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?

1 Answer
May 25, 2018

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:
https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvature_of_the_universe

#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.