An open universe is a universe with a constant negative curvature and a density parameter low enough so that it expands without bound. Open universes are in the shape of a hyperbolic space, although their name suggests that they can be in the shape of any open manifold or open set. This isn't always the case, as open universes with boundaries are closed manifolds.
Open universes are generally infinite in size like a flat universe, and can share the same end fate as one (a Big Rip). The exception to this are those with boundaries as described above or if they have a some type of closed topology despite the curvature of their space.
Open universes generally have much higher quantities of dark energy than flat or closed universes.
Open universes start off similarly to flat universes, through a Big Bang, as an extremely hot, dense, and curved infinitely large region of spacetime. This is unlike closed universes, as they have a center point from which they expand from.
They often expand more rapidly than flat or closed universes due to their lower density or additional dark energy, and this expansion easily prevents gravity from slowing its expansion.
Open universes will soon expand faster than light, overcome their fundamental forces, and tear themselves apart in a Big Rip scenario.
- Flat universe - A universe with 0 curvature, and follows Euclidean geometry.
- Closed universe - A universe with constant positive curvature, and is in the shape of a hypersphere.