The next time you see someone on the street corner shouting that the end is near, you can correct him. The end is not near—it is already upon us.

Our Milky Way galaxy currently pops out a litter of around ten stars every single year, with a variety of masses. The majority are small, because small = easy, but there are a couple of medium ones like our sun, and every once in a while a blazing giant. This process has been pretty steady for a couple billion years or so, but it's actually on the downswing. Our home used to be much more efficient at manufacturing stars; nowadays it's relatively lazy compared to its more productive years.

The ability of the universe at large to convert random blobs of gas and dust into stable nuclear reactors peaked long ago—more than nine billion years in the past. Before dark energy came into prominence, before structures even finished coalescing, the fat lady was already onstage warming up her vocal cords. It's been downhill ever since, with fewer and fewer stars coming online every day.

Indeed, the majority of stars ever to be born in the entire history of the universe, both past and future, have already formed. When it comes to the stellar output of our universe, this is all we're gonna get, folks.

We're both pretty sure and not exactly sure why the lights are going out. We know that the universe is expanding—and accelerating, too—so that's going to slow down the formation of structure, preventing the continual infall of new gas reserves into the galaxies. But star formation is sort of a complicated process, depending on lots of supercomplicated factors, so while it's no surprise that we're over the hill, we don't exactly understand why our downward slope is so steep.1

Still, our sun continues to burn, happily converting hydrogen into helium in its infernal core, providing warming light to Earth and all the other planets that don't care as much. Our solar parent is about halfway through its life cycle. There's plenty more hydrogen to spare inside the sun, but most of it won't make its way down to the core where it can do something useful. In about five billion years, the sun will bid farewell to the main sequence and begin its transformation into an ugly red monster.

But the Earth will be doomed long before then. The sun was dimmer in the past, to the dinosaurian delight. The uncomfortable consequence of that little nugget is that the sun will be hotter in the future. Imperceptibly it grows brighter. Not over the course of centuries or decades (sorry, this isn't the source of climate change), but over much longer timescales. In a few hundred million years, give or take, the sun will brighten to the point that the Earth's atmospheric temperature will trigger a runaway greenhouse effect, boiling the oceans.

Unrelatedly, as Edmund Halley didn't realize he realized, the moon is slowly spiraling away from the Earth. In the same amount of time, it will be too small in the sky for total eclipses to occur. So that's a handy astronomical bellwether: when eclipses stop, it's time to get packing. You may also have noticed that the oceans are boiling, but still.

While life on Earth will take a hit—microbes clinging to meager hope in polar ponds and subsurface streams—the body of Earth will be just fine, just a tad warmer, joining the rest of the planets in joyless rockishness. The so-called habitable zone, where Goldilocks finds it just right for liquid water to exist on a planetary surface, will steadily move outward, warming Mars and, in the very far future, the ice-locked moons of the outer worlds, providing a new, but relatively short-lived, home for life. If humanity is (a) still around and (b) emotionally/physically attached to the Earth, we could soothe the burning heat by lassoing an asteroid, sending it in a looping orbit to gently tug the Earth outward through the millennia and keep us right in the sweet spot. But that is definitely Somebody Else's Problem.

Over the course of a few billion years the Earth's core will cool down and turn solid, switching off our planetary defense screen (a.k.a. the magnetic field), rendering our atmosphere exposed to the onslaught of the solar wind. Plate tectonics will shut off too as the interior cools, eventually making the Earth, once vibrant with life, a wasteland.

Due to tiny gravitational interactions with Jupiter, Mercury's orbit is unstable and chaotic on long timescales. Within a few billion years it has a decent chance of simply ejecting from the solar system with no more warning than a “Later, guys!”2

Assuming we don't hitch a tow cable to the Earth and it stays in its current orbit, the onset of the sun's red giant phase is the offset of the Earth. When the sun swells it swells, with the outer tendrils of the engorged atmosphere reaching to our orbit. Once embedded in the outer layers of the demon sun, our home planet (and perhaps the only planet we lonely humans will ever know) will have a measly fifty days before friction drags it inward to complete, crushing oblivion. The only sign that our entire planet even existed—with its abundance of life and history and civilization and parking lots—will be a minuscule elevation of the metal enrichment in the star that killed its most beloved child.

There's a small chance that the temperamental fluctuations, mass loss events, and other outbursts during this twilight phase of the sun's life will simply destabilize, rather than consume, the Earth, allowing it to persist as an overroasted marshmallow orbiting the increasingly decrepit sun. So there's that small comfort if you need to sleep tonight.

