Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)

Chapter 9. The End of Everything

BLACK. NOTHING. EMPTY.

Everything is dark. No stars dot the inky sky, no galaxies can be seen.

They are all long since dead, gone, disintegrated as their very constituents have decayed into nothingness.

Nothing has occurred in the Universe for countless years. It is a cold, almost entirely empty void.

For trillions of trillions of years, this emptiness endures. But then, suddenly, in one tiny corner of the Universe no different from any other, a phase change snaps into existence. Like crystals growing in a saturated solution, this realignment in the very structure of space and time expands. It spreads outward at nearly the speed of light, enveloping more and more space.

What it leaves behind is . . . nothing. Or at least, nothing we can understand. Matter, energy, even space and time are destroyed, transformed in the wake of this quantum bubble.

When it is done, it has consumed the entire Universe. And what is left after that is something we may never know.

A NOTE ON EXPONENTIAL NOTATION

You may be familiar with what’s called exponential or scientific notation: using exponents to represent very large or very small numbers. So instead of writing out 10,000,000,000, it’s easier to refer to it as 1010: a 1 followed by ten zeros. Similarly, very small numbers are written using a negative exponent: 10−7 = 0.0000001 (the 1 is seven places to the right of the decimal point). This chapter is rife with scientific notation because the numbers involved will get staggeringly large very quickly. However, this introduces a slight prejudice in our barely evolved human brains that can trick even those of us familiar with the notation.

The number 1012 looks like it is only a little bit bigger than 1011, but it’s actually ten times larger (1 trillion versus 100 billion). Worse, 1020 looks only twice as large as 1010, but it’s actually 10 billion times larger! Even for someone experienced in this notation, it can be difficult to appreciate at a glance. Right now, the Universe is a little over ten billion years old: 1010. But far, far in the future, when it’s 1020 years old, the length of time spanned by 10 billion years will be a tiny, tiny fraction of the total age of the Universe. Keep this in mind, because by the end, even 1020 will be an infinitesimal amount of time compared to the length of our journey.

DEEP TIME

So far, we have examined a series of singular events that wreak havoc on our little planet: exploding stars of different flavors, catastrophic impacts, the death of the Sun.

Certainly, these events are exciting, and of course it’s the big flaming explosions that make headlines. A tree getting hit by lightning and burning to the ground might make the local newscast, but one that simply decays from within and falls over from rot after fifty years won’t even get noticed.

But while we might never get hit by an asteroid or fried by a GRB, the Earth is aging. Everything is aging. Even if we manage to survive the death of the Sun, how do we survive the aging of the Universe itself?

The answer may be bleak indeed: we don’t. While you’ve been reading this book, the Universe has aged. Maybe a week, maybe a few days if you’re a fast reader, and during that time the Sun has eaten through a few trillion tons of hydrogen, stars have exploded, and the volume of the Universe has increased. We’re all a little older, and so is the cosmos. After you’re done with this book and you put it on a bookshelf, it will age. It will always age. It’s inevitable: it will be a year older, a decade, a millennium. It will have decayed into dust by then, no doubt, but the atoms in that dust will age too. Someday they will be millions of years older, billions. Trillions.

And even that is a microscopic drop in the ocean of time. Time may very well go on forever, and a trillion years will be like the blink of an eye. The Universe will continue to age, and as it does, it will change. This change will be profound: it is more than just massive stars dying and galaxies colliding; the very nature of the Universe and the things in it will change fundamentally over stretches of time so long we don’t even have names for them.

What will the Universe look like in a trillion trillion trillion years? How about a trillion times that age?

Different. It’ll be different. But we’ll never know: we won’t be around to see it. And by that I mean nothing like humans at all, nothing like life as we know it. Not even matter as we know it will survive to see this stage of the cosmos.

Time and tide wait for no man. But deep time waits for nothing. Not even matter.

The only way to understand this look forward is to first take a look back, all the way back, to the beginning of the Universe. It may seem like a long time ago, but I promise you, soon it will seem like yesterday afternoon.

A VERY BRIEF HISTORY OF THE UNIVERSE

In the beginning, there was nothing.

Then there was everything.

DETAILS, DETAILS

Maybe that’s a little too brief. The details turn out to be important.

How we understand the beginning of the Universe—and its fate—depends on how we see it now. By carefully examining the clues coming down from space through our telescopes, we actually know a surprising amount about what the Universe has been doing the past few billion years. Perhaps even more surprising is how much we can extrapolate about what it will do in the future . . . and so far into the future that billions of years will seem as but a whisper, a mere tick of the cosmic clock.

The events described below will seem like science fiction, I suppose, since things get pretty weird at very short times after the Universe formed, and in the mind-numbingly long stretches of future time. But as far as we can tell, this is science fact, based on solid evidence. Like any examples of conjecture, there may be pieces we are missing that affect what really happened, or what really will happen. That is the nature of science; more observations and more information always lead to a refinement of the results. Science asymptotically approaches reality, and it’s hard to say just how far up the curve we are now.

But even with that caveat, the future of the cosmos is fascinating, if bleak. But as is the case in science and in stories, we have to start at the beginning.

Oh, and I better warn you: the very dawn and the very dusk of the Universe are times when things are completely different from what we see around us now. Be prepared to stretch your mind a bit.

The Universe is many, many things—it’s literally everything—but it’s also a damn odd place.

IN THE BEGINNING

Some 13.7 billion years (plus or minus 200 million years) ago, the Universe exploded into existence.

This mere statement causes a substantial amount of confusion. Astronomers refer to this event as the Big Bang, or, more accurately, we use the term Big Bang as a model for what we think happened. What’s the distinction? Well, for one, the event was neither Big nor was it a Bang. When it popped into being, the Universe was smaller than the size of a proton, so it wasn’t terribly big.113 And there wasn’t a bang either: it was more of a pop, or a snap.

By observing the Universe as it is now, we can run the clock backward and figure out what it was like in the past. What we discovered is that in the past, the Universe was hotter and more dense. The farther back in time you go, the hotter and denser it gets (the reverse is true as well: the older the Universe gets, the less dense and the cooler it gets). It gets smaller too: the Universe is expanding now (more on this in a moment), so in the past it was smaller. Eventually, you go back far enough to a time when the Universe was basically just a singular point: an infinitely small, infinitely hot, infinitely dense object.

Well, that’s weird. And it’s probably not even technically correct. As we turn the clock back, the Universe shrinks. At some point we see it as the size of a present-day galaxy, and then a star, and then a planet, and then a grapefruit, and then an atom. When it gets smaller than an atom, the weird world of quantum mechanics rears its head once again. One of the most fundamental rules of QM is that many characteristics of objects are related, and the more you know about one the less you know about another. The more carefully you measure an electron’s position, for example, the less you know about its velocity. The more you nail down one aspect of an object ruled by quantum mechanical processes, the slipperier another property becomes. It’s almost as if there is a cosmic censorship going on, happening at very teeny-tiny scales. The closer you look, the fuzzier things get.

