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The Real Armageddon

Will the Universe accelerate out of sight, or collapse in a fiery Big Crunch? Despite recent reports of acceleration, the far future cannot be predicted. But the prospects for life are more dismal than ever.

by Lawrence M. Krauss

A lot has changed in the past thirty years – and that’s as true of cosmology as it is of everyday life. New observing technologies and strategies have immeasurably increased our ability to probe and map the Universe on its largest scales. Theoretical developments in elementary particle physics have allowed us to reliably extend our picture of the Universe back in time, to a point far less than a second after the Big Bang itself.

In spite of these developments, however, we are now coming to realize that our ability to predict the future of the Universe is no longer so certain as astronomers thought a few years ago. Oddly enough, this new-found uncertainty about the destiny of the Universe itself doesn’t preclude us from making some definitive statements about the long-term future of life. And it doesn’t look good.

The startling development at the heart of all of this is the observation that the Universe is not decelerating, as it should do in any sensible scheme of things, but rather appears to be accelerating. The surprise of this discovery is based on an intuition familiar to anyone who has ever thrown a ball up into the air. The faster you throw it, the higher it goes. You can imagine that if you throw it up fast enough, the ball will not come back. Indeed, a ball thrown from the Earth’s surface with a velocity in excess of about 7 miles per second will escape from our planet’s gravitational field and continue to travel outward indefinitely (although in fact this velocity is not sufficient to escape from the pull of the Sun).

But, the key point here is that – even as the ball is escaping - its velocity is always slowing, in response to the work it has to perform to escape from the Earth. Gravity, at least for all normal matter, is attractive, and thus it takes work to pull objects apart.

This reasoning applies to the Universe as a whole. Since Edwin Hubble’s observations in the 1920s, we’ve known that the galaxies are moving apart from one another: the Universe is expanding. Traditionally we have argued that the ultimate determinant of the galaxies’ future is the amount of mass contained in and around galaxies. If this mass is large enough, its gravity should be strong enough to halt the outward movement of galaxies, causing them to eventually collapse together in a reverse of the Big Bang, a so-called Big Crunch.

Dust along the plane of the Milky Way

Will the Universe end in a fiery Big Crunch? Will this be the real Armageddon?

The new observation of an apparently accelerating Universe implies that some kind of cosmic ‘antigravity’ is at work. This may sound like the stuff of science fiction, but it has a basis in physics. If you endow empty space with energy, the gravitational effect of this energy has the strange property of producing a new repulsive force throughout all of space. While this theoretical insight has been known for over a generation, the common wisdom was that such energy in empty space must be precisely zero.

In fact, when Einstein first laid out the equations that govern the large-scale structure of the Universe in 1916, he introduced the possibility of universal repulsion in the shape of a quantity called the cosmological constant - without knowing its significance on the microscopic scale. However, he quickly dispensed with the idea, calling the cosmological constant his "biggest blunder".

If the current observations are correct, however, empty space contributes more energy to the expanding Universe than is associated with the rest mass of all the galaxies, stars and planets. It even exceeds the energy bound up in the vast amounts of dark matter that astronomers believe exists around and between galaxies. We currently have no sound theoretical understanding of why this should be the case. Nevertheless, we can ask what the consequences might be for the future - both of our Universe and of the life within it.

You might imagine that if the expansion of the Universe is accelerating, this implies that the Universe will go on expanding forever. But things are not that simple. We don’t know the source of the energy of the vacuum and so it may have properties we are currently unaware of. Here are three possibilities:

  • The vacuum energy decreases with time. The acceleration also slowly decreases, and ultimately the future of the Universe will once again be determined by the gravitational attraction of the matter within it.
  • The present inferred acceleration is later proved to be incorrect, but there is still a tiny amount of vacuum energy. You might suspect that once again an inventory of all the matter in the Universe should allow us to determine the ultimate long-term behavior of the expansion. Alas, this is not the case. If we found there was enough matter for us to be heading towards a Big Crunch, the energy in the vacuum can have unexpected effects. Suppose that empty space possesses an amount of energy only one-thousandth the amount needed to measurably affect the present expansion. This energy would still eventually forestall the ultimate collapse of a universe in which matter currently appears to have the upper hand.
  • Finally, and perhaps most unusual of all, once we realize that empty space has energy, nothing forbids this energy from being negative. If this is the case, then even an unimaginably small negative energy in empty space will ultimately cause the Universe to recollapse, independent of how much matter now exists therein.

