This Is the Way the Universe Ends

I doubt I am sticking my neck out when I say that most people will wake up on December 22, 2012, and find the world much as they left it the night before. Prophecies about the demise of our planet—or the end of civilization—come and go. So, oddly enough, do theories put forth by cosmologists describing the ultimate fate of the universe. It is a seemingly endless debate: Will the universe keep expanding forever? Or will it perhaps turn around, only to end in a Big Crunch, no less cataclysmic than the Big Bang that created it 14 billion years ago? Or will it, as appears increasingly plausible, meet an even more spectacular end, one that physics could not have predicted until just a few years ago? The long-term forecast for the universe has changed twice in the last decade. Here, I report the latest in cosmic eschatology.

For most of the twentieth century, the issue appeared to be simple. Cosmic expansion is slowed by gravity, so the question boiled down to whether gravity was strong enough to turn the expansion around. If the average density of matter in the universe was above a critical level, expansion should give way to contraction. All structures—stars, planets, and even atoms, would be destroyed in the final moments of the collapse. If, however, the density was below critical, the universe would expand forever. The stars would eventually run out of nuclear fuel, and the universe would descend into darkness, its temperature dropping ever closer to absolute zero. All we had to do to determine the fate of the universe, it seemed, was measure its average density. And for a good part of the last century, astronomical measurements put the density below critical, pointing to an icy end.

The story took a new turn in the late 1990s, when astronomers made a startling discovery. By studying the brightness of distant supernovae—those blazes of glory in which massive stars go out—they showed that instead of being slowed down by gravity, cosmic expansion is actually accelerating. This can be explained only if the universe is filled with some gravitationally repulsive stuff—and lots of it, so that its effect overwhelms the attractive gravitational pull of ordinary matter. The main contender for this role is the vacuum, or empty space. There is, of course, no shortage of it in the universe.

Vacuum, it should be pointed out, is not the same as nothing. According to modern theories of elementary particles, vacuum is a physical object; it can exist in different states and have energy. By the E = Mc² relation, each cubic centimeter of space should also have mass, which means it should produce a gravitational force. But here comes the most peculiar property of the vacuum: its gravity repels instead of attracts. The higher the mass density of the vacuum, the stronger the repulsive force it exerts.

In early cosmic times, soon after the Big Bang, matter was much denser than vacuum. And as a result, the repulsive gravity of the vacuum was too weak to counteract the attractive gravity of matter. Then, as the universe expanded, matter thinned out. There came a time when the average density of matter dropped below the density of the vacuum. It was at that point—about a billion years ago—that cosmic acceleration began. In the last billion years, the universe has roughly doubled in size.

Once started, the accelerated expansion continues forever. Thus, the prediction of an icy finale still holds, but some details need to be modified. In particular, as other galaxies accelerate away from our Milky Way, they cross the speed-of-light barrier and can no longer be seen. (Einstein’s ban on faster-than-light motion does not apply at large distances in an expanding universe.) This is a distinctive feature of an accelerating universe. One by one, galaxies will disappear from view, and in a few hundred billion years, astronomy will become a very boring subject indeed.

You might think that we now have the future of the universe firmly in hand. But hold on, we are not done yet. The most recent twist in cosmic futurology comes from string theory. This branch of physics postulates that the basic constituents of matter are not particles, as previously thought, but tiny vibrating loops. It is now our best candidate for the fundamental theory of nature. Physicists recently determined that string theory predicts the existence of an immense number of vacuum states besides the one that pervades our local universe, each with different physical properties. Some of these vacuums have positive and some have negative energies. This means that our vacuum does not have the lowest possible energy and must therefore be unstable.

Sooner or later, a tiny negative-energy bubble will form in our cosmic neighborhood (this could happen anywhere, even in your living room). The bubble will then expand, engulfing more and more space. It may be trillions of years before any such occurrence. Then again, we cannot exclude the possibility that an expanding negative-energy bubble is charging toward us at this very moment. It will come without warning: any light the bubble emits will not get to us much ahead of the bubble itself, since it expands at nearly the speed of light.

Once it arrives, all familiar objects will be turned into clumps of some alien form of matter. Under the pull of the bubble’s vacuum (which, thanks to its negative energy, has gravity that attracts instead of repels), as well as the tug of its own gravity, the matter inside the bubble will gradually stop expanding and begin to contract. Eventually, it will all collapse in a Big Crunch. Thus, as of this writing, the universe is going to end in fire. Stay tuned.

ALEX VILENKIN is professor of physics and director of Tufts’ Institute of Cosmology. He is the author of Many Worlds in One: The Search for Other Universes (Hill and Wang, 2006) and is the father of Alina Simone, J97, SMFA 97, who is the subject of “Siberian Express.”