We don’t like to think about it, but the world will end one day. It is likely to be engulfed by the Sun as it expands into a red giant star 6 billion years from now, but whether humans survive to see that day is up for debate (climate change…). With regards to the universe, however, it is unknown what will happen and when. Will it come to a violent end, or simply trail off into nothingness? And will it be 2.8 billion or 22 billion years from now, or even further into the future?
But how do we know that the universe will end at all? Why couldn’t it simply continue to be as it is now until eternity? Clues that suggest that the universe will end are hidden in the universe’s beginnings.
In 1927, Georges Lemaître observed that the universe was expanding, and this was confirmed by Edwin Hubble two years later. Up until this point, the prevailing theory of the universe was the static universe theory, which suggested that the universe was infinite and unchanging. Einstein had been a proponent of this theory, which was debunked by the discovery of expansion. Now, theories had to incorporate expansion, and one of the theories that arose was the Big Bang theory. Lemaître pointed out that the expansion of the universe could be traced backwards through time, getting smaller and smaller to a beginning where the universe was a single point, or a ‘singularity’. This was supported by the fact that galaxies were (and are) moving apart, as discovered by Hubble. In fact, the Big Bang theory has been supported by lots of observational evidence since, to the point where the basic premise of expansion is highly unlikely to be false.
So, the universe had a beginning, about 13.8 billion years ago. It is not constant and unchanging and therefore may someday cease to be. There are a number of theories about how the universe may end, some more dramatic than others.
The first theory involves the heat-death of the universe, or the ‘big freeze’. This theory suggests that eventually, as energy is spread thinner and thinner in an expanding universe, thermodynamic equilibrium will be reached leaving everything in the universe at a uniform temperature just slightly above absolute zero (-273°C/0K). This theory was proposed by Lord Kelvin (after whom the absolute temperature scale is named). When thermodynamic equilibrium is reached, mechanical work and gradients (which are needed for life) are impossible, so nothing can exist.
This theory is only possible if the universe continues to expand. However, many different things are possible if the universe continues to expand, and what will happen depends on the amount of matter in the universe. If the amount of matter remains below a certain threshold, heat death is the likely outcome. However, if the amount of matter is above this critical threshold, gravity acting on all this matter will cause the expansion to slow and eventually reverse. This is the ‘Big Crunch’. It predicts a reverse Big Bang; the universe will start to contract, getting smaller and smaller, while also getting hotter and denser, eventually collapsing into a singularity. If this does happen, it is then possible that there will be another Big Bang, potentially creating an oscillatory universe, where Big Bangs and Big Crunches happen alternately. This cyclic model allows for there to be an infinite sequence of universes expanding and contracting through eternity.
Throughout that 20th century, it was believed that the universe would end in one of these two ways, but which one was unknown. Scientists decided that they would try to estimate the amount of matter in the universe, to see if one theory was more likely than the other. However, this resulted in more uncertainty, as it turns out the amount of matter in the universe is very close to the critical threshold.
However, another discovery threw doubt onto both of these theories in the late 90s. Two groups of scientists independently found that not only is the universe expanding, but that is expanding faster and faster. So, the expansion of the universe is accelerating! This means that someday (probably 22 billion years from now), the pull of the expansion of the universe will become greater than the pull of gravity and all the fundamental forces. Since gravity is what holds everything together today, the universe will then rip apart, starting with galaxies, then black holes and stars and planets – the Big Rip. Eventually the universe becomes a soup of solitary, separated particles. One question that remains regarding this theory is the role of dark energy, which acts in opposition to gravity. Some accredit the accelerating expansion to dark energy and for the Big Rip to happen, dark energy must overcome gravity. However, the role of dark energy is poorly understood, as dark energy itself remains a hypothesis, unknown in the universe.
These three theories cover the most likely end of our universe, but there is one more, unusual theory that we cannot prepare for and could happen at any time. And quantum physics is the basis for this theory; according to it, the completely empty vacuum of space still has a level of energy, although small. But this could change if a ‘bubble’ of lower-energy vacuum appears and converts the rest of space to a lower energy. Again, according to quantum physics, if this is possible, it will inevitably happen; this bubble could be spontaneously generated by chance at any time, and would expand at the speed of light, converting the rest of the universe as it does so. This would certainly spell the end of the universe as we know it, although we do not know how the universe could change. It is possible that the fundamental rules of the universe would change; fundamental particles such as electrons and quarks could behave radically differently, perhaps not even forming atoms. And, to round things off nicely, this ‘big change’ could rewrite the properties of dark energy, causing it to pull the universe in on itself, triggering a big crunch on top of it all.
So, the future is looking cheery for the universe, but we can take comfort in the fact that at least three of these possibilities have a projected start time billions of years in the future. And, if the fourth does end up happening, it would happen so fast we couldn’t even react. All in all, there’s nothing that we can do, so why worry? Perhaps we should turn our attention to things we still have control over, things that will have dire consequences within our lifetimes: climate change. Perhaps, use the Sun while it is still burning, and switch to solar panels.