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
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.