If the cosmological constant (aka dark energy / vacuum energy) were zero, then an Omega greater than 1 would mean both positive curvature and a recollapse while an Omega less than 1 would mean both negative curvature and eternal expansion. This would make flatness (Omega = 1) very critical for the existence of large structures such as galaxies. But now that we believe the Omega for dark energy to be around 0.7, total Omega could be greater or less than 1 without causing recollapse. Wouldn't this make the "flatness problem" much less pressing?

I understand that dark energy density is independent of the scale factor and matter density changes as the inverse of the scale factor cubed, and that means that if total Omega were to deviate from 1 ever so slightly at the time of the Big Bang, we would not have a flat universe today, which we do. And I understand that that seems curious. But regardless, we could still have galaxies, couldn't we? In fact, wouldn't the expansion of the universe be slower if total Omega were greater than 1 (still assuming Omega_Lambda = 0.7), thus giving us even more time to form galaxies? Am I missing something?


1 Answer 1


The flatness problem would still be a problem.

The flatness problem is a pretty big problem today, but it was an even bigger problem in the early universe (notably at the times of baryogenesis and at the surface of last scattering). The current universe does appear to be dominated by some fluid that tends to make the universe evolve towards flat (dark energy), but at earlier times (when the universe was dominated by matter or radiation) this was not the case. At these times, the dark energy density was negligible, but the flatness problem still existed during these epochs.

Also to comment one point, you said "I understand that dark energy density is independent of the scale factor". Dark energy is the name given to the 'stuff' that causes the late time accelerated expansion. The cosmological constant is one possible explanation of what dark energy is. If dark energy is a cosmological constant, this would be true. But if dark energy is something else (quintessence, or pick your favorite exotic theory), then dark energy will evolve (e.g. change as the scale factor changes).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.