In order for the universe to remain flat, doesn't the contribution to curvature by mass have to be cancelled out by the contribution by dark energy? If the mass contributes to positive curvature, then dark energy would be there to make a negative contribution to curvature. And yet, without dark energy, the universe would be open and not flat (so that Omega=1), thus dark energy would have a positive contribution to curvature (as if it had mass which contributed to positive pressure)? Yet it makes the universe accelerate with negative pressure (thus a reason for having an open universe)? Wouldn't dark energy cause the big bang as well? Or is that type-2 dark energy, something that is even more mysterious? If one wrongly used SR to explain the recession of galaxies, one would have an effective "Hubble potential" = .5m(HD)^2, where HD is the Hubble velocity. This would make "Hubble potential" proportional to the square of the distance. For the General Relativistic case, would dark "energy" increase with the square of the distance (actually less than the square, since H is decreasing)? But if dark energy makes a positive contribution to the curvature (towards closed geometry), providing 2/3rds of Omega=1, how does it make galaxies diverge (as if the universe were open)? Or am I wrong in assuming that dark energy is the only thing causing the expansion? The radius of the curvature of the entire universe, i suppose remains the same (i.e. no curvature) - does it? I'm confused. How can a flat universe accelerate it expansion? If dark energy couples with matter, how can an accelerated expansion promote flat universe? How can a open universe be flat? How can a hyperbolic system be Euclidean? How can the universe be flat, if it has no sign of halting its expansion? How can it be open when Omega=1? Is this a mystery or just nonsense?