In practical terms—and I’m not sure what the word practical even means on scales like this!—this tells us that when the Universe was really, really small, there is very little we can know about what it was truly like. Our equations and understanding of physics tell us a great deal about the state of the Universe when it was a day old, an hour old, a second old . . . even a tiny fraction of a nanosecond old. But if you go back far enough in time, to when the Universe was literally 10−43 second old,114 our physics breaks down. There is a veil hiding the true beginnings of the Universe, farther back than which we can never directly see.

That’s another reason scientists prefer not to call this event the Big Bang. We don’t know much at all about the event itself, whether it was a bang or not. We can only figure out what happened right after it.

Still, you might wonder what happened before the Big Bang. This is a natural question, and there are two ways to think of it. One of them is that the question is meaningless. That may sound like a cop-out, but let me ask you this: what’s north of the north pole?

That question has no meaning, right? If you travel north, you get to the north pole, and you’re done. There’s no more north to go.115

Now remember that time itself was created in the Big Bang. Before then, there was no time, so there was no “before.” The question has no meaning.

That’s pretty strange, even for cosmology and quantum mechanics. It’s also unsatisfying. We’re used to things existing for a finite stretch of time, embedded in a bigger stretch of time. A symphonic concert might start at 7:00 p.m. and end at 8:24. But there existed things before the symphony started (the orchestra arrived at the concert hall, warmed up, filed on stage), and things continue after (the brass section empties their spit valves, the members leave the stage, they go home and watch reruns of Gilligan’s Island). So how can there be a beginning of time, a point on the timeline before which there is nothing?

There may be a way out of this conundrum. There are some theories that say that the Universe is not really all there is. There may be some sort of meta-Universe out there, some framework from which we are forever locked away, and our Universe is just a subset of it. This Universe existed before ours did and is much like ours, with the same or similar laws of physics controlling its behavior, including quantum mechanics. In the chapter on black holes we saw how particles can pop into existence spontaneously. There is a possibility that a tiny blip in the fabric of this other Universe’s space-time suddenly came into being, something like the creation of particles ex nihilo. Under some conditions, this herniated region of space and time would quickly collapse, but it’s also possible, in the realm of quantum mechanics, for this region to grow. Something like a black hole, it’s disconnected from the greater Universe around it, and becomes its own entity, its own Universe. Space and time and energy and matter simply spring into existence inside. After a few billion years it expands to the point where stars form, galaxies take shape, and on a planet somewhere lost in all that volume of space, a person reading a book is scratching his head and thinking the book’s author has lost his mind.116

At the moment, we don’t know for sure whether there was anything before our Universe existed, or if that idea even has meaning at all. But since the Big Bang theory was first postulated, we’ve learned quite a bit about what happened after that first 10−43 second.

A LITTLE AFTER THE BEGINNING: T + 1043 SECOND TO TODAY

We know that the Universe went through several different phases of its life prior to the one we live in now. In the very early Universe, when it was still unfathomably hot and dense, it consisted of a stew of bizarre subatomic particles held in sway by unfamiliar forces. As the Universe expanded and cooled, different types of particles were able to form and become stable (whereas before it was just too hot for them to exist, like an ice cube won’t last long on a frying pan). To make it easier on themselves, physicists divide the timeline of the Universe into different slices, different eras, based on what particles were present and what forces dominated at the time.

After just one microsecond (10−6 second), things had settled down enough for protons and neutrons to form from the thick soup of subatomic particles called quarks. After one second, one tick of the clock, was the period of nucleosynthesis,117 when conditions were similar to those in the core of a star. The heat and density allowed some protons and neutrons to come together and form stable nuclei. For about three minutes after the nucleosynthesis period started, the subatomic particles smashed into each other and created an entirely new kind of matter: helium (two protons plus two neutrons). Even a trace of lithium (three protons and three or four neutrons) was formed, but nothing heavier than that—the more complex reactions needed to make carbon and neon never got a chance to occur.

This left all the matter in the entire Universe divided into roughly 75 percent hydrogen, 25 percent helium, and a trace of lithium.

There was no calcium, no iron, no oxygen. For that matter (pun intended) there were no stars, no planets, no galaxies. Everything was pretty simple at this point, just a lot of extremely hot gas strewn into long filaments and ripples: fluctuations in the cosmic matter distribution created by fluctuations in the explosion of the Big Bang itself.

These streamers would soon begin to collapse under their own gravity. Under conditions still not fully understood, the matter would form the first stars at about T + 400 million years. Galaxies themselves formed around the same time, collecting along the matter filaments, creating a vast spongelike network of fantastic galactic clusters streaming throughout the Universe.

And so, after 13.7 billion years, here we are.

HOW WE KNOW WHAT’S SO

All of this is probably a little overwhelming. It may even strike you as ridiculous! It’s so far outside our comfort zone, our usual thought processes, that it might seem as if scientists are just making it all up.

I promise, we’re not. There is a logical series of steps that lead to our understanding of the early Universe.

One of the very first people to think about the Universe as a whole was a German astronomer named Heinrich Olbers. In the early 1800s, when Olbers was studying the sky, it was assumed that the Universe was infinitely old, and that it was infinite in extent. There was no reason to think otherwise. But as Olbers realized, this raises a problem. If the Universe is infinite, and populated with stars throughout its extent, then no matter what direction you looked eventually you’d be seeing the surface of a star. No matter how teeny-tiny a section of the sky you chose, a line drawn from you out into space in that direction must hit a star at some point. It might be a bazillion light-years away, but if the Universe is indeed infinite that is but a mere stroll compared to that infinity.

And that’s the problem. The apparent size of a star gets smaller as it gets more distant, of course, so it also appears fainter. But the drop in size and brightness is compensated by the Universe being literally infinite. The number of stars increases with distance, and in fact the number of stars increases at the same rate at which the brightness decreases. The two cancel out.118 So if the sky appears full of stars, literally, with no apparent space whatsoever between them, then the entire sky will glow with the same brightness as a star itself. To any observer inside such a Universe, it would be as if the sky were as bright as the Sun, everywhere you look.

Obviously, such a Universe would be uninhabitable. Also just as obviously, our Universe doesn’t behave that way.

This is what Olbers pointed out, and the conundrum is now known as Olbers’s paradox. This problem baffled people for some time, and the answer to the paradox came from a somewhat surprising source: Edgar Allan Poe.

Yes, that Poe. Besides writing scary stories and depressing poems like “The Raven,” he was quite a deep thinker. It occurred to him that perhaps the problem lay not in the Universe, but in our underlying assumption: what if the Universe were not infinite in space and/or time? If the Universe were finite in space, then you’d run out of stars at some distance from Earth. And if it were finite in time—that is, it had a beginning—then there simply hasn’t been enough time for the light from very distant stars to reach us. Paradox solved.

In fact, Poe was right. In his 1848 work Eureka, he wrote:

Were the succession of stars endless, then the background of the sky would present us an uniform luminosity, like that displayed by the galaxy—since there would be no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all. That this may be so, who shall venture to deny? I maintain, simply, that we have not even the shadow of a reason for believing that it is so.

