Hawking’s fascination was with cosmology, the science which deals with the origin, evolution and ultimate fate of the universe. Einstein – never one to think small – had in 1917 applied his theory of gravity to the biggest gravitating system he could imagine: the whole Universe. Like Isaac Newton before him, he was wedded to the idea of a “static” universe, in which the stars and galaxies hung in space, unchanging, for all time.
He therefore missed the message in his own equations, which was that the Universe was inherently restless and had to be in motion. Support for this came in 1929 when the American astronomer Edwin Hubble discovered that the Universe was expanding, its building blocks – galaxies of stars like our own Milky Way – flying apart from each other like pieces of cosmic shrapnel in the aftermath of a titanic explosion.
It appeared that the Universe had not existed forever but had been born in a “Big Bang”. The Big Bang marked the beginning of time (We now believe the Universe began 13.82billion ago). A Universe of finite age could be avoided, however, if, as the galaxies fly away from each other, new matter fountains into existence out of the vacuum, congealing into new galaxies to fill the gaps. Despite expanding, the Universe can still look the same at all times and so be infinitely old. But this “steady state” theory, championed by British astronomer Fred Hoyle, was dealt a killer blow by the discovery in 1965 of the “cosmic background radiation”, the “afterglow” of the big bang fireball.
This then was the scientific background as Hawking embarked on his post-PhD research and then made the first of his remarkable discoveries. The big question was: Was the Big Bang truly the beginning of the Universe? If the expansion of Universe is imagined running backwards like a movie in reverse, the Universe shrinks ever smaller. Squeezing material into a smaller volume, as anyone who has squeezed air into a bicycle pump knows, makes it hotter.
But, according to Einstein’s theory, this process has no limit. As the Universe dwindles to a point, its temperature and density sky-rockets to infinity. Such a point is known as “singularity” and it is a sure sign that a theory has been stretched into a regime where it is no longer valid. Since Einstein’s theory of gravity broke down in this way, it had nothing sensible to say about earlier times and so the Big Bang had to be the beginning of time.
However, there was still a possibility that a singularity might be avoided in Einstein’s theory and, with it, the identification of the Big Bang with the beginning of time. If the matter of the Universe were spread unevenly, this unevenness would become magnified as the backward-running universe shrunk ever smaller. Different parts of the collapsing Universe, instead of all piling up at one point, would miss each other and so not create a catastrophic singularity. Since Einstein’s theory of gravity would not break down, it would be possible to follow the history of the Universe to earlier times – before the Big Bang. Perhaps, for instance, the Universe had contracted down to a Big Crunch from which it had then bounced in the Big Bang.
While working on such matters, Hawking’s office-mate, Brandon Carter, happened to mention a talk he had attended in London given by a young mathematician called Roger Penrose. It seemed Penrose was using novel “topological” methods to investigate the formation of another type of singularity, one which formed at the heart of a “black hole” – the region of grossly warped space-time space left behind when a dying star shrinks catastrophically under his own gravity. A black hole singularity was a singularity in space rather than time but it had much in common with the singularity of the Bang. Hawking contacted Penrose and there began one of the most fruitful collaborations in 20th-century physics.
Between 1965 and 1970, the pair proved a range of powerful “singularity theorems”. The most important was that, under a wide range of general and highly plausible conditions, the singularity in the Big Bang was unavoidable. It formed no matter how the backward-running movie of the Universe went. According to Einstein’s theory at least, the Big Bang must have been a singularity – a true beginning of the Universe.