinates. The empty Milne universe model and models with dust, radiation and vacuum energy are exhibited. Discussing the existence of particle horizons we show that there is continual creation of space, matter and energy when conformal time is used in Friedmann-Robertson-Walker models with negative spatial curvature. 
Oyvind Gron and Steinar Johannesen
gr-qc