As Chaucer (reportedly) said rather long ago: “All good things must come to an end.” It seems that this is indeed the case for everything we observe all around us: lives of individuals come and go; civilizations rise and fall; and on an even larger scale, solar systems are born and burn out in spectacular supernovae. But what about our universe itself? Will there be a day when we must witness not simply the end of our own world, but the annihilation of our entire cosmos? Today, we’ll be taking glimpses at the end of existence itself – welcome to Passionately Curious.
To start, let’s posit one thing first: we don’t know how the universe will ‘end’, for two main reasons:
- We’re not actually sure if there’s ever going to be an ‘end’ to the universe to speak of.
- Whether or not (and if yes, how) this end is going to occur is largely determined by all of the stuff that’s inside our universe. The problem is: we don’t actually know exactly what kind of stuff is in our universe.
Before diving into the how and the why, let’s first explore another very important concept, relevant throughout physics: the second law of thermodynamics. Now, the second law of thermodynamics is often heavily abused in online debates by people who haven’t even the slightest clue what they’re talking about. As such, I feel it’s my duty as a proper physicist to try and set things right, beginning with explaining the second law (as I’ll be calling it from here on out).
The second law is actually rather simple. It only claims that the total amount of ‘entropy’ in a closed system must always increase1. In essence, what this means is that the amount of usable energy in a closed system will tend to decrease over time, typically being converted into useless heat energy. You will also often hear it phrased as “every closed system tends to a state of disorder,” or “the amount of order in a closed system always decreases over time.”2 Entropy can thus be seen as a measure of disorder.
What does this mean for our universe, then? It’s quite simple, really – if there is a finite amount of energy in our universe, eventually the second law will cause all of this energy to convert into meaningless heat. In fact, if nothing catastrophic happens to the fabric of reality itself (I’ll get to that in a moment), we would expect a finite universe to tend to greater and greater entropy, until eventually all that remains is a mass of photons floating about an otherwise empty universe.
But let’s take a step back, and catalogue the various possibilities cosmology predicts for the end of our universe – hopefully shedding some light on point 1 above. If we go back several months to my article on the edge of the universe, we already identified some important possibilities to consider: that is, whether the universe is infinitely old and/or large. Today, I’ll only be looking at the potential end of the universe if that universe is both spatially and temporally finite, as these are by far the most interesting cases, as well as the most relevant3.
For such a finite universe, point 1 is fairly uncontested; it will have some sort of end, at least. This end is determined, then, by the issue raised in point 2. The important thing to understand here is the fact that different kinds of ‘stuff’ exert different ‘forces’ on the fabric of our universe. Most importantly, matter and radiation ‘pull’ the universe inward, whereas something we call the Cosmological Constant (what this constant is exactly I’ll have to leave be, for now4) is ‘pushing’ the universe outward. Currently, the universe is expanding, implying that right now the force of the cosmological constant wins out against the forces exerted by all the matter and radiation in our universe. However, there’s another important factor that plays a role in the way our universe behaves: the curvature of the universe itself5.
Sadly, it’s not possible for me to explore this interplay in much detail here since this would require a good few pages of explanation and even more formulae6. For now, I’ll try and stick to descriptive explanations, so please bear with me. Basically, we’re left with a few options: the Big Chill, Big Crunch, Big Bounce, and Big Rip scenarios.
If the outward push of the stuff inside the universe is great enough and the universe is not positively curved (i.e. it does not fold back in upon itself), the universe will continue to expand outward forever. Though the universe itself will continue to exist for eternity (ignoring the possibilities of laws of nature that we have not yet discovered), the stuff inside suffers a much sadder fate.
