WHAT happened before the big bang? Cosmologists have long speculated that a universe much like ours could have collapsed in a big crunch and then bounced back into the universe we know. Now a theory that tries to reconcile the incompatible theories of general relativity and quantum physics has provided the first physically plausible model of how this could have happened.
General relativity explains gravity as being caused by distortions in the fabric of space-time. But physicists have struggled to also explain gravity in terms of quantum mechanics, leaving it the only force that still lacks a clear quantum description.
“You can reverse the big bang only so far before the universe becomes so dense that classical cosmology breaks down”
Advertisement
A theory known as loop quantum gravity (LQG) attempts to quantise gravity by suggesting that space-time is not as continuous as it seems. Instead, space-time is made of tiny interconnected loops, each only 10-35 metres in diameter, that form a smooth fabric much like a shirt’s fabric is smooth even though it is woven from separate threads. The curvature of LQG space-time creates the effects of gravity, just as in the theory of general relativity.
Abhay Ashtekar, Tomasz Pawlowski and Parampreet Singh of Pennsylvania State University in University Park have now applied the equations of LQG to the universe as a whole. Starting from the expanding universe we live in, they ran the equations backwards in time to see what happens if the expansion is reversed.
As space shrinks, matter and energy are crammed tighter and tighter. Up to a point, loop quantum gravity’s description of the process matches that of classical cosmology. However, in classical cosmology, you can reverse the big bang only so far. The universe eventually becomes so dense that the classical laws of physics break down.
At this stage, according to LQG, the extreme density undoes the fabric of space-time. Two years ago, this was shown by Martin Bojowald, who was then at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and is now at Penn State (91av, 20 March 2004, p 34). His mathematics did not go far enough, though, to work out the exact nature of the pre-big-bang universe.
Now, Ashtekar’s team has shown that when space-time comes undone, the loops resist further shrinking. “The quantum gravity effects make gravity repulsive, halting the collapse,” says Ashtekar. The breakthrough is in explaining what happens next: the loops soon rearrange themselves into a smooth fabric, and the universe bounces back in what we call the big bang.
This post-big-bang universe is strikingly similar to some cosmologies based on string theory, which also tries to reconcile gravity with quantum physics. According to those cosmologies, our universe is a 3D “brane” within a higher-dimensional space. Although the LQG universe has no extra dimensions and string theory’s brane-based approach cannot yet handle the pre-big-bang world, their agreement as to the post-big-bang universe is reassuring, says Joe Lykken, a particle theorist at Fermilab in Batavia, Illinois. “Finally we are all talking the same language.”
Ashtekar, Bojowald and others plan to refine the LQG model and hope that it will predict something experimentally testable, maybe even traces of the pre-big-bang past. “The big crunch does not completely erase memory of what the universe was like before,” says Ashtekar. Their work will be published in Physical Review Letters.
