This theory of an expanding universe was consistent not only with the second law of thermodynamics but also with Einstein’s theory of relativity. Hubble found that the farther away a galaxy is from us, the faster it is receding from us. This is now called Hubble’s Law, and it fulfills a prediction that was made on the basis of Einstein’s theory. The expanding universe theory also solved an old conundrum that had been frustrating scientists for decades: why the galaxies continued to stay apart from each other. Why had the force of gravity not pulled them together? The reason was that they had been hurled apart in a primordial explosion whose force continued to thrust them farther and farther away from each other. Astronomer John Barrow calls Hubble’s finding “the greatest discovery of twentieth-century science.”