He hath made the earth by his power,... and hath
stretched out the heaven by his understanding.
Jeremiah 51:15
"There has been a great deal of coverage in the popular media about a “Spectacular Cosmic Discovery” and this is mirrored by excitement at a more technical level about the theoretical implications of the BICEP2 results. Notice that the only other detection of the elusive B-mode signal is by POLARBEAR, but that is actually accounted for by gravitational lensing effects rather than being evidence of a primordial gravitational wave contribution.
The B-mode signal is so weak that it is to mind absolutely amazing that an experiment can get anywhere near measuring it.
But – and it’s a big “but” – we have to ask the question “How confident can we be that the signal detected by BICEP2 is, in fact, the imprint of primordial gravitational waves on the cosmic microwave background that cosmologists were hoping for?”
The answer to this question will depend on the individual, but I would say that to convince me the absolute minimum would be a detection of the signal in more than one frequency band. A primordial signal should not vary as a function of frequency, whereas foreground emission (likely to be from dust) would be frequency dependent.
Now BICEP2 only operates at one frequency, 150GHz, so the experiment on its own can’t satisfy this criterion but it could through cross-correlation with the original BICEP1 instrument which worked at 100 GHz and 150 GHz. In the discovery paper we find the
Additionally, cross-correlating BICEP2 against 100GHz maps from the BICEP1 experiment, the excess signal is confirmed with 3sigma significance and its spectral index is found to be consistent with that of the CMB.Here is the relevant plot, Figure 7 from the paper,
Well, the correct though the statement in the paper might be, it is clear from this (rather ratty) cross-correlation that there is actually no firm detection of the B-modes at all at 100GHz. In other words, the 100 GHz BICEP1 data may be consistent with BICEP2 but they are also consistent with zero. If we only have a detection at one frequency we have no compelling evidence that the signal is cosmological." Theoretical Cosmologist Dr. Peter Coles
