What does the Big Crunch refer to in cosmology?

The concept of the Big Crunch refers to a theoretical scenario in cosmology where the expansion of the universe eventually halts and reverses, leading to a gravitational collapse. This process suggests that the universe, having expanded from the Big Bang, could reach a maximum size and begin to contract. As gravity pulls everything back together, galaxies, stars, and ultimately all matter would converge back into a singular state. In this model, the universe wouldn't just cease to expand but would instead shrink down to an infinitely dense point, similar to its original state at the Big Bang.

Understanding this requires recognizing the dynamics of cosmic expansion driven by dark energy or similar forces and contrasting them with the gravitational forces that could cause a reversal into a contraction phase. The Big Crunch is thus significant as it presents an alternative fate of the universe, contrasting with scenarios like the Big Freeze, where expansion continues indefinitely.