Final answer:
The Big Crunch theory proposes the universe may collapse back into a singularity, but the concept is challenged by the presence of dark energy which causes accelerating expansion. The multiverse theory speculates on many universes with varying laws of physics, implying ours may be tuned for life as per the anthropic principle.
Step-by-step explanation:
The theory that postulates the possibility of our universe eventually returning to a singularity is often referred to as the Big Crunch theory. This is a scenario in which the expansion of the universe eventually reverses, and the universe collapses back into a singularity. However, the discovery of dark energy has cast doubt on this scenario, since dark energy is causing the universe to expand at an accelerating rate. Instead of a Big Crunch, we may face a future in which galaxies move apart in a lightless, lifeless realm as stars burn out and new star formation ceases.
The multiverse is another speculative idea that our universe is just one of many, each with its own set of physical laws. It is thought that if we are able to observe and measure the properties of our universe, it may be because its laws are tuned in a way that allows for our existence; this notion is known as the anthropic principle.