BICEP2 and Planck joint study: Gravitational waves remain elusive

Upgrades of NSF-supported telescopes at South Pole may aid in the search

A new joint analysis of data from two South Pole-based experiments–the BICEP2 telescope and the Keck Array, both supported by the National Science Foundation–and the European Space Agency’s Planck satellite, has found no conclusive evidence of primordial gravitational waves, despite earlier reports of a possible detection.

In early 2014, the BICEP2 team presented results based on observations of the polarized Cosmic Microwave Background (CMB), the legacy of light emitted only 380,000 years after the Big Bang, performed on a patch of the sky between 2010 and 2012. The team also used some preliminary data from the Keck Array. Their study revealed a signal that had never before been detected: so-called “curly B-modes” in the polarization observed over stretches of the sky a few times larger than the size of the full moon.

The BICEP2/Keck Array team presented evidence favoring the interpretation that this signal originated in primordial gravitational waves, sparking an enormous response in the academic community and general public.

However, there is another phenomenon that can produce a similar effect: interstellar dust in the Milky Way Galaxy.

A new paper, “A Joint Analysis of BICEP2/Keck Array and Planck Data,” submitted to the journal Physical Review Letters indicates that the interpretation of the earlier, much-publicized BICEP2 result as evidence for gravitational waves is no longer secure, once dust contamination is taken into account.

The Universe began about 13.8 billion years ago and evolved from an extremely hot, dense and uniform state to the rich and complex cosmos of galaxies, stars and planets we see today.

ESA’s Planck satellite observed this background across the whole sky with unprecedented accuracy, and a broad variety of new findings about the early Universe have already been revealed over the past two years.

But astronomers are still digging ever deeper in the hope of exploring even further back in time: they are searching for a particular signature of cosmic ‘inflation’–a very brief accelerated expansion that, according to current theory, the Universe may have experienced when it was only the tiniest fraction of a second old.

This signature would be seeded by gravitational waves, tiny perturbations in the fabric of space-time, that astronomers believe would have been generated during the inflationary phase.

Interestingly, these perturbations should leave an imprint on another feature of the cosmic background: its polarization.

When light waves vibrate preferentially in a certain direction, we say the light is polarized.

The CMB, an extraordinary source of information about the Universe’s history, is polarized, exhibiting a complex arrangement across the sky. This arises from the combination of two basic patterns: circular and radial (known as E-modes), and curly (B-modes).

Different phenomena in the Universe produce either E- or B-modes on different angular scales and identifying the various contributions requires extremely precise measurements. It is the B-modes that could hold the prize of probing the Universe’s early inflation.

“Searching for this unique record of the very early Universe is as difficult as it is exciting, since this subtle signal is hidden in the polarization of the CMB, which itself only represents only a feeble few percent of the total light,” says Jan Tauber, ESA’s project scientist for Planck.

The Milky Way is pervaded by a mixture of gas and dust shining at similar frequencies to those of the CMB, and this foreground emission affects the observation of the most ancient cosmic light. Very careful analysis is needed to separate the foreground emission from the cosmic background.

Critically, interstellar dust also emits polarized light, thus affecting the CMB polarization as well.

“When we first detected this signal in our data, we relied on models for Galactic dust emission that were available at the time,” says John Kovac, a BICEP2 principal investigator at Harvard University.

“These seemed to indicate that the region of the sky chosen for our observations had dust polarization much lower than the detected signal.”

Planck observed the sky in nine microwave and sub-millimeter frequency channels, seven of which were also equipped with polarization-sensitive detectors. By careful analysis, these multi-frequency data can be used to separate the various contributions.

The BICEP2 team had chosen a field where they believed dust emission would be low, and thus interpreted the signal as likely to be mainly cosmological.

However, as soon as Planck’s maps of the polarized emission from Galactic dust were released, it was clear that this foreground contribution could be much higher than previously expected.

In fact, in September 2014, Planck revealed for the first time that the polarized emission from dust is significant over the entire sky, and comparable to the signal detected by BICEP2 even in the cleanest regions.

