The fate of the Universe ultimately depends on how much stuff is in it, how fast it is expanding at the moment and scientists favourite fudge factor, dark energy (otherwise known as the cosmological constant). Now the nature of dark energy, or similarly dark matter, is unknown. In fact whether they exist at all has been called in to question. It may be our laws of gravity are just wrong. We call them “dark” energy/matter because we need them to explain what we see but we don’t know what they are. Tangent over, there are three possibilities for our Universe:
- It will expand at an ever faster and faster rate forever and ever.
- Gravity will pull everything in the Universe together and the Universe will collapse in on itself.
- Much like Goldilocks, we find a Universe that is just right. The Universe will expand forever but at an ever slower rate so that it effectively comes to a stop.
Now the ultimate fate of the Universe depends on which of these Universe we live in and you won’t be surprised to know we aren’t entirely sure which of these three we are in (honestly what do scientists know). We think we are in Universe 3 but we could be slightly either side in Universe 1 or 2. So out of 3 we’ve narrowed it down to… 3, good work. We’d best look at how the Universe will end in each of the three then.
If we are in Universe 3, known as a flat Universe because of it’s topological properties – best not to ask, then what will happen is the Universe will effectively stop expanding eventually but this doesn’t pose any particular threat to the Universe’s structure as such so we need to look at it’s contents. When stars die, some of them explode in a supernova which expels it’s contents far and wide, from which other stars can form. Ah the circle of life. But some aren’t big enough, some become white dwarfs which take a few billion years to cool eventually become a brown dwarf. Brown dwarfs are just big balls of stuff that are no longer hot or shining, a star corpse if you will, that selfishly keeps hold of most of it’s matter removing it from the circle of life. As gas and dust becomes more scarce the huge phoenix-like-supernova stars will become rarer and the stars will all go the dull brown dwarf root. We could say therefore that the Universe “ends” the day the last star goes out. Now this would be haunting on an unimaginable scale and the Universe may as well be over but there is still stuff happening so it’s not over. No, you may know some stars form black holes, the Universe’s vacuum cleaners (lets just appreciate the magnificence of that pun for a second). These hoover up gas and stars and well anything that gets too close. In fact it could be that the last star isn’t allowed to extinguish, it may be that it is ripped apart and swallowed by any of the now considerable number of black holes. These black holes would go on hoovering up the remaining brown dwarfs after the last star goes out so we could define the end as the point when the last of the matter and brown dwarfs is hoovered up by a black hole but no. See these black holes can hoover each other. They can collide and combine so that we might end up with one incredible super black hole. Is this the end? Not on your life or the Universe’s for that matter. See our esteemed friend Prof. Stephen Hawking, discovered that black holes evaporate over time. So here is where we end our Universe, when the last black hole has finely evaporated. Phew what a ride. I know what your thinking I am so emotionally drained I need a minute but we can’t stop here, this was just one scenario.
If the Universe is “open”, that is to say Universe 1 (where it expands ever faster forever) it is possible that the Universe would end in the same way as a flat universe above. But if the dark energy, whatever that may be, proves to be strong enough, it could be that eventually it overcomes gravity so stars and galaxies would be torn apart. Eventually the expansion could overcome the nuclear forces as well and atoms would be torn apart. Picture the end of Infinity War. This would be a pretty horrific end to the Universe, the end of all structure so that we just have an ever expanding spaced out sea of fundamental particles. But this would be a fairly peaceful end compared to the possibility of an ever stronger expansion that would rip even space-time apart as well. If that happened then all bets are off, reality is gone for sure.
That leaves us with our final Universe. a closed Universe. A universe that eventually begins to collapse in on itself. What happens is that gravity reigns in the expansion caused by the big bang and the Universe collapses in on itself. CRUNCH. A big crunch in fact. The Universe returns to the singularity from whence it came. That means that everything in the Universe and space and time are all crammed back into a “space” so infinitely small that it has dimensions of 0. I don’t know about you but I’m rather hoping for the slow peaceful, black hole evaporating death of a flat Universe which, good news, is what scientists have settled on as the most likely end… for now. But which ever it is, it’s billions of years in the future and we have many more imminent threats; nuclear war, superbugs, asteroid strike, the sun expanding and eventually enveloping the Earth… sleep well.