What is the flatness problem in cosmology?

The internal angles in a triangle usually add up to 180 degrees. But if that triangle is drawn on the surface of sphere then the angles add up to more than 180. On the surface of a saddle they sum to fewer than 180 degrees.

Measuring these angles in space suggests that the observable universe is flat – they add up to 180 degrees. That leaves two options. Either the universe does have curvature, but has expanded so much since the Big Bang that our bit of the universe has lost its curvature. Or the entire universe is flat.

The problem is that according to the original Big Bang theory there hasn’t been enough expansion since the Big Bang to smooth out the curvature of the observable universe. And the chances of the whole universe being flat are so astronomically low that it’s effectively impossible. So what causes the local flatness of the universe? That’s the flatness problem in cosmology.

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