|John Baez/UC Riverside|
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.
(read the rest of my article in the April 22 New Scientist)
This was my first news piece — as opposed to longer, feature articles — for the magazine.
Loop quantum gravity, or LQG, is the leading alternative approach to string theory in the efforts to “quantize” gravity. Both are still incomplete, speculative theories that have yet to find any experimental confirmation (though physicists hope that new experiments in astronomy and particle physics will change that in the next few decades). Quantizing means interpreting a phenomenon using the mathematical machinery of quantum theory, where all things — including matter and light — come in packets, or quanta, rather than in a continuum of possible amounts.
In string theory, the force of gravity would also come in tiny packets, called gravitons. But LQG goes one step further, and quantizes space and time as well. In a quantized world, your lunch break can’t be as short as you want: eventually, it will become so short that it can’t be any shorter — other than lasting exactly zero seconds. And in Zeno’s ancient paradox on infinitesimals, Achilles can’t get closer and closer to the tortoise he’s chasing: eventually he’ll have to choose whether to be at a quantum of distance from it, or to bridge that last gap in one quantum leap and touch the tortoise.
LQG postulates that, on the tiniest of scales, space and time are woven together out of “loops.” The loops are to space-time what elementary particles are to matter and light: the ultimate, indivisible units. They are so small that they lie beyond the reach of our senses — even beyond the reach of the most futuristic lab experiments — so space and time look to us like a smooth continuum, the way the fabric of a piece of cloth looks smooth from a distance.
Based on the assumption that LQG is true, Abhay Ashtekar, Tomasz Pawlowski and Parampreet Singh at Pennsylvania State University have now answered a long-standing question in cosmology: what was there before the big bang?
They start from the present picture of an expanding universe, and they use the equations of LQG backwards in time to revert the expansion, watching space collapse as if they were rewinding a movie. As space shrinks, the stuff in it crams in tighter and tighter. Here, LQG’s movie-in-reverse matches that of classical cosmology, which had originally led to the idea of a big bang. But in classical cosmology, you can play the movie backwards only so far, until the classical laws of physics stop making sense: temperature and density become infinite, in what physicists call a singularity.
At that point, according to the Penn State team, the extreme density undoes the fabric of space-time, which turns into “quantum foam.” But where the classical equations of cosmology go berserk, the LQG equations keep working beautifully, as physicist Martin Bojowald (now also at Penn State) had already discovered two years ago.
Hold the remote, though — the movie does not end here, as Ashtekar’s team found out. The threads soon rearrange themselves into a regular fabric, and the universe bounces back and starts expanding again. Beyond the quantum foam, in the possibly infinite expanse of pre-big bang history, lies another universe much like ours.
That was the movie in reverse. Now hit pause and play it forward again, and you’ll see the pre-big bang universe shrink, then implode in a “big crunch,” before bouncing back in what we call the big bang.