Big Crunch

The Big Crunch is a hypothetical scenario regarding the ultimate fate of our universe. It suggests that the expansion of the universe might eventually reverse. If this reversal occurs, the universe would begin to recollapse. This process would continue until the cosmic scale factor reaches absolute zero. Some scientists suggest this event could be followed by a new Big Bang. This would start a new era of expansion for the universe. While this is a major theory in cosmology, current evidence suggests it may not be correct.

The mechanism of a Big Crunch depends entirely on the density of matter. If the average energy density is high enough, gravitational attraction can overcome the expansion. This process is predicted by the FLRW cosmology, which uses the Hubble parameter and the cosmological constant. If gravity wins the tug-of-war, the expansion will stop. After the expansion stops, contraction must follow. As the universe shrinks, the contraction accelerates over time. This leads to a massive gravitational collapse similar to a black hole.

As the universe collapses, the environment becomes extremely intense. The space between objects gets smaller and smaller. This causes radiation from stars and high-energy particles to condense. This process leads to a phenomenon called blueshifting, where radiation moves to higher energy levels. The energy would become so intense that it could ignite the surfaces of stars. This would happen even before the stars themselves collide. In the final moments, the universe would become a single large fireball. It would reach a near-infinite temperature. Eventually, neither time nor space would remain.

Ideas about a collapsing universe have existed for centuries. In the 1700s, Richard Bentley asked Isaac Newton a question known as Bentley's paradox. He wondered if stars in a finite universe would all collapse into one point. Later, Albert Einstein studied the universe's movement. He initially favored a static model, which is a universe that does not change. To make his equations work, he added a cosmological constant. This was meant to act as an anti-gravity force to prevent collapse. However, Edwin Hubble later proved the universe was actually expanding.

Edwin Hubble's discovery changed everything in the 1920s. Working at the Mount Wilson Observatory, he measured the distances of galaxies. He paired these with measurements of red shifts from Vesto Slipher and Milton Humason. Hubble found a link between an object's red shift and its distance. This provided proof that the universe was not static. Because of this, Einstein abandoned his cosmological constant. He famously called his belief in a static universe his "biggest mistake."

Modern science has mostly moved away from the Big Crunch theory. In the late 1990s and early 2000s, researchers studied distant supernovas. They also mapped the cosmic microwave background, which is leftover radiation from the early universe. These observations showed that the expansion of the universe is actually accelerating. This acceleration suggests a "Big Chill" or a "Big Rip" is more likely. The 2011 Nobel Prize in Physics was awarded to those who helped make this discovery. However, some physicists suggest a Big Crunch could still happen if dark energy fluctuates.

Even if the Big Crunch is unlikely, it leads to the idea of cyclic universes. One version is the Big Bounce, where a Big Crunch triggers a new Big Bang. The Ekpyrotic model proposes that this happens through colliding "branes." These are parallel planes in a higher-dimensional space. Another theory is Conformal Cyclic Cosmology, proposed by Roger Penrose. He suggests the universe goes through cycles called aeons. Finally, Loop Quantum Cosmology proposes a "quantum-bridge" between expansion and contraction. These models all try to explain if the universe has a beginning or an end.