Planck versus BICEP2: Round One!

You may recall my scepticism about the recent announcement from the BICEP2 experiment about evidence from polarized microwave emission for the existence of primordial gravitational waves generated during a period of cosmic inflation.

Well, in between a couple of meetings this morning, I realised that there’s a paper just out onto the arXiv from the Planck Collaboration. Here’s the abstract:

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium. We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

There’s also a press release from the European Space Agency which includes this nice picture:


This study is at 353 GHz, compared to the 150 GHz of the BICEP2 measurements. Galactic dust emission increases with frequency so one would expect more of an effect in this Planck map than in BICEP2, but the fact that polarized foreground emission is so strong at these frequencies does give one pause for thought. The Planck data actually cover the whole sky, so the above map has clearly been censored; below you can see the actual region of the sky covered by BICEP2, so there is little or no direct overlap with what’s been released by Planck:


We’ll have to wait until later this year to see what’s going on in the masked regions (i.e. far above and below the Galactic Plane, where the dust emission is presumably weaker) and indeed at the 7 other frequencies measured by Planck. It’s all a bit of a tease so far!

Here’s what the press release says about BICEP2

In March 2014, scientists from the BICEP2 collaboration claimed the first detection of such a signal in data collected using a ground-based telescope observing a patch of the sky at a single microwave frequency. Critically, the claim relies on the assumption that foreground polarised emissions are almost negligible in this region.

Later this year, scientists from the Planck collaboration will release data based on Planck’s observations of polarised light covering the entire sky at seven different frequencies. The multiple frequency data should allow astronomers to separate with great confidence any possible foreground contamination from the tenuous primordial polarised signal.

P.S.  It’s gratifying to see the Planck Collaboration have used extragalactic Faraday Rotation measures to probe the Galactic Magnetic field as I suggested on this blog not long ago. The article that first advocated doing this with CMB maps can be found here.


8 Responses to “Planck versus BICEP2: Round One!”

  1. Good stuff Peter. Thanks.

    “Critically, the claim relies on the assumption that foreground polarised emissions are almost negligible in this region.”

    “…but the fact that polarized foreground emission is so strong at these frequencies does give one pause for thought.”

    It does, doesn’t it?

  2. […] hier, hier und hier). Regionen mit geringer Emission sind ausgeblendet, darunter leider auch jene, wo BICEP-2 sein Signal fand: Erst später im Jahr wird es komplette Himmelskarten der Polarisation geben, mit denen die noch […]

  3. Albert Says:

    This could still go either way. The blanking has been neatly done, probably by picking a suitable lowest contour?

  4. […] BICEP2 keek in één frequentie naar de hemel en wel bij 100 GHz, een frequentie die goed is om de CMB waar te nemen, maar niet voor gepolariseerd stof of synchrotronstraling. Met de Europese Planck satelliet is afgelopen jaren ook de CMB gemeten, alleen zijn er twee duidelijke verschillen met de BICEP2-meting: Planck keek naar de gehele hemel, dus inclusief de gebieden aan de hemel waar de Melkweg zich bevindt én het BICEP2-veld, én er werd niet in één frequentie gekeken, maar in zeven frequenties. De op die conferentie gepubliceerde kaart gepubliceerd van de polarisatie bij 353 GHZ door Planck toont ook de polarisatie in het BICEP2-veld en wat blijkt: er zou wel degelijk sprake zijn van polarisatie door stof van de Melkweg in dat gebied, ook al dacht het BICEP2-team dat het daar vrij zou zijn van storende ‘voorgrondruis’ van de Melkweg. Het gerucht gaat dat het team van BICEP2 heeft toegegeven dat er sprake is van vervuiling van het gemeten signaal met ruis van de Melkweg. Het betekent niet dat alles op het conto van de Melkweg kan worden geschoven, er zou nog steeds een bepaalde gedeelte veroorzaakt kunnen zijn door de primordiale zwaartekrachtsgolven. De inmiddels beroemde verhouding tussen de tensor van de zwaartekrachtsgolven en de scalar van de dichtheidsgolven zou dan niet r ≈ 0,2 zijn, zoals BICEP2 had gemeten, maar r ≈ 0,1. Het wachten is nu op meer gegevens van Planck, POLARBEAR, ACTpole en de Keck Array – concurrenten van BICEP2 – om dit te bevestigen of ontkrachten. :bron: Bron: Resonaances + In the Dark. […]

  5. Reblogged this on thecuriousastronomer and commented:
    This is intriguing, and well worth keeping an eye on what happens over the coming months……

  6. […] several other people’s blogs on this controversy, for example Peter Coles’ blog here and Matt Strassler here and here. As Peter and Matt’s blogs indicated, this controversy has […]

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