Gravitational waves stretched by inflation generate a faint but distinctive pattern,
called the B-mode signal, captured by the Bicep2. 

The Inflationary Theory

If the universe emerged from a sweltering hot and intensely dense primeval atom, as Lemaître called it, then as space swelled the universe should have cooled off. Calculations undertaken at George Washington University in the 1940s, and later at Princeton in the 1960s, showed that the Big Bang’s residual heat would manifest as a bath of photons (particles of light) uniformly filling space. The temperature of the photons would now have dropped to a mere 2.7 degrees above absolute zero, placing their wavelength in the microwave part of the spectrum—explaining why this possible relic of the Big Bang is called the cosmic microwave background radiation.

In 1964, two Bell Labs scientists, Arno Penzias and Robert Wilson, were at wits’ end, frustrated by a large ground-based antenna designed for satellite communications. Regardless of where they pointed the antenna, they encountered the audiophile’s nightmare: an incessant background hiss. For months they sought but failed to find the source. Then, Penzias and Wilson caught wind of the cosmological calculations being done at Princeton suggesting there should be a low-level radiation filling space. The incessant hiss, the researchers realized, was arising from the Big Bang’s photons tickling the antenna’s receiver. The discovery earned Penzias and Wilson the 1978 Nobel Prize.

The prominence of the Big Bang theory skyrocketed, impelling scientists to pry the theory apart, seeking unexpected implications and possible weaknesses. A number of important issues were brought to light, but the most essential was also the most basic.

The Big Bang is often described as the modern scientific theory of creation, the mathematical answer to Genesis. But this notion obscures an essential fallacy: The Big Bang theory does not tell us how the universe began. It tells us how the universe evolved, beginning a tiny fraction of a second after it all started. As the rewound cosmic film approaches the first frame, the mathematics breaks down, closing the lens just as the creation event is about to fill the screen. And so, when it comes to explaining the bang itself—the primordial push that must have set the universe headlong on its expansionary course—the Big Bang theory is silent.

It would fall to a young postdoctoral fellow in the physics department of Stanford University, Alan Guth, to take the vital step toward filling that gap. Guth and his collaborator Henry Tye of Cornell University were trying to understand how certain hypothetical particles called monopoles might be produced in the universe’s earliest moments. But calculating deep into the night of December 6, 1979, Guth took the work in a different direction. He realized that not only did the equations show that general relativity plugged an essential gap in Newtonian gravity—providing gravity’s mechanism—they also revealed that gravity could behave in unexpected ways. According to Newton (and everyday experience) gravity is an attractive force that pulls one object toward another. [But] the equations were [also] showing that in Einstein’s formulation, gravity could also be repulsive.

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?

During inflation (shown at left), a gravitational force pushed outward,
stretching the universe in a tiny fraction of a second. 

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 fieldremember, 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 by 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.

What’s certain is that current and future missions will provide ever more refined data that will sharpen the inflationary approach. Bear in mind that inflation is a paradigm, not a unique theory. Theorists have now implemented the core idea of the bang-as-repulsive-gravity in hundreds of ways (different numbers of inflaton fields, different interactions between those fields and so on), with each generally yielding slightly different predictions. The Bicep2 data has already winnowed the viable models significantly, and forthcoming data will continue the process.

This all adds up to an extraordinary time for the inflationary theory. But there’s an even larger lesson. Barring the unlikely possibility that with better measurements the swirls disappear, we now have a new observational window onto quantum processes in the early universe. The Bicep2 data shows that these processes happen on distance scales more than a trillion times smaller than those probed by our most powerful particle accelerator, the Large Hadron Collider. Some years ago, together with a group of researchers, I took one of the first forays into calculating how our cutting-edge theories of the ultra-small (like string theory) might be tested with observations of the microwave background radiation. Now, with this unprecedented leap into the microrealm, I can imagine that more refined studies of this sort may well herald the next phase in our understanding of gravity, quantum mechanics and our cosmic origins.

Inflation and the Multiverse

Finally, let me address an issue I’ve so far carefully avoided, one that’s as wondrous as it is speculative. A possible byproduct of the inflationary theory is that our universe may not be the only universe.

In many inflationary models, the inflaton field is so efficient that even after fueling the repulsive push of our Big Bang, the field stands ready to fuel another big bang and another still. Each bang yields its own expanding realm, with our universe being relegated to one among many. In fact, in these models, the inflationary process typically proves never-ending, it’s eternal, and so yields an unlimited number of universes populating a grand cosmic multiverse.

With evidence for the inflationary paradigm accumulating, it’s tempting to conclude that confidence in the multiverse should grow too. While I’m sympathetic to that perspective, the situation is far from clear-cut. Quantum fluctuations not only yield variations within a given universe—a prime example being the microwave background variations we’ve discussed—they also entail variations between the universes themselves. And these variations can be significant. In some incarnations of the theory, the other universes might differ even in the kinds of particles they contain and the forces that are at work.

In this enormously broadened perspective on reality, the challenge is to articulate what the inflationary theory actually predicts. How do we explain what we see here, in this universe? Do we have to reason that our form of life couldn’t exist in the different environments of most other universes, and that’s why we find ourselves here—a controversial approach that strikes some scientists as a cop-out? The concern, then, is that with the eternal version of inflation spawning so many universes, each with distinct features, the theory has the capacity to undermine our very reason for having confidence in inflation itself.

Physicists continue to struggle with these lacunae [(sic, a "gap" in our knowledge)]. Many have confidence that these are mere technical challenges to inflation that in time will be solved. I tend to agree. Inflation’s explanatory package is so remarkable, and its most natural predictions so spectacularly aligned with observation, that it all seems almost too beautiful to be wrong. But until the subtleties raised by the multiverse are resolved, it is wise to reserve final judgment.

If inflation is right, the visionaries who developed the theory and the pioneers who confirmed its predictions are well-deserving of the Nobel Prize. Yet, the story would be bigger still. Achievements of this magnitude transcend the individual. It would be a moment for all of us to stand proud and marvel that our collective creativity and insight had revealed some of the universe’s most deeply held secrets.