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.