Ever wondered how the mighty universe will end with all its vastness? The non-existence of all the planets, comets, stars, galaxies and of course humans and all other kinds of lives, nothingness permeating everywhere followed by the ultimate fall of the universe. The mere thought of such a phenomenon is appalling, especially when we haven’t even known enough about the mysteries of the universe. We are looking for life outside our solar system — a habitable place for humans before our solar system gives up on us. We definitely do not like to live with the anticipation of doom, for we revel in the present and tend to fantasize about good things in the future. However, scientists have come up with theories describing the eventual fate of the universe. So far, they have postulated four: Big Crunch, Big Bounce, Big Rip and Big Freeze.
What happens when you throw a ball high up in the air? It goes up until it runs out of the upward momentum and eventually gets pulled down by gravity. Similarly, when the Big Bang occurred – an explosion from a singularity point – the universe started expanding due to the outward momentum, and as long as the gravitational pull between the masses of galaxies, stars, gases and dust does not overcome the momentum, the universe keeps on expanding. Once the momentum dissipates enough for gravity to take over, everything gets pulled in and is shrunk back to a singularity. This is called the Big Crunch as the big universe is crunched back to a singularity. However, the mass-energy comprised of galaxies, stars, gases and dust is too little to cause the eventual crunch. There exists a mysterious and invisible dark matter, which makes up about 27% of the mass-energy of the universe that has gravitational property. The Big Crunch will, hence, occur once the dark matter combined with stars and galaxies overcomes the Big Bang’s outward momentum.
However, human imagination knows no limits. Scientists have come up with another theory which is an extension of the Big Crunch. After the universe has collapsed back to a singularity, there could be another Big Bang and hence the expansion and then the collapse. Just as a ball bounces off the ground, the universe will bounce off a singularity point. That is the Big Bang and the Big Crunch occurring in a continuous loop. This oscillatory behaviour is, thus, named the Big Bounce.
The aforementioned theories of the universe’s possible doom made sense until Edwin Hubble, an American astronomer, confirmed the theory of George Lemaître, an astronomer and physicist from Belgium, according to which the universe is actually expanding at an accelerating rate instead of slowing down. The accelerated expansion confounded the scientists. Therefore, to explain the phenomenon, they postulated that there exists an anti-gravity force that is pushing the universe further apart and called it dark energy. Though the exact properties of the dark energy are not known, their understanding is essential to know more about the universe. Right now, the most important question is whether it is constant throughout space or is it increasing with the expansion of space. For now, we know that about 68% of the mass-energy of the universe is made up of dark energy, which is surprisingly huge. This raises a question about why the universe was expanding at a slower rate initially, and to answer that scientists have postulated that dark energy has permeated the space since the beginning of the universe; it is just that the gravity had dominated it earlier to cause the slower rate of expansion.
The existence of dark energy has, thus, led to two more speculations about the universal apocalypse. Currently, dark energy does not have much effect on local levels i.e. stars and galaxies. However, if it keeps on increasing with space and time, it will not only distance galaxies and stars from each another but will also rip apart every galaxy, star and planet down to the last atom in due course. This is called the Big Rip, a very violent end of the universe. If that is to be true, it will take 22 billion years from now to happen.
It could be less dramatic if dark energy stays constant throughout space and the density of the universe remains less than or equal to the critical density – a quantity that ensures that the universe does not expand forever. In this case, the universe will keep on expanding at a slower or constant rate and eventually everything will be far apart. Then a time will come when all the stars will run out of fuel leading to the Big Freeze or Chill. For this end to occur, it will take about 100 trillion years from now.
Of the four possibilities, the Big Crunch and Big Bounce are invalidated by the discovery of an accelerated expansion of the universe and from the remaining two, Big Freeze is believed to be the most likely fate of the universe for two reasons. First, the universe density is found out to be almost equal to the critical density and second, there is some evidence that suggests the dark energy is constant throughout the space. However, the doom of the universe is very far in the future and we will not live to witness the apocalypse until we find other planets in the universe that could sustain human life.