While the nuclear drama unfolds in our solar system, a greater performance of two massive lovers plays out on a much larger stage. At about the same time our sun breathes its last, the Milky Way and Andromeda, racing together through the vastness of empty space though all these eons, will finally—and tragically—embrace. At first it will just be a touch as the outer spiral limbs encounter each other. But the headlong rush, as paramours across the world know, is inevitable. Over the course of hundreds of millions of years, just as our sun settles into long-term white dwarf retirement, the galaxies will merge into one. There will be multiple passes as the remnant galaxies swing back and forth through each other before finally settling down, and each encounter will trigger a new round of fresh star formation as gravitational ripples send shock waves racing through the once-quiescent nebulae.

But that embrace comes at a price. The onrush of new stars eats up the available supply of gas in a flash. Isolated, the galaxies could have steadily produced new stars far into the future without exhaustion, but fueled by hormones and newfound feelings, they drive their star formation to rates not seen since the reckless early days of cosmic history. The combined galaxy, finally merged after these countless years of eager anticipation, eats itself alive from the inside out.

If our cosmological near-term fate is a bit maudlin, well, I hope you're sitting down, because from here it only gets worse.

But first, a word of caution. The events I've foretold are coming relatively soon and are relatively certain to happen. We see what happens when our sun's cousins stop fusing hydrogen, because there's been enough time in the universe for us to be able to see it with our telescopes. We know what happens when galaxies collide, because the universe is littered with those galactic wrecks, and we've confirmed observationally that directions are locked in—Andromeda and the Milky Way are doomed to that fate.

These events will all play out over the next few billion years—not much longer than the baker's dozen that the universe has already been around. From the perspective of the intense exotic and nuclear ages, the current universe is impossibly old and cold. From the perspective of what I'm about to relate to you, our current epoch is barely a cosmic toddler. Just as how when we dug into our ancient past, the physics became increasingly murkier, so too will our predictions of the ancient future.

Our universe was outright bizarre billions of years ago. As we trundle on, it will become equally bizarre, inhabited by creatures we can only dream of now. Deep time plays deep tricks, and as the great cosmic web unravels itself from its current splendor, its constituents—the dark matter, the galaxies, the stars—will change and evolve. Over the course of hundreds of billions of years, far longer than the universe has currently existed, our cosmos will become almost entirely unrecognizable.

The story starting a hundred billion years from now is based on our current understanding of the universe, as flawed and possibly myopic as that may be. The further we go into the future, the weaker our confidence gets. To avoid littering the account with maybe and we think so and this one cosmologist estimates based on some questionable calculations, we need to make a deal. I'll start with the future history of our universe, assuming that dark energy continues to do its thing (accelerate the expansion of the universe), dark matter doesn't act up and do something weird, and all the physics we know hold across space and time. After that, we'll talk about some interesting alternatives.

But it doesn't really matter: every possible fate of the universe we can concoct, constrained by known observations, is equally miserable, just in its own unique, quirky way. It's the ultimate price for living in a changing, evolving universe. Once we realized that our cosmos is not static, that it was different in the past, we would inevitably begin forecasting its future. What an ironic twist from the dreams of Kepler: instead of the stars determining our fortunes, it's us who tell the stars their fate.

So let the misery commence.

Dark energy is cruel and implacable: it drives the expansion of the universe to ever-faster speeds without remorse or regret. Galaxies at the distant edges of the universe are already inaccessible to us—they are receding faster than the speed of light, which means even with the most powerful rocket conceivable, we could not hope to move there. The image of that galaxy is formed from light launched long ago; the actual object is now far removed from our reaching grasp. This is a normal and unsurprising feature of an expanding universe, but dark energy makes the whole thing worse, eventually causing even galaxies that are nearby (cosmologically speaking) to be wrested from our view.

Within a hundred billion years—ten times the current age of the cosmos—the entire observable universe will become…unobservable. Like losing your eyesight in old age, our view of the celestial realm grows further, redder, and dimmer. The distant galaxies that now populate with glee our deep-sky surveys won't technically disappear, but they'll grow so faint and so far redshifted that no telescope could ever distinguish their fading light from the background.

Virgo. Norma. The Great Attractor. Laniakea. The familiar structures of our nearby universe that have been slowly coalescing for billions of years will grind to a halt and reverse, the unmitigated outward pressure of dark energy driving them in the opposite direction. Only the truly gravitationally bound structures will remain. Our own Local Group, dominated by the triumvirate of the Milky Way, Andromeda, and Triangulum, together with a retinue of smaller dwarf galaxies, will persist, holding fast against dark energy's persuasive suggestions. Other clusters and groups will form their own island universes, separated from each other not just by vast gulfs of vacuum but by the limitations of the speed of light.