This was radical thinking for its time. While it was common in mid- to late nineteenth-century society to assume that the Universe had a beginning because it said so in the Bible, this was somewhat unsatisfying to a scientist. Poe changed that.

Less than a century later, the astronomer Edwin Hubble, together with other astronomers such as Vesto Slipher and Ellery Hale, made one of the most shocking discoveries in the history of science: essentially every galaxy they could observe appeared to be rushing away from us. This was so difficult to believe that it took years before there were enough observations to convince everyone, but the evidence was undeniable: the Universe itself is expanding.

This had profound implications. If galaxies were moving away from us, then they grew more distant as time went on. That in turn means they were all closer together in the past. If you run the cosmic clock backward far enough, then, at some point in the past, every galaxy, every bit of matter and energy in the Universe, would have all been in the same spot.

This meant the Universe had a beginning, a point in time when it all began. Matter and energy rushed outward from that point in time, expanding evermore. Albert Einstein had already been working out the general equations that govern the behavior of time and space when Hubble and his team discovered the cosmic expansion, and the news of the discovery electrified him. It was soon accepted by scientists that Einstein’s work was correct, and that the Universe itself could be described using mathematics.

Thus the Big Bang model was formed.

Over the years, the model has been reworked, refined, with parts added and others taken away. When an astronomer uses the term Big Bang, she doesn’t just mean that singular point in time 13.7 billion years ago; she is also implying a vast amount of other work done to make the model fit what is observed about the Universe. And, in fact, it is one of the most successful scientific theories of all time.119

One critical factor in confirming the Big Bang model is the finite speed of light. That may sound weird, but it’s this finite speed that allows us to see what the Universe was doing in the past. Imagine that the speed of light were infinitely fast. If we looked at a galaxy 10 billion light-years away, we’d see it as it is right now, at this very moment. It would probably look a lot like ours, and there’s not a whole lot about the Universe we could learn from it.

Instead, though, we have a wonderful characteristic of the Universe: light is not infinitely fast. It’s pretty fast, covering 186,000 miles every second (about a foot per nanosecond, if that helps you any), but the Universe is so big it takes a long time for that beam of light to make it here from some distant galaxy.

What this means is that we don’t see galaxies as they are right now; we see them as they were when they were younger. Telescopes are very much like time machines in this regard—the farther away we look in space, the farther back we look in time. How do we find out what the Universe was like five billion years ago? Easy: find galaxies that are five billion light-years away and take a look.

And why stop there? Our telescopes are huge, and our detectors sensitive. We have seen galaxies well over 12 billion light-years away, so we’re seeing them as they were when the Universe itself was about a billion years old. Because of this, we can actually see what galaxies looked like when they were young, and discover what happens when they age.

We can also detect and analyze the gas that lies between galaxies in the distant Universe, which in turn tells us even more about early conditions. In fact, radio telescopes tuned to the microwave part of the spectrum have detected a uniform hiss coming from all over the sky. This hiss is not noise: in a very real way it’s the cooled light from the fireball of the birth of the Universe. After about 100,000 years, the Universe had expanded and cooled enough that the matter became transparent to light, meaning that light could travel freely through it. Before then, a photon wouldn’t get very far before being absorbed by some bit of matter. This light, free to move across space, has since “cooled” as the Universe expanded, and has been able to travel to our waiting instruments.

These characteristics—and many, many more—have provided to us a wonderful series of clues on the way the Universe behaves. Because of this, we have a fairly good grasp on what the Universe was like almost all the way back to its birth, nearly 14 billion years ago.

But what about its future? Is it possible to take what we know about physics and astronomy and extrapolate the eventual fate of the cosmos?

Yes, it is. We can get a fairly good idea of what the Universe will be like in the next few billion years (for example, our local neighborhood will look surprisingly different pretty quickly). As we look farther into the future, though, our crystal ball gets cloudier, but given what we see and know we can guess in broad terms what will happen.

I’ll give it to you straight: things don’t look good for us. If we want to survive into the far future—and I mean far—we’ll have to change ourselves in such fundamental ways that I wouldn’t even consider the result to be human anymore. And even then, escape from the Universe’s eventual demise may be impossible.

And yet there is still hope. Maybe not for us, exactly, but for whoever comes next. Maybe there won’t be anyone next, but the Universe may yet get another chance to try.

THE AGES OF THE UNIVERSE

As the Universe ages, it changes profoundly. The time scales of these overall changes themselves change, getting longer as the Universe ages. When the Universe was young it changed rapidly. For example, one of the first big changes occurred just 10−35 second after it was created. Before this time, all the forces of the Universe—gravity, electromagnetism, and the nuclear forces—were combined in one unified force, and in fact this is called the Grand Unification Epoch. Today, these forces are as different as they can be, but when the Universe was incredibly hot and dense, they were indistinguishable. But just after that razor’s slice of time following the cosmic birth, the temperature and density dropped enough that the forces started acting differently.

It’s an incredibly short length of time, 10−35 second. But it was enough for the Universe to change profoundly. It went through many such changes: dropping in temperature and density such that particles like protons and electrons could form, and then dropping again such that these could come together to make more complicated elements, and then form stars, galaxies, and finally us.

There are lots of ways to divide up the Cosmic Epochs. A good way is to look at what’s producing the most amount of energy at that time. Right now, that would be stars. However, the stars will all eventually die out, and the current age will end. What then?

Astronomers Fred Adams and Greg Laughlin looked into this idea in great depth in their book The Five Ages of the Universe. As the title suggests, they found five ways to divvy up time in the Universe. Up until stars formed was the Primordial Era, which we just toured above. The current era of stars they dubbed the Stelliferous Era. After that is the Degenerate Era, then the Black Hole Era, and finally—forbiddingly—the Dark Era. The time covered by these eras is staggering, and difficult to grasp. While reading their book I had to constantly take a step back and laugh at the numbers. Maybe that was a defense mechanism on my part, like whistling past a graveyard.

Actually, that analogy is a little more on the mark than I’d like.

That’s just a gentle warning. We’re about to take the longest journey you’ve ever been on. It’ll last so long that even using scientific notation gets overwhelming. You’d better sit back and relax. You’re going to be reading this chapter for a long, long time.

THE STELLIFEROUS ERA: T + 108—1015 YEARS

The era of stars began with the birth of the first stars. It’s not known precisely when that happened, but the best estimates put it at about 400 million years after the birth of the Universe. Theoretical models show that it wasn’t until roughly then that the gas distributed throughout the Universe was cool and dense enough to collapse under its own gravity.

