Direct Evidence of Big Bang Inflation

Researchers with an experiment based at the South Pole have discovered the long-sought "smoking gun" for inflation.

Researchers with the BICEP2 experiment have set the world’s cosmologists buzzing with the announcement that they’ve detected the fingerprints of inflation — the exponential expansion that put the “bang” in the Big Bang.

B-mode discovery telescope

The BICEP2 telescope at twilight, which occurs only twice a year at the South Pole. The MAPO observatory (home of the Keck Array telescope) and the South Pole station can be seen in the background.
Steffen Richter / BICEP Collaboration

About 10 teams of researchers around the world have been actively looking for this signal, called primordial B-modes. But I have to admit that, of all the announcements that might have hit the air waves St. Patrick’s Day morning, the discovery of this polarization signal was not at the top of my list. Two teams did reach an important stepping point in this hunt several months ago, by finding another signal that could muck up the primordial data. But from work presented at the American Astronomical Society meeting this past January, I figured astronomers were at least a year away from the announcement made today.

I’m glad to be wrong.

B-modes are a particular pattern of polarization. As a wavelength of polarized light travels through space, it wiggles at a preferred angle to its direction of motion. If inflation happened, it should have sent gravitational waves rippling through spacetime. These waves would have imprinted the B-mode polarization pattern on the cosmic microwave background radiation (CMB).

Polarized photons explained

Gravitational waves created polarization patterns in the cosmic microwave background (CMB) by stretching and squeezing space — and therefore the plasma soup of primordial photons and electrons — as the waves passed. (A) Before a wave hits it from behind, a cross-section of space with an electron in the middle looks normal. But when the wave hits, the cross-section stretches and squeezes one way, then another, in an oscillating pattern (B). Instead of a uniform soup, the electron “sees” around it a universe a bit hotter in the squeezed direction and a bit colder in the stretched direction (C). Originally, a photon’s wave wiggles in all planes perpendicular to the photon’s motion (D and E, incoming crosses). When photons scatter off the electron, they become polarized, wiggling in only one plane (outgoing lines). The resulting pattern (F) is a sum of the cold and hot photons’ polarizations. But because photons from hotter regions have more energy, their pattern “wins out,” meaning the overall polarization is parallel to the hot regions (G).
Credit: Leah Tiscione / Sky & Telescope

There’s one other way to create a B-mode pattern in the CMB: when the gravity of large-scale cosmic structures works as a lens on the CMB, distorting its polarization pattern. But these lensed B-modes exist at an angular scale only one-tenth of the primordial ones. With a lot of careful analysis, researchers can weed these out.

The discovery of the primordial B-modes comes from the second round of the Background Imaging of Cosmic Extragalactic Polarization (BICEP) experiment. It’s among one of several projects observing the CMB in what’s called the Southern Hole, a patch of sky visible from Antarctica that’s a direct sightline out of our galaxy and into the cosmic depths. (See the sky map.)

The BICEP2 scope has an aperture of less than 30 centimeters (12 inches), but it doesn’t need to be big. Cooled to 4 kelvin, it gazes at a 20° patch of sky 24/7, detecting the CMB’s faint microwaves — and, crucially, how they’re polarized.

Using 3 years of data, the BICEP2 team meticulously analyzed their polarization measurements. They also compared their data with observations from BICEP1 and from the team’s new Keck Array, which is basically like five BICEP2s in one. It was this ability to combine three data sets that ultimately allowed the team to make the discovery.

polarization experiments' fields of view

Several projects are currently hunting for the polarization signature of inflation. Shown here are the fields of view for projects active as of fall 2013 (except for Planck, which is all-sky). Fields are approximate and distorted by projection at high declinations.
Credit: Gregg Dinderman / Sky & Telescope

After a year of intense work — including ruling out more than a dozen alternate explanations — the team is confident that they’re seeing the signal of inflation, on a scale of about 2° on the sky. In statistical terms, their signal is better than 5 sigma, which is the gold standard a detection has to meet before physicists accept it as a discovery.

“We are convinced that the signal is really coming from the sky, and that it’s coming from the cosmic microwave background,” says Clem Pryke (University of Minnesota), who headed up the analysis.

The other researchers present at the technical briefing were also swayed. “This looks as solid as any result that I’ve seen,” says Alan Guth (MIT), co-developer of the inflation paradigm. He and everyone (including the team) want other groups to confirm it, but the signal sure looks like it’s from inflation.

“I am extremely excited,” said gravitational waves physicist Scott Hughes (MIT), beaming. “Of course there are these implications for cosmology and inflation — just as a scientist it’s bloody awesome.”

The So What

Until now, astronomers have really only had one line of evidence to investigate whether inflation happened: the CMB’s speckled pattern of temperature variations. Studies of these patterns — particularly as seen by ESA's Planck satellite — support the simplest version of inflation.

