Why does the Universe expand the way it does? Why does it accelerate? Einstein’s equations offer an explanation of gravity that works on the scale we know, but do they work on the grandest scales of space and time? Humanity now has a way to find out. The General Theory of Relativity predicts the behaviour of gravity, and includes a term known as the cosmological constant.  Einstein added this term to make the universe static and unchanging, as he believed it was.  But when the expansion of the universe was discovered by Edwin Hubble, Einstein regarded it as ‘the greatest blunder...