What phenomenon does the Big Rip describe?

The Big Rip is a theoretical cosmological scenario that posits the ultimate fate of the universe in which the expansion of the universe, driven by dark energy, accelerates to the point where it overcomes all gravitational, electromagnetic, and nuclear forces. This would result in the tearing apart of all structures in the universe, from galaxies to individual stars, planets, and even atomic particles.

In this scenario, as the universe expands at an increasing rate, it would reach a point where the fabric of space-time itself becomes so stretched that galaxies are pulled away from each other and even the fundamental forces that hold matter together are no longer able to counteract this expansion. Ultimately, this would lead to a situation where everything in the universe would be ripped apart, making the Big Rip a dramatic end-of-the-universe concept rooted in our understanding of dark energy's impact on cosmic evolution.

The other choices describe different astronomical phenomena. The collapse of massive stars refers to a specific process leading to supernovae or black holes, the expansion of galaxy clusters is related to the large-scale structure of the universe, and the merging of black holes pertains to gravitational interactions rather than the broader cosmic fate envisioned in the Big Rip.