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)