^{Excerpted from THE END OF EVERYTHING (Astrophysically Speaking) by Katie Mack, published by Scribner, a Division of Simon & Schuster, Inc. Printed by permission.}

The question of how the world will end—fire or ice—has been the subject of speculation and debate among poets and philosophers throughout history. Of course, now, thanks to science, we know the answer: it’s fire. Definitely fire. In about five billion years, the Sun will swell to its red giant phase, engulf the orbit of Mercury and perhaps Venus, and leave the Earth a charred, lifeless, magma-covered rock. Even this sterile, smoldering remnant is likely fated to spiral eventually into the Sun’s outer layers and disperse its atoms in the churning atmosphere of the dying star.

But I’m a cosmologist. I study the universe, as a whole, on the largest scales. From that perspective, the world is a small, sentimental speck of dust lost in a vast and varied universe. What matters to me is a bigger question: how will the universe end?

Over the millennia since humanity’s first ponderings of its mortality, the philosophical implications of the question haven’t changed, but the tools we have to answer it have. Today, the question of the future and ultimate fate of all reality is a solidly scientific one, with the answer tantalizingly within reach. It hasn’t always been so. Debates once raged in astronomy about whether the universe might be in a steady state, existing unchanging forever. It was an appealing idea, that our cosmic home might be a stable, hospitable one: a safe place in which to grow old. The discovery of the Big Bang and the expansion of the universe, however, ruled that out. Our universe is changing, and we’ve begun to develop the theories and observations to understand exactly how. The developments of the last few years, and even months, are finally allowing us to paint a picture of the far future of the cosmos.

The best measurements we have are only consistent with a handful of final apocalyptic scenarios, some of which may be confirmed or ruled out by observations we’re making right now. Exploring these possibilities gives us a glimpse of the workings of science at the cutting edge, and allows us to see humanity in a new context. 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.

Many physicists get a little blasé about the vastness of the cosmos and forces too powerful to comprehend. You can reduce it all to mathematics, tweak some equations, and get on with your day. But the shock and vertigo of the recognition of the fragility of everything, and my own powerlessness in it, has left its mark on me. There’s something about taking the opportunity to wade into that cos-mic perspective that is both terrifying and hopeful, like holding a newborn infant and feeling the delicate balance of the tenuousness of life and the potential for not-yet-imagined greatness. It is said that astronauts returning from space carry with them a changed perspective on the world, the “overview effect,” in which, having seen the Earth from above, they can fully perceive how fragile our little oasis is and how unified we ought to be as a species.

For me, thinking about the ultimate destruction of the universe is just such an experience. There’s an intellectual luxury in being able to ponder the farthest reaches of deep time, and in having the tools to speak about it coherent-ly. When we ask the question, “can this all really go on forever?”, we are implicitly validating our own existence, extending it indefinitely into the future, taking stock, and examining our legacy. 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. Maybe this can be the meaning we seek.

We’re definitely getting closer to an answer. Scientifically, we are living in a golden age. In physics, recent discoveries and new technological and theoretical tools are allowing us to make leaps that were previously impossible. We’ve been refining our understanding of the begin-ning of the universe for decades, but the scientific exploration of how the universe might end is just now undergoing its renaissance. Hot-off-the-press results from powerful telescopes and particle colliders have suggested exciting (if terrifying) new possibilities and changed our perspective on what is likely, or not, in the far future evolution of the cosmos. This is a field in which incredible progress is being made, giving us the opportunity to stand at the very edge of the abyss and peer into the ultimate darkness. Except, you know, quantifiably.