Perseus cluster of Galaxies

The Perseus cluster of Galaxies is accelerating away from us, every hour the Universe grows a billion miles in every direction

A year ago, Michael Turner, of the University of Chicago, and I demonstrated that there are no observations one can hope to make in any finite time which will allow us to unambiguously determine the ultimate future of our expanding Universe. At least one cosmic mystery therefore appears to remain safe.

The only way out would be to develop a "Theory of Everything" that precisely predicts, in the absence of experimental input, all of the fundamental parameters of the Universe. But there are no signs whatsoever that such a development is in the cards.

We will therefore have to make sure that we, or our descendants, can hang around long enough to witness the final reel in this cosmic drama. However, other recent work suggests that while the Universe may expand forever, life cannot persist forever. (As any salesman for long term care insurance will happily tell you)

In fact, an accelerating universe turns out to be precisely the worst possible universe that anyone can inhabit - in the long term, at least. My colleague Glenn Starkman and I have recently described what our descendants would see in the far future. In an accelerating Universe, distant stars will slowly blink out of sight, as the space that is carrying them away expands faster than light-speed. In an imaginable time, about 150 billion years - during which stars will still be burning and one may imagine life existing around Earth-like planets - all of the galaxies outside our local supercluster will become invisible even to the most powerful telescopes. We will become ever more alone in the Universe.

Deep space

The human race will have ceased to exist before the fate of the Universe is revealed.

Worse still, in a universe dominated by vacuum energy, life must eventually run out of available stores of energy which it can use to survive. The vast energy of empty space is inaccessible for such use, and the accessible energy is carried away too quickly for us to collect. Like a mariner becalmed on an ocean of salt water who utters "water, water, everywhere, but not a drop to drink!" we will be adrift in a cosmic sea of energy without any to spare for our own metabolism.

What if the acceleration of the Universe eventually ceases - is there then any hope for life? In 1979, physicist Freeman Dyson proposed that if we were particularly careful, and hibernated for longer and longer periods, life could survive with a finite amount of available energy for an infinite amount of time. On the other hand, Starkman and I have more recently argued that prospects for intelligent life, at least, are less hopeful. Quantum mechanics will ultimately limit the ability of life to perform the computations necessary for consciousness. This issue is not yet settled, however. Dyson has not given up, and we are continuing our friendly debate.

We have lots of time for discussion. Even if the limits we have described on the long-term perdurability of life eventually come to pass, we are talking about literally billions of billions of billions of billions of times the present age of the Universe.

Well before these cosmic limits come to into play, life on Earth will have to face far more immediate challenges, associated with the evolution of our planet and the death of the Sun. Having the foresight and cooperation needed to deal with these global changes will be more demanding than any other challenge our civilization has ever faced. Hopefully we will be up to the task – and it will help to put us on track to tackle whatever future the Universe may have in store.

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Lawrence M. Krauss is the Ambrose Swasey Professor of Physics, Professor of Astronomy and Chairman of the Physics Department at Case Western Reserve University. The ideas discussed here are adapted from discussions in his most recently published book, Quintessence: The Mystery of the Missing Mass in the Universe, and also from his forthcoming title, Atom: An Odyssey from the Big Bang to Life on Earth. In February he was awarded the 2000 Public Understanding of Science Award from the American Association for the Advancement of Science, in Washington D.C.

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