What are the possible ends of the universe?
What are the Big Freeze, the Big Crunch, and the Big Rip/Crackup?
The Big Freeze is one scenario that could describe the end of the universe. In this scenario, everything in the universe keeps getting farther and farther apart because of the universe's expansion. When things get farther apart, they will become colder, because their atoms will no longer interact and cause fission and fusion, which create explosions, and thus energy and heat. So, in this scenario, when a star runs out of fuel, it dies, and that death does not support the beginning of another star, because there are no other particles nearby. So, one by one, the stars die, and they do not produce any heat, and the universe becomes very cold.
The Big Crunch is what will happen if the universe is more dense than a "critical density." If the universe is denser than this, eventually the gravitational force of the matter in the universe will cause the universe to stop expanding (overcoming the force of dark energy), and then the universe will begin to collapse. As it collapses, matter will be pushed closer and closer together, eventually becoming so dense that the universe will be made of a large set of black holes. The universe will collapse back into one singularity, and then, probably, another Big Bang will occur, and everything will start over.
The Big Rip is an end scenario that most scientists do not believe will happen. The current theory of the universe states that the its expansion is accelerating, but that it is accelerating at a constant rate. In the Big Rip scenario, the universe's acceleration is increasing as time goes on. Eventually, it will be accelerating so fast that the fundamental forces (like the force that holds atoms together) will not be able to have an effect, and everything will rip apart.
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