The Real Armageddon

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.

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