Most astronomers believe that the Universe will not go on forever and that it will have an end, just like it had a beginning. The fate of the Universe is widely debated among astronomers. Many different theories have been hypothesized revolving around the expansion of the Universe and how it's going to be the factor that makes the Universe come to an end. One thing is for sure though, it won't end anytime soon and Earth will be long gone.
The Big Rip
One theory on how the Universe could end is the Big Rip. This theory suggests that since the Universe is still expanding, it will expand so much to the point where it rips everything apart, particle by particle. As the Universe expands objects are slowly moving away from each other due to the expansion of the Universe. Instead of the Universe creating new space when it expands, it just stretches the existing space, causing things to get pulled apart. Right now this force is pretty weak because the gravity of the Universe is slowing its expansion, so only galaxies are moving away from each other right now. Although, as the Universe gets larger, gravity will have less of an effect against expansion. This will cause more things to begin to separate, until eventually individual atoms are being ripped apart. If this theory were correct the Universe would be nothing more than an open space of individual particles in 22 billion years from now (Woollaston, 2016).
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The Big Crunch
The Big Crunch is just about the opposite of the Big Rip. This theory states that the expansion force of the Universe will weaken and gravity will over power it, causing the Universe to collapse on itself. If this were to happen the Universe will take the reverse route of the Big Bang and everything would merge back together. Stars would merge with stars, galaxies would merge with galaxies, and so on until everything is compacted together into a gravitational singularity, where the gravity force is infinite in measurement. At this point things are the way they were before the Big Bang, and perhaps even another one could trigger. This theory is thought to be least likely to occur (Big Crunch, n.d.).
The Big Freeze
Another theory of the fate of the Universe is the Big Freeze. This theory is similar to the Big Rip in the sense that they both have the expansion of the Universe as the cause. This theory suggests that along with the matter of the Universe, the heat also gets distributed evenly throughout the whole Universe. In the far future, once the Universe has expanded enough, the heat will be so spread out that there will be very little to none, making the temperature so cold that atoms stop moving. During this process all the gas clouds that create stars and other celestial bodies will be spread out so thinly that nothing new can form. Eventually when everything dies in the Universe and nothing new is able to be created, the Universe will be in a dead state with individual atoms spread out and frozen. This theory is thought to be most likely from what is known about the Universe and physics. (Woollaston, 2016).
Here is a great video on these three theories: