"If that is the case, then we're not able to determine the precise origin of the universe," says theoretical physicist Martin Bojowald of Pennsylvania State University in University Park. "It would always remain a philosophical scenario."
For nearly a century, researchers have known that galaxies are flying apart from each other as if blasted outward by an explosion. According to general relativity, that motion is caused by the combined expansion of space and time, together called space-time, which pulls galaxies with it. If run in reverse, researchers believe, all matter and energy in the observable universe would become squished into an ultrahot, ultradense state—the big bang—as of 13.7 billion years ago. Unfortunately, general relativity hits a wall at the big bang because it allows matter and energy to become infinitely dense—a mathematical hiccup called a singularity.
Trying to peer back further in time, some researchers have turned to hypothetical theories combining gravity with quantum mechanics, which tells them what happens at short distances such as the size of the universe at the big bang. In an approach called loop quantum gravity, space-time consists of tiny pieces sort of like foam. Loop quantum gravity does away with the big bang singularity because these pieces can only contain a finite amount of energy, Bojowald says. As a result, the big bang is replaced with a big bounce, in which the universe collapsed under its own gravity in a prior phase and then began expanding, creating the universe we know.
Bojowald used the theory to construct a highly simplified model of the universe without galaxies or radiation. In this model, quantum mechanics caused the size of the universe to fluctuate by a small amount. In principle, Bojowald says, such quantum vibrations from before the bounce might have carried over and influenced those after the bounce.
Instead he found that quantum vibrations left only a trace impression on the expanding universe after the big bang—much too weak to detect, he reports in a paper published online by the journal Nature Physics. "The model is still very simple," Bojowald says. However, he says, if the result holds up it may call into question other models that have attempted to explain our universe as a result of repeated bounces.
Physicist Donald Marolf of the University of California, Santa Barbara, says the finding would be strengthened if it turned up in other models of quantum gravity, such as string theory. "No one has good control over this physics in any approach to quantum gravity," he says, "and it is important to explore a broad range of models and ideas."