Death from the Skies! (40 page)

And they won’t last forever anyway.

By 10
²⁰
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 10
³³
to 10
⁴⁵
years (the exact number is unknown, so for argument’s sake we can pick an intermediate time of 10
³⁷
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 10
³⁷
years. In another 10
³⁷
years, half more will disintegrate, and so on. After a few times 10
³⁸
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 10
³⁷
years in the future. White dwarfs will lose mass
¹³³
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 10
³⁸
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 10
⁴⁰
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.

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 10
⁴⁰
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.

Once again, though, we have to be careful when we talk about “slow.” When we have ten thousand trillion trillion trillion years to play in, “slow” can still happen. Given time enough, a black hole will completely evaporate through Hawking radiation.

A stellar mass black hole has a minimum mass of about three times that of the Sun. Pinging away particles one by one, it takes a long time to slog through six octillion tons of black hole: about 10
⁶⁶
years. To us, today, that seems like forever. But even that is the blink of an eye compared to the time it takes a supermassive black hole to blow away. The billion-solar-mass black hole that was once the Milky Way Galaxy (and Andromeda and several others from the Local Group) will take a whopping
10
⁹²
years
to evaporate to nothing.

And that’s it. I’m out of analogies. I give up. I was hoping to come up with something like, if the life span of the Universe up until now were a single beat of a hummingbird’s wing, then 10
⁹²
years would be like, well, like something that takes a
really
long time. But even comparing a single flap of a hummingbird’s wing to the current age of the Universe falls completely and hopelessly short of comparing the present age of the Universe to 10
⁹²
years. That’s just too long a time span. It crushes our sense of reality to dust. The closest analogy I could think of is to compare the mass of a proton to the mass of the entire Universe, but this analogy is useless. Analogies are supposed to make things easier to grasp, and who can grasp the mass of the proton, the mass of the whole cosmos, and then take the ratio?

Worse,
the analogy actually falls short of reality.
The ratio of 10
⁹²
years to the current age of the Universe is about 10
⁸²
, while the ratio of the mass of the Universe to the mass of a proton is 10
⁷⁹
. The analogy fails by a factor of 1,000.

So I give up. You’re on your own for analogies now.

But perhaps we’re done anyway. The most massive object in the Universe has evaporated away using the slowest process in the Universe. When it’s done, there’s not much left. The entire observable Universe will be only a million or two light-years across, and it will consist of countless electrons, positrons, neutrinos, a handful of exotic particles, and extremely low-energy photons. It will be an incredibly thin vacuum, far more rarefied than anything that exists today.

And that’s it. That’s all there is. Once the black hole is gone, everything familiar in our Universe will go with it.

The Universe will be dead.

The endless gulf of time stretches ahead of us now. At this point, our math breaks apart. The Universe is such a thin soup that it could be countless years before any two particles approach each other. And if they do, what will happen? If the two particles are both electrons, they repel each other and off they go in opposite directions. If one is an electron and the other a positron, they’ll attract each other, collide, and poof! They’ll make a pair of gamma rays that fly away.

But where will they go?

Every trace of the Universe we know today will be gone. No stars, no planets, no people. Not even matter. It will all have decayed away, eroded into an ethereally thin slurry.

10
¹⁰⁰
years, 10
^1,000
, 10
^1,000,000
. It’s all the same. Nothing ever happens, and nothing ever will. The Universe is dark, randomized, silent. And it will remain so forever.

Oh, but there’s that word again.
Forever.

As we’ve seen many times in this chapter, nothing is forever. Maybe not even Universal death.

There are some faint hopes for the ultimate fate of the Universe. Most involve the complete destruction of the Universe as we know it and the reconstruction of something entirely new and different.