Observational evidence has mounted for this date as well. Although we have never directly detected these pioneer stars—they would be so distant now that directly observing them would be nearly impossible—they had an effect on their environment, and that can be detected. These stars would have been made entirely of hydrogen and helium (and again, a trace of lithium), making them relatively simple compared to modern stars. Such a chemical composition made it possible for the early stars to be much more massive on average than current ones (the heavier elements in modern stars make them hotter, so they “switch on” at a much lower mass). Some models put these stars at well over 100 times the mass of the Sun. They flooded space with ultraviolet light, which ionized the hydrogen atoms around them, tearing the electrons off.

These electrons polarized the light from the stars: in effect, this means the waves of light coming from the stars were all aligned, like people in a room all facing the same direction.120 This polarization effect can still be detected today, and the observations agree with the theoretical models on the time when stars first appeared.

Also, at the ends of their short lives, these stars would have exploded as massive supernovae, scattering the Universe’s first heavy elements into the surrounding environs, from which the next generation of stars would form. The first stars probably created gamma-ray bursts when they exploded; these might yet be detected too.

We still live in the Stelliferous Era. Stars are the dominant feature of the Universe, and produce most of the energy we detect. As we saw in the last chapter, the available source of gas in the Milky Way to make stars will run out in the next few billion years, although some galaxies may use up their gas more slowly. But one way or another, the gas will eventually run out, and essentially no more stars will be born anywhere in the Universe.121

We know the Sun will last as a normal star for several billion more years before turning into a red giant, frying the Earth, losing its outer envelope, and then “retiring” as a white dwarf (chapter 7). But the length of time a star lives depends almost entirely on its mass. A star with a lot more mass than the Sun eats through its fuel far faster, and may only live a few million to a billion years. However, stars with less mass will live longer.

The lowest-mass star that can currently exist has about 0.08 times the mass of the Sun. Below that limit, the core isn’t hot enough or under enough pressure to fuse hydrogen into helium. This type of star is small (one-tenth the Sun’s diameter), dim (one one-thousandth the Sun’s luminosity), cool (with a temperature of about 5,000 degrees Fahrenheit), and red. Not surprisingly, these stars are called red dwarfs.

Imagine you take a large rock and hit it with a sledgehammer, shattering it. If you look at the pieces that remain you might see a few large pieces, a few more that are smaller, and a lot of little pebbles and shards. That is a natural size distribution in stars as well: when a cloud collapses and forms stars, only a few will be really big, some will be smaller, and more smaller yet. The vast majority will be the smallest type; it’s estimated that 75 percent of all the stars in the Universe are red dwarfs.

Although they have a small fraction of the mass of the Sun, red dwarfs are incredibly miserly with their fuel and can last far, far longer. A very low-mass dwarf can reasonably expect to shine for the next several trillion years.

This is longer than any other kind of star in the Universe. If we let the cosmic clock run forward, we see the last stars being born in a few hundred billion years. Very rapidly after that all the massive stars will be gone, since they don’t live long. The last core-collapse supernova in the Universe may occur only a hundred million years after the last massive star is born. This is a tick of the clock compared to how much time has elapsed in the Universe at that point.

Sometime not long after that, somewhere in the Universe, a star just barely too low-mass to explode will age and die, expanding into a red giant, blowing off its outer layers, and fading away as a white dwarf. It is part of a long, long line of such events: there are 100 billion galaxies in our Universe, each with an average of about 100 billion stars.

As time goes on, trillions of stars with lower and lower mass fade away and die. Stars with the lowest mass will take the longest, but they’ll all cross the finish line at some point.

If we wait a sufficiently long time—oh, say, a trillion years—all stars like the Sun will be long gone, and only the lowest-mass dwarfs will remain. You might think galaxies would be dim and red then, only illuminated by the tiny stars. Interestingly, though, galaxies may be as bright at that time in the distant future as they are today. We saw in chapter 7 that the Sun steadily increases in brightness as it ages. All stars do this, even red dwarfs. Calculations done by The Five Ages of the Universe authors Adams and Laughlin, together with their colleague Genevieve Graves, indicate that a star with one-tenth the Sun’s mass will live for about 10 trillion years. As it ages, it gets brighter and slightly hotter. What they found in their models, after adding up all the light from all the stars in the galaxy and then letting the galaxy age, is that the amount of light given off in toto by dwarfs increases roughly as quickly as light from the more massive stars fades as they die. In other words, the total light emitted by a galaxy will stay roughly constant for several hundred billion years, with the ever-brightening dwarfs picking up the slack as massive stars die off.

As the red dwarfs heat up, they will change color too. A hotter star gets bluer, and so too will red dwarfs. It’s possible that for a few dozen billion years the galaxy will shine with a demonic red hue, and then this will slowly morph to a vibrant blue.

But all good things . . . , as they say. Even dwarf stars eventually die. Unlike the Sun, which can only fuse fuel in its core, the smallest red dwarfs circulate their fuel. Like hot air rising and cool air sinking,122 the helium created in the core circulates upward and mixes with the rest of the star. As the hydrogen falls into the core it can fuse, forming more helium, which then mixes more with the star.

Eventually, the star runs out of hydrogen—and unlike the Sun, which just runs out of available hydrogen in its core, the dwarf totally runs out. Gone. Kaput. All that is left in the star is helium, and it lacks the mass to fuse it into carbon. The star cools, the helium contracts, and it becomes a pure helium degenerate white dwarf (see chapter 7 for details on this odd quantum state).

In seven or eight trillion years’ time, in the Milky Way (well, Milkomeda, after we collide with the Andromeda galaxy, and probably consume all the smaller galaxies in the Local Group as well) the last dwarf star will become a white dwarf. For trillions of years the galaxy will have glowed a beautiful blue, but that too shall pass.

Interestingly, in this late stage of the Stelliferous Era, some even lower-mass stars will still be able to shine. Because high-mass stars create heavier elements like iron and magnesium, stars forming later get imbued with these materials. Heavier elements make a star hotter (they absorb the light from the star, trapping the heat in), so lower-mass stars—perhaps even as lightweight as 0.04 solar mass—will be able to get fusion started in their cores. But again, we have to consider the span of time: even if these stars stave off turning into white dwarfs for 15 trillion years, that time will still eventually come. At some point, all stars in the Universe will be gone, having become white dwarfs, neutron stars, or black holes.

The tiny white dwarfs fade with time (neutron stars cool even faster). Eventually, the galaxy contains no stars actively fusing elements in their core at all. Over the next few trillion years these stars fade too. By the time the Universe is 100 trillion years old, the galaxies—and therefore the Universe itself—will be dark.

LOST HORIZON

In the distant future, not only will the Universe be darker, it’ll look a whole lot emptier as well.

Standing on a beach looking toward the horizon, you can see only so far out. The Earth curves downward, hiding more distant objects from view. The visible horizon is only a few miles away, and you cannot see things any more distant.

The Universe has a horizon too. Since it’s 13.7 billion years old, we cannot see any objects more than 13.7 billion light-years away. The Universe might be bigger than that, but the light emitted by any objects farther away than that distance has not had enough time to reach us, so we don’t see them.

