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.