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The Big Crunch is a possible scenario for the end of the universe, in which it collapses under its own gravity into a black hole singularity. However, recent observations suggest that the universe will continue to expand indefinitely. The key variable in determining the fate of the universe is Omega (Ω), which is the average matter density of the universe divided by a critical value of that density. If Ω is greater than 1, a Big Crunch is inevitable, but if it is less than 1, the universe will expand indefinitely. In a Big Crunch scenario, galaxies would eventually get so close together that they would become super-galaxies, hosting massive stars that would explode in less than a million years. Eventually, all matter would be consumed by a massive black hole.
The Big Crunch is a hypothetical cosmological event in which the entire universe collapses under its own gravity, ending up in a black hole singularity. The Big Crunch is one possible scenario for the eventual fate of the universe, along with others such as indefinite expansion (the Big Rip or Heat Death), and the Big Bounce, in which the universe would collapse but then expand again. Many cosmologists considered a Big Crunch to be a plausible outcome in the distant future for the universe until 1998, when dark energy, a mysterious force that caused the expansion of the universe to accelerate, making it more likely to expand indefinite.
The prevailing model for the origin of the cosmos is the Big Bang theory, which states that the present universe was formed in a gigantic explosion 13.73 billion years ago. After the Big Bang theory became well established and supported by many lines of evidence, scientists started saying, “Do we have a good model for the beginning of the universe – and its end?” General relativity, the theory devised by Albert Einstein in 1916, gave cosmologists the mathematical tools to examine the possible final states of the universe.
The most important variable in the question of the end of the universe and the possibility of a Big Brunch is the value of Omega (Ω), which is defined as the average matter density of the universe divided by a critical value of that density. If it is greater than 1, then the universe is closed, which means that the geometry of space can be compared to the interior surface of a sphere. If greater than 1, as in, the space is filled with enough matter, eventually a Big Crunch is inevitable. If it is less than 1, then the universe is open and will expand indefinitely. Recent observations suggest that it is less than 1, but this could always change with further knowledge.
It is a known fact that the universe is expanding. In a Big Crunch scenario, this expansion would eventually cease, then reverse, with the galaxies moving ever closer together. After a few tens of billions of years, the galaxies would eventually get so close together that individual superclusters of galaxies would no longer be clusters, and would instead become super-galaxies. The centers of these galaxies would host massive stars, similar to the Population I stars that existed in the early universe.
These stars would be so massive that they would explode in less than a million years; the typical lifespan of a star like the Sun is more like 10 billion years. Eventually, all matter would be so compressed that all life would cease and everything in the universe would be consumed by a massive black hole. Fortunately for our descendants, such an event now seems unlikely.
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