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 tim
e 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.
Death From the Skies!: These Are the Ways the World Will End... Page 28