1. Big Crunch: In this scenario, the expansion of the universe eventually comes to a halt and then starts contracting. The galaxies move closer together, the cosmic microwave backgroung radiation increases in temperature, and the universe becomes hot, dense, and eventually reaches a singularity, similar to the Big Bang but in reverse. For the universe to end in a big crunch, the matter density of the universe must be sufficiently high. However, current observation suggest the universe is dominated by dark energy which causes expansion of the universe to accelerate, making a big crunch unlikely according to our current understanding.
2.Another Bang or Cyclic Universe: In this scenario the expansion of the universe continues until all stars have run out of fuel and black holes have evaporated due to a phenomena known as Hawking Radiation. At that point, the universe would consist only of extremely dilute matter and radiation. Depending on the fate of dark energy, the universe might either reach a state of maximum expansion and then collapse and restart in another Big Bang, or may continue to expand forever. Some cosmological theories propose cyclic models, where the universe goes through phases of expansion and contraction, leading to a series of Big Bangs and Big Crunches. However, these ideas are speculative and require new physics to support them.
The actual fate of the universe remains uncertain and is one of the unsolved questions that continues to fascinate cosmologist. Continued observation, improved understanding of dark energy and development of new theoretical frameworks should provide future insight into how the universe will ultimately meet its end.
