Have you ever lost touch with a longtime friend? Someone so close you were planning on naming your kids after each other, but then a sudden move or change in lifestyle separates you. At first it's not much—you still meet up for drinks. But as the years go on and you focus on other priorities and other relationships, you realize you haven't spoken to her in ages. You don't even keep track of her on social media, man. Before you know it, you're not even sure where she lives anymore, or the name of her kids. Did she even have kids?

It's inevitable, but it's the way of things: once a pair is separated, it's hard to bring them back, especially if dark energy is involved. First the cosmic web dissolves, then the galaxy itself. Each observable patch of universe past 10³⁰ years of age consists of a single object, whether it is a brown or black dwarf, a planet, a neutron star, or a black hole. That's it. Each macroscopic object with an entire observable universe to call its miserable own.

And over time, even those macroscopic objects dissolve, and it's the physics of the subatomic world that govern their fate.

In yet another symmetry in this story, the exotic, turbulent early eras in our cosmos shaped the eventual growth of the familiar structures in the present-day universe. In the unfathomably distant future, when the universe has groaned into a near-dead, endless winter, those once-magnificent structures unwind themselves, breaking smaller and smaller, eventually returning themselves to the particle constituents that—for a glorious epoch in the history of the universe—once made themselves into something great.

Dust to dust, as they say.

It's not so much that nothing interesting happens in the multiple-quadrillion age of the future universe, but that everything that does happen is so much slower and colder and relies more on pure chance than intentional action. All the forces of nature are still there and still operating, but anything unstable (for really serious definitions of the word “unstable”) has long since vanished. But now we must adjust our definition of “stable” too. Objects that we consider permanent are anything but, given the extremity of the timescales involved. And that's the opportunity for the universe to engage in some, like I said, interesting games.

Take, for example, the noble proton. It is by all accounts as stable as your marriage (uh, let's assume). You could hold a single proton in your hand, and you'll get tired of holding it long before the little bugger does anything.

But we're not exactly sure just how stable the proton is over ludicrously extreme lengths of time. Remember the GUTs? The wish-we-understood Grand Unified Theories combining the strong, weak, and electromagnetic forces in a single happy home? How at the outset of the big bang, the dissolution of the GUT may have triggered the inflationary epoch? Yeah, good times, good times.

Well, in some of those fancy GUT theories, the proton can just up and vanish if it so pleases. It's just rare enough that we'll basically never see it happen. In fact, it's exactly such processes that might provide an alternate route to explaining why there's more matter than antimatter in the universe, so the vanishing proton is not an altogether crazy notion.1

If the proton decays, then somewhere between 10³⁶ and 10⁴³ years from now (uh, excuse the rather large uncertainty, but the math is starting to get a bit sketchy here), then all macroscopic objects that aren't named “black hole” will simply dissolve. The neutrons aren't long for the universe either, in case you were wondering: the same physics that would transform a proton could also do so to a safely bounded neutron, the unsuspecting fool.

In a bit of a reprieve against the gloom, the gradual decay of protons is able to (gently) heat any old dead stars once more. Not much, just a few hundred watts of heat. So not enough to run, say, a toaster oven, but far, far warmer than anything else that's going on in the universe (i.e., nothing at all). Proton by proton, a white dwarf sheds mass and converts back to its primordial state: pure hydrogen in a relatively dense ball.

It's not nearly dense or hot enough for traditional nuclear fusion—that ship sailed a long time ago—but untraditional nuclear fusion is still fair game. It's very simple according to the rules of quantum mechanics: put two atomic nuclei together and wait for an exceedingly long time, and by pure random chance they might combine, providing a brief spark of energy in the process.

For reference, through this stage of life, any white dwarfs have a surface temperature of less than a tenth of a degree above absolute zero.

But the grinding continues, eventually reducing the mass of the white dwarf so much that it simply unglues itself—there's not enough stuff to hold it together. A slowly expanding and dissolving thin cloud of hydrogen is its one and only fate, a fate shared by its nuclear star cousins, planets, and brown dwarfs. Not that it would ever find out, since those objects would be in their own personal patches of the universe.

If the proton doesn't decay, which is a very reasonable possibility, then the above scenarios still play out by other, more exotic processes. They get to stay as “recognizable objects” for a lot longer, however, which is small comfort, somewhere into the range of 10²⁰⁰ years.

I know I'm breezing through all this cosmological history like it's nothing, but I think it's important to remember that these objects, while dim and desiccated, live extremely, fantastically, overwhelmingly long lives. With each paragraph, we're leaping from epoch to epoch, jumping multiple multiples of the current age of the universe.

