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Physics + MathPhysics & Math

Can Fossils of Inflation Provide Quantum Gravity Clues?

The Nature of RealityThe Nature of Reality

As a science writer, it’s alarmingly easy to overstate a scientific discovery’s importance. I often must go out of my way to dial back the enthusiasm, mercilessly editing out words like “groundbreaking” and “revolutionary” after I lose control in a first draft. But if the recent discovery of

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polarization in the cosmic microwave background (CMB) radiation is confirmed, it will be very hard to exaggerate its significance.

“This is so big that we haven’t figured out how big it is,” said Michael Turner, director of the Kavli Institute for Cosmological Physics, in a recent

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video hangout with members of the team behind BICEP2 , the experiment that recently garnered the kind of breathless headlines I have so often tried to tame. BICEP2 detected a distinctive polarization pattern in the CMB that is thought to be the imprint of the dramatic inflation that morphed our universe from “tiny” to “cosmic” in the first 10 -35 second after the Big Bang. If confirmed, it will be the first direct evidence of cosmic inflation and our earliest ever glimpse into the action of the newborn universe.


Perhaps most tantalizing to researchers, it might also reveal the rules that unite the physics of very small things with the physics of very large things. That’s because, just after the Big Bang, a huge amount of matter and energy were packed into a very small space. To understand what happened in these moments, physicists must invoke both Einstein’s theory of gravity and quantum physics, the theory of matter at the smallest scales. But in situations like this, when both theories have something to say, the results yield what a non-scientist might call “gibberish.” Physics therefore needs a theory of “quantum gravity” to act as a Rosetta Stone, capable of translating from the language of one theory into the language of the other theory in a non-gibberish-inducing way.

Months before the announcement, Nobel-winner Frank Wilczek and theorist Lawrence Krauss were already thinking about the implications for quantum gravity of such a detection. “Measurement of polarization of the Cosmic Microwave Background…from Inflation in the Early Universe would firmly establish the quantization of gravity,” they wrote in 2013. Quantization—the idea that some physical properties, like energy, are discrete, like grapes, and not continuous, like grape jelly—is the fundamental tenet of quantum physics. The BICEP2 results, if validated, suggest how the idea of quantization can be extended to gravity by showing how the universe’s primordial gravitational waves, the ripples in space-time that transmit gravity, can be traced to quantum effects inside the tiny, expanding early universe.

Furthermore, the BICEP2 results could help scientists rule out certain models of inflation. “There are two big classes of [inflation models],” says Colin Bischoff, a researcher on the BICEP2 team. Bischoff explains that the early universe seems to have expanded quickly enough that the evidence supports one of these model types more than the other. This would allow theoretical physicists to focus their attention on the correct class of inflation models, which some hope will have a more direct bearing on understanding the quantization of gravity at the highest energy scales.

There is also hope that the BICEP2 results could help guide physicists in their search for supersymmetry , a speculative extension of the standard model that predicts that every subatomic particle variety should have a (still undiscovered) mirror-image “superpartner.” Supersymmetry plays a huge role in theories of quantum gravity, because many of the approaches become intractable unless you include the mathematics of supersymmetry, including the approaches that predicted the polarization from inflation. “I’d say that there’s more evidence that supersymmetry might be correct after the discovery of gravitational waves from the Big Bang,” said theorist Lawrence Krauss, discussing the BICEP2 results in a recent interview .

Of course, the best evidence for supersymmetry would be the direct detection of one of the many superpartner particles that the theory predicts—but to date none have been detected, to the consternation of many, including Krauss, who thought the Large Hadron Collider would see superpartners before it saw the Higgs boson. If particle physics results defy expectations from supersymmetry but astronomical results conform to those expectations, it would likely mean that physicists are missing something important.

Astrophysics gives us access to conditions far more extreme than anything researchers can reproduce in the laboratory. The ability to explore these frontiers of nature drew University of Chicago researcher Abby Vieregg to this field. In the recent Kavli Foundation discussion, Vieregg said, “We’re probing the universe at a time when it was really, really energetic, so at an energy that’s way bigger than we can ever make on Earth. Way bigger than anything you can make at the LHC…13 orders of magnitude bigger in energy.”

“In high energy physics, I think that people expect that at that grand unification scale there is going to be new physics, stuff that we don’t really understand yet,” says Bischoff. These high energies are precisely what is needed to explore the smallest scale of matter, space and time. It’s possible that the oldest event in the universe, the Big Bang, may hold the secret to new physics.

Go Deeper
Editor’s picks for further reading

BICEP2: 2014 Results Release
On the BICEP2 project page, find links to the original papers covering BICEP2′s results as well as FAQs, data products, and informational videos.

Huffington Post: Toward a Quantum Theory of Gravity?
In a two-part essay, physicist and writer Mario Livio discusses BICEP2 and the search for quantum gravity. (Read Part 2 here .)

Of Particular Significance: If It Holds Up, What Might BICEP2′s Discovery Mean?
Theoretical physicist Matt Strassler blogs on the implications of the BICEP2 results.