Prior to this posts, I've discussed the history of the universe, as we currently can observe and understand it, up to present day (give or take a few million years). If you missed those, here's Part I and Part II, which will catch you up. But really, as cool as the history of the universe is (and it's pretty neat), I wrote those posts so that I could write this one, about what happens next. Fair warning, it's fairly bleak and existential, albeit fascinating. Now, with Halloween around the corner, I've got a scary story for you: how the universe will (likely) end.

Right now, we're still in a very young universe, in what is called the stelliferous era. Stars, especially Population I stars that are so good for life like us, abound in the universe, and light and matter are everywhere. There is no shortage of fuel for new stars, and supernovae will result in new elements being scattered across the cosmos and the creation of stellar nursery nebulae. We are living in the universe's golden years, and we got on near the beginning of them. But even now, something insidious is happening, if you look for it. We don't know exactly what it is, but we have a name for it: dark energy.

Dark energy is the name cosmologists have for the mysterious force that seems to be pushing the universe apart. Sometime in the last few billion years, it become the dominating factor in the universe's expansion, and continues to push things apart which aren't currently being pulled together by gravity. And it's only getting stronger. The more empty space between things, the stronger the force pushing space apart, and the greater the expansion of space between them. The universe has been expanding since the Big Bang, it that expansion continues to accelerate. As the gaps between galactic clusters and groups become larger, the space between them expands at an ever-faster rate, pushing everything even further apart. Even now, we can see the foreshadowing of a darker future.

However, long before dark energy becomes the terrible scourge that it will one day be, we have other, more local things to worry about. The sun is going to keep getting brighter, and burning hotter as time goes on. Assuming we haven't been wiped out by an asteroid or supernova radiation, in around 600 million years the increased heat and light from the sun will start disrupting normal cycles in the Earth, bring about halt to plate tectonics. This will wipe out nearly all plant life, as carbon dioxide will not be able to be recycled through volcanic and tectonic activity. By one billion years after the present day, the Earth will have lost much of its surface water from runaway greenhouse effects due to the ever-increasing brightness of the sun. If we're still around, humanity will likely have abandoned it for another planet. Mars will star looking cozier, and by one and a half billion years, it will be smack-dab in the middle of the sun's habitable zone, where Earth used to reside. The sun continues its march to burn bigger, brighter, and hotter. Soon, Mars will no longer be safe either. But for us, something more interesting will happen before our sun hits the next phase of its life.

Our closest neighbor galaxy (excluding the so-called "dwarf galaxies" that we have orbiting us) is Andromeda. Currently, we're on a collision course with it, as the massive gravity of both our galaxy and it pull us inexorably together. And, at four billion years from the present day, we will finally collide. This will be much less dramatic than one might thing, as it will take an rather large amount of time to happen. However, some stars, even whole solar systems, could be flung out into intergalactic space (becoming the awesomely-named hypervelocity stars) and the supermassive block holes that exist at the center of each galaxy will merge into a bigger, badder black hole (for a bigger, badder galaxy). Astronomers have a name for this bigger, badder galaxy: Milkomeda.

Less than a billion years after this, our sun, having consumed all of its hydrogen, will enter the final stage of its life, expanding into a red giant. If humanity is still around and has not left the confines of our solar system, it would be in our best interests to begin doing so.

The sun will continue to expand. Over the next two to three billion years, it will subsume Mercury, Venus, Earth, and possibly Mars as it marches onward in its relentless growth. Finally, as it reaches its maximum radius of over 250 times its current size, it will shed much of its mass in a nova, and all that remains of the sun will by a tiny, much less luminous white dwarf star of less than half the sun's current size. From here, considering the Solar System is toast and we no longer have immediate local concerns, we're going to skip ahead by a lot.

