The End of Everything (Astrophysically Speaking)

by Katie Mack

We are a species poised between an awareness of our ultimate insignificance and an ability to reach far beyond our mundane lives, into the void, to solve the most fundamental mysteries of the cosmos.

Acknowledging an ultimate end gives us context, meaning, even hope, and allows us, paradoxically, to step back from our petty day-to-day concerns and simultaneously live more fully in the moment.

I have not yet found a serious suggestion in the current cosmological literature that the universe could persist, unchanged, forever. At the very least, there will be a transition that for all intents and purposes destroys everything, rendering at least the observable parts of the cosmos uninhabitable to any organized structure.

“High redshift” is long ago when the universe was young; “low redshift” is more recent. Redshift 0 is the local, present-day universe; redshift 1 is seven billion years ago. At the high end, redshift 6 is a universe only a billion or so years into its life, and the very beginning of the universe, if we could see it, would have a redshift of infinity.

Even with the most pessimistic estimates, any Big Crunch event can only occur many billions of years in the future—our universe has been around for 13.8 billion years and with respect to the possibility of future collapse, it is definitely no more than middle-aged.

As galaxies get closer together and merge more frequently, galaxies across the sky will burst with the blue light of new stars, and giant jets of particles and radiation will rip through the intergalactic gas. New planets might be born along with those new stars, and perhaps some will have time to develop life, though the terrifying frequency of supernovae in this chaotic, collapsing universe might irradiate the new planets clean. The violence of the gravitational interactions between galaxies and between central supermassive black holes will increase, flinging stars out of their own galaxies to end up caught in the gravity of others.

A Big Crunch–fated universe is called a “closed” universe, because two parallel laser-cannon beams would eventually bend toward each other—it’s the same kind of thing that happens to lines of longitude on a globe. What’s happening in the cosmic case is that there’s so much matter in a closed universe that all of space is curved inward. A perfectly balanced universe is “flat” because the beams would just stay parallel forever, in much the same way two parallel lines would stay parallel on a flat sheet of paper. A universe with way more expansion than gravity is called an “open” universe, and in that case, as you may have already guessed, the two laser beams would diverge from each other over time.

The purpose of the cosmological constant was to save the universe from catastrophic collapse. Or more accurately, from having catastrophically collapsed already. Being an expert, as he was, in all things gravitational, Einstein knew that all the data available pointed to the uncomfortable conclusion that gravity should have destroyed the universe long ago. This was 1917, half a century before widespread acceptance of the Big Bang theory, when the cosmos was still largely thought to be static and unchanging. Stars could live and die, matter might slightly rearrange, but space was space—it was just a background on which other things happened. So when Einstein saw that there were stars in the night sky, apparently stationary, he knew the universe was in trouble. Every one of those stars, he figured, should be gravitationally attracting every other star, and slowly drawing together over time. It doesn’t help for the other stars to be really distant, either; gravity is an infinite and purely attractive force.

A cosmological-constant-induced apocalypse is a slow and agonizing one, marked by increasing isolation, inexorable decay, and an eons-long fade into darkness. In some sense, it doesn’t end the universe exactly, but rather ends everything in it, and renders it null and void.

Knowing that the universe is about 13.8 billion years old, logic would tell you that the particle horizon must be a sphere of radius 13.8 billion light-years. But that’s assuming a static universe. In actual fact, since the universe has been expanding all that time, something just close enough to send its light to us 13.8 billion years ago is now much farther away—approximately 45 billion light-years. So we can define the observable universe to be a sphere of about 45 billion light-years in radius, centered on us.

The distance at which galaxies are currently moving away from us faster than light is surprisingly close, given how far we can actually see. We call it the Hubble radius, and it’s around 14 billion light-years from here.

And for many billions of light-years, the more distant the galaxy is, the smaller it looks. As you would expect. But somewhere in the vicinity of the Hubble radius, that relationship reverses. Beyond that distance, the farther away something is, the larger it appears! This is super convenient for us astronomers, of course, as it allows us to see structure and details in galaxies that are extremely distant from us, and that in a sensible universe would look like infinitesimal points. But if we think about it too much, it still seems like an utterly unreasonable way for geometry to work.

