Stephen Hawking and Penrose proved that the Universe must have had a beginning in time, on the basis of Einstein's theory of General Relativity.  The implication of this is that near the beginning of time, when the Universe was sufficiently small, the laws of quantum mechanics would have applied.  Earlier, Penrose had shown that black-holes produce singularities, mathematical points where certain physical quantities attain infinite values.  Hawking now showed mathematically that the big-bang must have arisen from a singularity.