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Less than a mile from the South Pole, the Dark Sector Lab’s Bicep2 telescope (at left) searches for signs of inflation. (Steffen Richter / Harvard University)

Listening to the Big Bang

Just-reported ripples in space may open a window on the very beginning of the universe

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(Continued from page 1)

The gravity of familiar objects, such as the Sun, Earth and Moon, is surely attractive. But the math showed that a different source, not a clump of matter but instead energy embodied in a field uniformly filling a region, would generate a gravitational force that would push outward. And ferociously so. A region a mere billionth of a billionth of a billionth of a centimeter across, filled with the appropriate energy field—called the inflaton field—would be wrenched apart by the powerful repulsive gravity, potentially stretching to as large as the observable universe in a fraction of a second.

And that would rightly be called a bang. A big bang.

With subsequent refinements to Guth’s initial implementation of repulsive gravity by scientists including Andrei Linde, Paul Steinhardt and Andreas Albrecht, the inflationary theory of cosmology was born. A credible proposal for what ignited the outward swelling of space was finally on the theorists’ table. But is it right?

Testing Inflation
At first blush, it might seem a fool’s errand to seek confirmation of a theory that ostensibly operated for a tiny fraction of a second nearly 14 billion years ago. Sure, the universe is now expanding, so something set it going in the first place. But is it even conceivable to verify that it was sparked by a powerful but brief flash of repulsive gravity?

It is. And the approach makes use, once again, of the microwave background radiation.

To get a feel for how, imagine writing a tiny message, too small for anyone to read, on the surface of a deflated balloon. Then blow the balloon up. As it stretches, your message stretches too, becoming visible. Similarly, if space experienced dramatic inflationary stretching, then tiny physical imprints set down during the universe’s earliest moments would be stretched across the sky, possibly making them visible too.

Is there a process that would have imprinted a tiny message in the early universe? Quantum physics answers with a resounding yes. It comes down to the uncertainty principle, advanced by Werner Heisenberg in 1927. Heisenberg showed that the microworld is subject to unavoidable “quantum jitters” that make it impossible to simultaneously specify certain features, such as both the position and the speed of a particle. For fields suffusing space, the uncertainty principle shows that a field’s strength is also subject to quantum jitters, causing its value at each location to jiggle up and down.

Decades of experiments on the micro­realm have verified that the quantum jitters are real and ubiquitous; they’re unfamiliar only because the fluctuations are too tiny to be directly observed in everyday life. Which is where the inflationary stretching of space comes into its own.

Much as with your message on the expanding balloon, if the universe underwent the stupendous expansion proposed by the inflationary theory, then the tiny quantum jitters in the inflaton field—remember, that’s the field responsible for repulsive gravity—would be stretched into the macro­world. This would result in the field’s energy being a touch larger in some locations, and a touch smaller in others.

In turn, these variations in energy would have an impact on the cosmic microwave background radiation, nudging the temperature slightly higher in some locations and slightly lower in others. Mathematical calculations reveal that the temperature variations would be small—about 1 part in 100,000. But—and this is key—the temperature variations would fill out a specific statistical pattern across the sky.

Beginning in the 1990s, a series of ever more refined observational ventures—ground-, balloon- and space-based telescopes—have sought these temperature variations. And found them. Indeed, there is breathtaking agreement between the theoretical predictions and the observational data.

And with that, you might think the inflationary approach had been confirmed. But as a community, physicists are about as skeptical a group as you will ever encounter. Over the years, some proposed alternative explanations for the data, while others raised various technical challenges to the inflationary approach itself. Inflation remained far and away the leading cosmological theory, but many felt the smoking gun had yet to be found.

Until now.

Ripples in the Fabric of Space
Just as fields within space are subject to quantum jitters, quantum uncertainty ensures that space itself should be subject to quantum jitters too. Which means that space should undulate like the surface of a boiling pot of water. This is unfamiliar for the same reason that a granite tabletop seems smooth even though its surface is riddled with microscopic imperfections—the undulations happen on extraordinarily tiny scales. But, once again, because inflationary expansion stretches quantum features into the macrorealm, the theory predicts that the tiny undulations sprout into far longer ripples in the spatial fabric. How would we detect these ripples, or primordial gravitational waves, as they are more properly called? For the third time, the Big Bang’s ubiquitous relic, the cosmic microwave background radiation, is the ticket.

Calculations show that gravitational waves would imprint a twisting pattern on the background radiation, an iconic fingerprint of inflationary expansion. (More precisely, the background radiation arises from oscillations in the electromagnetic field; the direction of these oscillations, known as the polarization, gets twisted in the wake of gravitational waves.) The detection of such swirls in the background radiation has long been revered as the gold standard for establishing the inflationary theory, the long sought smoking gun.

On March 12, a press release promising a “major discovery,” issued by the Harvard-Smithsonian Center for Astrophysics, North American ground control for the Bicep2 mission, sent breathless rumors churning through the worldwide physics community. Perhaps the swirls had been found? At the press conference on March 17, the rumors were confirmed. After more than a year of careful analysis of the data, the Bicep2 team announced that it had achieved the first detection of the predicted gravitational wave pattern.

Subtle swirls in the data collected at the South Pole attest to quantum tremors of space, stretched by inflationary expansion, wafting through the early universe.

What Does It All Mean?
The case for inflationary theory has now grown strong, capping a century of upheaval in cosmology. Now, not only do we know the universe is expanding, not only do we have a credible proposal for what ignited the expansion, we’re detecting the imprint of quantum processes that tickled space during that fiery first fraction of a second.

But being one of those skeptical physicists, albeit one who’s excitable too, let me conclude with some context for thinking about these developments.

The Bicep2 team has done a heroic job, but full confidence in its results will require confirmation by independent teams of researchers. We won’t have to wait long. Bicep2’s competitors have also been in hot pursuit of the microwave swirls. Within a year’s time, maybe less, some of these groups may report their findings.

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