Death From the Skies!: These Are the Ways the World Will End...

by Philip C. Plait

  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 1066 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 1092 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 1092 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 1092 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 1092 years to the current age of the Universe is about 1082, while the ratio of the mass of the Universe to the mass of a proton is 1079. 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 DARK ERA : T + 1092- ∞ YEARS

  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.

  10100 years, 101,000, 101,000,000. It’s all the same. Nothing ever happens, and nothing ever will. The Universe is dark, randomized, silent. And it will remain so forever.

  REBIRTH?

  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.

  You might consider that a drawback.

  But the alternative is the boring Universe where nothing ever happens. So let’s see what we’ve got.

  The Big Bang was a singular happening. Somehow, all matter, energy, space, and time were generated in that one event, forming the Universe as we know it.

  But where did that event come from?

  As discussed earlier, there are some theories that there is a meta-Universe, someplace other that exists outside of our framework of space and time. It developed a little quantum hernia, and this formed our own Universe. If that’s really the case, then the death of our Universe isn’t that big a deal in context. The other Universe may still be there, and it’s possible that it budded off countless other universes too. These may all have vastly different laws of physics (maybe in one the speed of light is a few miles per hour instead of 186,000 miles per second, and in another the electron has more mass than the proton, instead of the other way around as in ours). It’s also possible that our own Universe is doing this all the time—even now, tiny offspring universes are popping into existence in other “places,” outside what we can see and investigate. However, according to everything we understand about physics, we can never physically learn anything about these other universes, so for all practical purposes they don’t exist.

  Of course, it’s conceivable that in the next, oh, quintillion years or so our understanding of physics may change. I’ll readily grant that! But for now there’s not much we can say about this.

  But maybe we’re starting off on the wrong premise. Maybe we should ask: was the Big Bang actually the first cause? Or was there some other event that jump-started the cosmos?

  There is another idea, still in its infancy, called the ekpyrotic universe (Greek for “from [or out of] fire”). According to this idea, the Universe is already incredibly old. At first, it was basically a giant void, with nothing interesting happening (much like the way we left it after 10100 years). According to this theory, there exist other universes with characteristics similar to those of ours now, but these other universes are outside our view. They exist in eleven dimensions instead of the four with which we are familiar (the three dimensions of space and one of time), and they float around in this extradimensional space. Called branes, short for membranes, these are all self-contained universes like ours in many ways, generally minding their own business.

  But sometimes they collide.

  You can picture these universes as parallel plates floating around. When they smack into each other, they shake up the contents rather vigorously. The theory predicts that the universes would get violently disturbed, with energy and matter being heated up tremendously, and space itself set to expand.

  Sound familiar?

  This may sound a little like fantasy, but it’s all part of a set of very complicated but scientifically based math and physics theories. No one has any idea if these theories really are viable alternatives to the Big Bang model, or if it’s just so much fantasy. But the ideas are internally consistent, and are being studied very seriously.

  If they pan out, then there is some hope for the Universe, or at least for some meta-Universe: it means other universes exist, and they might be habitable. We can never reach them, so that’s too bad for us, but maybe other species in those universes can survive.

  And there is some hope for us as well. What happens once can happen again, especially if you wait long enough.

  In 10100 or 101,000 years, however long it takes, another brane may collide with ours. When it does, it may spark a reignition of the Big Bang, kick-starting our Universe once again. When that happens, pretty much everything that happened in the Universe before that point will be destroyed: kindling, if you will, for the fires of a new Universe. Again, that’s too bad for us, but it does mean that there is a cyclical nature to reality, and a chance for life to rise anew.

  Again, cold comfort. But it’s a possibility.

  DOWN THE LONG STAIRCASE

  Yet another fate may wait in store for us in this far distant future, and we have even less of an idea of what will exist after it unfolds.

  Objects have what is called an energy state. It’s a bit like climbing a set of stairs. On the bottom step you are at the lowest energy state, and at the top you’re at the highest. There are energy states in between too. It take
s (muscle) energy to move up to higher states, and you give up that energy when you go back down.134 Sitting at the bottom, you’re as low as you can go, and you stop.

