We think we know how it all started. In 1929, Edwin Hubble proved that the universe was expanding by finding that galaxies were moving further away from us. If galaxies are moving away from each other, that must mean they started out much closer; as a singularity. The expansion of this high density state is believed to be the start of the universe, or the big bang.
But how will it all end? First, we need to define ‘the end’. When could we consider the universe to be officially ‘dead’? One view could be that the universe ends when we end. When all humans die out, we take the idea of the universe with us. A more accepted definition is that the universe ends when nothing can happen anymore. This outcome could be caused by the maximisation of entropy. This refers to the second law of thermodynamics — “all closed systems tend to maximise entropy” — where entropy is the degree of disorder or randomness in the system. In this case, the universe is our closed system, in which all available energy will one day be spread out evenly between all matter.
How does this spread happen? Well, it happens every single time you, or anyone, or anything does something. The radiation of light from the sun, the supernovae of stars, the processing of this article by your brain. At its core, every action requires the transfer of energy. This transfer leads to the spreading out of energy, or the maximisation of entropy.
This end result (maximisation of entropy) seems convincing, but how do we get there? There are many different speculations about how entropy could be maximised, but which one is correct? It all depends on gravity and dark energy. In 1998, we discovered that the universe is expanding at an accelerated rate. However, we don’t know the cause. Our best solution? Give it an intriguing name: dark energy. Gravity is the opposite of dark energy. All matter in the universe has its own gravity. This opposes the expansion of the universe by pulling it inwards. In the end, it is the winner of this cosmic battle between gravity and dark energy that will decide the fate of the universe.
Possibility 1: Dark energy wins: the big rip! This means that the expansion of the universe will continue to accelerate forever. As of now, gravity is strong enough to hold galaxies and solar systems together even though the universe is expanding, but after a certain point, space may expand too fast for gravity to keep up. First, galaxies and solar systems will be pulled apart as their gravity is too weak to hold them together. Then, smaller entities such as planets will be separated as well. This separation will continue until the atomic level, where electrons and quarks will be torn apart. Eventually, it will become impossible for particles to interact, which means nothing can happen anymore even though entropy might not be maximised. The universe will be ripped apart.
Possibility 2: Gravity wins: Big Crunch/Big bounce! Eventually, gravity will overcome the expansion caused by dark energy, and will become the dominant force affecting the size of the universe. Everything will be pulled back together and black holes will devour all matter as they merge together. Eventually, this black hole will devour itself, and the universe will revert back into a singularity. But there’s more! Depending on the nature of black holes, this new singularity could be identical to the one we had before the big bang. This would mean that the universe operates in cycles, and countless big bangs and crunches happen throughout many repetitions of this cycle. Either way, entropy is maximised.
Possibility 3: A tie? Heat death: Everything stays how it is right now. Dark energy and gravity will never fully overcome each other. The universe will keep expanding and the transfer of energy will keep happening until entropy is maximised. All the energy in the universe will be spread out over countless lightyears and nothing will ever happen anymore. The last stars and black holes will die out, leaving the universe cold and dead. This seems to be the most probable scenario at the moment.
The maximisation of entropy seems to be a common theme here. Therefore, if one of these possibilities is correct, time itself should end when the universe ends. How? Entropy is a quantity that goes hand in hand with time. As “time’s arrow” moves forward, entropy increases. So what happens when entropy is maximised? What use does time have now? Well, nothing. Time is rendered a useless quantity, so it ‘ends’ in a way.
Clearly, we don’t want all this to happen. To be frank, the end of the universe is so far away that calling it forever is acceptable, but what if I said you could help delay the death of the universe?
How could you help, you say? Well, just do nothing. For entropy to be maximised, something needs to happen, but if you just don’t do anything, and just be lazy, you are technically delaying the death of the universe, albeit by an unimaginably insignificant amount of time.