As a discipline within physics, the study of cosmology isn’t really about finding meaning per se, but it is about uncovering fundamental truths. By precisely measuring the shape of the universe, the distribution of matter and energy within it, and the forces that govern its evolution, we find hints about the deeper structure of reality. We might tend to associate leaps forward in physics with experiments in laboratories, but much of what we know about the fundamental laws governing the natural world comes not from the experiments themselves, but from understanding their relationship to observations of the heavens. Determining the structure of the atom, for example, required physicists to connect the results of radioactivity experiments with the patterns of spectral lines in the light from the Sun. The law of universal gravitation, developed by Newton, posited that the same force that makes a block slide down an inclined plane keeps the Moon and planets in their orbits. This led, ultimately, to Einstein’s general theory of relativity, a spectacular reworking of gravity, whose validity was confirmed not by measurements on Earth, but by observations of Mercury’s orbital quirks and the apparent positions of stars during a total solar eclipse. Today, we are finding that the particle physics models we’ve developed through decades of rigorous testing in the best Earthly laboratories are incomplete, and we’re getting these clues from the sky. Studying the motions and distributions of other galaxies—cosmic conglomerations like our own Milky Way that contain billions or trillions of stars—has pointed us to major gaps in our theories of particle physics. We don’t know yet what the solution will be, but it’s a safe bet that our explorations of the cosmos will play a role in sorting it out. Uniting cosmology and particle physics has already allowed us to measure the basic shape of spacetime, take an inventory of the components of reality, and peer back through time to an era before the existence of stars and galaxies in order to trace our origins, not just as living beings, but as matter itself.

Of course, it goes both ways. As much as modern cosmology informs our understanding of the very, very small, particle theories and experiments can give us insight into the workings of the universe on the largest scales. This combination of a top-down and bottom-up approach ties in to the essence of physics. As much as pop culture would have you believe that science is all about eureka moments and spectacular conceptual reversals, advances in our understanding come more often from taking existing theories, pushing them to the extremes, and watching where they break. When Newton was rolling balls down hills or watching the planets inch across the sky, he couldn’t possibly have guessed that we’d need a theory of gravity that could also cope with the warping of spacetime near the Sun, or the unimaginable gravitational forces inside black holes. He would never have dreamed that we’d someday hope to measure the effect of gravity on a single neutron. Fortunately, the universe, being really very big, gives us a lot of extreme environments to observe. Even better, it gives us the ability to study the early universe, a time when the entire cosmos was an extreme environment.

But to the question of where it’s all going, there isn’t just one accepted answer—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. Based on their prominence in ongoing discussions among professional cosmologists, there are five leading possibilities: the Big Crunch, the spectacular collapse of the universe that would occur if our current cosmic expansion were to reverse course; two 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; vacuum decay, the spontaneous production of a quantum bubble of death that devours the cosmos; and finally, 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.

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. 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 the cosmos uninhabitable to any organized structure. 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.

Let’s start with the Big Crunch scenario. For as long as we’ve known that the universe started with a Big Bang and it is currently expanding, the logical next question has been whether it will turn around and come back on itself, ending in a catastrophic Big Crunch. Starting with some very basic and reasonable physics assumptions, there appear to be only three possibilities for the future of an expanding universe, and they are all fairly direct analogs to what can happen to a ball thrown into the air.

In the usual case, the ball goes up for a while, responding to the initial push, but immediately starts being slowed in its ascent by the gravitational pull of Earth and, eventually, slows so much that it stops dead in the air, reverses course, and falls back to Earth. But if you were to throw the ball incredibly fast—specifically, 11.2 km/s, the escape velocity of the Earth—you could in principle give the ball so much of a push that it leaves Earth entirely, slowing down slightly all the while, and only comes to rest infinitely far in the future. If you throw it even faster than that, it’ll be completely unbound from the Earth and just coast away forever.

The physics of an expanding universe follows very similar principles. There’s the initial push (the Big Bang) that set off the expansion, and from that point onward the gravity of all the stuff in the universe (galaxies, stars, black holes, etc.) works against the expansion, trying to slow it down and pull everything back together again. Although gravity is the weakest force in nature, it’s also infinite in range and always attractive, so even distant galaxies must pull toward each other. As in the baseball example, the question boils down to whether or not the initial push was enough to counteract all that gravity. We don’t even have to know what the initial push was; if we measure the expansion speed now, and also measure the amount of matter in the universe, we can determine whether its gravity will be enough to make the expansion eventually stop. Alternatively, if we can infer the expansion speed in the distant past, we can determine how the expansion is evolving over time by comparing that number against the expansion speed today.

