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The recent results of the BICEP 2 experiment published on March 17th 2014, has generated a lot of media attention, with the general consensus being that "this is a major discovery" perhaps leading to a Nobel Prize for some.

But for those that are still skeptical, what are the competitors to this experiment, and when are their results expected to be published?

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Competing experiments that are also trying to detected large scale B-mode polarization in the CMB include:

South Pole Telescope

https://pole.uchicago.edu/spt/science/index.html

POLARBEAR

http://www.mcgillcosmology.ca/polarbear

http://bolo.berkeley.edu/polarbear/

Atacama B-mode Search

http://www.princeton.edu/physics/research/cosmology-experiment/abs-experiment/

ACTpol (Atacama Cosmological Telescope)

http://arxiv.org/abs/1006.5049

http://www.princeton.edu/act/

CLASS (Cosmology Large-Angular Scale Surveyor)

http://cosmos.pha.jhu.edu/bennett/class.html

PIPER (Primordial Inflation Polarization Explorer) balloon experiments http://science.gsfc.nasa.gov/665/research/

ESA Plank satellite

http://www.esa.int/Our_Activities/Space_Science/Planck

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  • $\begingroup$ There is also Spider. $\endgroup$ – user10851 Oct 14 '14 at 19:25
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The Planck experiment looks to be "in tension" (e.g. it conflicts) with BICEP2's results: Planck has previously reported an upper bound $r<0.11$ for the parameter that BICEP2 reports at 0.20 (see for example, http://arxiv.org/abs/1303.5082). Planck is expected to publish new results this summer fall.


21 Sep 2014 update: Planck has now published their galactic dust measurement results.

The situation to date:

  • BICEP2 assessed (via indirect means) that polarized emissions from galactic dust ("foregrounds") constituted at most a small fraction of the signal they observed at 150 GHZ in the multipole region $l=30-150$, so that all (or at least most) of the signal was cosmological in origin and therefore Nobel-worthy evidence for inflation. (The larger the foreground, the lower the value of the tensor-to-scalar ratio $r$; BICEP2's reported $r=0.20$ corresponds to no foreground.)
  • However, Planck's new results put the level of those foreground signals substantially higher than BICEP2's estimates. (Planck's result is not a direct measurement at BICEP2's 150GHz frequency, where Planck cannot match BICEP2's sensitivity, but a calibrated extrapolation from 353 GHz data, where the dust signal is much stronger.) In fact, it is possible that Planck's foreground accounts for all of BICEP2's observations, which would mean that BICEP2 detected no cosmological signal at all.
  • Currently, Planck and BICEP2 are jointly analyzing their combined data sets to determine how much, if any, of BICEP2's signal can be attributed to cosmological inflation. The results of their joint analysis are expected by year end 2014.

31 Jan 2015 update:

The joint Planck-BICEP2 analysis is now complete, with the paper available here. The result: BICEP's claim of detecting primordial B-mode polarization at a level of $r = 0.16-0.20$ is withdrawn, and replaced by a 95%-confidence upper bound $r<0.12$, which is almost exactly the same as Planck's upper bound $r<0.11$ before BICEP2 reported last year.

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    $\begingroup$ BICEP2's response to the apparent conflict is "Our measurements don’t disagree. Constraints on the gravitational-wave background level r reported from Planck and previous experiments are not from measurements of B-mode polarization. Instead, they come from the CMB temperature measurements which show surprisingly low power at the largest scales, implying little room for an additional contribution from tensors in the context of the simplest models. (continued below) $\endgroup$ – DavePhD Mar 18 '14 at 14:41
  • $\begingroup$ (continued from above) B-mode measurements like ours aim to directly measure the inflationary gravitational-wave pattern itself at the degree angular scales where it should peak. The tension between the high level of B-mode polarization we see and the apparent low power at large scales may be a statistical fluke, but many possible extensions to the simplest model could also relieve this apparent tension." source: bicepkeck.org/faq.html $\endgroup$ – DavePhD Mar 18 '14 at 14:42
  • $\begingroup$ @DavePhD: Interesting. This portion of the conclusion of the BICEP2 "Detection" paper acknowledges the "tension": $\endgroup$ – Art Brown Mar 18 '14 at 16:33
  • $\begingroup$ "Subtracting the various dust models and re-deriving the r constraint still results in high significance of detection. For the model which is perhaps the most likely to be close to reality (DDM2 cross) the maximum likelihood value shifts to r = 0.16 +0.06 -0.05 with r = 0 disfavored at 5.9σ. $\endgroup$ – Art Brown Mar 18 '14 at 16:34
  • $\begingroup$ These high values of r are in apparent tension with previous indirect limits based on temperature measurements and we have discussed some possible resolutions including modifications of the initial scalar perturbation spectrum such as running. However we emphasize that we do not claim to know what the resolution is." $\endgroup$ – Art Brown Mar 18 '14 at 16:35

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