In fact, it’s actually worse than that. The Universe is expanding; the fabric of space is literally being stretched out. Objects farther away appear to be receding from us at greater speeds. If you look out to a great enough distance, galaxies appear to be receding from us at the speed of light. We cannot detect such galaxies: their light is approaching us at the same speed that space is expanding. Like running on a treadmill, that light can’t get anywhere, so it never reaches us.123

And it’s even worse than that. In 1998, it was discovered that not only is the Universe expanding, but its expansion is accelerating. Not only is the Universe getting bigger, the amount it’s getting bigger is bigger every day.

This has a rather depressing outcome for the distant future. Because the Universe is accelerating, galaxies that are currently inside our horizon (because they are receding from us at less than the speed of light) will eventually move outside our horizon (because they will accelerate relative to us to beyond the speed of light).124 This means that over time distant galaxies will fade as the expanding Universe sweeps them out of our view. As time creeps ever forward, galaxies that are closer to us now will slip away, and the cosmic horizon will close around us like a noose.

However, it won’t tighten too much. Space expands, but this expansion can be counteracted by gravity. You might expect that, say, two stars orbiting each other will get farther apart as space expands between them. However, that’s not the case. Since the two objects have gravity, and they are bound to each other—that is, their gravity holds them together—space doesn’t expand between them. It’s just another peculiar outcome of relativity and the way space-time behaves.

This means that even though the Universe is expanding, and even accelerating, the cosmic horizon won’t continue to shrink forever. The Local Group of galaxies—the Milky Way, Andromeda, and a dozen or two smaller galaxies—are gravitationally bound to each other. We know that by 10 billion years from now we will have merged with Andromeda, and over time we’ll gobble up all the remaining smaller galaxies as well. Some calculations show that the cosmic horizon will shrink down to encompass just the Local Group’s volume of space in something like 100 billion years, still during the Stelliferous Era. By that time, the Local Group will be one giant elliptical galaxy.125 From our point of view, we will see closer galaxy groups like the Virgo Cluster fall over the horizon, but our own will always be visible.

Eventually, our view will be extremely limited: as far as we’ll be able to tell, the entire Universe will consist of our one massive galaxy with literally nothing outside of it. Any new species that evolves during this time will have no clue that, once upon a time, a Universe far vaster than their own once teemed with galaxies and stars.

What will their cosmology be like?

The tightening of the horizon will occur far sooner than all the stars in the galaxy will die out, hundreds of billions of years compared with tens of trillions. Still, it’ll be a harbinger of things to come, of a Universe growing increasingly darker.

It should be noted that the acceleration of the cosmic expansion does mean one thing for sure: the Universe will not recollapse. Before the acceleration was discovered, it was still a matter of some debate whether the Universe would expand forever or whether the combined gravity of all the matter in it would slow, stop, and eventually reverse the expansion. But the discovery of the acceleration pretty much put an end to that debate. The Universe will expand forever, ever faster, while (somewhat ironically) our view of it will get smaller and smaller, until we have our own private Universe just a few million light-years across.126

THE FAULT LIES IN THE STARS

What does this mean for us, for humans? To a good approximation, it means that we have about 100 trillion years to get our affairs in order. After that, we won’t have enough light to read our books by. Things’ll get boring.

Assuming that anything resembling humans still exists a thousand times the current age of the Universe from now, there are ways to extend the stars’ reign. Physically colliding stars—literally smacking them into each other—to make new ones will help. But how long can you do that? If you decide you need a star like the Sun, you can smush together a few dwarfs and get a star that shines for another few billion years. Remember, though, that the Universe is trillions of years old by this point. A billion years is a pittance in comparison. When the Universe is 100 trillion years old, our descendants will be out of fuel, out of stars, and out of luck.

The time scales here are forbidding. When we reach this point in the age of the Universe, galaxies will have lived the vast majority of their lifetime populated only by dwarfs. Think of it this way: currently, our galaxy has only been around a tiny fraction of its potential life span. Right now, as you read this, despite the Universe being over 13 billion years old, 99.9 percent of the galaxy’s life still lies ahead of it.

We think of the Universe as being relatively unchanging, but in fact we live in a very special epoch compared with the dim future. By the time the last dwarf fades away, the galaxy will look back at the time stars like ours could exist in the same way you look back at the time you were a month old.

And after all that, we’re only just getting started. We’re about to enter a realm where even 100 trillion years is a single breath of time.

THE DEGENERATE ERA: T + 1015—1040 YEARS

When the last normal, fusing stars die, the only objects left in the Universe that can generate energy will be white dwarfs, neutron stars, black holes, and degenerate low-mass objects that lack the capability to fuse hydrogen in the first place, called brown dwarfs.127 Because the Universe is dominated by these objects, this time period is called the Degenerate Era.

In visible light, the Universe will be pretty dark at this point. However, it won’t be completely dark, since there will be a few scant sources of light.

White dwarfs will fade; when they are at about 10,000 degrees Fahrenheit they will shine with the same color as the Sun, getting redder as they age. When they reach a temperature of about 800 degrees Fahrenheit they radiate mostly in the infrared and will become invisible.

Every now and again a black hole may pass close enough to a white dwarf, neutron star, or brown dwarf to shred it and consume the debris. An accretion disk will form and shine brightly, but only as long as the black hole eats. Once the meal is gone, the light source shuts off (this may provide a temporary source of energy for any future beings looking to stay alive, but it really is only a short-term solution).

Brown dwarfs will have their moments as well. These failed stars give off visible light for a short time after they form because of their internal heat, but their lack of core fusion means they have no ongoing source of energy. Eventually they cool and glow faintly in the infrared.

But they still can get a second chance. Collisions between stars are incredibly rare in the present-day Universe because stars are so small compared to the distances between them. However, the word rare has less meaning as time stretches on. Something that has an incredibly small chance of happening in 13.7 billion years may become a virtual certainty over 100 trillion.

The Degenerate Era will actually last much longer than this, in fact, so collisions between stars will happen frequently once you grasp that time scale. When two brown dwarfs merge, their mass will be just above the fusion limit, so a relatively normal star could result. In fact, if the collision is a little bit off-center, then matter from the two objects could be stripped off, forming a disk around them. It’s entirely possible that planets could form from this material; is it too hard to imagine life forming under such circumstances? Their view of the Universe would be far, far different from ours. Their skies would be entirely dark except for the one sun burning during the day. No stars, no galaxies, no ribbon of milky gas streaming across the sky. What myths and legends would arise on such a planet?128

At any one time, perhaps a hundred or so of these odd stars will shine in a galaxy. But again, these new stars would shine briefly, then suffer the same fate as the Sun did all those forbidding trillions of years in the past.

There will be other, brief flashes of light. A collision between two white dwarfs could result in an object whose mass is so high that it collapses into a neutron star or even a black hole. A Type I supernova may result, which to any denizens of this future era would be even more blindingly bright than to us: there will be literally nothing against which to compare it.

It’s also possible that two low-mass white dwarfs could merge to form an odd type of “normal” star, much as the colliding brown dwarfs will, but again, this is a short-lived object (a mere few billion years!) and will fade with time.

