Take a good look as it won't be the same on Monday. COBE was first to measure the amplitude

*As*of the primordial scalar fluctuations in the CMB. More recently, WMAP and Planck and other experiments pinned down the so-called spectral index

*ns*(x-axis in the plot), which measures the departure of the scalar fluctuation spectrum from scale invariance. In the vanilla model of inflation, these two parameters are related to three parameters of the inflationary potential: the absolute value, and the first and second derivatives during inflation. To connect all dots we need another observable - the amplitude

*At*of the primordial tensor perturbation spectrum, or equivalently, the tensor-to-scalar ratio

*r=At/As*(the y-axis of the plot). This observable is directly related to the absolute value of the potential during inflation. To measure the tensor amplitude, one needs to see a special pattern in the CMB polarization spectrum - the so-called B-mode (more precisely, the primordial B-mode; the non-primordial B-mode, induced by gravitational lensing effects on the CMB, was observed by the SPT and POLARBEAR experiments). The rumor is that BICEP2 - a small-scale experiment down at the South Pole measuring the polarization of the CMB - pinned down the primordial B-mode. Various rumors quote the measured value of the tensor-to-scalar ratio between 0.06 and 0.2. For the former value, it is hard to understand how BICEP2 could obtain a statistically significant discovery, while the latter value is in tension with the results from WMAP and Planck. We'll see on Monday, when the press conference of BICEP2 is scheduled. If this is true, we learn that the energy scale where inflation happened is around 2*10^16 GeV. This would be the first direct evidence for a new particle physics scale between the electroweak and the Planck scales.

One final remark: newspapers are spinning this story as the discovery of gravitational waves. Right, there is a connection: the primordial B-mode amplitude originates from fluctuations of the metric at the time when CMB photons decoupled from matter. So finding the B-mode can be viewed as another (after the Hulse-Taylor binaries) indirect confirmation of the existence of gravitational waves. But the discovery of the primordial B-mode in the CMB is much much bigger than that.

For more rumors and counterrumors see here and here and here. Another rumor is that Guth and Linde will be present at the press conference on Monday.

## 34 comments:

Hi Jester, I thought I'd point out that the South Pole Telescope (SPT) actually won the race to detect non-primordial B-modes in 2013 (i.e. ahead of POLARBEAR). SPT's first article is here.

Ooops, sorry for that, corrected. Thx.

right, already corrected.

Is there a hard source that says the announcement will actually be by BICEP? The press release only seems to talk about some major discovery, and would also be consistent with the discovery of an inhabited exoplanet or some other mundane topic ;)

What, blogs are not hard enough sources for you ??? ;) It's certain it's going to be an announcement by BICEP, what is not completely clear is the statistical significance of the discovery.

"This would be the first direct evidence for a new particle physics scale between the electroweak and the Planck scales"

Are you saying that this evidence confirms that the SM breaks at the GUT scale, but nowhere in between the EW and the GUT scales?

No. All I can say is that there is some new physics (at least the inflaton) between the EW and Planck scales. There may be more things, I cannot exclude that.

SPT did not detect primordial GWs, therefore if BICEP sees any - this will be in my opinion the first evidence of tensor to scalar ratio, also the first evidence of quantum aspect of gravity: http://arxiv.org/pdf/1211.4678.pdf

The scale does not say whether GUT was broken down to SM - there is NO direct correlation between tensor to scalar ratio and particle physics yet, it only says that there is a new scale of physics - scale of inflation. If inflation can be embedded with GUT, or SM or MSSM, that is a separate story altogether.

"This would be the first direct evidence for a new particle physics scale between the electroweak and the Planck scales"

What about gravity? Isn´t that a truly BSM new physics scale?

Could the same analytical methods be used for reading tea leaves?

Thanks Adam, I thought so.

Many believe (myself included) that the desert hypothesis is likely wrong for many reasons.

Jester thanks for your note. How do you get the scale 2 x 10^16 GeV?

For slow-roll inflation you can solve for V and V' knowing As, and r, see e.g. Eqs. 13-14 in the Planck paper 1303.5082.

Just a minor point -- the B-mode actually results from the action of the gravitational wave (metric fluctuation) *AT* the last-scattering surface. Although the wave is generated by quantum fluctuations during the inflationary epoch, it propagates unimpeded and shakes the plasma. When decoupling happens, CMB polarization (E and B modes) are generated.

True. I modified that sentence to make it more clear.