The stelliferous era will last for a very long time. There will be other stars like the sun being born all throughout it, and the end of our own star, while dark, would certainly not be the end of life in the universe, or even human life. We would have plenty of time to move to new places in the galaxy, or possibly even travel between galaxies. However, the insidious pull of dark energy cannot be stopped. And we'll find that other galaxies we may want to visit keep moving further and further away from us as space itself grows larger between us and them. In one hundred billion years, galaxies that are outside what is known as our local group will be forever inaccessible, as even moving at the speed of light the space between us and them is too vast, and expanding too rapidly. Our local group is still gravitationally bound and the pull of that attraction keeps the dark energy at bay, and around that same time, all those galaxies will have merged into one, much like Milkomeda. Humanity, if it has survived, will now be incapable or escaping this new supergalaxy for another. Light emitted from us will now never reach those other distant galaxies, and the light we see from them will only be billion-year-old echoes that will slowly fade over time.

But hey, look on the bright side (literally)! We're still in the stelliferous era, and we've got plenty of stars. New stars are still forming, the our local group supergalaxy is doing just fine. So what if we can't reach any other galaxies ever now? Well, that is a bit disheartening, but life can still flourish, and the universe is far from the desolate place that it will eventually be. But we're getting there.

Our local cluster continues to coalesce over the next few hundred billion years, and we get some truly wicked black holes. Keep them in mind, because they'll outlive almost everything else. Finally, our descendants, if we have any, or maybe some other intelligent life that has evolved, will look to the sky outside of our megagalaxy and see that those distant other points of light, the other galaxies we'll never be able to reach, have begun to fade away. By two trillion (2,000,000,000,000) years after the Big Bang, the light they emit will have been so redshifted by the expanding universe that its wavelength will be longer than the observable universe, and will be completely undetectable. Any civilization that rises after this point will assume that there is only the local cluster, and will be unable to find any evidence or a Big Bang or an expanding universe, now that the last distant galaxies' light has faded and the cosmic microwave background radiation has redshifted into imperceptibility. New stars continue to be born and die, and the universe moves on, heedless of the loss that has occurred.

The stelliferous era lasts a long time. There's plenty of fuel for stars out there, and while they'll mainly be Population I stars with high metalicty (astronomy-speak: "metals" are any elements heavier than Hydrogen and Helium, "metalicity" is the amount of "metals" in something), new stars will continue to form and burn for almost a hundred trillion years (100,000,000,000,000 years, or 7,300 times longer than the universe has currently existed). However, the supply of fuel for stars is not limitless, and whenever a star burns, it consumes Hydrogen and Helium and produces heavier elements that can't be used well in other stars. Eventually, star formation will altogether cease. Throughout the night sky, light will slowly fade as old stars wink out of existence in fantastic novae, and new ones fail to take their place. White dwarves, red dwarves, and neutron stars are all that remain to still give faint light, and black holes will give off the nearly imperceptible ghostly glow of Hawking Radiation. Now, about one hundred trillion years after the Big Bang, we have entered the degenerate era.

Things are pretty bleak from here on out. There's no happy ending to this story, only a gradual, creeping existential dread. As we sink slowly into the degenerate era, the last vestiges of stars wink out, beginning with the red dwarves, which, while they burn slowly and relatively cool, only last around ten trillion years. Over the course of tens to hundreds of trillions of years, the universe becomes dark. No new light sources appear, excepting the occasional white dwarf that can come about from chance collisions. These don't last as long as anyone would like, though, and the universe becomes a cold, desolate, and lightless place, populated only by the husks of dead stars and lifeless rocky or gaseous planets with no luminosity. They exist along side the black holes, which continue to abide, and will for a very, very long time.

As the degenerate era continues, former galaxies disperse as chance passings of masses fling lighter planetoids out into deep intergalactic space. As the galaxies shrink, the relatively "lighter" planetoids grow larger until only around one percent of the original objects in the galaxy remain. The Earth, if it had somehow survived until this point, would now be flung out into deep space, with nothing at all around it for thousands or millions of light-years, or it would likely be swallowed by a black hole if it remained. The last remaining real structure to the universe has disbanded.