If two neutron stars collide, then they will form a black hole with a gamma-ray burst to announce the merger (see chapter 4). But this fades within days, and the black hole itself will be dark, one among many millions of others orbiting a dark galaxy.

And it will get even darker. Time piles up. After trillions, quadrillions, quintillions of years, even the brown dwarfs go away. They merge to form normal stars that eventually die, or they get ejected from the galaxy entirely. In fact, after this length of time, the galaxy will have a hard time holding itself together. In the far distant future, the galaxy itself will evaporate.

GALACTIC PERCOLATION

Stellar collisions129 are the culprit for this next stage of galactic evolution. A moving object has energy, and this energy can be transferred to another object (which allows us to do things like play pool, throw a baseball, hold a book, and so on). When two stars pass close to each other, they can exchange energy by interacting gravitationally. In general, what happens to two stars as they pass each other depends on their mass (it also depends on the sizes, shapes, and directions of their orbits, but we’re being general here). The higher-mass object gives away some of its orbital energy to the lower-mass object. An orbit with lower energy is smaller, so the higher-mass star will sink closer to the center of the galaxy, while the lower-mass star will move outward. Over many such encounters, lower-mass stars “evaporate” away; they get ejected from the galaxy to wander the depths of intergalactic space.

The higher-mass stars drop to the center of the galaxy, where an unpleasant host awaits them: a supermassive black hole (see chapter 8). Eventually, all the higher-mass stars in the galaxy will get eaten by the black hole.130

This process has been seen on much smaller scales: globular clusters—gravitationally bound spherical collections of roughly a million stars—are packed tightly enough with stars that collisions of this sort are more frequent. In all globular clusters, even after only a few billion years, the more massive stars tend to be closer to the cluster center, with lighter stars farther out.

The time scale for galactic evaporation is about 1019 to 1020 years (10 quintillion to 100 quintillion years), making this process currently undetectable in galaxies.

But the Universe is still young. Patience.

Incidentally, over this length of time, the odds of a star getting extremely close to the Sun go up, and close in on 100 percent. Even by the beginning of the Degenerate Era (T + 1015 years), it’s likely another star will have passed close enough to the Sun to dislodge the Earth from its orbit and eject it from the solar system (of course, any star that passes that close is likely to eject the outer planets as well—and by this time, Mercury and Venus will have been swallowed by the red-giant Sun, so Earth will be the innermost planet). Given enough time, planets even closer to their stars will go; even by halfway through this era, it’s very unlikely that any planet orbiting any star anywhere will not have been ejected from its system. By the time the galaxy itself has evaporated through stellar collisions, there may be ten times as many planets as stars roaming intergalactic space, frozen to their cores and utterly uninhabitable.

And they won’t last forever anyway.

PROTON DECAY

By 1020 years after the Universe formed, galaxies will be dark and mostly dispersed. Black holes, neutron stars, white dwarfs, and brown dwarfs will roam the Universe (such as we can still see of it, owing to the smaller cosmic horizon), and illumination will drop to a feeble whisper of what it once was.

But even this ignominy is not quite the end.

Matter, it turns out, may not last forever. We already know that many types of atomic nuclei and subatomic particles decay. Uranium is radioactive: over time, a uranium nucleus will spontaneously split apart into lighter elements (a process called fission), and give off a tiny bit of energy. The time for any given nucleus to fission is random, but if you take a whole pile of them and take data as they decay, statistically you start to see trends. You can measure how long it takes half the sample to decay, for example, and that number is pretty consistent. For one kind of uranium, it takes 4.5 billion years for half the sample to decay and become lead. This length of time is then uranium’s half-life. If you start with a pound of uranium, you’ll have half a pound in 4.5 billion years, and the other half will be lead. Wait another 4.5 billion years and half the remaining uranium will turn to lead (leaving you with a quarter pound of uranium). In another 4.5 billion years you’ll have an eighth of a pound. And so on. Eventually, it will all turn to lead, but you have to be patient.

Individual particles like neutrons decay too, in this case with a half-life of about eleven minutes. This only happens if they are alone, free to roam space; in a nucleus neutrons are stable (they like the company, one supposes). But when they decay, they create a little shower of smaller particles and energy.

Until recently, protons were thought to be stable forever. But “forever” takes on a different meaning when dealing with the time scales of the death of the Universe.

Protons are theorized to decay into lower-mass particles extremely rarely, on average after about 1033 to 1045 years (the exact number is unknown, so for argument’s sake we can pick an intermediate time of 1037 years). Currently, no protons have been unequivocally seen to decay,131 but scientists are fairly sure they will. Given time.

Time is all we have here. In a given sample of protons—like, say, a white dwarf—half the protons will decay in 1037 years. In another 1037 years, half more will disintegrate, and so on. After a few times 1038 years or so they’ll all be gone.

Like any other subatomic decay reaction, when a proton decays, it creates smaller particles and energy. By this time, almost all protons will exist inside other objects—white dwarfs, brown dwarfs, neutron stars. When they decay, the net result is that energy is released, heating up the object a bit.

So long after the last light of fusion has burned out, long after all the material objects in space have cooled to nearly absolute zero, we find another source of energy: heating from proton decay.

It’s feeble, to be sure. Very, very feeble: in a given white dwarf, the energy released by proton decay is only about 400 Watts. My microwave oven needs more power than that! In fact, the entire galaxy, even if full of such decay-powered objects, will only shine with less than a trillionth of the power with which the Sun shines now. Worse, the light it emits will be incredibly low-energy, well into the radio range of the electromagnetic spectrum.

If we were to make a leap of faith (and this isn’t a leap, it’s a trans-galactic hyperspace jump) and assume that some form of life is still around deep into the Degenerate Era, then they had better figure out a way to go green. The amount of energy available to them will be incredibly small. They won’t even be able to make a bowl of popcorn.132

And they’ll run out of time too. Every time a proton decays inside a white dwarf or a brown dwarf, the star loses that much mass. It’s not much each time—protons are pretty small—but time has a way of adding up 1037 years in the future. White dwarfs will lose mass133 and will eventually evaporate entirely. As they lose mass they go through some weird stages. When they have roughly the mass of Jupiter, for example, they will have the same density as water (when white dwarfs first form they are millions of times denser) and will be made almost entirely of hydrogen; all the more complex elements will have fallen apart as their protons decayed. The temperature of the object will be so low that it will be frozen, a ball of hydrogen ice 100,000 miles across.

Eventually, this too will go away as the protons inside it disappear.

Even neutron stars will undergo this evaporative process. Having more protons inside, they’ll take longer than white dwarfs to disappear. They’ll be warmer too: they’ll shine at −454 degrees Fahrenheit. Today that’s considered extremely cold, but in the year 1038 they’ll be the hottest objects in existence.

And they, too, shall pass.

Eventually, they’ll lose mass through proton decay as well. At some point, their gravity will decrease enough that neutron degeneracy cannot be maintained, and the star will suddenly expand into something like a white dwarf. This won’t help it, though; we know what happens from there.

