I want you to imagine a star with a nuclear fire raging in its heart, but with water ice clouds circling its frozen surface.3
Even those stars will eventually sputter out. It's difficult to tell when the long autumn will come to our universe, because as you may have noticed, the formation, lives, and deaths of stars are a little bit complicated.
The most pessimistic scenario gives only a trillion years until the last star in our universe is born. That's essentially no time at all. More optimistic predictions, trying every trick in the book to keep the fire lit, give a scale a hundred times longer. Either way, eventually the great nebulae of our galaxy will be too thin. Interactions that might trigger a rapid collapse and the birth of a new star will be too rare. And when they do happen by random chance, the energetics will be too low to trigger high enough densities for continued nuclear reactions.
It's in that same time frame, a hundred trillion of these cold, dim years, that we expect the last star to finally burn out. The far-future descendants of today's generation may be far smaller, with far feebler nuclear reactions, but the new molecular mixture of later generations of stars can shorten, rather than lengthen, their lifetimes. Still, longevity is meaningless on these timescales. Even a star that can make it to a ripe old age of ten trillion can't compete against the inexorable march of time.
The universe will simply stop caring about stars.
When the last star sputters into oblivion, it will be the last nuclear fusion reaction that the cosmos will ever produce, save for a few increasingly rare, catastrophic collisions between the dead cinders that remain.
Once the stars fall, the universe will be ruled by the degenerates.
These are the remnants, the has-beens, the never-weres: the sad, sorry states that befall all stars. When a star “dies,” it doesn't necessary go poof and vanish (unless, of course, it blows up, which can happen). There's almost always an object remaining. Much smaller and more pitiful than its progenitor, but still there.
A star like our sun will eventually leave behind a carbon and oxygen ball about the size of a planet—a white dwarf. At the present epoch, these are brilliantly hot objects, which makes sense since they used to be the hearts of stars. When they are first exposed, they blast their environs with hard X-rays, but that fades after a mere ten thousand years. They still remain blazing hot for eons, but now we're in timescales where eons come and go with ease. Eventually they cool and solidify, and when carbon turns solid, it naturally arranges itself into interesting crystalline patterns, which you may know by a more familiar name: diamonds.
The smaller stars, unable to turn helium into anything heftier, simply sputter out without much fanfare, leaving behind a lump of inert helium: a shrug, muttering to the universe, “Eh, I give up.”
The most massive stars will be long gone a hundred trillion years from now, but their leftovers remain scattered around the ruined, disfigured clump of our galaxy. Neutron stars, the more massive cousins of the white dwarfs, are a couple of suns’ worth of pure neutrons (hence the name) crammed into a sphere the size of a city. For both these neutron stars and white dwarfs, they're supported against gravitational calamity not by any nuclear fires but by the simple refusal of electrons and neutrons to cram themselves too tightly together, a wonderfully quantum phenomenon known as degeneracy pressure.4
Despite their cold hearts—or maybe because of it—they will persist through the coming death of light.
And then there are the black holes. First considered a mathematical curiosity—a freak show generated by general relativity but not found in nature—they turned out to be…found in nature.5 When nothing can fight against gravity, not even the resisting pressures of crowded neutrons or electrons, the insatiable urges of gravity drive everything into an infinitely small point—the singularity—encased in the one-way boundary of the event horizon.
There are the supergiant supermassive ones, like our friend Sagittarius A*, whom we met in chapter 8, and there are a far greater number of smaller ones floating around the galaxy, the remains of the most massive stars after they spent their fuel. Their numbers will only increase with time.
Last are the most pathetic degenerates of all, the brown dwarfs: loose collections of hydrogen and helium, too big to be called planets but too puny to ignite fusion reactions and name themselves among the stars. The galaxy, even today, is littered with these dim, abandoned half-wits. No solar system to call—or make—a home, these vagabonds wander the dark reaches of the galaxy, hardly ever interacting with or even encountering another object for millions of years at a time. And in this era ruled by the degenerates, still they travel, aimlessly drifting among the tattered remnants of what was once an energetic, bustling metropolis.
