The Demise of a Discovery: BICEP2 and Gravitational Waves

By Sarah Scoles | February 2, 2015 11:28 am
The BICEP2 telescope at twilight. Credit Steffen Richter, Harvard University

The sun sets on the BICEP2 telescope in Antarctica. Credit Steffen Richter, Harvard University

Last week, BICEP2 scientists — who in March announced evidence of cosmic inflation, a potentially Nobel-worthy find — threw handfuls of dust on the grave of their own results. The official paper [pdf], just published on the BICEP website, tells the story of how they mistook cosmic dust for “primordial gravitational waves,” and why everybody needs to calm down and stop trying to bury inflation, too.

What’s inflation?

Just 10-35 seconds after the Big Bang, cosmologists (or at least most of them) believe the universe expanded in hyperdrive — faster than it ever has since and faster than it ever will again. This ballooning, called inflation, smoothed everything out. It turned the cosmos into the roughly homogenous place we see today, and perhaps created other universes that add up to the sci-fi-sounding “multiverse.”

But it’s difficult to find direct evidence that inflation actually happened (after all, it was a long time ago). That’s where B-modes, which the BICEP2 team saw, come in.

What’s a B-mode?

You can’t spread a universe out that spectacularly and not expect to leave some stretch marks. Inflation would have made waves in the so-called “fabric of the universe” — spacetime itself. These gravitational waves would have left their fingerprint on the light leftover from the Big Bang. This light is called the cosmic microwave background (CMB).

The gravitational waves’ fingerprint — a twisting of the light’s orientation — looks a bit like yours: a swirling, cirrus-like pattern called “B-mode polarization.” Today, with our fancy telescopes, we finally have the ability to scan for that fingerprint. And in March of 2014, researchers using BICEP2, a South-Pole telescope, claimed they’d found it.

So what’s the problem?

Inflation doesn’t actually have a unique fingerprint. Dust in our own Milky Way twists the signal the exact same way. So do distant, massive galaxies. This “lensing” by galaxies is well understood, and the BICEP2 scientists subtracted it out, but it turns out they hadn’t done such a robust job cleaning up the dust. Immediately after the BICEP2 announcement, other scientists began to raise doubts: Maybe their B-modes came only from dust.

And now for a different metaphor.

To find out how “loud” the gravitational-wave B-modes are, scientists have to subtract lensing B-modes (easy) and dust B-modes (harder).

Imagine that your neighbor turns her radio to FM station 107.5 loud enough so you can hear it; a car across the street also blasts 107.5; and you stream it from your laptop. The song will be louder than if you alone were playing it. If you want to know how much of the sound comes from the car, you have to subtract the sound coming from your own computer. Easy enough. But you also have to subtract your neighbor, and to do that, you have to go talk to your neighbor and find out how loud their radio is.

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image by ESA/Planck

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image by ESA/Planck

Talking to the neighbors

The BICEP2 team didn’t have a good map of the dust’s contribution to the total B-modes, because such a map didn’t exist until the Planck telescope made one. Planck is a precise CMB cartographer, and it can see multiple frequencies — specifically ones around 353 MHz, where dust is strongest. (Dust displays itself differently at different frequencies.)

But Planck’s results weren’t yet available when the BICEP2 team wanted to have a press conference. They believed their B-modes came from a place where the dust wasn’t very thick. They didn’t think it was a problem.

But they were wrong. And that’s what the new paper, the old controversy, and the continuing saga are about.

The BICEP2ers joined forces with Planckers and added in another South Pole telescope — the Keck Array — for good measure. Here’s what they found:

The good news

Planck sees essentially the same B-modes in the same places as BICEP2 did. The fingerprints are a match.

The bad news

Planck’s data shows more dust in the way, which means the dust’s contribution to the total B-mode signal is bigger than the BICEP2 team thought. When they subtracted the well-known lensing and the newly-known dust signals from the total, there was almost no signal left — definitely not enough signal to declare a detection of gravitational waves.

In the radio analogy, if you find out the neighbor’s radio is louder than you thought, the faraway car’s radio must be softer than you thought — maybe so soft you aren’t actually hearing it at all.

The news by the numbers

Scientists measure how “loud” gravitational wave B-modes are using a ratio called r.

r measures, to put it crudely, how “powerful” inflation was. It compares primordial gravitational waves to “density perturbations,” or lumpiness in the young universe. The stronger gravitational waves are, the more forceful inflation was, and the bigger r is.

The whiz-bang BICEP result from March 2014 claimed that r was a whopping 0.2 (OMG, I know). Scientists had expected it to be (and early Planck results suggested it was) closer to 0.1. Inflation was more epic than we thought! And we already thought it was pretty epic! was the science world’s excited reaction to that first result.

