The End of Everything: (Astrophysically Speaking)

by Katie Mack

  These days, I’m pretty solidly a theorist, which is probably better for everyone. This means I don’t carry out observations or experiments or analyze data, though I do frequently make predictions for what future observations or experiments might see. I work mainly in an area physicists call phenomenology—the space between the development of new theories and the part where they’re actually tested. That is to say, I find creative new ways to connect the things the fundamental-theory people hypothesize about the structure of the universe with what the observational astronomers and experimental physicists hope to see in their data. It means I have to learn a lot about everything,VII and it’s a heck of a lot of fun.

  SPOILER ALERT

  This book is an excuse for me to dig deep into the question of where it’s all going, what that all means, and what we can learn about the universe we live in by asking these questions. There isn’t just one accepted answer to any of this—the question of the fate of all existence is still an open one, and an area of active research in which the conclusions we draw can change drastically in response to very small tweaks in our interpretations of the data. In this book, we’ll explore five possibilities, chosen based on their prominence in ongoing discussions among professional cosmologists, and dig into the best current evidence for or against each of them.

  Each scenario presents a very different style of apocalypse, with a different physical process governing it, but they all agree on one thing: there will be an end. In all my readings, 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. For this purpose, I will call that an ending (with apologies to any temporarily sentient bursts of random quantum fluctuationVIII that may be reading this). A few of the scenarios carry with them a hint of possibility that the cosmos might renew itself, or even repeat, in one way or another, but whether some tenuous memory of previous iterations can persist in any way is a matter of rather intense ongoing debate, as is whether or not anything like an escape from a cosmic apocalypse could in principle be possible. What seems most likely is that the end for our little island of existence known as the observable universe is, truly, the end. I’m here to tell you, among other things, how that might happen.

  Just to get everyone on the same page, we’ll start with a quick catch-up on the universe from the beginning until now. Then we’ll get on with the destruction. In each of five chapters, we’ll explore a different possibility for the end, how it might come about, what it would look like, and how our changing knowledge of the physics of reality leads us from one hypothesis to another. We’ll start with the Big Crunch, the spectacular collapse of the universe that would occur if our current cosmic expansion were to reverse course. Then come two chapters of dark-energy-driven apocalypses, one in which the universe expands forever, slowly emptying and darkening, and one in which the universe literally rips itself apart. Next is vacuum decay, the spontaneous production of a quantum bubble of deathIX that devours the cosmos. Finally, we’ll venture into the speculative territory of cyclic cosmology, including theories with extra dimensions of space, in which our cosmos might be obliterated by a collision with a parallel universe… over and over again. The closing chapter will bring it all together with an update from several experts currently working on the cutting edge on which scenario looks most plausible now, and what we can expect to learn from new telescopes and experiments to settle the question once and for all.

  What that means for us as human beings, living our little lives in all this inconsiderate vastness, is another question entirely. We’ll present a range of perspectives in the epilogue, and address whether or not sentience itself could have any kind of legacy that endures beyond our destruction.X

  We don’t know yet whether the universe will end in fire, ice, or something altogether more outlandish. What we do know is that it’s an immense, beautiful, truly awesome place, and it’s well worth our time to go out of our way to explore it. While we still can.

  I. Exactly how those rewards are doled out, and to whom, is not the part they have in common.

  II. This view is also espoused, though not explored in philosophical detail, in the classic early-2000s TV series Battlestar Galactica.

  III. apocalypsi?

  IV. We do this by bouncing it. Really. First we cool the neutrons to almost absolute zero, then we slow them to jogging speed, then we bounce them up and down like a Ping-Pong ball on a paddle. And this also tells us something about dark energy, the mysterious something that makes our whole universe expand faster. Physics is wild.

  V. String theorists produce a lot of these theories. (String theory is a blanket term for theories that try to bring together gravity and particle physics in new ways, but most of the work done to develop it now relies on mathematical analogs rather than anything pertaining to the “real” world.) Sometimes when I’m in string theory talks, I have to resist the urge to raise my hand and clarify that none of these calculations pertain to our universe, just in case anyone in the room is as confused as I first was when I started attending string theory talks.

  VI. This is, of course, one of the most fun things I’ve ever worked on, hence this book. I’m not sure why I like it so much. It may be a bad sign.

  VII. And we’re talking about the universe here, so I really do mean EVERYTHING.

  VIII. Please stick around until Chapter 4, when the Boltzmann Brain community will get their proper due.

  IX. Technically it is called a “bubble of true vacuum,” which, to be fair, also sounds pretty darn ominous.

  X. Another spoiler: it’s not looking great.

  CHAPTER 2: Big Bang to Now

  Beginnings imply and require endings.

