The Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment at the South Pole found a pattern called primordial B-mode polarization in the light left over from just after the big bang, known as the cosmic microwave background (CMB). This pattern, basically a curling in the polarization, or orientation, of the light, can be created only by gravitational waves produced by inflation. “It looks like a swirly pattern on the sky,” says Chao-Lin Kuo, a physicist at Stanford University, who designed the BICEP2 detector. “We’ve found the smoking gun evidence for inflation and we’ve also produced the first image of gravitational waves across the sky.”
Indeed, the tensor-to-scalar ratio of 0.20 that BICEP2 measured is considered to be significantly larger than that expected from previous analyses of data. But the BICEP2 researchers said in their press conference yesterday that they believe certain tweaks could be made to an extension of the ΛCDM cosmological model that could make the two results agree.
"But these tweaks would be tremendously ugly....and in fact, I believe that if both Planck and the new results agree, then together they would give substantial evidence against inflation!" exclaims Turok, further explaining that "[we] must be careful before we treat them as true".
It is important to realize that a scale-invariant spectrum of adiabatic fluctuations as the origin of structure in the universe was already postulated a declare before the development of inflationary cosmology , and that ANY scenario which produces an almost scale-invariant spectrum of adiabatic fluctuations agrees equally well with the observations. This point is generally recognized, but it is then often claimed that since inflation produces a nearly scale-invariant spectrum of gravitational waves, the discovery of such a spectrum will confirm inflation. However, as will be emphasized here, this logic is incorrect.
Taking all this together I have to say that I stick to the point of view I took when I first saw the results. They are very interesting, but it is far too earlier to even claim that they are cosmological, let alone to start talking about providing evidence for or against particular models of the early Universe. No doubt I’ll be criticized for trying to put a wet blanket over the whole affair, but this is a measurement of such potential importance that I think we have to set the bar very high indeed when it comes to evidence. If I were running a book on this, I would put it at no better than even money that this is a cosmological signal.
The BICEP2 instrument detects radiation at only one frequency, so cannot distinguish the cosmic contribution from other sources. To do so, the BICEP2 team used measurements of galactic dust collected by the Wilkinson Microwave Anisotropy Probe and Planck satellites, each of which operates over a range of other frequencies. When the BICEP2 team did its analysis, the Planck dust map had not yet been published, so the team extracted data from a preliminary map that had been presented several months earlier. Now a careful reanalysis by scientists at Princeton University and the Institute for Advanced Study, also in Princeton, has concluded that the BICEP2 B-mode pattern could be the result mostly or entirely of foreground effects without any contribution from gravitational waves. Other dust models considered by the BICEP2 team do not change this negative conclusion, the Princetonteam showed (R. Flauger, J. C. Hill and D. N. Spergel, preprint at http://arxiv.org/abs/1405.7351; 2014).
"We know that galactic dust emits polarized radiations. We see that in many areas of the sky, and what we pointed out in our paper is that pattern they have seen is just as consistent with the galactic dust radiations as with gravitational waves," Spergel told AFP last week.
"I think in retrospect, they should have been more careful about making a big announcement," he said.
The bottom line is that they do still see some evidence for gravitational waves affecting the light from the early Universe, but it doesn’t look like it rises to being statistically significant, and it’s certainly not as strong as they first thought.
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