A cosmological model in which the Universe is infinite and will expand forever.
A cosmological model of the Universe that at an infinite time in the future will possess an infinite volume with hyperbolic geometry; it also has no center and no boundaries.
a universe with too little matter (gravity) to stop the expansion (it has an ``open future'').
A model of the universe in which gravity is not strong enough to bring the universe to a halt; it expands forever. In this model the geometry of spacetime is such that if you go in a straight line, you not only can never return to where you started, but even more space opens up than you would expect from Euclidean geometry.
A model of the universe in which it will expand forever. In this model, there isn't enough matter with its accompanying gravitational forces to stop the current expansion.
A universe with a density less than (or equal to) Friedmann’s critical value. Gravity cannot prevent such a universe from expanding forever.
A geometrical model of the universe in which the overall structure of the universe extends infinitely in all directions. The rules of geometry in an open universe are like those that would apply on a saddle-shaped surface.
