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Big Bang finding challenged

nature.com

60 pointsd0mine11 comments

From the abstract of the paper: "The expected amplitude of the dust polarization power spectrum remains uncertain by about a factor of three. The lower end of the prediction leaves room for a primordial contribution, but at the higher end the dust in combination with the standard CMB lensing signal could account for the BICEP2 observations, without requiring the existence of primordial gravitational waves."

Two things to note. First, they aren't saying that is is wrong, only that it could be wrong. Second, the source of this doubt appears to come from a single professor at Princeton.

Not just one. This has been mounting for a while: http://resonaances.blogspot.se/2014/05/is-bicep-wrong.html http://resonaances.blogspot.se/2014/05/follow-up-on-bicep.ht... http://resonaances.blogspot.se/2014/05/weekend-plot-bicep-li...

Note also Steinhardt's opinon piece: http://www.nature.com/news/big-bang-blunder-bursts-the-multi...

> "Not just one."

But you linked to random blogspot articles, and Steinhardt is the guy who the first source is referencing. His name is mentioned in the byline. He's the guy from Princeton I mentioned. All of this is mainly coming from him, and those blogs you linked are the result of his statements (aka opinion sprawl).

The "random blog posts" are by Adam Falkowski, a particle physicist who has been following the controversy over BICEP2 from the start. They document how opinion has been swinging over time.

Steinhardt is not "the guy who the first source is referencing". The two references in the first source are

1) http://arxiv.org/abs/1405.5857 , by Mortonson and Seljak, both at LBNL and UC

2) http://arxiv.org/abs/1405.7351 , by Faluger, Hill and Spergel, from IAS, NYU and Princeton, respectively.

As you may have noticed, none of them is Steinhardt.

Steinhardt, an early advocate/expounder of inflation, now has a major bone to pick with the theory. His opinion piece is more nuanced than attacking the BICEP result directly; his major point and criticism is directed at inflation.

Whether BICEP has resolved B-modes or not, his main point persists. Whether he's right or not is yet another separate issue, but he has compelling points to consider.

If BICEP's foreground subtraction contains an error, the situation will be resolved soon after Planck's release later this year. The already-running follow-on to BICEP2 will provide an independent check.

Physicist here: the BICEP2 findings have a fair chance of being borne out, but I do feel that the BICEP group was a bit premature claiming definitive detection.

The doubts of independent groups, and the need for further experiments to confirm the results is the scientific method and community working as intended.

Ultimately, its mostly been the sensationalistic media coverage that makes the situation seem more controversial than it really is.

But isn't it the case that anything we're capable of measuring here, given the relatively short available sample of data we'd be able to obtain (decades), would be inside any margin of error when comparing that to the age of the universe (13.8 billion years according to Big Bang)?

I don't see how it follows that we shouldn't be able to measure phase differences in the CMB just because the CMB is a lot older than our measurements.

With the original announcement came a "five sigma" or so claim. Was that misleading or not? (was it plain wrong?) I mean, I'd expect from hearing something like that that unless the underlying theory changes the hypothesis is almost certainly true. Or is the uncertainty figure given in a more limited sense?

If you are making a systematic mistake (not necessarily an error, could also be an approximation that doesn't hold) in your analysis, the "5sigma" does not really take that into account.

Struggling to come up with a simple example. Imagine you collect some data, analyse it and see a "3sigma" effect. You decide to collect more data to see if the effect keeps getting bigger or goes away. After collecting a lot more data you get "5sigma". This could be because it is real, or because you have an "unknown unknown"

If you had a systematic (as oppose to a random effect) shift in your analysis, collecting more data will make you more sure that there is an effect. Even though all you are seeing is the effect of the systematic shift.

For almost all analyses in particle physics (or astro or ...) we spend a huge part of our time evaluating "systematics", it isn't uncommon that this part of the analysis takes a lot longer than the nominal result. Unfortunately this only protects you against known unknowns. You can't take into account things you don't think of/check for.

I suppose if your estimator is biased and not consistent -- due to some sort of omitted variable -- you can end up with "significant" estimates that are completely removed from reality. Great explanation at http://eranraviv.com/blog/bias-vs-consistency/.

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