  Atoms behave this way: electrons zipping around an atomic nucleus have certain energy states available to them, with none in between (just as you can’t stand on the four-and-a-halfth step; it doesn’t exist, so you have to be on either the fourth or the fifth step). This is one of the most basic ideas of quantum mechanics.

  Perhaps the Universe behaves this way as well. We think of the vacuum of space as being, well, a vacuum. Empty. Devoid of matter and energy, and therefore at its lowest energy state.

  But this may not be the case: we know that space bubbles and boils with energy at extremely small size scales (this is the basis for Hawking radiation, in fact). So what if we’re not at the lowest energy level, the lowest energy state? What we’re experiencing now would then be a “false vacuum state,” and we might take that final step down, dropping to a lower state.

  The starting point for this drop is difficult to predict. Maybe it’s a quantum effect, again like Hawking radiation: somewhere, someplace in the Universe, a teeny-tiny bit of the Universe suddenly drops to the lower state. According to the theory, this one event acts as a trigger, pushing regions around it to drop into the lower state as well (imagine standing on the second-to-last step from the bottom with ten other people; you jump down and drag everyone else with you). A cascade starts, with more and more bits of the Universe dropping to the lower state.

  This tunneling event, as it’s called, would expand outward in a sphere at very nearly if not at the speed of light. It’s a bit like a sugar crystal growing in a supersaturated solution; once you start it someplace, the other sugar molecules attach themselves there, growing rock candy.

  In this case, though, the rock candy is the collapse to the true vacuum, and the sugar molecules are actually the fabric of space itself. The damage wrought is literally total.

  Inside this expanding bubble of vacuum collapse, the laws of the Universe change. Space and time themselves are rewoven, becoming something entirely new, something the nature of which we cannot even begin to guess. Anything caught in this wave will be utterly destroyed.

  And there is literally nothing to stop it. The entire Universe sits on the second-to-lowest state, so once poked by the expanding bubble, everything will collapse. Every star, every planet, every black hole, every human.

  That would be pretty bad, were it to happen today. Odds are it won’t; the chances of this happening are extremely small. But if this event were to wait, say, 10200 years, would that be so bad? I argue it would be good. By that time the Universe will be dead, stagnant, with nothing to show for all those years of activity. A collapse of the false vacuum to the true vacuum would possibly reenergize the Universe, giving it a second chance for life.

  So there is some hope. You and I and even the entire Universe as we know it won’t be there to witness it, much less survive it.

  But afterward, a new Universe will be created, sparkling and clean and ready for a new start. In this case—and also that of the ekpyrotic universe, and maybe even other processes we haven’t even yet conceived of—there is a chance that instead of a bleak and dim future, filled with nothingness for all eternity, there will be a rebirth of the Universe, and the rebirth of possibilities.

  And if it happens once, it might happen again in another 10200 years, or 101,000. And again, and again. Endlessly.

  Rather than dealing out death and mayhem, destruction and chaos, the Universe will cyclically clean itself out, reboot, and set everything in motion once again.

  Each time, perhaps, the laws will be different, and the characteristics of that future infinite parade of universes will be grandly set apart from what we know today. And despite our prejudices, the Universe appears to have no set of rules on how things need to be for complex chemistry, for life, to arise.

  We don’t know for sure if there are aliens in our own Universe now, though the odds favor such a possibility: there are 200 billion stars in the galaxy, and hundreds of billions of galaxies in the Universe.

  And so I wonder: can we now multiply those odds by the number of potential universes that lie ahead as well?

  If that’s the case, then the Universe provides us a near-infinite number of do-overs, something I find very uplifting. It may seem that the Universe spends all its time trying to kill us, but in the end—the very end—there may yet be Life from the Skies.

  EPILOGUE

  What, Me Worry?

  STILL HERE?

  Good. It’s been quite a ride, but I hope that while you read this book you weren’t vaporized, crushed, irradiated, flung out into deep space, spaghettified, or had any of your protons decay.

