@article{Hawking1966TheOO,
title={The occurrence of singularities in cosmology},
author={S. Hawking},
journal={Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences},
year={1966},
volume={294},
pages={511 - 521}
}

Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences

It is shown that singularities of space-time are inevitable if the Einstein equations hold, if matter has normal properties and if the universe satisfies certain reasonable global conditions. The singularities would be in the past and would, in principle, be observable. Observation to determine whether such singularities actually occurred would provide a powerful test of the Einstein equations in strong fields. The singularity would not necessarily constitute a beginning of the universe.