How will our universe end?

The mystery of our universe and its end has left scientists baffled since it was discovered that our solar system was the smallest part of a cosmos larger than was every previously thought possible. While the earth is predicted to vaporize in about 6 billion years, the universe will continue long after that. The issue with finding an answer to how our universe will eventually end is that with such a large part of our universe being made up of the elusive dark matter, and the potential ‘end’ of the universe being trillions of years into the future, it is difficult to come up with one definitive theory. So far, there are three major competing theories hypothesizing potential ways that the universe will end.

The Big Freeze theory grounds itself in the field of thermodynamics, the study of heat. In the universe, events, processes and more generally, everything, occurs due to a heat difference between different sources. This theory suggests that since heat always moves, eventually heat will be evenly distributed throughout the entire universe. At this point, also referred to as ‘heat death’, all stars will run out of fuel and die, all matter will decay and the only thing remaining would be a few particles that would also over time be shifted away by the expansion of the universe. Even the largest stars that collapse into black holes would eventually give off Hawking radiation, eventually evaporating these too. A rather bleak theory, this suggests that the universe will eventually end up cold and empty.

In many ways the Big Crunch theory is a direct opposite to the Big Freeze. Thanks to the theory of relativity, and the consequent discovery of cosmic microwave background radiation, it was discovered that the universe is expanding. As a result of this discovery it has been speculated that although the universe is expanding right now, it will eventually reach a threshold where there is so much matter in the universe that gravity becomes the dominant force, causing the universe’s expansion to slow down, stop and then to contract. The universe would contract faster and faster, becoming denser and hotter as it does so, until all matter finally implodes in on itself in a final singularity. This is often though to be in effect a reverse big bang. However, this theory is less widely though because of a more recent discovery that makes this theory improbable, the rate at which the universe is expanding is increasing.

The Big Rip is the final major theory entirely grounds itself in the activity of dark matter. Dark matter is thought to be responsible for the universe’s expansion, and since its density remains constant despite the universe growing, it is thought that more and more dark matter ‘pops’ into existence in order to keep up the rate of expansion. Oddly enough, this does not contradict the fundamental law of conservation of energy. This law states that in an isolated system, the total amount of energy will remain constant. This law is conserved because as energy and momentum are dependent on spacetime, if spacetime stays the same, total amount of energy remains the same but since spacetime changes, so does total energy. It is suggested that eventually, so much dark energy will have popped into existence, that its density would be above that of ordinary matter so the forces of expansion from the dark matter will overcome the gravitational forces of ordinary matter. This will essentially rip the universe apart, with larger objects of a lower density like planets and stars being ripped apart first, then humans and other living creatures and finally atoms will be destroyed before the universe will finally be entirely ripped apart.

All of these theories have their merits and provide good explanations for what will one day happen to our universe, but none yet provide conclusive evidence. What we can be sure of, is that by the time that these doomsday scenarios might happen, after trillions of years humans will be so far evolved that we probably will not still be able to call those living at that time humans anymore, if life still continues to exist for that long.

Sources
http://www.bbc.co.uk/earth/story/20150602-how-will-the-universe-end
www.wired.co.uk/article/how-will-universe-end
https://www.sciencedaily.com/terms/supercooling.htm http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

By Yuval, Year 11