I’m not sure whether my scepticism about the BICEP2 results is just a sign of my old age, so it’s time for a quick (and, it goes without saying, totally unscientific) straw poll to see what people think. Feel free to add comments through the box as well!
What other mechanisms have been proposed? I thought cosmic defects/strings due to phase transitions were the main other candidate, but three different people who have worked in the area (Dani Figueroa, Mark Hindmarsh and David Spergel) now say their contribution would be an order of magnitude too small. If there are other viable mechanisms I’d genuinely like to know …
Peter: I can’t see how anyone could vote for “definitive proof of inflation” at this stage. But it’s certainly strong enough evidence to move me from the camp of mild inflation sceptic to accepting that this is really now the null hypothesis. Accepting the apparent V=m^2phi^2 potential as a working hypothesis, certain testable consequences follow – notably negligible running (curvature of the primordial power spectrum). This is certainly of interest, since BICEP and Planck are more closely consistent if you assume nonzero running (not that they are strongly inconsistent). If future data reduce the upper limit on running substantially, that will further increase confidence in simple inflation models. But that confidence should already be pretty high after today.
As far as I can see, primordial magnetic fields can generate scalar, vector and tensor modes. And the B mode polarisation C_l that results seems to look much like the Bicep2 result, continuously rising toward larger l. B mode is caused by Fraday rotation so there will be a frequency dependence unlike for gravity waves.
Diont really understand your point about decaying vector mode…
Yes, they do generate all modes but the spectrum depends on what you put in; without a convincing model of magnetogenesis you have probably got freedom to fit anything you like!
My point about decaying vector modes is only relevant if you want them to source galaxy formation..
This is now out of my league, but at first glance, their foreground estimation doesn’t seem very convincing if they only project/model contamination rather than try to measure it using different frequency bolometers in each pass.
Their lack of data/power on low angular scales is also odd, but I don’t understand how these medium angular range experiments work.
Their findings about inflation seem to come largely from those four bandpowers on the GW bump. It is amazing that these four data points are the only data (with confidence limits) that we have so far to use to directly probe the inflation.
But aren’t they going to replicate their results before they jump into conclusion about proving anything? Because we are not talking about the cosmic variance where there is only one universe to test. We are talking about a small patch of the sky which may have a very different B mode pattern than other patches of the sky of the same size.
A number of ongoing experiments, including: ABS; Polar Array; PolarBear, Quiet II, SPTpol; ACTpol; Spider; EBEX; and of course Planck. Not all of them cover the same range of spherical harmonics though.
In fact the rumour is that BICEP2 was rushed out because of fears that the competition were getting close. We’ll wait and see…
I’m not convinced. I don’t have any issue with the B-mode polarization itself, but I think it’s going way too far to claim that this is proof positive of primordial gravitational waves and inflation as proposed by Guth.
[…] There has been a great deal of excitement 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 results. Having taken a bit of time out last night to go through the discovery paper, I think I should say that I think all this excitement is very premature. In that respect I agree with the result of my straw poll. […]
Gravitational waves are incompatible with special relativity. Special relativity can explain orbital degeneracy. Energy is lost by heat due to ordinary tidal forces. In GR, distribution of matter is not considered, it is approximated as centers of mass. This is a fatal flaw. Just consider matter distribution and the relativity of mass. Wait for a direct detection that will never come. SR has been proved in laboratories, not gravitational waves. Inflation is just a pet theory to fill a lack of knowledge about quantum gravity and the structure of black holes (and the cause of the big bang). The detection of B-modes is a great achievement, but please, do not interpret things you don’t understand. It sounds more like religion than science…
1. Inflation is not based on any physical evidence (free parameters). In fact, it breaks the known and verified laws!
2. There is no evidence of gravitational waves.
3. The singularity is just a mathematical aberration.
Another giant leap of faith into the darkness? or into Swiss cheese nonsense? :o)
Even assuming the signal is really there in the sky, there are four additional steps in the chain of logic:
1. Is the signal really in the background? (And not e.g. galactic foreground.)
2. Is the background signal really promoridal? (And not added since then, by e.g. Alfven currents.)
3. Is the priomoridal signal really due to gravity waves? (As opposed to, say, magnetic fields.)
4. Are the gravity waves really due to inflation?
It’s presuasive evidence, because I think each of the above is more likely than not to be “yes”, but 0.5^4 does not rise to the level of “convinced”.