The reason for this reversal is related to the reason we can see things that are currently moving away from us faster than light. In the past, when the light was emitted, they were closer.

Distant galaxies being dragged out of the Hubble radius by cosmic expansion will become lost to us. Galaxies whose distant past we can see now will slowly fade into darkness like ancient decaying photographs. In our own cosmic neighborhood, after the Milky Way and Andromeda merge, our little Local Group of galaxies will become more and more isolated, surrounded by darkness and the dying primordial light. All across the cosmos, invisible to us, other groups and clusters of galaxies will merge to form giant elliptical clumps of stars, burning brightly in the initial violence of the collisions but fading eventually to embers, whose glow will never reach beyond their own pool of expanding, emptying space.

When the stars have all faded to darkness, the ultimate decay sets in. Black holes begin to evaporate.

While we’ve never seen a proton decay experimentally, we have reason to believe that can happen too, if you’re willing to wait something like 1033 years.

higher entropy is also linked to higher temperature. This makes sense if you think of the difference between a block of ice and a cloud of steam. In order to be ice, the water molecules have to be arranged in a crystal structure, whereas the particles in steam are free to move around in three dimensions. But even just cooling the steam a bit reduces its entropy because the particles are moving less: they’re more constrained, or less disordered.

Every attempt to bend some part of the world to our will creates disorder somewhere else, often in the form of heat.

Could hiding entropy behind black hole event horizons be the perfect crime? Whatever other part of physics you have to break, don’t bet against the Second Law of Thermodynamics. The solution to the entropy problem of black holes turned out to change everything we thought we knew about black holes and absolutely nothing about entropy. You can’t hide entropy in black holes, because they have entropy of their own. Which means they have a temperature (they create heat). Which means they are not black at all.

Fortunately, if we need weirdness in physics, we can always rely on the quantum realm to serve us up something good. In this case, Hawking made use of the quantum weirdness of virtual particles—pairs of positive- and negative-energy particles popping into and out of existence from the vacuum of space itself.XI The idea is that this spacetime popcorn is happening all the time, everywhere, but usually it has no effect on anything because the two particles will appear and immediately annihilate against one another, both going back to being nothing again. But, Hawking said, near a black hole, you could have a situation where the negative-energy virtual particle falls past the horizon, leaving the positive-energy virtual particle so bereft that it becomes real and wanders away. The mass of the black hole would reduce a little as it absorbs that bit of negative energy, and the same amount of positive energy would appear to radiate off the black hole’s horizon. Because these virtual particles are always popping up everywhere in space, any black hole that’s not actively pulling in matter from its environment should be gradually bleeding off mass through this evaporation process all the time.

The way it works is this: when the universe reaches a state of steady exponential expansion, you can define a radius (from wherever you are) beyond which the rest of the cosmos is forever hidden. It’s a true horizon in the sense that nothing beyond it could ever reach you. It turns out that this horizon, like a black hole’s horizon, also has an entropy associated with it, and thus a temperature. The difference is that instead of the heat going out like it does with a black hole, it goes in. The temperature is very small—something like 10-40 degrees above absolute zero—but when everything else has decayed, this radiation is all that’s left to contain all the entropy of the universe. When the universe gets to this pure de Sitter state, it is a maximum entropy universe. From that point on, there is no way for the universe’s total entropy to increase, which means, in a very real sense, the arrow of time is… gone.

That point about how it’s an inescapable law of the universe that the entropy always increases? That technically only applies on average over sufficiently large scales. On the quantum scale, or even on large scales if you wait long enough, unpredictable fluctuations will, from time to time, spontaneously shift some part of the system into a lower-entropy state at random. The larger the system, the less likely it is that fluctuations could do much of anything at all, but in a universe that is in an eternal expansion and contains only a cosmological constant, there’s a lot of time and space for waiting around, and even extremely low-probability events happen. It’s unlikely that a whale and a bowl of petunias could suddenly pop into existence in completely empty space, but, in principle, if you wait long enough, it could happen.

