Physics The Missing Epoch New calculations extend Einstein’s general theory of relativity into the universe’s first few moments

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

Illustrations by Thomas Fuchs January 2013, ScientificAmerican.com 17

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