But that's only because we're counting in years, the length of time it takes for the Earth to orbit around the sun. By the degenerate era, there are no Earths orbiting any suns, so this timekeeping device is just another relic of a long-dead age. In general, it makes sense to think of the passage of time as “the interval between interesting events.” The cycles of the seasons on Earth are interesting—and regular—enough to qualify, for humans. When we looked at the initial moments of the big bang, interesting things would occur in a tiny blink, but to the frenzied subatomic processes involved, it was several lifetimes.

Now at the far end of the scale in the endless cosmological winter, life in the universe is much slower and much colder than it is today, which makes it seem like an eternity between events, the same way that from the point of view of a hypothetical thinker living in the inflationary epoch, our present universe is embarrassingly slothlike. But to the denizens of this far-flung era, life is just…normal.

Even the black holes don't make it, given these hilarious lengths of time. This seems like a good time to inform you that as best we can tell, black holes aren't exactly 100 percent black. Just almost entirely, but not quite black. Due to a strange and kind-of-understood quantum mechanical process at the event horizon, dubbed Hawking Radiation in honor of the Stephen who figured it out, black holes emit a very tiny amount of light. You'll thank me for sparing you the gory technical details (especially since most popular descriptions of this process don't really get to the heart of the physics),2 and because black holes are a mere supporting actor in our story.

The prime takeaway is this: black holes emit radiation and lose mass. Slowly. Like, the equivalent of one photon per year slowly. But hey, when you've got 10⁵⁰ years to play around with, you're going to spit out some significant mass. So all the black holes, each isolated in its own special observational bubble of the universe, decay.

Somewhere in the ballpark of 10¹⁰⁰ to 10²⁰⁰ years (but who's really keeping track now?) every single macroscopic object that is or will be formed will be gone. Dissolved, disassociated, disintegrated. Even protons and neutrons, the venerable baryons that were forged in the first dozen minutes of the big bang, will whittle away to other, more fundamental particles.

All that's left, after these countless eons (even though we're trying our hardest to count them), is a cold, thin soup of photons, neutrinos, electrons, and a few stray other fundamental particles. Some positrons inhabit the cold, dark depths, a leftover of the decay of protons. These positrons can “find” (for lack of a better term) a stray electron and bind to it using the force of, get this, gravity. Orbiting a common center of mass at a distance of, say, a light-year, these exotic creatures will be the last higher-order structures known in the universe. Eventually, of course, the orbits of this positronium decay, and the particles annihilate each other in a rare flash of light.3

Completely unglued from each other, the particles become their own isolated universe, in a repeat of the process that happened for larger objects in the degenerate era. One photon, or one electron, or one neutrino, alone in its entire observable cosmos, slowly losing energy and approaching absolute zero.

With no heat differences, with no hot springs to contrast with cold flows, the ability to do work is nullified. No work means no consumption, no computation, no cognition. If any form of life makes this far, it too eventually grinds to a halt.

The grim endgame: the heat death of the universe.

That…that can't be it, right? That's the fate of the universe as predicted by modern physics? That's all we get, a slow winding down of energy differences and the dismemberment of structures? We've worked so hard over the past few centuries to plumb the deepest mysteries of the cosmos, and this is all we can show for it?

Sadly, as depressing as this scenario is, it's a simple extrapolation of the physics of the universe as we know it. If you know where your car is and how fast you're driving, you've played the same game to figure out when you'll get to that party. Except there's no party here, just a miserable end to an enfeebled cosmos.

It's easy to get caught up in the melodrama of the long-term fate of our universe because the outcome is just so dang morose. I'm guilty of it too, though I don't know what else you were expecting—the title of this chapter is “The Long Winter,” after all. Still, we shouldn't get too comfortable with this telling. There are a lot of assumptions and conditionals baked into our current forecasts, and it makes it all seem so safe and predictable and boring and simple.

And if there's one thing the universe has taught us these past few centuries, it's that complexity has a way of taking its revenge.

For one thing, dark energy. The accelerated expansion depends on dark energy behaving like a cosmological constant, applying its accelerating pressure with dumb eternal insistence. But we're not sure if it really is constant—at best we hope to measure it to within a few percent accuracy in the coming decades. Even if we were to apply all our methodological might and constrain the properties of dark energy to one part in—let's go crazy here—a thousand, that still won't be enough. Given the protracted timescales involved in discussing the ultimate fate of the universe, tiny variations can add up.

