Some say the world will end in fire, some say ice. Here's what the scientists think. Artenol photo illustration

Cosmic nailbiter

The fate of the world: an update

By Alex Vilenkin

 

The world as we know it began some 14 billion years ago in a huge explosion called the “Big Bang.” The universe is still expanding after that primordial blast, as evidenced by the distant galaxies we observe moving away from us at very high speeds. Will the expansion continue forever? Or will it perhaps turn around, only to end in a big crunch, no less cataclysmic than the initial explosion? This question has been at the focus of cosmological research for nearly a century, but the answer has proved elusive. In fact, the long-term forecast for the universe has changed twice in the last decade alone. Here, I report the latest in cosmic eschatology.

It seemed initially that the issue was very simple. The cosmic expansion is gradually slowed down by gravity, so the question boils down to whether or not gravity is strong enough to turn the expansion around. It all depends on how much matter there is in the universe.

If the average density of matter is above a certain critical value, the expansion will halt and will be followed by contraction. The temperature will then steadily rise, until it reaches very uncomfortable levels. Any creatures who manage to survive until this late epoch will end their days like lobsters in boiling water. All structures – stars, planets and even atoms, will ultimately be destroyed in the final moments of the collapse. Alternatively, if the density is below critical, the universe will keep expanding forever. The stars will eventually run out of nuclear fuel, and the universe will descend into darkness, with its temperature dropping ever closer to absolute zero.

It thus appears that all we have to do to determine the fate of the universe is to measure its average density. For a good part of the last century, astronomical measurements indicated that the density is below critical, pointing to the icy alternative.

Picking up speed

The story took a new turn in the late 1990s, when astronomers made a startling discovery. By studying the brightness of distant supernovae, they showed that, instead of being slowed down by gravity, the speed of 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 job is the vacuum, that is, empty space. There is, of course, no shortage of it in the universe. Einstein suggested a long time ago that each cubic centimeter of space could carry a nonzero energy. Then, by the E = MC² relation, it should also have a nonzero mass and should produce a gravitational force. But here comes the most peculiar property of the vacuum. It follows from Einstein’s general theory of relativity that the gravity of the vacuum should be repulsive. The observed cosmic acceleration is explained if the mass density of the vacuum is about twice the average density of matter.

Once started, the accelerated expansion continues forever. Thus, the prediction of an icy future 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 100 billion years, when this process is complete, astronomy will become a very boring subject!

You might think that we now have the future of the universe firmly in hand. But hold on, we aren’t done yet.

Read the rest of this story in the Winter 2016 issue of Artenol. Order yours today

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