  We’ve covered a lot of scary ground (not to mention space and time). It seems as if the whole cosmos is trying to snuff us out. In a sense, it is—there’s danger aplenty in the Universe—but we have to take a practical view here. We have to appreciate the vastness of space and time, and our ability to manipulate events around us.

  Asteroid impacts provide an excellent example of practical versus theoretical danger. They have done ferocious damage to the Earth in the past, and our relatively fragile economic system could be destroyed by far smaller impacts than the one that did in the dinosaurs. To understand the actual danger, we have to balance the idea that they don’t happen very often with the knowledge that they do in fact sometimes happen.

  In your daily life, this may not present much of a problem (except for when you lie awake at night and your brain, unfettered by the common sense available during the day, is wondering if tonight is the night). But in the case of impacts, we can actually prevent them from happening. It would cost a lot of money (hundreds of millions to test various ideas, and hundreds of millions more to implement them), time, and effort. Can we afford to start worrying about this now? Can we afford to wait?

  Scientists are asking these questions, because they have to ask Congress for a lot of money to be able to do anything about them. And what Congress decides influences your money (other governments may become involved as well). To make sure we get the right answers, we really have to understand the issues. Astronomers are looking at a theoretical danger and finding a practical action to take against it.

  I hope this book has cleared some of that up. I studied gamma-ray bursts for many years, and I personally am not at all worried about them. Nor do I fret over the death of the Sun, the eventual decay of the Universe, or a black hole slipping through the solar system and snacking on the odd planet or two. That’s because I understand the odds of these events actually taking place, and they are vanishingly small. There’s no need to worry about them, whereas an asteroid strike or a particularly nasty solar coronal mass ejection can do a lot of damage. Even then we have it within our power to minimize their impact.

  In an effort to make all this a little easier to digest, here is a table that gives the odds, the potential damage, and our ability to prevent the disasters described in this book. What you’ll see is that chapters 1 and 2—asteroid impacts and solar events—cover the only two events we can do anything about. While they may not happen tomorrow, they will happen, and it’s in our best long-run interest to do something about them.

  Following the table is a description of how I came to those conclusions. Bear in mind that astronomy is a field of science, and that means that things change as better data and better ideas come along. Don’t assume any of this is written in stone.

  Of course, in 1040 years or so, even stone will be long gone.

  EVENT DAMAGE ODDS OF FATALITY (PER LIFETIME) PREVENTABLE?

  Asteroid impact Local for a small rock, global for a big one 1 in 700,000 Almost 100% preventable

  Identify potential impactors, then blow them up or push them out of the way

  Solar flare/CME Collapse of power grid, potential ozone depletion 0∗ Not preventable, but mitigable

  Build robust power grids

/>   Supernova Ozone depletion, radiation 1 in 10,000,000 Not preventable

  Gamma-ray burst Ozone depletion, radiation, setting planet on fire 1 in 14,000,000 Not preventable

  Black hole Destruction of Earth 1 in 1,000,000,000,000 Not preventable

  Alien attack Humanity wiped out by aliens; space bugs give us runny noses ? Preventable, assuming we colonize the galaxy first; otherwise, forget it

  Death of the Sun Earth cooked to a crisp 0† Not preventable, but we have a long time to go yet

  Galactic doom Ice ages, radiation, eaten by supermassive black hole 0† Not preventable, but again, none of these will happen on a human time scale

  Death of the Universe Decay of all matter, collapse of false vacuum 0† Not preventable, but dwarfs any time scale we can imagine

  ∗ Fatilities are very unlikely from a solar event, but they can still cause extensive damage.

  † These events all take billions of years (at least!) to unfold, so the chances of their happening during your lifetime are zero, but they are inevitable over longer times.

  ASTEROID AND COMET IMPACTS

  Of all the woes facing us from space, this is the one that is nearly 100 percent preventable. Scientists and engineers have viable ideas on how to stop big impacts—ones big enough to do significant damage. The real problem is in finding these objects in time, and even that is improving as more surveys of the sky find more objects. However, it’s physically impossible to find every single potential impactor; some come from so far out in the solar system that we simply cannot see them until they are on the way here.