The Big Bang theory: it's a repeat

New Books by Martin Bojowald and Sir Roger Penrose say the Big Bang was not just the start of our universe, but the end of its predecessor.

Like great works of art, even the most astonishing scientific discoveries can lose their impact through overexposure. Take the notion that our universe began in a Big Bang around 14 billion years ago. Has there ever been a more mind-boggling scientific statement? Yet through constant repetition, it has gone the way of The Ride of the Valkyries, leaving most who hear it completely unmoved.

Most, but not all. Children can still appreciate the concept of the Big Bang for what it is, and ask awkward questions like "So what was there before the Big Bang?" Until recently, such impertinences could be batted away with the standard scientific answer that because "everything" includes time itself, it simply makes no sense to ask what happened "before" the Big Bang. The more common - and honest - response from many parents was doubtless "How should I know ?"

Both those responses are in danger of losing their credibility with the publication of two new books by leading theorists who believe the Big Bang needs to be rethought. They do not doubt it happened, but they do doubt whether it marked the beginning of Everything.

For many people, this will mark a return to good old-fashioned common sense. For the textbook account of the Big Bang would have us believe that until around 14 billion years ago, there was not merely just a vacuum but quite literally nothing at all: no space, time or anything. The new theories dispense with this appalling absence, replacing it with a more comforting view of the Big Bang as merely the latest in an endless cycle of cosmic birth and death.

The two theorists arrive at their conclusion by rather different routes. As an expert in quantum gravity at Penn State University in the United States, Prof Martin Bojowald spends his time combining the two great theories of physics: quantum theory, which rules the sub-atomic world, and Einstein's theory of gravity, known as general relativity. This is a notoriously difficult challenge, yet one that must be taken on if we are to understand the nature of space and time at the birth of the cosmos, when it was incredibly compact.

Prof Bojowald's work is exceptionally technical, but in his new book cycle, he explains how it seems able to probe back before the Big Bang, and hints at the existence of a previous universe.

Until recently, this was thought impossible, as the equations of general relativity go haywire at the moment of the Big Bang, giving crazy answers to even the simplest questions. Put simply, this is because at the Big Bang all the energy of the universe becomes crammed into an infinitely small volume of space and time. There have, however, long been suspicions that this so-called singularity problem might be solved if Einstein's theory were combined with quantum theory. That is because the famous uncertainty principle prevents the existence of an infinitely small volume of space and time, smearing it out into merely an incredibly small volume.

According to Prof Bojowald, quantum gravity can do more than just solve the singularity problem. It also suggests that there was something before the Big Bang, and that our universe did not burst literally out of nowhere. Indeed, he argues the smearing-out of space and time has the power to turn the attraction of gravity in the earlier universe into a powerful anti-gravitational effect, providing the propulsive force that triggered the birth of our own universe 14 billion years ago. Thus our own expanding universe may be the spawn of a collapsing and dying predecessor.

Sir Roger Penrose of the University of Oxford is renowned for his role in understanding why Einstein's theory of gravity breaks down at the Big Bang. He has spent much of his life trying to resolve the singularity problem and sets out his findings in Cycles of Time. His approach has been to turn what we know about our own universe into insights about what came before. And he too now believes the Big Bang was just part of an endless cycle, in a kind of cosmic Groundhog Day.

For Prof Penrose, one of the key features of our universe is its inexorable decline into a state of ever greater disorder. We are all familiar with cups smashing into a myriad fragments, but we would be dumbstruck if a whole load of fragments were to gather themselves up and assemble themselves into a cup. Indeed, we can rely on the emergence of disorder - or "entropy" - to tell us the direction of time. If we see footage of, say, a wrecked car putting itself back together, we know the film is running backwards.

To cut a long story short, Prof Penrose argues that this feature of our universe links its beginning and its end - which may in fact be the same thing. As such, what we regard as the birth of our universe, the Big Bang, may also be regarded as the death of the universe before our own. And so, like Prof Bojowald, he concludes our universe is just the latest in an endless cosmic cycle.

It is tempting to dismiss this as idle academic speculation. But Prof Penrose has come up with a testable prediction of his ideas. It exploits the power of the most violent events that can occur in any universe, including our own: the collision of black holes.

Black holes are objects whose gravity is so strong not even light can escape their clutches. Astronomers believe that gigantic black holes lurk at the centre of most galaxies, and when they collide they release so much energy that it triggers tsunamis in the very fabric of space and time.

According to Prof Penrose, the upheaval generated by colliding black holes in one universe could appear as bursts of energy in the early moments of its successor. These, in turn, would show up as ring-like patterns in the heat left over from the last Big Bang.

Since the 1990s, astronomers have been drawing up ever more detailed maps of this cosmic heat. Together with Prof Vahe Gurzadyan of the Yerevan Physics Institute, Armenia, Prof Penrose has been analysing this data. And he now claims to have found the ring-like patterns predicted by his theory.

You can be sure Prof Penrose's claim will be given a tough time by his fellow researchers. Even so, it may well prove to be the first rumble of an intellectual Big Bang that will explode our current conception of creation.

Robert Matthews is a visiting reader in Science at Aston University, Birmingham, England