First evidence of Quantum Gravity? Ask the dust

Rise and fall of the biggest discovery of the century highlights the importance of open, collaborative science.

On 17 March 2014 BICEP2, a South Pole based experiment aimed at studying the very first moments of the universe, made a sensational announcement. They claimed to have detected for the first time the signature of an extremely rapid expansion of space that occurred right after the universe’s birth. This expansion, also called inflation, is believed to be responsible for the existence of large-scale structures like clusters of galaxies, as well as to explain why the properties of the universe appear to be the same for all observers. If confirmed, the existence of inflation would represent the first evidence of a fundamental connection between gravity (general relativity) and quantum physics.

But was it really a fast-growing baby universe that BICEP2 observed? Or something more mundane like the effect of dust? To collect the light of the CMB (cosmic microwave background, the elusive echo of the Big Bang), BICEP2 had to look with telescopes through the window glass of our Galaxy. And it turns out that this window is not as clean as previously thought, the dirt being small dust particles. These dust particles modifiy a particular property of light called polarization, which incidentally is the same proxy used to detect the existence of an inflation period. The specific signature that the scientists are after is called B-mode polarization, and could either have been imprinted by the gravitational waves associated with inflation right after the Big Bang, which occurred about 13.5 billion years ago, or created by the presence of dust as the ancient light of the CMB finally passed through the Milky Way before reaching the telescopes.

Planck sky map of B-mode polarization generated by dust. The BICEP2 field is indicated at bottom left of right panel

In a new paper, the collaboration behind the european satellite Planck reported that the amount of foreground dust present in the line of sight of BICEP2 instruments is enough to explain the observed signal, previously attributed to cosmological inflation. While it could still be that a significant part of the observed polarization signal is coming from inflation, it would require two very different, unrelated phenomena to contribute at a very similar level. Which is unlikely. The absence of evidence is not evidence of absence, but any extraordinary claims about the very first instants of the universe will have to be backed by extraordinary proof.

“The light-blue rectangles are what Planck actually sees and attributes to dust. The black line is the theoretical prediction for what you would see from gravitational waves with the amplitude claimed by BICEP2. As you see, they match very well. That is: the BICEP2 signal is apparently well-explained by dust” Caption from S. Carroll’s blog

More work needs to be done before the dust is settled (literally), but interestingly it seems that the final word on this particular result will have to come from a joint effort between Planck and BICEP2. The two groups have complementary data: BICEP2 is looking at a very specific location of the sky and at a very specific frequency, but with high sensitivity to polarized radiation. Planck is looking to the whole sky and at 9 different frequencies. Overall, only Planck can characterize the galactic dust foreground, while BICEP2 has the sensitivity to detect the presence of a B-mode polarization from cosmological inflation. That’s why the two competing teams finally decided to join forces and collaborate on a paper where they will jointly analyze the combined data. While this fascinating story reminds us that trial and error will always be part of the scientific process, it also stresses that open science and collaboration are the future of scientific research.

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