According to the second law, eventually every single atom of matter in the universe is converted into heat energy: photons. As the universe continues to expand, these photons lose little bits of energy to the void of space, as well as being diluted further and further over the ever-increasing vastness of the now-empty universe, until in the end even these lose so much energy as to become wholly insignificant. This is what is known as the Big Chill, or the heat death of the universe – though not an actual ‘end’ in the usual sense of the word, it certainly means that eventually, the universe will be a very cold, empty, and sad place to be. This is what cosmologists believe is the most likely outcome for our own universe, too, which seems to be backed up by the fact that we observe the universe expanding.
The next scenario is, in my eyes, much more spectacular. If the ‘stuff’ inside the universe is too heavy, and thus pulls inward harder than the cosmological constant can (eventually) push outward, meaning that after an initial period of expansion, the universe will start to again collapse inward, leading to a catastrophic anti-Big Bang at the end of the line: the Big Crunch, where all matter, energy and space-time is again compressed into a single point. Though it must be spectacular to witness, it would hardly be pleasant, and there’s little telling what would happen after. There’s even a possibility of the remnants of such a Big Crunch initiating a new Big Bang, potentially creating an endless cycle of universes.
The Big Bounce, on the other hand, is something quite special. It starts out with a positive speed of expansion that is exponentially decreasing, rather than increasing, but upon reaching a certain minimum expansion speed, it will ‘bounce’ back around, and from that point on continue to expand outward forever to infinity much like a Big Chill universe.
Finally, the Big Rip would be an unlikely, though not impossible, dramatic end to existence in our universe. Rather than a gently increasing expansion rate, it’s possible for the rate of expansion to increase at a great rate. Now, it’s important to understand that every point in space is always expanding all the time, albeit very slowly. Were it not for the forces keeping your body together, all of its pieces would slowly drift apart due to this expansion of the very space between them. However, what would happen if this speed became absurdly great long before the heat death of the universe? It’s possible for space to expand so fast that traditional forces can no longer keep up: every composite object in the universe would be disintegrated particle by particle, all of them shooting away from each other at breakneck speeds. This is why this scenario is called the Big Rip: everything is quite literally torn apart by the sheer power of the expansion of space itself.
And that concludes our tour through the Gallery of Potential Ends to Our Universe. We really have it all, from the most likely “everything is going to end in a dark, cold, and lonely place” to “death by squish” to “death by being torn apart” to the delightfully silly “we’re going to squish you first and then either freeze you or tear you apart atom by atom. Surprise!”
If you’re at all worried by any of these: please, don’t be. From what we can see, it seems that the universe is still expanding at a rather leisurely pace, meaning that no matter which scenario of “death by space” we’re in, it isn’t bound to happen for a long, long time. As in, hundreds-of-billions-of-years. By then, the human race is in all likelihood going to be too busy being extinct to bother with such trivial matters as the end of the universe. For now, we advise you to enjoy the finer things in life, such as understanding the universe, good food, and Netflix series – the end of the universe is still a long ways away.
1. Or, in very rare exceptions, stay the same.
2. I highly recommend you watch this amazing video by Veritasium.
3. Remember that things get very confusing once you’re talking about infinite timescales. After all, if there’s an infinite amount of time, it’s not unreasonable to assume that every single possibility has already played out an infinite number of times – though it gets even more confusing if we consider that there’s an infinite amount of possible things to happen, so we can’t possibly make any meaningful statements about whether or not all of them have happened or not. Well, I’ve succeeded in even making my own brain hurt – if you want to learn more about infinities (and dividing them by each other) go pester your local mathematician.
4. As usual, Wikipedia is going to help you out in a pinch here: Link.
5. If you want a little bit more background on how curvature of the universe would work, exactly, I’d again recommend you have a look at my article concerning the edge of the universe. In this article, I explore the geometry of the universe a bit more in-depth.
6. For those of you interested regardless, I’d recommend either Ryden’s book I’ve listed in the references (though I’d avoid the Pearson New International Edition), specifically chapters five and six, or try your luck browsing through cosmology articles on Wikipedia.
Ryden, B. (2014). Introduction to Cosmology. Edinburgh: Pearson Education Limited.