By the end of the Degenerate Era, an incredible 1040 years in the future, all the galaxies will not only be dead, but their corpses desecrated. There won’t be a single proton left anywhere in the Universe. There will be no more stars of any kind at all. No white dwarfs, no neutron stars . . . not even planets, which will have evaporated long before the white dwarfs did.

All that will remain are extremely low-energy photons, a few subatomic particles that don’t decay (electrons, positrons, neutrinos) . . . and black holes.

THE BLACK HOLE ERA: T + 1040-1092 YEARS

Black holes survive the Degenerate Era because of one simple reason: they aren’t made of matter.

Chapter 5 covers black holes in detail, but basically a black hole is an object that is so dense that its escape velocity is equal to or exceeds the speed of light. Once a black hole forms, no information can come out of it, and it’s essentially cut off from the Universe. Any matter it was once made of, or any matter that falls in, is gone. Since there are no protons, there is nothing to decay. They therefore persist.

At the end of the Degenerate Era, all that’s left are black holes and an extraordinarily thin soup of radiation and subatomic particles. After 1040 years, we have entered the Black Hole Era.

Black holes can have masses as low as three times that of the Sun and as large as the monster supermassive black holes in the centers of galaxies that, in our current era, contain from a million to a billion solar masses.

During the time of galactic evaporation in the Degenerate Era, a curious thing happens. Out in the suburbs of the galaxy, black holes will be the most massive objects that still exist. Normal stars today can have far more than three solar masses—the most massive have about 130 solar masses or a tad more—but they will have long since exploded. The only objects left in the Degenerate Era are neutron stars (top mass: 2.8 solar masses), white dwarfs (top mass: 1.4 solar masses), and the far less massive brown dwarfs. Since the most massive objects tend to sink and the lighter ones float away in the evaporation process, after the process is complete the galaxy will really consist of (1) a single, central supermassive black hole that has eaten many of the smaller stellar mass black holes that dropped into it, (2) quite a few (perhaps millions) of stellar mass black holes that have dropped down toward the center of the galaxy but have not (yet) been consumed, and (3) a bunch of lower-mass objects at large distances, many of which will have physically left the galaxy completely.

During the galactic evaporation process, the central black hole may have consumed 1 percent to 10 percent of the galaxy’s mass in all. So, for a galaxy that started off with a hundred billion stars, the black hole at the core will end up with a billion or two solar masses by the end of the Degenerate Era.

Not all galaxies live alone, though. As pointed out, the cosmic horizon will shrink, but only to the point where gravity offsets it. Some galaxies exist in clusters like the Local Group, but far larger. The Virgo Cluster is the nearest galaxy cluster, and it has perhaps two thousand galaxies gravitationally bound to it. In a process similar to the evaporation of a single galaxy, the Virgo Cluster will evaporate as well, given enough time. When it’s all done, the cluster will consist of a single galaxy with a mass of about 10 trillion times the mass of the Sun. Eventually that MonoVirgo galaxy will evaporate, and the black hole in its core will have a mass of a hundred billion times the Sun, or possibly more.

However, because our horizon will be so close, we’ll never be able to observe that black hole. We’re stuck with our one-billion-solar-mass hole in the center of our galaxy. And you’d think that would be that. Once a black hole, always a black hole.

Well . . . almost always.

Also as discussed in chapter 5, black holes too can evaporate. The process is called Hawking radiation, after the physicist Stephen Hawking, who first postulated it. Although it is still theoretical—we don’t have any black holes handy on which to test it—it’s grounded in well-understood physics. The basic principle is that black holes can radiate away their mass in the form of subatomic particles because of weird quantum effects. The process is in general excruciatingly slow, and it goes even slower the more massive a black hole is.

Once again, though, we have to be careful when we talk about “slow.” When we have ten thousand trillion trillion trillion years to play in, “slow” can still happen. Given time enough, a black hole will completely evaporate through Hawking radiation.

A stellar mass black hole has a minimum mass of about three times that of the Sun. Pinging away particles one by one, it takes a long time to slog through six octillion tons of black hole: about 1066 years. To us, today, that seems like forever. But even that is the blink of an eye compared to the time it takes a supermassive black hole to blow away. The billion-solar-mass black hole that was once the Milky Way Galaxy (and Andromeda and several others from the Local Group) will take a whopping 1092 years to evaporate to nothing.

And that’s it. I’m out of analogies. I give up. I was hoping to come up with something like, if the life span of the Universe up until now were a single beat of a hummingbird’s wing, then 1092 years would be like, well, like something that takes a really long time. But even comparing a single flap of a hummingbird’s wing to the current age of the Universe falls completely and hopelessly short of comparing the present age of the Universe to 1092 years. That’s just too long a time span. It crushes our sense of reality to dust. The closest analogy I could think of is to compare the mass of a proton to the mass of the entire Universe, but this analogy is useless. Analogies are supposed to make things easier to grasp, and who can grasp the mass of the proton, the mass of the whole cosmos, and then take the ratio?

Worse, the analogy actually falls short of reality. The ratio of 1092 years to the current age of the Universe is about 1082, while the ratio of the mass of the Universe to the mass of a proton is 1079. The analogy fails by a factor of 1,000.

So I give up. You’re on your own for analogies now.

But perhaps we’re done anyway. The most massive object in the Universe has evaporated away using the slowest process in the Universe. When it’s done, there’s not much left. The entire observable Universe will be only a million or two light-years across, and it will consist of countless electrons, positrons, neutrinos, a handful of exotic particles, and extremely low-energy photons. It will be an incredibly thin vacuum, far more rarefied than anything that exists today.

And that’s it. That’s all there is. Once the black hole is gone, everything familiar in our Universe will go with it.

The Universe will be dead.

THE DARK ERA : T + 1092- ∞ YEARS

The endless gulf of time stretches ahead of us now. At this point, our math breaks apart. The Universe is such a thin soup that it could be countless years before any two particles approach each other. And if they do, what will happen? If the two particles are both electrons, they repel each other and off they go in opposite directions. If one is an electron and the other a positron, they’ll attract each other, collide, and poof! They’ll make a pair of gamma rays that fly away.

But where will they go?

Every trace of the Universe we know today will be gone. No stars, no planets, no people. Not even matter. It will all have decayed away, eroded into an ethereally thin slurry.

10100 years, 101,000, 101,000,000. It’s all the same. Nothing ever happens, and nothing ever will. The Universe is dark, randomized, silent. And it will remain so forever.

REBIRTH?

Oh, but there’s that word again. Forever.

As we’ve seen many times in this chapter, nothing is forever. Maybe not even Universal death.

There are some faint hopes for the ultimate fate of the Universe. Most involve the complete destruction of the Universe as we know it and the reconstruction of something entirely new and different.

You might consider that a drawback.

But the alternative is the boring Universe where nothing ever happens. So let’s see what we’ve got.

