How will it end?

We have come a long way, dear readers. We learned how the Universe began. We explored the two theories of structure formation. We followed the lives of stars. We even explored the mysterious surroundings of black holes and supernovae. There are so many wonderful things in the Universe that no lecture course or article series will ever encompass them all, but I hope that I managed to shed some light on the darkness that pervades the infinite space.

There are two more topics that I wanted to talk about. Each of them may serve as an ending to this article series, albeit in a different way. The first of these two is the end of the Universe. After all, if all that is had a beginning, it should have an end, shouldn’t it?

It turns out that the answer, as usual, is not very easy to give. Much like when exploring the Big Bang and the beginning of the Universe, it is impossible to make observations of those moments. What is worse, whereas observational results can help us constrain the properties of the Universe at early epochs rather tightly, the future of our cosmos is much harder to figure out. As a result, there have been, and still are, numerous theories about what may happen. I will present three of them – “The Big Crunch”, which is also related to the “Bouncy Universe” model; “The Big Rip”, which is the one favoured right now; and “Heat Death”, which may be considered a property of the Big Rip, but not just that.

When Edwin Hubble discovered his famous law that other galaxies are moving away from us, people realised that the Universe is expanding. This led to the formulation of the Big Bang model of cosmology, which is the accepted theory of how the Universe began. What scientists did not know was whether the expansion of the Universe is constant, accelerating or slowing down. For a while it seemed that the appropriate cosmological model required matter (whether ordinary or dark) to make up the largest part of the density of the Universe. If that was the case, then equations showed that the expansion would gradually slow down until finally coming to a stop. This is because all particles attract each other and thus create an inward force felt by every object, which opposes expansion. One could say, comparing this situation with a space shuttle trying to escape Earth’s orbit, that the total energy of the Universe is not positive, therefore it is not possible for it to expand forever. Such a comparison is, strictly speaking, incorrect, but if it helps you visualise the situation, then it’s good enough. There are, however, two possibilities here: either the density of matter is large enough to make the Universe recollapse or it is low enough so that the expansion continues indefinitely, finally stopping after an infinite time. The latter case is not exactly the same as that of a Universe which has a “positive energy”, because such a Universe would expand to an infinite size, whereas a “zero energy” Universe would stop at a finite size. More on this later.

If the matter is dense enough to make the Universe start contracting, the situation in it will be, on the largest scales, very similar to what happened during the expansion, but in reverse. The Hubble constant will become negative. Objects will start moving toward us, rather than away from us. The sky will start glowing, because all the photons with radio, microwave and infrared would gain energy and shift into the visible range. Of course, later on, they will shift further into ultraviolet, x-rays and gamma rays, so we won’t be able to see them anymore. Even later on, galaxies would start merging everywhere and structure would disappear on large scales. After that, the stars would start colliding, until finally the whole Universe would become one huge ball of ultra-dense matter. This ball would most likely continue collapsing, as its pressure would not be enough to counteract the already very fast infall of outer layers. What would happen then is anybody’s guess, really, as we do not yet know enough about black holes and similar objects to speculate on what would happen to them in an environment of reversed expansion.

The above is the Big Crunch scenario. One suggestion of how its finale might proceed is that once all the mass of the Universe collapses into a singularity, it will explode creating a new Big Bang and a new Universe. This is known and the “Bouncing Universe” model, but as with most other models, it is no more than a theoretical postulate.

The second model is one that conforms to the current understanding of the Universe. Since we now know (or think that we know) that the main part of the density of the Universe is caused by the mysterious dark energy, this has important implications to its future. The density of dark energy does not decrease as the Universe expands. Therefore, while at the early days of the Universe, dark energy contributed only a tiny fraction to its density, currently it comprises more than 70% and in the future, this value will only increase. If a Universe is dominated by dark energy, it will expand exponentially, much like during the inflation period in the very early Big Bang.

What does this bode for us, or, more precisely, our descendants and any other objects in the Universe? First of all, more and more distant objects will disappear beyond the edge of the observable Universe and other objects will grow more distant. Secondly, the cosmic background radiation will move to radio waves and gradually become undetectable. So the night sky will become far less interesting to look at. So what? Well, some time later the “pull” of dark energy will become so strong that our own Galaxy will be torn apart and stars will fly away. The Earth will be long gone by then, but any other planets will leave their stars as well – the force created by dark energy will have become stronger than gravity. And it will continue growing stronger still, eventually overcoming intramolecular forces, electromagnetic interaction between atoms and even the strong force in atomic nuclei. No known force in the Universe will be able to resist its expansion – everything will turn dark and cold.

The third and final scenario I will present has been proposed before any of the other two, as it does not depend on any particular cosmological model. In the mid-19th century, a physical property was discovered that allowed scientists to explain why time flows forward, but not backward. It’s called entropy, and the main law governing it says that as time goes by, entropy will increase in any closed system. Entropy is rather difficult to visualise and there are numerous interpretations about what it can represent. For the purposes of this article, the important one is that entropy represents the amount of energy in a system that can be used to do work. The more entropy, the less such energy. As work can only be done when there are energy differences in a system, the impossibility of doing work suggests that the system is in energetic equilibrium – no part has energy greater or smaller than any other part.

Since the Universe is the most perfect closed system we know, it is conceivable that after some time the whole Universe will reach a state of maximum entropy – the planets will erode, stars will burn out and even the most massive black holes will evaporate due to Hawking radiation. In such a Universe, there would be absolutely nothing. It probably sounds very strange, but that is the model. Of course, it has certain problems – for example, there might be possibilities of contacting other Universes, some of them may even have reverse time flows. Also we are not sure whether black holes would actually evaporate. Furthermore, there is one interesting aspect to this theory. Every system, even in a state of maximum entropy, fluctuates around its equilibrium position a little. Most of these fluctuations are very tiny, but some may be large enough to spontaneously generate various particles. Again, most of these particles will be just pairs of photons, electrons or something like that. Yet sometimes, even large collections of particles will appear. Given enough time (this is already counted in quadrillions, if not more, years), one may get an airship materialising. Or a Red Army choir singer. Or a new Universe. Yes, it is theoretically possible, certain even, that a new Universe will appear sooner or later in an energy equilibrium. It might even be that our Universe is such a fluctuation in the eternal void.

Now now, don’t panic, okay? First of all, we are not yet sure of what is going to happen. Secondly, no sensible scientific theories predict any significant changes to the Universe as a whole earlier than the Sun ends its life and destroys Earth in the process. And finally, even if all this is certain – there’s nothing we can do about it, so enjoy your lives.

Here you go – this was the penultimate article in the series. The last one should come within a week or two.