How long the degenerate era will last is a point of some contention among physicists and astronomers. It all depends on whether or not protons, one of the basic building blocks of matter, decay into lighter particles. If the proton is an unstable structure, experimentally, we know that its half-life is at least 10³⁴ years, or 10,000,000,000,000,000,000,000,000,000,000,000 years in long form (in words: 10 duodecillion years). So, by that time, protons, something we take for granted as being fundamental to everything, have begun to fall apart. Matter itself, as we know it, has become unstable and can no longer exist. Even if humanity, or some other species, has managed to survive to this point, for 2.7 sextillion times the current age of the universe, they could survive no longer, as the very material that made them up is now decaying (also, there isn't really any energy to keep them going, it's all gone with the stars). As all matter decays around them into its smallest possible components, the black holes abide. We have entered the black hole era.

In the black hole era, black holes are the only thing of substance dominating the universe. Even if protons do not decay, all that will be left of matter will be cold and without any non-kinetic energy (as it hurtles through the vast, empty, and lightless universe). Black holes will reign over the near-dead universe as the last vestiges of energy, the lords of gravity. But they are not eternal. Though their lifespans are measured in such absurd numbers as even a googol (10¹⁰⁰, or a 1 followed by one hundred zeros) years, they do die. Through a process called Hawking Radiation, they slowly lose energy and mass. Now, in the black hole era, with matter scattered to the dark cosmos, they cannot absorb more matter to offset their loss, and they slowly evaporate. As a final salute to the stelliferous and lit universe that spawned them, as they die each can give off light comparable to that of a star, once again bringing a small glimpse of what was to a now largely black universe. As the last black holes die out, the universe will become permanently dark. The black hole era ends, and the dark era begins.

Gravity is no longer a force to be reckoned with as we leave the black hole era. Matter is disparate, and existing only its its lightest forms—neutrinos, electrons, and positions. Photons, redshifted to oblivion by the increasingly expanding universe, stream unimpeded through the darkened universe, giving the dark era the alternate name the photon age, though that name implies a luminescence that simply doesn't exist. The universe is dark, it is lifeless, and it is empty. Matter is spread too thin, and atoms themselves no longer exist (unless protons do not decay, in which case matter slowly decays into iron, creating the awesomely-named iron stars, which drift cold and uncaring though the cosmos, and eventually collapse into black holes, which, when they evaporate, bring us back to the beginning of this paragraph). From a googol years onward, the dark era drifts onward, and the universe slowly reaches its lowest possible energy state. This is called the heat death of the universe, and it is the unavoidable doom that awaits us in the far future. The universe had a beginning in the Big Bang, and so to it must have an end.

What happens after the heat death or the universe? Who knows? This is a time over 10²⁵⁰⁰ years in the future, and its possible that the universe will remain in that state forever, with nothing more happening as dark energy expands it into infinity. A scenario called "The Big Rip" may occur, whereupon the universe, in its expansion, tears itself to pieces, ripping the very fabric of spacetime into nothingness. The current vacuum state could be a false vacuum, and the universe could decay into a lower-energy state, which could trigger a new inflationary period or a new Big Bang. Given long enough time frames, its also possible that quantum tunneling could give rise to another Big Bang in an absurd 10¹⁰⁵⁶ years (a one followed by 10⁵⁶ zeros. More understandable, one hundred followed by a thousand trillion trillion trillion trillion zeroes. This number is so beyond comprehension I can't even think of a good way to express it). We really don't know, though. One thing is for sue, though: nothing can survive to that point. The universe, as we know it, or even as it was known in as late as the black hole era, is no more. Whether it be eternal heat death, or a new Big Bang, our universe has had its run, and its time is up.

Really, though, there's some positive stuff to this: we're here in the really early universe. Just a scant few billion years into the hundred-trillion-year-long stelliferous era. We exist in a young universe, and we have the chance to go out and explore. We are around at a time when, theoretically, we can still reach other galaxies. We can see other galaxies! The cosmic background microwave radiation is still fresh, so we can detect how our universe started. We have a lot a time ahead of us before things get bad, and we should make the most of it. Just because humanity can't survive the eventual darkest night of the far-off future doesn't mean that now we shouldn't be out there seeing and doing everything we can while the universe allows us. The death of all things is inevitable, but that has never stopped us before, and it shouldn't now. Let's go out and find everything there is to discover in the universe, while it lasts.