The Big Bang was a singular happening. Somehow, all matter, energy, space, and time were generated in that one event, forming the Universe as we know it.

But where did that event come from?

As discussed earlier, there are some theories that there is a meta-Universe, someplace other that exists outside of our framework of space and time. It developed a little quantum hernia, and this formed our own Universe. If that’s really the case, then the death of our Universe isn’t that big a deal in context. The other Universe may still be there, and it’s possible that it budded off countless other universes too. These may all have vastly different laws of physics (maybe in one the speed of light is a few miles per hour instead of 186,000 miles per second, and in another the electron has more mass than the proton, instead of the other way around as in ours). It’s also possible that our own Universe is doing this all the time—even now, tiny offspring universes are popping into existence in other “places,” outside what we can see and investigate. However, according to everything we understand about physics, we can never physically learn anything about these other universes, so for all practical purposes they don’t exist.

Of course, it’s conceivable that in the next, oh, quintillion years or so our understanding of physics may change. I’ll readily grant that! But for now there’s not much we can say about this.

But maybe we’re starting off on the wrong premise. Maybe we should ask: was the Big Bang actually the first cause? Or was there some other event that jump-started the cosmos?

There is another idea, still in its infancy, called the ekpyrotic universe (Greek for “from [or out of] fire”). According to this idea, the Universe is already incredibly old. At first, it was basically a giant void, with nothing interesting happening (much like the way we left it after 10100 years). According to this theory, there exist other universes with characteristics similar to those of ours now, but these other universes are outside our view. They exist in eleven dimensions instead of the four with which we are familiar (the three dimensions of space and one of time), and they float around in this extradimensional space. Called branes, short for membranes, these are all self-contained universes like ours in many ways, generally minding their own business.

But sometimes they collide.

You can picture these universes as parallel plates floating around. When they smack into each other, they shake up the contents rather vigorously. The theory predicts that the universes would get violently disturbed, with energy and matter being heated up tremendously, and space itself set to expand.

Sound familiar?

This may sound a little like fantasy, but it’s all part of a set of very complicated but scientifically based math and physics theories. No one has any idea if these theories really are viable alternatives to the Big Bang model, or if it’s just so much fantasy. But the ideas are internally consistent, and are being studied very seriously.

If they pan out, then there is some hope for the Universe, or at least for some meta-Universe: it means other universes exist, and they might be habitable. We can never reach them, so that’s too bad for us, but maybe other species in those universes can survive.

And there is some hope for us as well. What happens once can happen again, especially if you wait long enough.

In 10100 or 101,000 years, however long it takes, another brane may collide with ours. When it does, it may spark a reignition of the Big Bang, kick-starting our Universe once again. When that happens, pretty much everything that happened in the Universe before that point will be destroyed: kindling, if you will, for the fires of a new Universe. Again, that’s too bad for us, but it does mean that there is a cyclical nature to reality, and a chance for life to rise anew.

Again, cold comfort. But it’s a possibility.

DOWN THE LONG STAIRCASE

Yet another fate may wait in store for us in this far distant future, and we have even less of an idea of what will exist after it unfolds.

Objects have what is called an energy state. It’s a bit like climbing a set of stairs. On the bottom step you are at the lowest energy state, and at the top you’re at the highest. There are energy states in between too. It takes (muscle) energy to move up to higher states, and you give up that energy when you go back down.134 Sitting at the bottom, you’re as low as you can go, and you stop.

Atoms behave this way: electrons zipping around an atomic nucleus have certain energy states available to them, with none in between (just as you can’t stand on the four-and-a-halfth step; it doesn’t exist, so you have to be on either the fourth or the fifth step). This is one of the most basic ideas of quantum mechanics.

Perhaps the Universe behaves this way as well. We think of the vacuum of space as being, well, a vacuum. Empty. Devoid of matter and energy, and therefore at its lowest energy state.

But this may not be the case: we know that space bubbles and boils with energy at extremely small size scales (this is the basis for Hawking radiation, in fact). So what if we’re not at the lowest energy level, the lowest energy state? What we’re experiencing now would then be a “false vacuum state,” and we might take that final step down, dropping to a lower state.

The starting point for this drop is difficult to predict. Maybe it’s a quantum effect, again like Hawking radiation: somewhere, someplace in the Universe, a teeny-tiny bit of the Universe suddenly drops to the lower state. According to the theory, this one event acts as a trigger, pushing regions around it to drop into the lower state as well (imagine standing on the second-to-last step from the bottom with ten other people; you jump down and drag everyone else with you). A cascade starts, with more and more bits of the Universe dropping to the lower state.

This tunneling event, as it’s called, would expand outward in a sphere at very nearly if not at the speed of light. It’s a bit like a sugar crystal growing in a supersaturated solution; once you start it someplace, the other sugar molecules attach themselves there, growing rock candy.

In this case, though, the rock candy is the collapse to the true vacuum, and the sugar molecules are actually the fabric of space itself. The damage wrought is literally total.

Inside this expanding bubble of vacuum collapse, the laws of the Universe change. Space and time themselves are rewoven, becoming something entirely new, something the nature of which we cannot even begin to guess. Anything caught in this wave will be utterly destroyed.

And there is literally nothing to stop it. The entire Universe sits on the second-to-lowest state, so once poked by the expanding bubble, everything will collapse. Every star, every planet, every black hole, every human.

That would be pretty bad, were it to happen today. Odds are it won’t; the chances of this happening are extremely small. But if this event were to wait, say, 10200 years, would that be so bad? I argue it would be good. By that time the Universe will be dead, stagnant, with nothing to show for all those years of activity. A collapse of the false vacuum to the true vacuum would possibly reenergize the Universe, giving it a second chance for life.

So there is some hope. You and I and even the entire Universe as we know it won’t be there to witness it, much less survive it.

But afterward, a new Universe will be created, sparkling and clean and ready for a new start. In this case—and also that of the ekpyrotic universe, and maybe even other processes we haven’t even yet conceived of—there is a chance that instead of a bleak and dim future, filled with nothingness for all eternity, there will be a rebirth of the Universe, and the rebirth of possibilities.

And if it happens once, it might happen again in another 10200 years, or 101,000. And again, and again. Endlessly.

Rather than dealing out death and mayhem, destruction and chaos, the Universe will cyclically clean itself out, reboot, and set everything in motion once again.

Each time, perhaps, the laws will be different, and the characteristics of that future infinite parade of universes will be grandly set apart from what we know today. And despite our prejudices, the Universe appears to have no set of rules on how things need to be for complex chemistry, for life, to arise.

We don’t know for sure if there are aliens in our own Universe now, though the odds favor such a possibility: there are 200 billion stars in the galaxy, and hundreds of billions of galaxies in the Universe.

And so I wonder: can we now multiply those odds by the number of potential universes that lie ahead as well?

If that’s the case, then the Universe provides us a near-infinite number of do-overs, something I find very uplifting. It may seem that the Universe spends all its time trying to kill us, but in the end—the very end—there may yet be Life from the Skies.