Alan Guth, Andrei Linde

Founding fathers rejoice. Among the many notables who came to be part of Monday's announcement were the inflation theory's prime originators, Alan Guth (left) and Andrei Linde. Said Linde, "To see this confirmed within my lifetime — it's so wonderful. I had to get on a plane and fly here."
Credit: Camille M. Carlisle / Sky & Telescope

But having B-modes in hand is another ballgame. “This is not something that’s just a home run, but a grand slam,” says Marc Kamionkowski (Johns Hopkins University), one of the theorists who first suggested inflation-triggered B-modes might be detectable in the CMB. “It’s the smoking gun for inflation.”

The B-modes carry with them specific information about the size of the gravitational waves, when inflation happened, and how much energy inflation involved. So having an actual detection in hand shrinks the theoretical playing field — and not just a little, both Kamionkowski and Guth stress.

From the BICEP2 results, it looks like inflation happened roughly 0.5 × 10-37 second after the Big Bang, says Kamionkowski — but he cautions that’s from a quick calculation he did on a scrap of paper.

The measurement also suggests that inflation might have had something to do with the unification of three of the four fundamental forces of nature — the strong, weak, and electromagnetic. The energy level implied by the BICEP2 data — we’re talking 2 × 1016 GeV, according to Guth, or roughly a trillion times the energy of the Large Hadron Collider — matches the energy of grand unified theories, or GUTs. That’s an idea theorists have toyed with since the 1970s, but the BICEP2 result is the missing link they’ve sought for decades.

The data also tell us something about the size of the gravitational waves. This information comes in the form of the ratio of gravitational waves (which are a type of density perturbation) to the run-of-the-mill density fluctuations in the CMB. The technical term for this number is the tensor-to-scalar ratio, where the gravitational waves are tensors and the “normal” density fluctuations are scalars.

The BICEP2 team came up with a ratio of about 0.2, which means the gravitational waves were “pretty big,” Kamionkowski says. (Sorry, I don’t have an ironclad number for you.) The Planck team had come up with an upper limit of 0.11 from their data, but Pryke says that, while there’s a bit of tension here with his team’s result, the discrepancy is not much to worry about. It could be solved by simple extensions to the standard cosmological model, for example. They don’t know yet.

The results do not tell us what set inflation in motion, only that it happened. Nor do they answer the question of whether inflation is eternal, setting off an endless series of big bangs and creating pocket universes. This cosmological landscape is usually referred to as the multiverse. (You can read my in-depth discussion of the search for evidence of multiple universes in the December 2012 Sky & Telescope.) However, it’s hard to tune inflation such that pocket universes don’t happen, Guth points out.

B-mode sky map

The "curly" B-modes of polarization in the cosmic microwave background. Each line is a measure of polarization at one point on the sky. When the larger E-mode polarization is subtracted, this is what's left. Nearly all of it is the signature of quantum-gravitational chaos in the first instant of the Big Bang. This is an actual map of the sky near the south galactic pole, about 15° tall. The strongest curl patterns (emphasized with colors) are a couple of degrees wide, roughly the size of your thumb held at arm's length against the sky.
Credit: Harvard University / BICEP2 team

A few “smaller” results that have been lost in the inflation hubbub:

1. This is the first hard evidence that gravity is quantized, or comes in discrete packets as light does. The gravitational waves that produced the B-modes were born as quantum fluctuations in gravity itself, then stretched during inflation’s faster-than-light-speed expansion. “I think this is the only observational evidence that we have that actually shows that gravity is quantized,” says cosmologist Ken Olum (Tufts University). “It’s probably the only evidence of this that we will ever have.”

2. This is the first detection of gravitational waves’ action on matter other than their source. Astronomers have observed neutron stars spiraling inward toward each other just as they should if the system was radiating gravitational waves, but they’ve never seen these waves affecting other matter in the cosmos.

3. This is the first detection of Hawking radiation. Hawking radiation is usually associated with the slow evaporation of black holes, as photons emitted from the event horizon. But the observable universe also has a horizon. Hawking radiation should be coming from this horizon, and also from every horizon in the universe — in other words, from every point in the universe, says cosmologist Max Tegmark (MIT). Today the cosmic horizons are huge and their Hawking radiation is utterly insignificant. But in the universe’s first fraction of a second, the horizons were tiny and sharply curved. The gravitational waves announced today are these horizons’ Hawking radiation.

Other teams will be working arduously to confirm the BICEP2 result. Planck’s polarization measurements aren’t expected until later this year, and the last word from the team was that those results wouldn’t include primordial B-mode analysis. But Planck's all-sky coverage might reveal B-modes on larger angular scales than BICEP2 can, and also show something called the "reionization bump," a result of primordial B-modes being rearranged by intervening ionized material, says Planck scientist Charles Lawrence (JPL). Whether Planck can do it at all, though, is for now uncertain.