Indeed, over the past few years some small tensions have arisen between measurements of Hubble's constant when using early-universe probes (like the relic cosmic microwave background) and contemporary-universe tools (like supernova). That's quite possibly explained by operator error on the part of astronomers, but it could be a sign that something fishy is going on in the world of dark energy.4

If dark energy is constant, it's not necessarily a good thing, so don't get too excited. For example, if dark energy is actually increasing with time—a scenario called phantom dark energy because that sounds totally awesome—then the expansion of the universe will become overwhelmingly coercive, ripping apart clusters, galaxies, and even solar systems. In short order, the small patches of vacuum inside atoms tear them apart, dissolving structures in a cascade of doom. If that's the case, then we only have about another ten billion years or so before our entire universe rips itself apart at the seams.

While violent, at least it's quick.

This scenario puts a big red line under the phrase “we don't know what dark energy will do in the future.” As time goes on and accelerated expansion continues apace, the universe will look more and more like the inflationary epoch of old. And we think that rapid expansion transformed the cosmos and flooded it with all the cool subatomic toys and gadgets that we love today. Perhaps an encore performance is in store for the far-future universe, refreshing and reigniting the feeble flickering candle of our fate?

Dunno.

What about dark matter? You know, the stuff that makes up most of the stuff? If it's anything like we suspect it to be, then it does occasionally interact with regular matter and even itself. Over time, this drains energy from the dark matter particles, allowing them to settle into any nearby gravitational wells, like a brown or white dwarf. Continued interactions can keep them warm (for very minimalistic definitions of the word “warm”) through the twilight of the degenerate era. It's not much to go on, but when faced with the ultimate heat death of the universe, we've got to count our blessings.

Or the universe could just up and change in a flash. Seriously, poof, it's gone and replaced with something else. Don't put the book down, I'm not kidding around. Here's why it's a very real possibility: it's already happened! It would just be another phase transition, like the one that sent the strong nuclear force (and before that, gravity) splitting off from the remaining forces. During that energetic, exotic process, the universe transformed from one state with a certain population of particles and fields to a completely different one.

If you lived through that transition, you wouldn't: the forces and interactions that you depended on would be up and gone, replaced with strange and unfamiliar new species.

Here's the kicker: what if the phase transition of the universe isn't done? What if the current universe, with its four fundamental forces and array of leptons and hadrons, isn't the true ground state of the governing equations? What if the universe got “stuck”?

Imagine skiing downhill, racing to the bottom of the mountain—the ground state—and watch out for that rock! and taking a tumble, getting jammed on a slight rise. You can see the rest of the mountain below you, but you're not moving. You're stable, but only in a meta sense—a swift enough kick (avalanche, abominable snowman, I'm not really familiar enough with winter sports for more examples) would send you tumbling down to the true base of the slope. But if that swift kick doesn't come, you can just chill out and relax; you're not going anywhere.

So maybe—emphasizing that word as hard as I can—the universe is in just such a state. Metastable, it can maintain its current arrangement of forces and physical constants for a very long time. Indeed, it's already managed to do so for more than thirteen billion years. So everything looks nice and comfy…for now. All it would take to trigger a new vacuum decay is a random blip or jiggle in the wrong place at the wrong time. Good thing there's nothing in the vacuum of space-time providing a minimum energy level capable of destabilizing the local patch of reality.

Oh, right. Vacuum energy. Microscopic quantum fluctuations. If you the skier started shaking uncontrollably, a violent jerk might send you continuing on your downward way.

Maybe the universe has already done it. In such a nucleation event, just as in any other phase transition, the cosmos reconfigures itself from a single point in an outwardly expanding Sphere of Doom. Traveling at the speed of light, there's literally no way to see it coming. By the time it overwhelms you, it's already replaced all your electrons, photons, and anything else-ons with…with whatever comes next, I guess.

Calculations are rough here, since they depend on physics beyond the standard model. If we stick to what we know so far (it's worth a shot, I guess), the stability of the universe depends on the nature of the Higgs field, since that field was involved the last time the cosmos underwent a phase transition, giving us the clean separation between the weak nuclear and electromagnetic forces. And you thought we were done with exotic subatomic physics.

Now that the Higgs boson has been confirmed to exist, thanks to the tremendous rock-smashing powers of our particle colliders, looking at how the Higgs particle behaves gives us insights into its future. Is it done, stable for all eternity in its ground state? Or does it have more room to fall? Current measurements of the Higgs put us right on the line of metastability, which is of little comfort.5

Hey, at least the universe isn't unstable (but we knew that already).