Any remaining planets, having survived the deaths of their parent stars, have long since been ripped from their decaying homes and forced into odd, random trajectories. While each solar system is extremely isolated—even in our tidy galactic suburbs in the present day, our nearest neighbor star is twenty-five trillion miles away—given enough time (and don't worry, the universe has plenty of that to spare), stars will eventually pass by each other. When they do, the gravitational interactions will pluck an unlucky planet or two from their cozy, stable, familiar orbits and send them flying off to face the deep darkness alone.
Within a quadrillion years, when the white dwarf remnant of the sun has faded to a handful of degrees of absolute zero, no bound solar systems remain throughout the universe. The galaxy/universe is now split roughly evenly between the brown dwarfs and white dwarfs (now more properly called black dwarfs), with a small fraction of rogue planets and a few neutron stars and black holes. That is the entirety of macroscopic objects, all scattered randomly, all completely, totally isolated.
An occasional flash of light illuminates the decay when two degenerates collide, igniting in a brief but intense supernova or flaring star, a reminder of what the universe was once capable of. But this is no more than a handful of embers, a mere echo of the hundreds of millions of burning torches that once enlightened the Milky Way.
Over time the galaxy, alone in its pocket of the ever-expanding observable universe, begins to dissolve. One by one, the same rare near misses that detach planets from their parents occasionally give a stellar remnant a burst of energy, sending it flying away from the galaxy altogether into the vast emptiness beyond. Once a remnant is free from the chains of gravity that kept it orbiting within the galaxy, dark energy can apply its wicked influence on it. And by now, with the universe in such an advanced age, dark energy is by far the greatest force in the cosmos. One little taste is all it takes for the remnant to be ripped away from its home, flung to impossible distances, literally never to be seen again.
Over the course of a few tens of quintillions of years (that's about eight billion times the current age of the universe—our numbers here are quickly growing preposterously large), up to 90 percent of the galactic members will be thus ripped into cosmic seclusion. The remainders, the most massive and the most lucky, that managed to cling to the home that gave birth to them suffer an even worse fate. Their orbits around the central supermassive black hole—now swelled far larger than its present-day size—emit gravitational waves.
Any orbiting body will do so, even here, even now. But gravity, being by far the weakest force of all, doesn't usually enter into our attention except through extremely accurate observations. But once again, given enough time, the universe can make the imperceptible obvious. Orbit by orbit, the remaining objects slowly, agonizingly, spiral in toward the doom. Due to the pathetic strength offered by gravity, this takes an eon of eons, so much so that we've ran out of Greek prefixes.
In a poignant symmetry, we started the story of our universe with exponential notation to express the intense action happening in mere fractions of a fraction of a second. Now, at the opposite end of the life of our universe, processes take so long to complete that we need to return to that notation.
In this case, within
1030 years the universe will be composed of only solitary objects. All orbits, whether remnants around the central supermassive black hole or binary pairs, will have decayed. If a brown dwarf or neutron star hadn't managed to escape the galaxy, by now it will have fallen into the gaping maw of the central black hole.
Imagine life arising—or, if you prefer, some form of consciousness clinging to persistence—on one of the surviving rogue planets or brown dwarfs. Since dark energy operates so efficiently in this ancient universe, you are permanently marooned. Even if another object were within the speed-of-light limitations of communication, its light—assuming it even emitted anything—would be so feeble there'd be no hope of detection. Your entire cold, sluggish existence, limited to a few degrees of absolute zero, would be confined to a single, solitary object, completely and crushingly alone in the vast expanse of nothingness that surrounded you. No star, or companion planet, no anything to signal the existence of anything else. Unless you had some memory of the distant past, of what used to be, would you even know the rest of the universe existed?
And then things get weird.
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 becau
se 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.
Your Place in the Universe Page 25