Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice. -Robert Frost
Sure, if all we’re talking about is the Earth, that one’s a no-brainer. It’s fire.
Our Sun, very slowly, is burning up its Hydrogen fuel into Helium, through the process of nuclear fusion. But over the course of billions of years, it starts to burn its remaining fuel at an ever increasing rate. Over the next billion or two years, the Sun’s output will slowly increase, forcing the Earth’s temperature to creep upwards, ever so slowly.
The temperature will rise until it’s hot enough that — believe it or not — the oceans boil. Not from lava vents, as shown above, but from the Sun heating the oceans up past their boiling point!
And if that weren’t dastardly enough, the Sun won’t be done with us yet.
Although it will continue to burn hotter and hotter for billions of years into the future, at some point, five to seven billion years from now, the Sun will run out of Hydrogen to burn, at which point it will start burning Helium, and will become a red giant star. At this point, the Sun will expand so greatly that it will likely engulf the already charred Earth.
But that is but one world in our vast Universe. What if we look far into the future, and ask about the fate of it all? In other words, how will the Universe end?
Let’s start by showing you what the Universe looks like today.
Start by looking up. Each point of light you see in the night sky is — with the exception of the occasional planet — its own star. Some of these are actually multiple stars together; many of them (possibly most of them) have their own planets orbiting them. And, as is just faintly visible in the image above, you can also see this dim streak of light across the sky, known as the Milky Way.
The thousands of stars you can see with your naked eye are only a tiny fraction of the hundreds of billions of stars making up our galaxy, which stretches across the sky for tens of thousands of light years.
But even our galaxy, as massive and expansive as it is, is not all there is to this Universe. In fact, it isn’t even close. If we look farther out, we find that galaxies are absolutely everywhere, and in many cases, we can find hundreds (or more) of them together in one place, forming gigantic galaxy clusters, like the Coma Cluster, shown below.
While the Universe we live in happens to be expanding, things like our Solar System, our Galaxy, and Galaxy Clusters (like Coma, above) are not. Sure, the Coma Cluster is expanding away from us, but the cluster itself is what we call gravitationally bound.
In short, if gravity is strong enough to keep you bound together, it doesn’t matter what the expanding Universe is doing: it isn’t powerful enough to tear these bound things apart.
So, you ask, what will stay bound together, and what will get expanded away.
This is a supercluster of galaxies! Nearly 10 million light years across and containing thousands of galaxies, the Perseus Supercluster teeters on the brink of what it means to be gravitationally bound. (It is, but just barely.)
If we look to even larger and larger scales, though, know what we find?
The largest scales, where you can see filaments connecting the different-sized clusters of galaxies to one another, are not bound to one another!
In fact, we know that — thanks to dark energy — the farther away an object is from us, the faster it will continue to expand away from us!
But this is assuming one very important fact: dark energy is a constant.
Now, this is in excellent agreement with observations. We have measured that whatever dark energy is, it is very, very consistent with a cosmological constant, and if we want it to be anything different (such as to either increase or decrease in strength over time), there are tight constraints on how quickly it could possibly be changing.
But, to be certain, the only way is to wait for the future to unfold and see what happens. Other things are possible, even though our best measurement shows us the middle case, above, where the things currently bound together stay bound, and the things expanding away from us now will eventually disappear.
What does this mean for us, and the fate of the Universe as we know it?
This, above, is a pretty accurate representation of our Local Group. This is our little, nearby cluster of galaxies: the stuff gravitationally bound to us. Over the next few billions of years, all of these galaxies will eventually merge together, forming a giant elliptical galaxy, while the other galaxies in the Universe will continue to expand away from us at an ever increasing rate.
But, what if dark energy isn’t a constant like we presently think it is? What are the other two options?
The reversal strategy — leading to a recollapse of the Universe — is by far the biggest stretch. Not only would dark energy need to stop doing what it’s currently doing, it would need to start doing the opposite. If this were the case, we’d see galaxies accelerate away from us, and then suddenly begin decelerating, stop, and reverse direction back towards us. Not only is this observationally troubling, the only theoretical ways to make it happen are completely contrived to achieve this “Big Crunch” fate as an end result. (Are you reading this, Paul?)
But there is another possibility: that dark energy will continue to get stronger. This is a fun one to think about. (I don’t think it’s particularly likely, but it is one of the most interesting possibilities out there.) Let’s take a look.
A cosmological constant means that the expansion rate is constant. In other words, for every Megaparsec (about 3 million light years) distant an object is, it expands away from us at about 60 km/sec. Wait until it’s 30 million light years away, and it appears to move away at 600 km/s. Wait until it’s 3 billion light years away, and it recedes at 60,000 km/s. (You get the picture.)
But if dark energy continues to get stronger over time, that means the expansion rate itself will continue to rise and rise, unchecked, until gravity itself is challenged.
The super-galaxy that the local group will become? As the expansion rate continues to rise and rise, the individual stars in this galaxy will start to get caught up in the ever-increasing expansion.
At first, the outermost stars — the ones least tightly bound to the galaxy — will be the only ones expelled. But eventually, even the innermost stars will get kicked out, leading to a Universe of scattered, expanding star systems formed from the remnants of our once-grand galaxy.
But it gets worse. The expansion rate doesn’t stop accelerating just because we destroyed the galaxy. Each star system, with its planets, moons, comets, asteroids, gas, dust, and possibly multiple stars, gets ripped apart too! First the outermost objects get flung out of each Solar System, but eventually even the innermost planets are cast off into interstellar space, away from their host stars and away from one another.
And at this point, you may think, it couldn’t possibly get any worse.
Are you ever in for a rude surprise.
The expansion rate will eventually become so large that everything, from stars to planets to individual atoms, will be torn apart. In the last fraction-of-a-second of the Universe, even subatomic particles will be ripped out of the atoms themselves, as the Universe ends in an unprecedented catastrophe known as The Big Rip.
Current observations disfavor both the Big Rip and Big Crunch scenarios, and find that — to the best accuracies we can measure — dark energy really is a cosmological constant.
But now, when someone asks you about the Fate of the Universe, oh, what a story you’ll have to tell!