Within a few hundred billion years, the remaining members of the Local Group will complete their merger, initiated in the now dim and distant past, forming a single large but deformed galaxy, alone in the night.

Within that same time, the cosmic microwave background, that rock-solid bastion of evidence for the big bang, will fade into literal obscurity. It's already pretty cold—just a few degrees off absolute zero—but, lucky for us, clearly visible in the microwave. But with a couple hundred billion more years under its belt, the CMB will be exhausted, redshifting to frequencies so long that not even a telescope the width of the observable universe could detect them. It will, for all intents and purposes, vanish.

If any new life arises in those distant and dark times, they perhaps will never know of their true heritage. No relic radiation to signal the big bang. No distant galaxies to measure a cosmic expansion. They'll think they live in an essentially static universe, unchanging in space and time. If any Fred Hoyle analog arises in that civilization, he'll be right. The cosmological models of our past, of a relatively small universe encompassing the entirety of the galaxy, fixed in time, will turn out to be observationally indistinguishable from the expansion that we know today.

They might get lucky if they're patient observers, perhaps by noticing a peculiar pattern in the redshifting of stars ejected from their galaxy, and deduce that their cosmos was at one time far smaller. And the populations of stars remaining in that single, lone galaxy will indicate that perhaps the universe was different in the unimaginable past. But the universe as we have come to know it will be as alien and unfamiliar as the Planck epoch is to us now.

It makes you wonder if we're missing anything big with our current observational limits, but that's a little too uncomfortable to ponder, so we'll leave it at that.

Indeed, by this time stars as we know them may become a thing of the fanciful past: a tale of a distant golden age to thrill and excite the youngsters around the campfire. As we make the big leap from measuring time in billions of years to trillions of years, the remaining megagalaxy that sits alone in its otherwise empty universe begins to change from the vibrant, rich, colorful panoply that we know today.

For one, stars like our sun will slowly stop appearing and altogether die out. A life span of ten billion years may seem like a long time—and, don't get me wrong, it is—and for us in the present, that's a good fraction of the age of the entire universe. But ten billion is just 1 measly percent of a trillion. As the cosmological eons stretch out to incredible lengths, sunlike stars are as effervescent as fireflies on a warm summer night. The most massive stars, capable of detonating in brilliant supernova explosions, could be missed in a blink of the cosmic eye. And they'll become more rare overall—as star formation slowly grinds to a halt, the capacity for a galaxy to manufacture such massive nuclear beasts dwindles. There simply isn't enough gas in the right places at the right times to build them.

It will be the death of color. White and blue stars will, one by one, disappear from the cosmic stage. Abundant red and blue nebulae will disperse. With nothing left to spin them, the delicate filigrees of planetary nebula will dissolve. The only remaining stars will be small, cool, and red. Indeed, for every star you see in the night sky, there are a thousand in the same patch that you can't. Your eyes are liars—they're not telling you about the true nature of the galaxy. Most stars are too wimpy to be seen with the naked eye, but they're ridiculously abundant.

They already dominate the galaxy, and in a few trillion years, they'll be all that's left. Sole inheritors of a denuded empire.

The small stars win out over long times because they're the economy cars of the galaxy. Their low mass means that gravity is less insane than in bigger stars, so fusion reactions occur at a slightly less eager pace. Their interiors are also constantly churning, drawing fresh reserves of hydrogen into the depths, where it can keep the nuclear fire stoked. Proxima Centauri, another star you can't see with the unaided eye despite the fact that it's so proxima that it's our nearest neighbor, will keep chugging along for another four trillion years. In the game of galactic races, always bet on the tortoises.

All stars grow steadily brighter as they age, and since these dwarfs are so numerous, the galaxy of the far future will be roughly as bright as it is today, with its output dominated not by a handful of searchlights but by innumerable cheap LED flashlights.

Stranger things begin to appear. As heavier elements continue to pollute the intergalactic waterways, it changes the makeup of new stars. Not only will big stars become more rare, smaller stars—far smaller than the dwarfs of today—will become possible. The more heavy elements a nebula carries, the more efficiently it can cool itself, allowing chunks of itself to squeeze down tightly and make smaller stars—less than a twentieth of the size of our sun, or about half the mass of the current smallest known star.

A star that small is hard to comprehend, because it's simply infeasible in our current epoch. But given enough time, it'll be common. You'd be surprised to find anything else. And with the heavy elements mixing around its bulk, a star that small could potentially reach surprisingly cool temperatures.

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.