Instants after the big bang, the universe underwent a burst of rapid expansion known as inflation. In this period, according to standard cosmology, tiny ripples of energy seeded galaxies and the other large-scale structures we see today. But no one can explain how the ripples formed in the first place. Three physicists now say the key to this riddle lies in quantum gravity, a still tentative theory in which gravity would display the same fuzzy “uncertainty” typical of subatomic physics.
Standard cosmology, based on Einstein's general theory of relativity, cannot explain the origin of the ripples, because it breaks down at very small scales. During the infinitesimally brief period before the start of inflation, called the Planck era, the entire known universe was stuffed into a region many orders of magnitude smaller than an atom. If pushed that far back, relativity makes nonsensical predictions such as infinite energy densities.
To extend the reach of Albert Einstein's theory to such extreme regimes, researchers have developed a theory called loop quantum gravity. Beginning in the 1980s, Abhay Ashtekar, now at Pennsylvania State University, rejiggered Einstein's equations to make them quantum-friendly. Among the consequences are that space itself, instead of being a smooth backdrop, would consist of discrete units called loops and that its microscopic structure could fluctuate among multiple simultaneous states. In recent years physicists have also found that if loop quantum gravity is correct—a big if because experimental evidence is still lacking—then the big bang would really have been a “big bounce” from an earlier collapsing universe.
Ashtekar's team now says that by extending loop quantum gravity techniques it has bridged the gap between the big bounce—which is in the Planck regime—and the onset of inflation and that it can explain those all-important ripples without which you and I would not be here. The ripples, the researchers calculate, would be the natural outcome of quantum fluctuations existing at the time of the big bounce.
The team's predictions, however, differ slightly from those of “vanilla” inflation in a way that could be tested in future surveys of cosmic structure, Ashtekar says.
These results, to appear in Physical Review Letters, provide “a self-consistent extension of inflation all the way to the Planck scale,” Ashtekar says.
The conclusion that quantum gravity might have left an imprint on today's large-scale cosmic structures is “quite surprising and beautiful,” says Jorge Pullin of Louisiana State University, an expert on loop quantum gravity who was not involved in the research.
Neil Turok, director of the Perimeter Institute for Theoretical Physics in Ontario, says that the team still needs “artificial assumptions,” which it pushes back from the onset of inflation to an earlier time. Loop quantum gravity “has many interesting ideas,” Turok says, “but it is not yet a theory one should take too seriously as making predictions.”