# Flat universe and accelerating expansion

As current data suggests that the universe is flat, a flat universe would imply a slowing expansion rather than an accelerating one which we observe. Neither in an open nor a closed universe there is such a thing as accelerating expansion. Dark energy is thought to be responsible for that, but wouldn't that drive $\Omega$ away from 1, although we still measure it to be 1?

• This happens because the geometry is defined by the total mass/energy density, but the time evolution of the scale factor is defined by the equation of state of the stuff making up that density. Dark energy has a different equation of state to matter. Specifically the matter density scales as $1/a^3$ while the dark energy density is constant. You might be interested to read How does the Hubble parameter change with the age of the universe? and the links within it. – John Rennie Apr 12 '15 at 17:17