Maybe the end isn't an end at all. Quantum mechanics teaches us that reality is ruled by random chance. An electron can just so happen to be on the opposite side of a wall the next time you look at it. Two protons can just so happen to cohabitate the same volume, and voilà, you have a fusion reaction. But the larger and less quantumish an object or system, the less you expect it to behave weirdly. I can lean against a wall all day long without expecting to pass through it spontaneously. I can sit on my couch all day long and not occupy its same volume (hopefully).

Even in the not-quantum world, gamblers still rule the day. For example, there's nothing in the laws of known physics to prevent all the air molecules in the room you're sitting in to spontaneously end up crammed into a tiny corner, leaving you to asphyxiate helplessly in the vacuum. The only reason thoughts like this don't keep physicists up at night is that these conditions are exceedingly, exceedingly, exceedingly rare. There are so many more ways for the air molecules to be jumbled around the room compared to the number of ways they can be crammed in the corner that the random jostling and jiggling at the molecular level almost always leads to an air-filled room.

That was the briefest summary of the concept of entropy that I could concoct, so consider yourself spared a more long-winded metaphor. Entropy itself is a way to count the number of ways a bunch of particles can rearrange themselves, and the second law of thermodynamics—that entropy always goes up in closed systems—comes from the fact that there are way more disorderly states (like air spread evenly throughout a room) than orderly ones (crammed into a corner).

When you throw out a possibility like bodies spontaneously jumping through walls or air molecules conspiring against you, a proper physicist would immediately scoff and say, “Pshaw, yes, it's technically possible, but not likely in a bajillion years.”

Well, now we're dealing with a bajillion years. The absurd and unlikely are bound to happen. Given an infinity of time, anything that could happen must happen. What does this mean for the long-term fate of the universe? It's hard to say because we're operating far outside the normal bounds of known and generally accepted physics. It could mean that a new universe—big bang and all—simply pops into existence through a new inflationary event triggered by a random fluctuation. That universe would be effectively cut off from its parent, with its citizens blissfully ignorant of what came before their own bang.

This new universe—which might or might not have its own set of physical laws—would eventually lead to the formation of new cosmo-babies, on and on and on. That would imply that our big bang was neither the first nor the last of those dramatic events, but simply one bead along an infinite glittering strand of…beads, I guess.

It could mean that a random patch of the universe might spontaneously decrease in entropy, so much so that a complex structure—say, for example, something like a brain capable of something like conscious thought—would get to contemplate its lonely existence before subliming back into the mean. Preposterous? Yes, but in ten to the hundred to the hundred years, the preposterous becomes plausible.

If the universe is infinite in size, or at least capable of infinitely generating new inflation events, and if matter can only arrange itself in a finite number of ways (which just might be true due to the quantum limits on measurements), then that means that all possible combinations and permutations of arranging matter in the universe have been realized. Infinity is a tough concept to deal with, and scenarios like this certainly aren't helping. In this picture, not only is every possible organization of galaxies, stars, planets, rocks, and molecules brought to fruition somewhere (or somewhen), each possibility is realized an infinite number of times.

That means there's a literal copy of this exact situation, either of me sitting in my pajamas typing this sentence, or you wearing who-knows-what reading it. If the universe is infinite in size, the nearest copy is somewhere out there, well beyond our observable horizon (thankfully). If the universe is infinite in time, then this scenario has already occurred and is fated to happen again. An infinite number of times. Cripes, this is getting embarrassing.6

I'll be the first to admit that this picture is a bit hard to swallow, but we should remember that, well, the universe doesn't care what we think about the issue, and it's the (extreme) logical conclusion if we're to take our most modern theories at face value.

Or maybe we're just wrong about all of it. It's not like it hasn't happened before.

We don't know if inflation is correct. We don't know how the rules of quantum mechanics can be extended to incomprehensible timescales. We don't know if the technology of entropy can be applied to the whole entire universe, let alone over the course of an exceedingly exponential number of years.

And don't even get me started on braneworld cosmologies or string theories or whatever the kids are calling it these days. The more hypothetical the physics, the more room for creative explorations of the end state (states?) of the universe.

Our knowledge of the universe at 10¹⁰⁰ years isn't much different from our knowledge at 10⁻¹⁰⁰ seconds: woefully incomplete. In both cases it's the energies involved. In the young cosmos, the temperatures are so high and pressures so extreme that the physics of the familiar are melded together into some strange chimera that eludes understanding. In the remote future, temperatures are so low and processes so agonizingly slow that the statistical rules that govern our daily lives lose their identity. In both cases the universe is extreme, exotic, and potentially unknowable. At its core, after centuries of searching, we don't know how the universe began or how it will end—or if those are even reasonable scientific questions to ponder.

But at least there is symmetry.