Thinking about cosmology

If the universe's expansion speed does not exceed the escape velocity, then the mutualgravitational attraction of all its matter will eventually cause it to contract. If entropycontinues to increase in the contracting phase (see Ergodic hypothesis), the contraction would appear very different from the time reversal of the expansion. While the early universe was highly uniform, a contracting universe would become increasingly clumped. Eventually all matter would collapse into black holes, which would then coalesce producing a unified black hole or Big Crunch singularity. If the density of the universe is greater than the critical density, then the strength of the gravitational force will stop the universe from expanding and the universe will collapse back on itself—assuming that there is no repulsive force such as a cosmological constant. Conversely, if the density of the universe is less than the critical density, the universe will continue to expand and the gravitational pull will not be enough to stop the universe from expanding. This scenario would result in the Big Freeze, where the universe cools as it expands and reaches a state of entropy. One theory proposes that the universe could collapse to the state where it began and then initiate another Big Bang,so in this way the universe would last forever, but would pass through phases of expansion (Big Bang) and contraction (Big Crunch).Another scenario results in a flat universe