Cosmologists are agreed that the universe began with a big bang 13.7 billion years ago. People naturally want to know what caused it. A simple answer is nothing: not because there was a mysterious state of nothing before the big bang, but because time itself began then – that is, there was no time “before” the big bang. The idea is by no means new. In the fifth century, St Augustine of Hippo wrote that “the universe was created with time and not in time”.
Religious people often feel tricked by this logic. They envisage a miracle-working God dwelling within the stream of time for all eternity and then, for some inscrutable reason, making a universe (perhaps in a spectacular explosion) at a specific moment in history.
That was not Augustine’s God, who transcended both space and time. Nor is it the God favoured by many contemporary theologians. In fact, they long ago coined a term for it – “god-of-the-gaps” – to deride the idea that when science leaves something out of account, then God should be invoked to plug the gap. The origin of life and the origin of consciousness are favourite loci for a god-of-the-gaps, but the origin of the universe is the perennial big gap.
In his new book, Stephen Hawking reiterates that there is no big gap in the scientific account of the big bang. The laws of physics can explain, he says, how a universe of space, time and matter could emerge spontaneously, without the need for God. And most cosmologists agree: we don’t need a god-of-the-gaps to make the big bang go bang. It can happen as part of a natural process. A much tougher problem now looms, however. What is the source of those ingenious laws that enable a universe to pop into being from nothing?
Traditionally, scientists have supposed that the laws of physics were simply imprinted on the universe at its birth, like a maker’s mark. As to their origin, well, that was left unexplained.
In recent years, cosmologists have shifted position somewhat. If the origin of the universe was a law rather than a supernatural event, then the same laws could presumably operate to bring other universes into being. The favoured view now, and the one that Hawking shares, is that there were in fact many bangs, scattered through space and time, and many universes emerging therefrom, all perfectly naturally. The entire assemblage goes by the name of the multiverse.
Our universe is just one infinitesimal component amid this vast – probably infinite – multiverse, that itself had no origin in time. So according to this new cosmological theory, there was something before the big bang after all – a region of the multiverse pregnant with universe-sprouting potential.
A refinement of the multiverse scenario is that each new universe comes complete with its very own laws – or bylaws, to use the apt description of the cosmologist Martin Rees. Go to another universe, and you would find different bylaws applying. An appealing feature of variegated bylaws is that they explain why our particular universe is uncannily bio-friendly; change our bylaws just a little bit and life would probably be impossible. The fact that we observe a universe “fine-tuned” for life is then no surprise: the more numerous bio-hostile universes are sterile and so go unseen.
So is that the end of the story? Can the multiverse provide a complete and closed account of all physical existence? Not quite. The multiverse comes with a lot of baggage, such as an overarching space and time to host all those bangs, a universe-generating mechanism to trigger them, physical fields to populate the universes with material stuff, and a selection of forces to make things happen. Cosmologists embrace these features by envisaging sweeping “meta-laws” that pervade the multiverse and spawn specific bylaws on a universe-by-universe basis. The meta-laws themselves remain unexplained – eternal, immutable transcendent entities that just happen to exist and must simply be accepted as given. In that respect the meta-laws have a similar status to an unexplained transcendent god.
According to folklore the French physicist Pierre Laplace, when asked by Napoleon where God fitted into his mathematical account of the universe, replied: “I had no need of that hypothesis.” Although cosmology has advanced enormously since the time of Laplace, the situation remains the same: there is no compelling need for a supernatural being or prime mover to start the universe off. But when it comes to the laws that explain the big bang, we are in murkier waters.
Paul Davies, a physicist, writer and broadcaster, and director of the Beyond Center for Fundamental Concepts in Science at Arizona State University.