Adrian Berry  
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The Big Rip

Dark Energy May Tear us Apart

Anew and somewhat sinister theory about the long-term future of the universe is emerging. It has been called the ``Big Rip'', and it forecasts that our bodies will be literally torn apart.

Its leading proponent Robert Caldwell of Dartmouth University calls it a ``pretty fantastic possibility'', but he and his colleagues cannot see how it can be avoided if the present acceleration of the cosmos continues.

The idea goes like this: the universe, driven by that mysterious force called ``dark energy'', or repulsive gravity, is flying apart. The furthest galaxies are moving ever further from us.

The Big Rip - a fantastic possibility?

But the rate of expansion is itself accelerating. Imagine a car that somehow goes 10 mph faster for every mile it travels. This is the present state of the universe. Yet since the rate of acceleration is itself increasing, there comes a time when the car goes 10 mph faster every half-mile that it travels.

The accelerating acceleration continues unchecked. Eventually the increased speed of 10 mph comes every 100 yards, and at last every foot. Finally parts of the car fly apart from each other. The bumpers go tearing off in one direction, the engine in another. In a word, car and passengers explode. ``The expansion becomes so fast that it literally rips apart all bound objects,'' says Caldwell.

There is no need for immediate panic, since this dreadful sequence of events will not become noticeable for another 20 billion years.

At this point all galaxies beyond our own, traveling much faster than light, will have flown so far away from us that they become invisible. (This is not a violation of Einstein's special relativity. The galaxies are not flying through space. In the expanding universe, space itself expands.) Our Milky Way galaxy alone can be seen.

Then the Milky Way begins to fly apart, and at this point there are only 60 million years left. Long before this, of course, other events will have destroyed our Sun and planets. But imagine some part of humanity inhabiting the planets of another stellar system.

When there are only three months left, these planets and their parent Sun explode. ``There's about 30 minutes left before atoms and their nuclei break apart,'' says Caldwell, ``but it's not quality time. We're not sure what happens after that. On the face of it, it would look like time ends.''

Yet perhaps there may be a more cheerful way of looking at the matter. Whenever a new scientific finding or physical theory appears, one's instinct is to ask whether it is of any practical use. Knowledge only becomes truly useful when it is applied.

Repulsive dark energy, although we know next to nothing about its cause, comprises 73 per cent of all the energy in the cosmos. It exists everywhere in the vacuum of space.

One day we will surely learn to how to exploit it and acquire it for our own use. It may prove to be the most perfect means of propelling starships. Discovering how to do this may be one of the most challenging problems of the 21st century.

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