This is called Poincaré recurrence. If you have an infinite amount of time to work with, any state the system can be in is a state it WILL be in again, an infinite number of times, with a recurrence time determined by how rare or special that configuration is. In one rather arresting example, physicists Anthony Aguirre, Sean Carroll, and Matthew Johnson once calculated that if you were willing to wait something like a trillion trillion times the age of the universe, you could watch an entire piano spontaneously assemble itself in a seemingly empty box.

A post–Heat Death universe is, essentially, a very large, very slightly warmed box, with statistical mechanics stepping in to provide the random fluctuations. If the Big Bang is a state the universe has been in once, and the post–Heat Death universe is eternal (so eternal that, having lost the arrow of time, past and future are meaningless), there’s no reason a Big Bang can’t fluctuate out of the vacuum to start the universe anew.

This possibility is of particular interest to cosmologist Andreas Albrecht, who has written about what he calls the de Sitter Equilibrium state. The basic idea of this equilibrium version of de Sitter space is that the origin of our universe and everything that happens in it can be thought of as the result of random fluctuations out of an eternally expanding universe containing only a cosmological constant. From time to time, a universe fluctuates out of the heat bath into a very low entropy starting state, and then evolves forward (with increasing entropy) until it gets to its own Heat Death, decaying back into the background de Sitter universe. And from time to time, the fluctuation doesn’t produce a Big Bang, it just re-creates last Tuesday—specifically, that moment when you stubbed your toe on the kitchen table and spilled an entire cup of coffee on the floor. That moment. And every other moment of your life. And everyone else’s.

The Boltzmann Brain problem is the assertion that this unfortunate brain, doomed to quantum-fluctuate back into the vacuum almost instantaneously after its creation, is so vastly more likely to occur than a whole universe that, if we want to use random fluctuations to build our universe, we have to accept that we’re much more likely to be just imagining the whole thing.

Nonetheless, we probably have a good 101000 years or so before we and all other thinking structures fade from the possibility of memory.

The problem is that the difference between a Heat Death–fated universe and one headed for a Big Rip might literally be unmeasurable. If dark energy is a cosmological constant, the equation of state parameter w equals -1 exactly, and we get a Heat Death. If w is at all lower than -1, even one part in a billion billions, dark energy is phantom dark energy, capable of tearing the universe apart.

This discovery was revolutionary, and perhaps one of the most important in the history of astronomy, in that it let us finally measure the scale of the universe around us. It meant that anywhere a Cepheid could be seen, we could get a reliable distance and start to make a usable map. By measuring how quickly a Cepheid pulsed, and how bright it looked from here, Leavitt could tell you with great precision how bright it really was, and thus how distant.

That limit, about 1.4 times the mass of the Sun, became known, appropriately, as the Chandrasekhar Limit. Any white dwarf that gains enough mass to exceed that limit is immediately doomed to explode spectacularly as a supernova. And now that we know that the physics of the explosion is always the same, we know how bright a Type Ia supernova is intrinsically, and can therefore figure out its distance.

For the last several years, measurements of the Hubble Constant from supernovae have been giving us a number around 74 km/s/Mpc—that means that a galaxy one megaparsec away (that’s around 3.2 million light-years) is receding from us at around 74 km/s. One twice as far away is moving, relative to us, about twice as fast. But we can also measure the Hubble Constant indirectly, by carefully studying the geometry of the hot and cold spots in the cosmic microwave background. When we measure it that way, the number we get is closer to 67 km/s/Mpc. Even though these observations are looking at very different epochs of cosmic history, each of them can tell us the expansion rate today. In a universe made of what we think it’s made of, both methods of determining the Hubble Constant really ought to give us the same number. And they don’t.

massless particles can’t travel at anything but light speed.

The bad news is that this consistent picture of the Standard Model also tells us that our Higgs vacuum—the perfectly balanced set of laws that govern the physical world—is not stable.

