Dust and Rumors: Gravitational Wave Signal Still Legit?


In March, astronomers announced a revolutionary discovery: they had observed evidence for primordial gravitational waves and, by extension, uncovered evidence for inflation — the rapid period of cosmic expansion immediately after the Big Bang.

Two months later, however, rumors have been whispered suggesting the team who made the discovery may have done so in error. Now, the BICEP2 researchers have struck back, saying that they stand by their results and that any rumors about doubt in the team are simply not true.

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Is this all just a storm in the cosmic teacup?

Gravitational Waves and the Origins of the Universe

The BICEP2 telescope is located at the South Pole and is used to study the ubiquitous cosmic microwave background (CMB) radiation that can be seen right at the edge of our observable universe. The CMB is a key piece of evidence for the Big Bang and can be thought of as the echo (or the “afterglow”) of the earliest energetic creation of our Universe nearly 14 billion years ago.

While measuring slight temperature variations in the CMB, known as “anisotropies,” cosmologists have been able to gain valuable insights to the characteristics of the early universe and have even been able to precisely gauge the Universe’s age.

By studying the polarization of this primordial microwave radiation, BICEP2′s mission is to seek out observational evidence for "B-mode polarization" in the CMB caused by gravitational waves. Gravitational waves are predicted by Einstein’s theory of general relativity equations, but a century after he penned that theory, Einstein’s space-time ripples have been notoriously difficult to detect.

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To huge fanfare on March 17, the BICEP2 team announced that they’d not only detected gravitational waves etched into the CMB, but that the waves’ origins can be traced back to the moments just after the Big Bang.

This is amazing news in its own right, but there’s more.

If these gravitational waves originated from the inflationary period — when the Universe was many orders of magnitude smaller, fractions of a fraction of a fraction of a nanosecond after the Big Bang — these waves must have a quantum origin. In other words, these results suggest quantum gravity may indeed be real, potentially marrying two key physics theories that just don’t seem to fit: the Standard Model (which describes all the fundamental forces apart from gravity) and general relativity (that describes gravity, but fails to consider quantum phenomena).

Planck Support?

An announcement of this magnitude, which encompasses the discovery of gravitational waves and provides evidence for the inflationary model of our universe and potentially unites two apparently ununitable theories, is bound to draw a healthy dose of scientific scrutiny. This is especially true as the BICEP2 results have yet to emerge from the peer review process — a scientific ‘quality control’ of sorts before the research is accepted for publication in one of the major scientific journals.

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As soon as the gravitational wave media storm subdued, members of the scientific community came forward with their concerns about the BICEP2 results. Although the team reassured the world that they had taken all the proper precautions to ensure the polarization signal was indeed coming from the CMB and not from the obscuring dust of the Milky Way, the definitive proof of this won’t come until another group can confirm the BICEP2 results.

One project of particular interest is the European Planck space telescope that is also studying the CMB and is currently working to subtract polarizing interference from dust in our own galaxy.

On May 6, the Planck team released preliminary observations of the magnetic configuration of the Milky Way’s field after analyzing the polarization of light caused by galactic dust grains. As the BICEP2 telescope is peering through this polarizing galactic dust, did the BICEP2 team properly account for this obscuring dust and subtract its effect?

Polarizing Rumors

Unfortunately in that Planck announcement, the patch of sky studied by BICEP2 was not included and, during analysis, the team had to use a preliminary foreground polarization map and did not have access to detailed Planck data.

In a blog posted to the website Résonaances on Monday, CERN particle physicist Adam Falkowski hinted of news that even members of the BICEP2 team were concerned that they hadn’t properly corrected for galactic dust.

Referring to the preliminary polarization map of the entire sky issued by the Planck team, Falkowski said that BICEP2 scientists didn’t properly account for the polarized dust plotted on the map and that “some experts claim that the polarized galactic dust emission can account for most of the BICEP signal. The rumor is that the BICEP team has now admitted to the mistake.”

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This rumor and accusations of doubt in the team didn’t go unnoticed by BICEP2 co-principal investigator and cosmologist Clement Pryke, of the University of Minnesota, Twin Cities. The rumor that the BICEP2 team thinks they made a mistake in the subtraction of foreground dust polarization subtraction “is totally false,” Pryke told Science Now. He added: “We stand by our paper.” The BICEP2 paper has submitted to the arXiv preprint service.

Co-principal investigator John Kovac of Harvard University added that due care was taken by his team and that they are well aware of the limitations of their analysis. "We tried to do a careful job in the paper of addressing what public information there was, and also being upfront about the uncertainties," he told New Scientist. "We are quite comfortable with the approach we have taken."

So what’s next? It seems the BICEP2 team are sticking to their guns whereas the rumors will probably still buzz like static until another team confirms the BICEP2 gravitational wave detection. Fortunately, we probably won’t have to wait for long until the discovery (or not) is set in stone.

In October, Planck scientists plan on releasing their CMB polarization data that could either support or conflict with the BICEP2 gravitational wave results. They also aim to release a detailed map of the obscuring galactic dust polarization, including the region of sky covered by BICEP2. But directing comparing Planck data with the BICEP2 results will likely not provide definitive proof, however, so results from other ground-based observatories -- such as the POLARBEAR experiment in Chile or the Keck Array -- will need to be analyzed.

As we await further investigation, it's best to be patient for the scientific process to take its course before letting vague rumors blur the public’s perception of this (potentially) historic BICEP2 result.

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