Ray tracing in FLRW flat space-times
  In this work we take moves from the debate triggered by Melia et al [9]. and
followed by opposite comments by Lewis and Oirschot [10,11]. The point in
question regards the role of the Hubble horizon as a limit for observability in
a cosmological setting. We propose to take the issue in a broader way by
relating it to the causal character of the Hubble surface and to the tracing of
null trajectories, focusing on both three-fluids and generalized Chaplygin gas
models. The results should make clear that light rays reaching a comoving
observer at R(t_0)=0 have never travelled a distance greater than the areal
radius of the horizon until t_0.
