What We Cannot Know: Explorations at the Edge of Knowledge

by Marcus du Sautoy

It seems that about 7 billion years ago something dramatic happened. Up to this point, the expansion appeared to be slowing down, as one would expect as the gravitational force of the matter in the universe exerted a braking effect. But at this point, halfway through the current life span of the universe, the expansion rate changed character and started to increase, accelerating as if something had suddenly put its foot on the pedal. The fuel driving the acceleration is what scientists call dark energy. It seems that in the first half of the universe’s existence, the density of matter was enough to assert a slowing gravitational pull, but as the universe expanded this density decreased to such a point that the underlying dark energy was strong enough to take over. Dark energy isn’t thought to be something whose density decreases with expansion. It is a property of space itself.