In the meantime, the excitement is palpable. As Tegmark put it, “I think this is one of the most important discoveries of all time.” (See his blog post from the event for why.)

Here is the BICEP team's website for their papers, detailed information about the data, explanations, images, and videos.

Senior editor Alan MacRobert contributed to the reporting for this news article.

12 thoughts on “Direct Evidence of Big Bang Inflation

  1. Robert L. Oldershaw

    Please bear in mind that a very large number of stellar and galactic scale black holes undergoing violent interactions immediately following the Big Bang could also have produced the gravitational waves that billions of years later yielded the B-mode polarization that was observed recently.

    In science, it is important not to dogmatically insist on one interpretation when other equally valid interpretations are available.

    Observational evidence will eventually help us to decide the best way to interpret the physics of the B-mode polarization.

    We are in the very early stages of this exciting new research.

  2. Anthony Barreiro

    Tom, I believe the colors in this image represent temperature variations, red being warmer and blue colder (the opposite of most astronomical images!). —– And Camille, thank you very much for this clear and intelligible report. When I heard this story on the news I knew that Sky and Telescope would explain it in greater depth but at a level an interested layperson could understand. Previous articles in Sky and Telescope and on the website have given me enough background to put this exciting news in context. The findings of quantum gravity, gravitational waves, and Hawking radiation are immensely important, too! —– I wonder, since COBE, WMAP, and Planck all found temperature variations at every possible angular dimension, might all-sky polarization maps find similar variations in polarization at larger angles, and if so what would this tell us? —– We live during interesting times!

  3. Peter WilsonPeter Wilson

    Robert is correct, “it is important not to dogmatically insist on one interpretation when other equally valid interpretations are available.” But, according to the article, they’ve ruled out, “more than a dozen alternate explanations.” With all due respect to the many intelligent commentators here, we are not going to settle this. We have no choice but to take it on faith that Robert is wrong and they are right: no other valid interpretations will do. Again: Inflation was conceived to erase large-scale density fluctuations, but later was discovered to have the exact opposite effect of magnifying quantum-scale density fluctuations to just the right size. Wow! But how? How does Inflation erase large-scale fluctuations while simultaneously magnifying microscopic ones? Don’t bother answering, Camille, I’m sure it involves math that is way over my head. I’ll just take in on faith.

  4. Rod

    An observation about this report. Perhaps I missed this in the report but inflation depends upon quantum gravity which has no laboratory experimental verification. Also inflation sidesteps the 1st and 2nd laws of thermodynamics so can create the universe out of nothing and is the ultimate free lunch. Q: What experimental evidence in the lab shows the 1st and 2nd laws do not apply in quantum mechanics? We still need to have caution about BICEP2 claims here I feel.

  5. Anthony Barreiro

    Rod, good questions. My knowledge of thermodynamics, quantum mechanics, and particle physics ranges from elementary to virtually nonexistent, but I don’t think that inflation posits creating something out of nothing. Rather during the first moment of creation, the universe was inconceivably hot and dense. As the universe expanded and started to cool, there was a phase transition, analogous to steam condensing to water or water freezing to ice, and this released the tremendous amount of energy that powered inflation for a tiny fraction of a second. —– My understanding is that inflation initially was a post-hoc hypothesis needed to explain the fact that today’s universe appears roughly the same at large dimensions in every direction and seems to extend much farther than we can see in any direction. The fact that we’ve now found observational evidence of a predicted aftereffect of inflation is as close as we’re going to get to experimental evidence. We will never be able to construct a particle accelerator with conditions as hot and dense as the first moment of the universe. We would have to squeeze the entire universe into the size of a hydrogen atom, and there’s currently no funding for that project in the NSF budget. Not even CERN is planning anything that ambitious.

  6. Joe S.

    I’ve always thought that the cosmic background radiation was predominantly from the UV photons emitted when the universe had cooled sufficiently to allow the hydrogen plasma to recombine into neutral hydrogen, so that the photons no longer interacted; then the expansion of the universe red-shifted them into the microwave. I believe that moment is often called the "last scattering surface." My question is how could the photons from any time during or at the very end of inflation (10^37 sec) have survived after propagating through the quark plasma then the hydrogen plasma for roughly 300,000 years? This one has me stumped.

  7. Rod

    Anthony, thanks for the comments. So what we have is the quantum fluctuation in an area <= Planck length that gives birth to the universe and during inflation a phase transition takes place that evolves into everything we see today including the 1st and 2nd laws of thermodynamics and all physical constants and other natural laws. This is indeed an amazing creation story.

  8. Joe S.

    Ok – the article says the inflation-generated gravity waves were big. I guess they were still rattling around at the time of recombination and they polarized the usual CMB photons. We’re not seeing radiation from the time of inflation – i knew that had to be wrong. My internal story is now recalibrated.

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