2008-06-28

Prof Stephen Hawking has come up with a new idea to explain why the Big Bang of creation led to the vast cosmos that we can see today.

Astronomers can deduce that the early universe expanded at a mind-boggling rate because regions separated by vast distances have similar background temperatures.

They have proposed a process of rapid expansion of neighbouring regions, with similar cosmic properties, to explain this growth spurt which they call inflation.

But that left a deeper mystery: why did inflation occur in the first place?

Now New Scientist reports that an answer has been proposed by Prof Stephen Hawking of Cambridge University, working with Prof Thomas Hertog of the Astroparticle and Cosmology Laboratory in Paris.

Prof Hawking is best known for his attempts to combine theories of the very small, quantum theory, and that of gravity and the very big, general relativity, into a new theory, called quantum gravity.

Quantum mechanics is awash with strange ideas and can shed new light on inflation, which came in the wake of when the universe itself was around the size of an atom.

By quantum lore, when a particle of light travels from A to B, it does not take one path but explores every one simultaneously, with the more direct routes being used more heavily.

This is called a sum over histories and Prof Hawking and Prof Hertog propose the same thing for the cosmos.

In this theory, the early universe can be described by a mathematical object called a wave function and, in a similar way to the light particle, the team proposed two years ago that this means that there was no unique origin to the cosmos: instead the wave function of the universe embraced a multitude of means to develop.

This is very counter intuitive: they argued the universe began in just about every way imaginable (and perhaps even some that are not). Out of this profusion of beginnings, like a blend of a God’s eye view of every conceivable kind of creation, the vast majority of the baby universes withered away to leave the mature cosmos that we can see today.

But, like any new idea, there were problems. The professors found that they could not explain the rapid expansion - inflation - of the universe, evidence of which is left behind all around us in what is called the cosmic microwave background, in effect the echo of the big bang, a relic of creation that can be measured with experiments on balloons and on space probes.

Now, in a paper in Physical Review Letters with Prof James Hartle of the University of California, Santa Barbara, they realised that their earlier estimates of inflation were wrong because they had not fully thought through the connection between, on the one hand, their theoretical predictions and, on the other, our observations of the echo.

At first, they found that the most probable history of the cosmos had only undergone "a little bit of inflation at the beginning, contradicting the observations," said Prof Hertog. Now, after a correction to take account of how the data we have on inflation is based on only a view of a limited volume of the universe, they find that the wave function does indeed predict a long period of inflation.

"This proposal, with volume weighting, can explain why the universe inflated," Prof Hawking tells New Scientist. By taking into account that we have a parochial view of the cosmos, the team has come up with a radical new take on cosmology.

Most models of the universe are bottom-up, that is, you start from well-defined initial conditions of the Big Bang and work forward. However, Prof Hertog and Prof Hawking say that we do not and cannot know the initial conditions present at the beginning of the universe. Instead, we only know the final state - the one we are in now.

Their idea is therefore to start with the conditions we observe today - like the fact that at large scales one does not need to adopt quantum lore to explain how the universe (it behaves classically, as scientists say) - and work backwards in time to determine what the initial conditions might have looked like.

In this way, they argue the universe did not have just one unique beginning and history but a multitude of different ones and that it has experienced them all.

The new theory is also attractive because it fits in with string theory - the most popular candidate for a "theory of everything."

String theory allows the existence of an" unimaginable multitude of different types of universes in addition to our own," but it does not provide a selection criterion among these and hence no explanation for why our universe is, the way it is", says Prof Hertog.

"For this, one needs a theory of the wave function of the universe."

And now the world of cosmology has one. The next step is to find specific predictions that can be put to the test, to validate this new view of how the cosmos came into being.

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