Wherever the Higgs field is now in its potential, it’s given us a perfectly livable, comfortable universe. We have constants of nature that are nicely compatible with bound particles and solid, life-compatible structures. If another state is possible, lower down the potential, all that is at risk.

Because the true vacuum is the more stable state, the universe “prefers” it, and will revert to it if given the slightest chance, just like a pebble will roll down a slope if it’s placed on one. As soon as the bubble appears, the Higgs field all around it is suddenly being shaken down to the valley floor. It’s as though that first event knocks free every precariously balanced pebble near it, and then the avalanche spreads. More and more space succumbs to the true vacuum state. Anything unfortunate enough to be in the bubble’s path is first hit by the intensely energetic bubble wall, approaching at about the speed of light. Then it undergoes a process that could only be called total and complete dissociation, as the forces that previously held particles together in atoms and nuclei can no longer function.

The true vacuum cancels the universe entirely.

If our vacuum really is metastable, strictly speaking, the bubble has to show up eventually.)

If we conclude that our vacuum really is metastable, this may be incompatible with the theory of cosmic inflation. The quantum fluctuations during inflation, or the ambient heat afterward, seem like they should have been sufficient to trigger vacuum decay in the first moments of the cosmos, negating our very existence. Clearly, that didn’t happen. Which suggests either we don’t understand the early universe, or vacuum decay was never possible at all.

We have good reasons for being suspicious. Compared to the other forces, gravity is an oddball. Not only does it look totally different from a mathematical point of view, it’s way too weak. Sure, when you get together enough mass for a galaxy, or a black hole, it seems fairly strong. But in daily life, it’s easily the weakest force you encounter. Every time you lift a coffee cup you’re overcoming the gravitational pull of the entire planet. It takes putting the mass of the Sun into something the size of a city before gravity can even begin to compete with the atomic and nuclear forces holding atoms together.

For a while now, some physicists have suspected that the incongruous weakness of gravity might be forcing them to a similar conclusion. Maybe there’s nothing wrong with the strength of gravity. Maybe there’s something wrong with the universe that’s making gravity seem weaker than it really is. What could make gravity seem weak? The solution might end up being surprisingly mundane. It’s leaking. Into another dimension.

We can calculate the amount of entropy in our observable universe, and we can look back through cosmic history to determine what it must have been at early times if it has been steadily increasing over the lifetime of the cosmos. The result is that the universe must have started at a shockingly low-entropy—highly ordered—state when our own cosmic history began. This is a deeply uncomfortable idea for a lot of cosmologists. How did the entropy get set so low at the beginning? It’s as if you walk into a room you’re sure no one has ever been in before and you find rows and rows of dominoes lying on the floor, overlapping as if they’ve just toppled upon each other in sequence. How did they all get so carefully set up in the first place?

Penrose’s model also contains the intriguing possibility that some imprint of the events that occurred in past cycles might appear in astronomical observations, showing up as features in the cosmic microwave background. In fact, Penrose and his collaborators have claimed that evidence for such features can already be seen in the data, though this has been met with skepticism. Whether or not these possible CMB hints will someday be seen as a compelling sign of a pre–Big Bang universe is yet to be determined.

Someday, deep in the unknown wilderness of the distant future, the Sun will expand, the Earth will die, and the cosmos itself will come to an end. In the meantime, we have the entire universe to explore, pushing our creativity to its limits to find new ways of knowing our cosmic home. We can learn and create extraordinary things, and we can share them with each other. And as long as we are thinking creatures, we will never stop asking: “What comes next?”

Whatever legacy-based rationalization we use to make peace with our own personal deaths (perhaps we leave behind children, or great works, or somehow make the world a better place), none of that can survive the ultimate destruction of all things. At some point, in a cosmic sense, it will not have mattered that we ever lived. The universe will, more likely than not, fade into a cold, dark, empty cosmos, and all that we’ve done will be utterly forgotten. Where does that leave us now?