Question

What does Flat Universe mean?

I hear the term "Flat Universe" a lot, and I never quite figured out what it meant. Does this mean that the universe is similar to a flat plane?

Answer 1

It means that Euclidean geometry is followed:

• If three points in the universe are marked, the three angles add up to `180^circ`
• Parallel lines remain at the same distance apart forever.
• Pythagorus theorem applies to the universe.

Also, it means that each 'slice' of the universe is flat (imagine a cube split into smaller cubes, each slit being `x, y and z` planes in space).

The other terms are open and closed, closed being represented the most as a sphere and open universes as a continuous hyperbola. Images show common shapes and geometry of a plane of space for each of the three universe types:

`Omega_0="density parameter"="average density of the universe"/"critical energy density"`

A flat universe means that the universe will either expand forever at a continuously decreasing rate but never reaching 0, or expand slowly then speed up (but not accelerate). The value of `Omega_0` can actually be measured.