What would it look like if it happened? If the collapse is just the reverse of the expansion, you’d expect that a crunching universe would look a lot like the cosmos did just after the Big Bang. But it’s not really a reversable process. As the universe has evolved over time, it has taken what was, at the very beginning of the cosmos, a fairly uniform collection of gas and plasma and used gravity to collect that gas into stars and black holes. The radiation produced by stars and black holes is even hotter than the final stages of the Big Bang, and if the universe re-collapses, all that energy gets condensed too. The collected radiation from stars and high-energy particle jets, when suddenly condensed to even higher energies by the collapse, will be so intense it will begin to ignite the surfaces of stars long before the stars themselves collide. Nuclear explosions will tear through stellar atmospheres, ripping apart the stars and filling space with hot plasma. 

The alternative to re-collapse is eternal expansion, which, like immortality, only sounds good until you really think about it. On the bright side, we’re not doomed to perish in an apocalyptic cosmic inferno. On the, well, dark side, the most likely fate for our universe turns out to be, in its own way, much more upsetting. A universe whose expansion is accelerating is, paradoxically, one in which the influence exerted by the things in it is shrinking. Galaxies will become more and more isolated, surrounded by darkness and the dying primordial light. All across the cosmos, 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.

Eventually, each new, dying supergalaxy will be utterly alone. Nothing will again approach to bring in a fresh supply of gas to fuel new stars. The stars already shining will burn out, exploding as supernovae or, more often, sloughing off outer layers to become slow-burning relics, gradually cooling for billions or trillions of years. Black holes will grow, for a time. Some will engulf galaxies’ worth of dead stellar remnants; some will stall in their growth, with no new matter approaching close enough to be consumed. When the stars have all faded to darkness, the ultimate decay sets in. Black holes begin to evaporate. This slow fade is just the beginning of the ultimate end: the Heat Death. The term “heat” is a technical physics term, not meaning “warmth” but rather “disordered motion of particles or energy.” And it’s not the death of heat, but a death by heat. It’s the disorder in particular that kills us. Eventually, when the stars have burned out and the parti-cles have decayed and the black holes have all evaporated, the universe is basically empty space with only a cosmological constant in it, expanding exponentially.

Things could be worse. As dark energy goes, a nice, steady, predictable cosmological constant is something of a best-case scenario. Other possibilities are not ruled out, and one of them, phantom dark energy, leads to something more dramatic, more immediate, and, in a sense, much more final: the Big Rip.

Dark energy is often assumed to be a cosmological constant that stretches space out, accelerating cosmic expansion by imbuing the universe with some inherent inclination for swelling. If dark energy is a cosmological constant, its defining feature is that the density of dark energy in any given part of space is constant over time, even as space expands. If you happen to be a clump of matter in the universe, and you would like to form a nice stable gravitationally bound galaxy, you can rest assured that once you get enough matter together to build something, dark energy won’t ruin all that hard work. Unless, that is, the dark energy is something more powerful than a cosmological constant.

Dark energy is both the dominant component of the cosmos and a sure sign of some new physics beyond our current understanding. Depending on what dark energy turns out to be, it might violently and inescapably destroy the universe, much sooner than anyone ever imagined. Why wait for the slow fade of a Heat Death, if you can have a dark energy apocalypse as sudden and dramatic as the appropriately named Big Rip? Not only would it be a kind of destruction from which there is no escape, it would be one that could tear apart the very fabric of reality, render-ing any thinking creatures in the cosmos helpless as they watch their universe being ripped open around them. This alarming possibility is hardly an outlandish fringe idea. In fact, the best cosmological data we have not only fail to rule it out as the fate of our cosmos, but, from some perspectives, slightly prefer it.

We probably shouldn’t panic just yet, however. In fact, the one thing that all the universe-ending scenarios we’ve discussed have in common is that they definitely aren’t coming around any time soon. As far as we can tell from our best understanding of physics, we have at least tens of billions of years before even the most extreme version of a sudden Big Crunch reversal could occur, and no Big Rip could be less than a hundred billion years off. A Heat Death, considered by most to be even more likely, would be so far into the cosmic depths of the future that we hardly have terms to describe it. This is not necessarily the case for the other two scenarios that I haven’t discussed—vacuum decay or the Bounce. But if the universe is going to end, one way or another, I concede that we may as well make our peace with it. There is perhaps some solace in the fact that whatever happens, it’s not our fault.--KM