Universe mostly forgets its past during cosmic rebirth

• 18:59 02 July 2007
• NewScientist.com news service
• Saswato Das

Some cosmologists think that our universe has been cycling through an endless series of big bangs and big crunches. If so, it implies the universe is doomed to repeat the same thing over and over. A new study, however, suggests that with each big bang, the universe mostly forgets its past and starts anew.

The accepted wisdom in modern cosmology is that it is meaningless to ask what came before the big bang. That's because the big bang is what physicists call a "singularity" – a moment at which the equations of physics break down.

"No one is happy with the big bang singularity,"

says Martin Bojowald, a theorist at the Pennsylvania State University, University Park.

Bojowald works on loop quantum gravity (LQG) – a theory that seeks to unify the otherwise incompatible theories of general relativity and quantum mechanics. In LQG, space-time is made of tiny interconnected loops, each only 10 ^-35 metres across, that form a smooth fabric much like a shirt's fabric is smooth even though it is woven from separate threads.

Bojowald and his colleagues have run the equations of LQG backwards and shown that they can avoid the singularity. They showed that as the universe collapses, it reaches a point at which it bounces back in a big bang, and the process repeats.

Cosmic 'forgetfulness'

Does that mean that one day we can, either mathematically or via observations, know about the pre-big bang universe? To answer this question, Bojowald developed a simple LQG model to determine the limits of what we can know. In his model, he assumed that the physical properties of the universe were the same everywhere and that the kind of matter it contained did not interact with itself. The model included gravity but not radiation.

The model showed that most, but not all, of the information about what came before the big bang gets irretrievably lost through the big bang transition. And in a perpetual cycle of big bangs and crunches, this information loss means no two universes are ever the same. Bojowald calls this "cosmic forgetfulness".

Cosmologist Paul Steinhardt of Princeton University says that Bojowald's model is right in principle. "It's important to lose some information, but not everything," he says. Thomas Thiemann of the Max Planck Institute for Gravitational Physics in Golm, Germany, says that although some of Bojowald's assumptions may turn out to be too simple, the model is "the cleanest derivation of a pre-big bang scenario that any physical theory has delivered so far".

Journal reference: Nature Physics (DOI: 10.1038/nphys654)

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.