    Ann Leckie, Ancillary Justice

  I love stories about time travel. It’s easy to quibble about the physics of time machines or to balk at the various paradoxes that come up. But there’s something appealing about the idea that we might somehow find a trick that will open up the past and future to our knowledge and intervention, to allow us to step off this runaway train of “now” barreling inexorably toward some unknown fate. Linear time just seems so restrictive, even wasteful—why should all that time, all those possibilities, be lost to us forever, just because the clock has ticked forward a few degrees? We may have grown accustomed to strict chronological oppression, but that doesn’t mean we have to like it.

  Fortunately, cosmology can help. Not in any practical sense, of course—we’re still talking about a relatively esoteric branch of physics that will in no way enable you to get back the umbrella you left on the train yesterday. But rather, in the sense that your life remains the same but absolutely everything else about existence is forever changed.

  To a cosmologist, the past is not some unreachable lost realm. It’s an actual place, an observable region of the cosmos, and it’s where we spend most of our workday. We can, while sitting quietly at our desks, watch the progress of astronomical events that happened millions or even billions of years ago. And the trick isn’t special to cosmology, but inherent to the structure of the universe in which we live.

  It all comes down to the fact that light takes time to travel. Light speed is fast—about 300 million meters per second—but it’s not instantaneous. In everyday terms, when you switch on a flashlight, the light coming out of it covers about one foot per nanosecond, and the reflection of that light off whatever you’re illuminating takes just as long to get back to you. In fact, when you look at anything, the image you see, which is just the light coming off it that reaches your eye, is a little bit stale by the time it gets to you. That person sitting across the café from you is, from your perspective, several nanoseconds in the past, which may go part of the way toward explaining their wistful expression and outdated fashion sense. Everything you see is in the pa
st, as far as you’re concerned. If you look up at the Moon, you’re seeing a little over a second ago. The Sun is more than eight minutes in the past. And the stars you see in the night sky are deep in the past, from just a few years to millennia.

  The concept of this kind of light speed delay might already be familiar to you, but its implications are profound. It means that as astronomers, we can look into the sky and watch the evolution of the universe happen, from its early beginnings to the present day. We use the unit “light-year” in astronomy not just because it’s conveniently huge (about 9.5 trillion kilometers, or 5.9 trillion miles), but also because it tells us how long light has been traveling from the thing we’re looking at. A star 10 light-years away is 10 years in the past, from our perspective. A galaxy 10 billion light-years away is 10 billion years in the past. Since the universe is only about 13.8 billion years old, that 10-billion-light-year-distant galaxy can tell us about the conditions of our universe when it was still in its youth. In that sense, looking out into the cosmos is tantamount to looking into our own past.

  There’s an important caveat to this, and I would be remiss if I didn’t mention it. We technically can’t see our own past at all. The light speed delay means that the more distant a thing is, the farther in the past it is, and that relationship is strict: not only can we not see our own past, we can’t see those distant galaxies in the present, either. The more distant something is, the farther away it is on a timeline of the cosmos.

  Figure 1: Light travel times. We sometimes express distances in light-seconds, light-minutes, and light-years because it makes it clear how long the light has been traveling to us, and thus how far into the past we’re looking. (None of the illustrations here are to scale!)

  So how do we learn anything useful about our own past if we’re only seeing the past for some other galaxy, long ago and far away? It comes down to a principle so central to cosmology that it is literally called the cosmological principle. Simply stated, it’s the idea that for all practical purposes, the universe is basically the same everywhere. Obviously this isn’t true on human scales—the surface of the Earth is pretty importantly different from deep space or the center of the Sun—but on the kind of astronomically large scales on which whole galaxies can be tallied up as individually uninteresting specks, the universe looks the same in every direction, and is made of all the same stuff.I This idea is closely related to the Copernican Principle, which is the erstwhile heretical notion stated by Nicolaus Copernicus in the sixteenth century that we do not occupy a “special place” in the cosmos, but are just at some generic spot that may as well have been chosen at random. So when we look at a galaxy a billion light-years away, and see it as it was a billion years ago, in a universe that was a billion years younger than our universe is here and now, we can be pretty confident that the conditions here a billion years ago would have been fairly similar. This can actually be observationally tested, to a degree. Studies of the distribution of galaxies throughout the cosmos find that the uniformity implied by the cosmological principle holds up everywhere we’ve looked.

  The upshot of all this is that if we want to learn about the evolution of the universe itself, and the conditions our own Milky Way galaxy grew up in, all we have to do is look at something far away.