So, the Planck and BICEP2 teams joined forces, combining the satellite’s ability to deal with foregrounds using observations at several frequencies–including those where dust emission is strongest–with the greater sensitivity of the ground-based experiments over limited areas of the sky, thanks to their more recent, improved technology. By then, the full Keck Array data from 2012 and 2013 had also become available.

“This joint work has shown that the detection of primordial B-modes is no longer robust once the emission from Galactic dust is removed,” says Jean-Loup Puget, principal investigator of the HFI instrument on Planck at the Institute d’Astrophysique Spatiale in Orsay, France.

“So, unfortunately, we have not been able to confirm that the signal is an imprint of cosmic inflation.”

Another source of B-mode polarization, dating back to the early Universe, was detected in this study, but on much smaller scales on the sky.

This signal, first discovered in 2013, is not a direct probe of the inflationary phase but is induced by the cosmic web of massive structures that populate the Universe and change the path of the CMB photons on their way to us.

This effect is called ‘gravitational lensing’, since massive objects bend the surrounding space and thus deflect the trajectory of light, much like a magnifying glass does. The detection of this signal using Planck, BICEP2 and the Keck Array together is the strongest yet.

As for signs of the inflationary period, the question remains open.

“While we haven’t found strong evidence of a signal from primordial gravitational waves in the best observations of CMB polarization that are currently available, this by no means rules out inflation,” says Reno Mandolesi, principal investigator of the LFI instrument on Planck at University of Ferrara, Italy.

In fact, the joint study sets an upper limit on the amount of gravitational waves from inflation, which might have been generated at the time but at a level too low to be confirmed by the present analysis.

“Our analysis concludes the amount of gravitational waves can probably be no more than about half the level that would have produced the full BICEP2 B-mode signal,” says Pryke.

“The new upper limit on the signal due to gravitational waves agrees well with the upper limit that we obtained earlier with Planck, albeit indirectly using the temperature fluctuations of the CMB,” says Brendan Crill, a leading member of both the Planck and BICEP2 teams from NASA’s Jet Propulsion Laboratory in the USA.

“But gravitational waves might still be hiding in the data, and the search is definitely on.”

To this end, the Keck Array added two new telescopes at 95 GHz in late 2013, and over the past year has produced new maps at that frequency that are nearly as deep as the ultra-deep BICEP2 maps, but in which polarized galactic dust power is expected to be around five times lower. This Keck Array 2014 dataset is expected to dramatically improve constraints on inflationary gravitational waves; its results are expected this spring.

The BICEP and Keck Array series of telescopes are operated by NSF’s Division of Polar Programs, which manages the U.S. Antarctic Program. The BICEP and Keck Array series of experiments is funded by NSF and led at Harvard by Kovac; at the University of Minnesota by Clem Pryke; at Caltech and JPL by Jamie Bock; and at Stanford University and SLAC by Chao-Lin Kuo. Other major collaborating institutions include Cardiff University, CEA, NIST, University of British Columbia, UC San Diego, University of Chicago, and the University of Toronto.

The South Pole telescopes have been used to search for B-mode polarization in the CMB with progressively increasing sensitivity, using relatively small cryogenic telescopes and the most advanced available detector technologies. The telescopes operate from NSF’s Amundsen-Scott South Pole station where they benefit from an ultra-dry atmosphere that is the closest a ground-based telescope can get to being in space. BICEP1 operated from 2006-2008. The present joint analysis with Planck uses data from BICEP2, which operated from 2010-2012, and data taken by the Keck Array in 2012 and 2013, all at a frequency of 150 GHz.

The program is ongoing and just last month, the Keck Array added a third frequency (220 GHz) to further improve separation of dust and CMB in its 2015 observations. And the team has just commissioned a new telescope, BICEP3, at the South Pole within the past two weeks. With a dramatic increase in its detector count, BICEP3 will further accelerate progress in the search for inflationary gravitational waves.

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