“It’s all started with a Big Bang”… but how the Universe will end, or when, we don’t yet know. What we do know is that one day the end will come, one way or another. At the moment cosmologists are talking about four different possibilities for the “end of the world”.
But before you start to panic, remember: it is probably not going to happen tomorrow or the day after. Or, even, in the next many billion years. So no need to worry just yet.
At the moment the Universe is expanding and, moreover, the rate of expansion is increasing. What that means is the distances between the galaxies get bigger, while the galaxies themselves and the objects they contain remain the same. Moreover, the further the galaxies are, the faster they seem to be running away from us. Will this accelerated expansion continue indefinitely? Slow down? Speed up? Well, that depends.
In simple terms, the ultimate fate of the Solar System, the Milky Way and everything in the Universe depends on who wins the battle: the expansion of the Universe or the gravity that pulls everything together. If the amount of matter in the Universe is greater than a certain critical value, the gravitational attraction between the “bits” of matter will be able to stop the expansion. If there is less matter than the critical value, the Universe will continue to expand.
But the accelerated expansion hints that things are way more complicated than that. That it’s not just the initial impulse from the Big Bang that causes the galactic runaway, but also some other substance, the so-called dark energy. What it is exactly, we do not yet know. But it accounts for about 70% of all the energy in the Universe, so it’s pretty important. Therefore it might be the stuff that we know nothing about, and not the stuff we observe, that will determine our ultimate fate.
With all that in mind, let’s look at our possible futures. They are Big Freeze, Big Rip, Big Crunch and Vacuum Decay. Sounds comforting, huh?
Big Freeze (also known as Big Chill or Heat Death)
At the moment it looks like we are heading for the Big Freeze. In this scenario the Universe will keep expanding, the galaxies will keep getting further apart until they get completely isolated. In each such galaxy new stars will stop forming and the whole structure will get colder and dimmer and gradually fade away. Eventually all that’s left will be cold planets, dead stars and black holes. The latter will slowly evaporate through a process called Hawking radiation. No life will be able to exist in such an environment. Nice!
Another, more dramatic way the dark energy might lead to a cosmic destruction is called the Big Rip. If the rate of expansion of the Universe continues to accelerate out of control, then not only galaxies will be pulled apart, but also planetary systems, stars, planets and all the matter. One way to think about it is this: the galaxies will be running away from each other so fast, that the cosmic event horizon, i.e. the boundary outside which things are moving away faster than light, will quickly shrink, eventually getting as small as the Milky Way, then as small as the Solar System, then as small as the Earth… Until finally the cosmic horizon will become smaller than the tiniest subatomic structure – the matter will be torn apart. Ouch!
This is one of the oldest End-Of-The-World theories and, judging by the latest observations, an unlikely scenario.
The idea of the Big Crunch is that if the gravitational pull overcomes and reverses the expansions, the galaxies will start to rush back towards each other. The Universe will get smaller and the background radiation and the radiation emitted by the stars and other objects will get harder. The temperatures will rise so that the surfaces of the stars will ignite. Eventually, all the matter will get packed into a small super dense and hot region. Sort of Big Bang in reverse.
The last scenario emerges from the way the Universe is put together on a fundamental level.
It turns out that by analyzing elementary particles that make up matter and carry fundamental forces (or, rather, fields that these particles represent) we can tell whether the whole Universe is stable or metastable. More specifically, the answer comes from an elementary particle discovered in 2012, called Higgs boson. The state of the Universe depends, physicists tell us, on the mass of the Higgs boson.
According to the recent measurements, the Universe we live in is metastable. One day, any day really, some tiny bubble somewhere in space might spontaneously flick into a stable, lower energy state. The laws of physics inside the bubble will be different from those that exist in our metastable Universe. This bubble will expand at a speed of light and probably destroy the Universe itself on the way. Luckily, it will happen so fast that we will not feel a thing!
Just to get a sense of what the vacuum decay is about, imagine a bottle of water. Normally, the water freezes at 0 degrees Celsius. But if you have a very pure water and keep the bottle still, the water might actually remain liquid even if you cool it down to almost -50 degrees Celsius. This is called supercooling. The supercooling is possible if there are no nuclei, like impurities in the water, for the ice crystals to start growing on. Now if you shake a bottle or drop something into the water, it will freeze immediately. Just like the supercooled water, our Universe is stable for now. But one violent shake – and that’s it. Luckily, no cosmic events we know of are energetic enough to shake the Universe. But (thank you, quantum mechanics, for this) there are other ways.
So what is the ultimate fate of the Universe? To be honest, we don’t yet know. But there is plenty of time to figure it out. Maybe.
If you want to learn more about the end of the Universe, its beginning and everything in between, come and see our presenter-led inflatable star dome shows. You can also book an inflatable planetarium for your Space Birthday Party or a corporate event. Happy learning!