THE END

Astronomers are now fairly sure that the Universe was born 15 billion years ago in the Big Bang. Since then, the Universe has been constantly expanding, stretching the very fabric of space itself. But will it ever die? And if so, how?

There are three alternative scenarios for the future fate of the Universe. If the expansion continues forever then the Universe is destined for the Big Freeze, gradually pulled apart into a cold desolate wasteland of dying stars and black holes. If space stops stretching and springs back on itself, it will shrink until galaxies start colliding in the Big Crunch - terminating in the mother of all black holes. However, there is a more peaceful option. The final scenario involves the Universe gradually slowing down to a halt. With the whole Universe balanced precisely, cosmic catastrophe would be averted and space would be saved. At least for a while. Eventually the Universe would succumb to the Big Freeze, it would just take a lot longer.

The fate of the Universe is basically a battle fought between the inward pull of gravity and the outward push of expansion. So astronomers are trying to calculate the strength of these forces. The amount of gravity the Universe has to wield against this expansive onslaught depends on how much stuff there is out there in space. Anything with a mass has its own gravity. Even you yourself have a gravitational force that attracts everything else around you, including other people. The bigger you are, the stronger this force is, and so, the Earth, being the most massive thing around, completely overwhelms the tiny forces that we personally possess. So to calculate the fate of the Universe, we must weigh it to find its density.

In astronomy, the density of the Universe goes under the symbol Ω (or Omega), the last letter of the Greek alphabet, meaning 'the end'. The precise amount of matter needed to gradually halt the expansion of the Universe is known as the critical density, where Ω=1. If Ω is 1 then the future of the Universe is a gentle, serene stop. Smaller than 1 and we're heading for the quick Big Freeze, a universal ice age. Larger than 1 then it's Crunch time. So our destiny depends on our density. Recent results show that Ω is at least 0.3. That figure includes the mysterious dark matter that lurks invisibly in space and has yet to be fully understood.

It has also been proving difficult to measure the precise expansion rate of the Universe. Recently it's been discovered that this expansion is accelerating under the grip of a previously undetected force, called dark energy, that is helping to pull the Universe apart. Until both these dark questions are answered, the fate of the Universe still hangs in the balance.

However, although the end of the Universe may look pretty black, this impending cosmic catastrophe is not our most pressing concern. In about 4 billion years the Sun will expand and engulf our planet Earth. Also around the same time, our nearest galactic neighbour, Andromeda, will start to crash into our own galaxy, the Milky Way. Life on Earth must escape into space if it is to survive.