  It also means that cosmology doesn’t really have a well-defined concept of “now.” Or rather, the “now” you experience is highly specific to you, to where you are and to what you are doing.II What does it mean to say “that supernova is going off now” if we see the light of it now, and we can watch the star explode now, but that light has been traveling for millions of years? The thing we’re watching is essentially fully in the past, but the “now” for that exploded star is unobservable to us, and we won’t receive any knowledge of it for millions of years, which makes it, to us, not “now,” but the future.

  When we think of the universe as existing in spacetime—a kind of all-encompassing universal grid in which space is three axes and time is a fourth—we can just think of the past and the future as distant points on the same fabric, stretching across the cosmos from its infancy to its end. To someone sitting at a different point on this fabric, an event that is part of the future to us might be the distant past to them. And the light (or any information) from an event that we won’t see for millennia is already streaming across spacetime toward us “now.” Is that event in the future, or the past, or, perhaps, both? It all depends on perspective.

  As mind-bending as it is to contemplate if you’re used to thinking in a 3D world,III to astronomers, the noninfinite speed of light is a fantastically useful tool. It means that instead of looking for mere clues to the distant past of the cosmos—its traces and remnants—we can just look at it directly and watch it change over time. We can peer at the universe at an age of just three billion, during the renaissance of star formation, when galaxies were bursting with light (if not art and philosophy), and we can see how that shine has dimmed in the intervening eons. We can look even farther back, and see matter swirling into supermassive black holes in a universe less than 500 million years old, when starlight had only just begun to penetrate the darkness between galaxies.

  Soon, with new space telescopes, we will be able to observe some of the first galaxies to form in the cosmos—those that formed when the universe was only a few hundred million years old. But if those galaxies were the first, what happens if we look farther back than that? Can we look so far away that there are no galaxies yet? We have plans to do so. Radio telescopes being built now may be able to see the material the first galaxies were born from, by exploiting a fortuitous interaction between light and hydrogen. By looking directly at the hydrogen, the matter that will one day become stars and galaxies, we can watch the very first structures in the universe form.

  Figure 2: Diagram of light moving through spacetime. In this diagram, time moves forward in the upward direction, and we’re showing only two dimensions of space instead of all three. The positions of four objects that are stationary in space are represented by the vertical dotted lines, marking the same location at different times. The “light cone” is the region we can see in the past from the observatory—it encompasses everything close enough to us that the light has had time to reach us since it was emitted. We can see a galaxy a billion light-years away as it was a billion years ago, but we can’t see what it looks like “now,” because the “now” version of that galaxy is outside our light cone.

  But what if we look back even farther? What if we look back to the time before stars, before galaxies, before hydrogen? Can we see the Big Bang itself?

  Yes. We can.

  SEEING THE BIG BANG

  There’s a popular picture of the Big Bang as some kind of explosion—a sudden conflagration of light and matter from a single point that billowed out through the universe. It wasn’t like that. The Big Bang wasn’t an explosion within the universe, it was an expansion of the universe. And it didn’t happen at a single point, but at every point. Every point in space in the universe today—a spot on the edge of a distant galaxy, a piece of intergalactic space just as far in the other direction, the room in which you were born—every one of these points was, at the beginning of time, close enough to touch, and at that same first moment, rapidly tearing away from one another.

  The logic of the Big Bang theory is pretty simple. The universe is expanding—we can see that distances between galaxies are getting larger over time—which means that the distances between galaxies were smaller in the past. We can, as a thought experiment, rewind the expansion we see now, extrapolating back across billions of years, until we reach a moment when the distance between galaxies must have been zero. The observable universe, encompassing everything we can see today, must have been contained within a much smaller, denser, hotter space. But the observable universe is just the part of the cosmos we can see now. We know that space goes on much farther than that. In fact, based on what we know, it’s entirely possible, and perhaps probable, that the
universe is infinite in size. Which means that it was infinite at the beginning too. Just much denser.

  This is not easy to picture. Infinities are tough that way. What does it mean to have infinite space? What does it mean for an infinite space to be expanding? How does infinite space get infiniter?

  I’m afraid I can’t help you with this.

  There is simply no easy way to hold infinite space in a finite brain. What I can say is that there are ways to deal with infinities in mathematics and physics that make sense and don’t break anything. As a cosmologist, I work from the basic assumption that the universe can be described with math, and if that math works out, and is useful for approaching new problems, I go with it.IV Or, more precisely, if the math works out and a somewhat different assumption (e.g., that the universe is not quite infinite but is so big that we can’t possibly ever perceive its edges) also works but makes no difference to our experience or anything we can measure in any way, we may as well stick with the simpler assumption for now. So: infinite universe. We can work with that.