Now, though, the team have faced the dreary reality that dust accounted for most of the signal. Maybe all of the signal. Maybe r is 0, meaning there were no gravitational waves, there was no inflation, and we must trudge back to the whiteboard to figure out what the hell happened at the beginning of the universe.

But don’t pick up that dry erase marker just yet. There are a lot of r’s between 0 and 0.2. The smaller r is, the harder those cirrus-y fingerprints are to detect, and the longer and harder we’ll have to look for them.

Now that the dust has settled, the search can continue.

CATEGORIZED UNDER: Space & Physics, Top Posts
MORE ABOUT: Big Bang, cosmology
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  • Jim Wylie

    They needed to take a page out of the climate science play book. Those guys managed to remove astronomical amounts of noise to arrive at a signal measured in tenths of a degree!

    • NavySubNuke

      It is amazing how easy it is to find the “right” answer when you “know” what it should be and can manipulate the data to match….

  • Ron Nussbeck

    Jean-Loup Puget, principal investigator of Planck’s high frequency instrument, said in a statement from the ESA.
    “So, unfortunately, we have not been able to confirm that the signal is an imprint of cosmic inflation.” It is my Hypothesis that in the Primordial Universe first moments that Light did not accelerate it’s speed past the current level, there were no gravitational waves creating Inflation. The sudden expansion of the Universe was caused by a Phantom particle that still can be found today entering the Universe. At the very earliest moment as subatomic particles flowed into existence they did not carry a charge either positive or negative and none of the forces of Nature had been imprinted on them, a blank slate so to speak. The Phantom particle created outside our Primordial Universe entered each subatomic particle at the same time and in one moment released their energy across the entire Primordial Universe. Light never had to exceed the Laws of Physics, it was our minds being incapable of understanding what had happened at that point, the energy was now in every particle. The energy released by the Phantom in each and every subatomic particle was a perfectly controlled, a massive detonation. Hypothesis, particles entered the Universe without a charge, Phantom enters leaving no trace evidence because it was created outside our Universe and annihilates in a calculated event sending material in every direction with it’s corresponding charge and laws of physics encoded within each particle.

    • YeahRight

      What’s with this delusion that you are smart?

      • Ron Nussbeck

        YeahRight, this post was a test, it would take a person with an I.Q. of 180 to comprehend it’s content. Since widely publishing my comment many Physicist have now discarded the Inflation theory as lacking a basis in fact and lacks mathematical solvability. This post was never intended for you to understand, it’s like a coded message for intellectuals…. Hope I did not disturb your thought process.

        • sadoul1

          Fellow intellectual here … still don’t understand the origin of this Phantom Particle theory. What is the mathematical basis for this hypothesis? What evidence lead you down the path to promulgate this conclusion that our physics is missing a particle at the origin of time? What particular variable or phenomenon?

          • Ron Nussbeck

            Trying to explain the Phantom particle in classical terms requires an understanding of proton spin. To say spin is a particles rotation is more metaphorical than Literal. Spin is a quantum quantity of complex probabilistic property and always equal to one half. A proton is a mixture of phantom particles appearing and disappearing in an energy annihilation. This keeps it’s rotation, alignment, spin and color pure. Extra dimensions, as many as 2 would solve gravities weakness and give credence to the Phantom particle being produced outside our Universe. Clearly, the phantom could explain the rapid expansion of the Universe beyond the speed of light. It also could lead directly to the understanding of Dark energy. The hypothesis that the Phantom particle imprinted the four forces on matter in the first moments of the primordial Universe leaves the Inflation theory just hot air.
            Finally, phantom particles are quantum mechanics control mechanism to charge each atom keeping the Universe from decaying. The reason for a Phantom particle created in another dimension keeps the 1st law of thermodynamics from being violated. I very well expect CERN to find new unexplained particles with much higher energies when it restarts in June. The reason for my belief in the Phantom particle is my study of Quasars using spectral imaging and Electron imaging. As material enters the Event Horizon it disassembles particles into their smallest pieces of matter and antimatter. As the material enter in the Event Horizon in mass quantities the Black Hole over eats and belches back material that was annihilates in a Gamma Burst.

        • Tim Tian

          I guess you mean completely comprehend, because my last IQ test result was 130, and I understand enough to challenge several assumptions. Not that IQ is an accurate measure of actual intelligence.

  • YeahRight

    There never was a discovery. A “discovery” in science is a measurement that has been confirmed in multiple independent ways.

  • Don Crawford

    When you start with the wrong bases for understanding, you never discover the Truths beings sought. Example, the universe is eternal, unbounded and without beginning or end. This Ancient Wisdom has been with humanity since its vey inception on this earth. So, the big bang theory is an erroneous beginning which leads to further and further errors in scientific thinking.

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