Did pre-big bang universe leave its mark on the sky?

• 10 April 2008
• From New Scientist Print Edition.
• Stephen Battersby

WHAT happened before our universe began? According to two theoretical physicists, if there was a universe before ours then it should have been remarkably similar to this one, with the same basic ingredients and properties.

It may even be possible to see a faint picture of our parent universe imprinted on the sky.

Questions about a time before the big bang were once thought to be meaningless, because according to Einstein's general theory of relativity, the universe began at a singularity - a mathematical point with infinite density at which all calculations break down.

However, physicists now believe that the theory of relativity is limited and the effects of quantum mechanics would have blurred out the singularity just a little, so at a crucial moment the density of matter and radiation was not infinite. If this was the case, it becomes possible to try to work out what led up to that moment.

In cosmological models based on a theory called loop quantum gravity, our universe has a parent.

Loop quantum gravity attempts to meld relativity with quantum mechanics by describing space-time as a constantly rearranging fabric of interconnections. On the smallest scales, around 10^-35 metres, that fabric is a tangled mess, but on much larger scales the space and time of our universe look smooth.

The theory predicts that when this fabric is scrunched up, it becomes bouncy. So if the universe before ours was contracting, it would have reached a point of maximum density and then bounced out again in our big bang.

So what would this predecessor have been like? To find out, Parampreet Singh of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, and Alejandro Corichi of the National Autonomous University of Mexico, Morelia, applied the equations of loop quantum gravity to a highly simplified model of the universe.

They found that the properties of space, such as the quantity of matter and energy it contains, hardly change when the universe goes through the big bounce. "For the simple model considered, the universe is almost exactly the same on other side," says Singh.

It raises the possibility that we could see an imprint of the universe before ours. Singh suggests that the seeds of large-scale structures in our universe, such as superclusters of galaxies, would have been present on the pre-big-bang side. The pattern of those seeds might be preserved in the cosmic microwave background radiation - the relic radiation left behind by the big bang. "If this conclusion holds true, then it is possible that we are going to see signatures of the pre-big-bang universe," says Singh.

“Seeds of structures in our universe, such as superclusters of galaxies, would have been there on the pre-big-bang side”

A pioneer of loop quantum gravity, Carlo Rovelli at the Centre for Theoretical Physics in Marseille, France, likes the work. "It is quite remarkable that we can begin to address these questions and find the first tentative answers," he says.

But the calculation doesn't convince Martin Bojowald, another loop quantum gravity theorist, based at Pennsylvania State University in University Park. He disagrees with their interpretation of the mathematics and also points out that loop models so far are still very simple.

Bojowald thinks that the universe before the big bang could have been a very different place, perhaps without even the familiar smooth, classical space-time of our world.

Corichi and Singh's paper will appear in Physical Review Letters.