Eventually, the universe will die. The universe is constantly expanding via a strange phenomenon called dark energy, which is also believed to be what permeates the universe. Until 1998, scientists believed the universe was constantly expanding and contracting. Not only did they realize that the universe constantly expanding and not contracting at all, they realized that this expansion was accelerating. No one knows exactly what precise properties are shown by dark energy but they lead us to speculate three potential scenarios for the end of the universe.
The first scenario is called the Big Rip. In this theory, the concept is that space will eventually accelerate to a speed that is so fast, that gravity cannot compensate and everything will be molecularly decimated. The space between galaxies expands which causes them to move apart because the effect of gravity is not very strong. The space within galaxies also expands, however the presence of gravity is strong enough to keep everything close together. Eventually, gravity will be unable to hold galaxies together and they will be torn apart. Next, individual large bodies such as black holes, stars, and planets will die and their molecular structure will be dissolved. Once everything has settled down and space continues to accelerate, space will eventually reach faster than the speed of light and even atoms will disband. If this occurred, it would be impossible for any one particular particle to interact with any other particle.
The second way is called the Heat Death/Big Freeze theory. This theory states that instead of a spontaneous explosion or a large event, matter slowly decays and becomes radiation. The universe has the tendency to evolve to the highest entropy. Entropy is when matter or energy all evolves into a state of uniformity. While the universe expands and gets bigger, matter begins to decay and spread out which cause the effect of entropy to take place. After a very long time, all of the gas clouds necessary to become stars will decay causing the universe to become very dark. Black holes will begin to disintegrate after all of the stars disappear due to the effect of Hawking Radiation. After this finishes, only a faint gas of photons and light particles will remain before even this decays and leaves the universe as static and perpetually empty. However, theoretically it is possible that after a very long time this state of highest entropy can spontaneously decrease which would give birth to another universe.
The third and final proposed theory is called the Big Crunch & Big Bounce. This theory is considered to be the most optimistic scenario. If the amount of dark energy in the universe is less than originally thought, gravity will become the dominant force in the universe causing its expansion to slow and eventually halt. Once the expansion of the universe stops, everything in the universe will rush inward. As the universe becomes smaller, temperatures begin to increase everywhere as well – even hotter than most stars; and as a result, these stars begin to be cooked from the outside. Moments before the big crunch happens, atoms rip apart and black holes will begin to devour everything in the universe. Once this finishes, all black holes in the universe come to close to form one extremely massive black hole. In the final moments before the big crunch, this massive black hole would devour everything that’s left, including itself. This theory believes that this has happened countless times before and infinitely expands and contracts creating countless universes.
