Much ado about dust?

If a physics announcement makes the evening news, it must be an epoch-making discovery. And initially, it looked like that in March of this year, when an American team of scientists reported to have discovered gravitational waves in the cosmic microwave background for the first time ever. Critical colleagues believe, however, that the signal could have a different cause and could be generated, at least partly, by the galactic dust in the Milky Way. This is also implied by a new publication of the Planck collaboration.

So far, the cosmic inflation is only a theory: It states that a split second after the Big Bang and only for a tiny moment, the universe expanded faster than with the speed of light, thereby increasing in size by at least a factor of 1026. This fantastic assumption could explain some of the crucial properties of our universe, but evidence is lacking. However, according to cosmologists, the presence of a particular swirly pattern in the polarization of the cosmic microwave background would be a strong indicator. If a cosmic inflation had occurred, the tiny gravitational waves caused by the Big Bang would have been pulled apart strongly during the inflation phase and thus should be preserved in the microwave background until nowadays.

The 512 detectors of the South Pole Telescope “Background Imaging for Cosmic Extragalactic Polarization” (BICEP2) have been looking for that swirly pattern for more than two years, mapping the microwave radiation with a frequency of 150 GHz coming towards the earth from a 380 square degrees spot in the sky. The signal should be extremely weak, if measurable at all. Therefore, the location and the spot in the sky have been carefully chosen: far away from the galactic plane of the Milky Way, clear and free of atmospheric disturbances. If anything, the scientists would be able to prove the distortion of space-time postulated by Einstein from this place on the earth. And at first, it looked as if the BICEP2 collaboration had been successful: In March, after two years of data analysis, the researchers broke the news that they had discovered the long-sought evidence and thus to have confirmed the inflation theory.

But meanwhile, international colleagues have doubts whether the conclusions drawn from the measurements are accurate. In May, the Excellence Cluster Universe invited its member Prof. Viatcheslav Mukhanov from the Ludwig-Maximilians-Universtität Munich to hold a public lecture. The internationally renowned cosmologist referred to tensions between the theory and the BICEP2 results. But what might be the problem?   Very likely, the problem is caused by interstellar dust in our galaxy. "The thermal radiation from dust particles that align in galactic magnetic fields is polarized", says Prof. Hans Böhringer from the Max Planck Institute for Extraterrestrial Physics, who is also a member of the Excellence Cluster Universe. "Since the galactic magnetic fields themselves are swirled, it is not surprising that the resulting polarization pattern contains vortices."

Apparently, the interpretation of the BICEP2 data strongly depends on how much dust is allocated to that 380 square degrees spot. The collaboration led by John Kovac only used very limited own data to estimate the dust contribution. The team had to rely on theoretical models as well as results from other experiments. Since June 2014, the BICEP2 results are published in Physical Review Letters where the scientists present their results more cautiously. The scientists state that their "models are not sufficiently constrained by external public data to exclude the possibility of dust emission bright enough to explain the entire excess signal".

A new publication of the Planck collaboration from September 2014 confirmed that the dust is not a negligible quantity: "We can say, that the detected spiral pattern in large parts arises from dust particles rotating in the galactic magnetic fields", says PD Dr. Torsten Ensslin from the Max Planck Institute for Astrophysics (MPA), who heads the Planck team at MPA and is a member of the Excellence Cluster Universe. "Whether the BICEP2 telescope has really observed signs of gravitational waves, is open." Both collaborations are now working together to jointly analyse their data. "In the microwave range, galactic astrophysics and cosmology meet each other, and experts from both are needed to understand the data," says Torsten Ensslin. It remains to be seen whether the news from March was more than much ado about dust.

Original publications:
Planck intermediate results XXX: "The angular power spectrum of  polarized dust emission at intermediate and high Galactic latitudes"

BICEP2 I: "Detection of B-Mode Polarization at Degree Angular Scales by BICEP2", Phys. Rev. Let. 112, 241101

Planck intermediate results XIX: "An overview of the polarized thermal emission from Galactic dust"

M. J. Mortonson, R. Sejak: "A joint analysis of Planck and BICEP2 modes including dust polarization uncertainty"

R. Flauger, J. Collin Hill, D. N. Spergel: "Toward an Understanding of Foreground Emission in the BICEP2 region"

BICEP2 I: "Detection of B-Mode Polarization at Degree Angular Scales by BICEP2"

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