A result of r=0.20 +/- 0.07 plus or minus 0.01 in either number, gives rise to an almost 3 sigma signal of some non-zero value of r, would have a two sigma confidence interval that would overlap with the Planck data. "Proof that gravitational waves exist" is a pretty good way to popularize a finding that r>0 even if your MOE is too large to say much more than that.

This high end result for the central value would, when viewed together with the current Plank data in light of the high margin of error of the result, tend to favor scenarios (with the other parameter being at a best fit 0.97 value from the existing data) such as a r that is exactly on the concave/convex boundary (i.e. flat, which nicely complements the overall topology of space-time seen in CMB data) and also "natural inflation" scenarios, while disfavoring very simple Higgs inflation models motivated by apparently very low r values seen so far.

Why would BICEP be so hot to trot on a result that might very well be less than 3 sigma and hasn't had time to be carefully reconfirmed?

BICEP needs to scoop the Planck polarization data to be relevant. The Planck polarization data is due to be released in a matter of weeks and will probably confirm the BICEP result at an unknown but sub-5 sigma level.

Waiting until it has enough data to report a 3-4 sigma result and fudging close judgment calls in analysis in favor of a higher result with a higher margin of error, rather than holding out for a low margin of error and the same level of confidence in the result, means letting your find be simply a confirmation of the Planck polarization data result with slightly different central and MOE values. This doesn't win prizes or grants the way that being the first experiment to show strong, sub-discovery evidence of a non-zero value for r does.

Hi Jester, At his blog Sesh mentions that the rumored result would mean that the inflation field \phi takes values larger than the Planck scale. If so, can we conclude that the discovery would imply some new physics between weak and Planck scales as you have done?

Sesh says that while the "energy scale" is always less than the Planck scale, the field value for \phi is above Planck scale for r=0.2.

Sesh's blog is at http://blankonthemap.blogspot.co.uk/2014/03/b-modes-rumours-and-inflation.html

Andrew, I sort of doubt that they would use the word "major discovery" if the signal were just 3 sigma. I think that people underestimate BICEP2's sensitivity and also the time they have spent. It's been running almost for 5 years or something like that. They may still be wrong for some reason but it is utterly unfair to call their presentation "premature" etc.

I'm not sure if the point about trans-Planckian values is new; we knew before BICEP, or even before Planck, that in single-field inflation we need trans-Planckian inflaton vev to ensure slow-roll conditions. Now, it's true that BICEP only tells us the energy density during inflation is 10^16 GeV, not that the inflaton mass is 10^16 GeV.

What system of units do you use when talking about the inflation energy density measured in GeV?

April fool's day - even before April 1st. The cosmology community is soooo desperate to get some solace from the data after the Planck - No-primordial-non-gaussianity..

I do not think there is any iota of reasons to believe Bicep will release any data and improve the data just within 3 Months of their publication in January ( where they quoted B-mode consistent with zero), http://iopscience.iop.org/0004-637X/783/2/67/pdf/apj_783_2_67.pdf

Forget B modes, there could be a new discovery of a worm hole, new aliens wants to do chit-chat with the Planetary beings.., there could many major discoveries on card..

Ervin, the inflaton potential in the Lagrangian has units [Energy]^4 (in the only legitimate unit system where c=hbar=1). BICEP says V~\rho~(2*10^16~GeV)^4

To comment on my comment before: reading into Sesh's post he says that the large tensor-to-scalar ratio more generally disfavors all small-field inflation models. That's interesting, I need to read more about this.

Thanks. I thought you were referring to natural units,just wanted to make sure.

Large "r" does not mean Large VEV inflation, Sesh himself worked on with Hotchkiss - a possibility of large 'r' with small VEVs - or Sub-Planckian VEV inflation.

Anonymous: " ...just within 3 Months of their publication in January ..." That was BICEP1 = zero with big error bars. This is BICEP2

"It's certain it's going to be an announcement by BICEP"

Again...who's certain? Where did the information that it's going to be a BICEP announcement come from?

deep throat

deep throat

I would say that this rumored result, if correct, would be a much more direct detection of gravitational waves than for the Hulse-Taylor system. (In fact, the mechanism here is not so different from that used in interferometers: it's the primordial matter which serves as test masses.) The issue as far as gravity waves go is not so much indirectness as the problem of eliminating possible confounding effects.

Looks like the rumor was right (they have a non-zero tensor to scalar ratio, 0.20 with ~1/3 of this value as uncertainty), but the website is unavailable now - probably due to the load.

"they find a best-fit value of the scalar-tensor ratio of r = 0.20 +0.07/-0.05,"

See 16 March 2014 05:15

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