**Guest blog by Liam McAllister, Cornell University.**

The BICEP2 team has just announced a remarkable discovery (FAQ): they argue that they have detected, at very high significance, the imprint of **primordial gravitational waves** on the polarization of the cosmic microwave background. Moreover, the signal they see is very strong. If they are right, this is The Big One.

BICEP on site at the South Pole |

BICEP's map of the CMB:

Vorticity in the CMB according to BICEP |

**How should we interpret this result, and what are its implications?**

If the BICEP measurement really is a detection of primordial gravitational waves, and if we interpret this finding in the context of the overwhelmingly favored theory for producing primordial gravitational waves — namely, inflation — then the implications of this finding are staggering. I find it hard to imagine a more powerful, more transformative experimental result anywhere in fundamental physics, short of a discovery of extra dimensions or of a violation of quantum mechanics.

Let me now temper this excitement with a number of cautionary remarks, and then explain why a detection of inflationary gravitational waves would be so important.

**Is it real?**

The first caution involves the experiment itself. Before the community has a chance to pore over the BICEP analysis, and before this result is confirmed or refined by another experiment, it is difficult to be certain that the BICEP team has detected primordial gravitational waves, let alone primordial gravitational waves from inflation. First of all, what they actually see are polarized photons. They have made a beautiful map of the polarization of a small patch of the CMB, and have then carefully teased out the B-mode component of the polarization. (The B-mode is the curl — more precisely, divergenceless — component of the polarization field— see the earlier post by Luboš for more details and references.)

At this stage, the natural questions that a naive theorist like myself will ask about the experiment include: have they correctly measured a B-mode on the sky, or could the observed B-mode be the result of a systematic effect in the instrument? For example, could there be an overlooked coupling that converts E-modes, which are bright and were seen long ago, into a trace of B-modes? Next, if there really is a B-mode in the CMB, is it primordial, or could it come from polarized foreground sources? Foregrounds should look different at different frequencies and in different parts of the sky, so given enough measurements one should be able to remove this ambiguity. BICEP claims to have done so, and I have no specific concern to share.

Suppose now that BICEP has indeed seen primordial B-modes. How does this become a measurement of gravitational waves? The connection is that gravitational waves propagating at the time that the CMB decoupled from protons and electrons (at $$$t \approx 380,000$$$ yr) leave a distinctive imprint, inducing vorticity in the polarization field. Gravitational waves are by far the most plausible explanation for primordial B-mode polarization, so

*unless*the BICEP signal is due to an instrumental problem or a foreground, it is very likely a consequence of primordial gravitational waves. For the purpose of the rest of this discussion, I will suppose that BICEP really has detected primordial gravitational waves, even though some sort of confirmation will be essential.

**What do we learn?**

**Direct detection of gravitational waves**

On its own, even without any framing in the context of inflation (see below), the detection of primordial gravitational waves is a spectacular event of historic significance. Gravitational waves are a central prediction of general relativity, and we have had

*indirect*experimental evidence for their existence for many years: most notably, the Hulse-Taylor binary pulsar PSR B1913+16, discovered in 1974, is gradually losing orbital energy at exactly the rate predicted based on losses to gravitational radiation:

But seeing gravitational waves directly is quite another matter! Seeing them imprinted in the universe on the largest scales is icing on the cake: while a pulsar counts as an exotic source with strong gravitational fields by the standards of terrestrial life, the gravitational waves seen by BICEP were generated on an absolutely epic scale, a fraction of a second after the Big Bang.

**Implications for inflation**

The impact of the BICEP discovery is greatly heightened if we interpret primordial gravitational waves in the context of the leading model of the early universe,

**inflation**. Inflation provides a compelling framework for understanding the origin of the anisotropies in the CMB, and the theory makes a number of clear predictions:

The universe on large scales should be approximately

- Homogeneous,
- Isotropic, and
- Flat.

- correlated on super-horizon scales, and should be approximately
- Gaussian,
- Adiabatic,

as well as approximately, but not exactly, - Scale-invariant.

**Every one**of these predictions agrees beautifully with observations. Here is the Planck 2013 CMB power spectrum: the curve corresponds to the maximum-likelihood model.

A beautiful fit, to be sure.

But inflation makes one more prediction: there should be a spectrum of primordial tensor (gravitational wave) perturbations, with amplitude determined by the

**energy scale**at which inflation occurred:\[

V^{1/4} = 2.2\times 10^{16}\GeV \times \zav{\frac{r}{0.2}}^{1/4}

\] Here \(V\) is the energy density at the time of inflation, and $$$r$$$, the tensor-to-scalar ratio, measures gravitational wave perturbations normalized to the (well-measured) size of the scalar perturbations.

But why do theorists care so much about this final prediction — why isn't this just one more successful prediction of a framework that many of us already think is broadly correct? I will go through the implications of this result in roughly increasing order of subtlety/ambiguity.

**Implication 1: inflation at the GUT scale**

The first reason is the energy scale. The BICEP measurement, taken at face value, gives us a direct window on processes with energy around $$$ 10^{16}\GeV $$$, which is

**12 orders of magnitude**higher than the center of mass energy of the LHC. (Go on, read that again.) Zel'dovich said that the universe is the poor man's accelerator, but we now see that it far surpasses the most expensive devices built on Earth.

Moreover, this is (obviously) a positive measurement, where gravitational waves, rather than their absence, is what has been detected. All previous experimental probes of physics above the \(\TeV\) scale have been negative: most notably, limits on proton decay place bounds on processes around the GUT scale, but do not directly reveal phenomena at that scale. This distinction is important: despite decades of theoretical arguments about the nature of physics at extremely high energy scales (meaning, well above the \(\TeV\) scale), on the experimental front we have had to settle for knowing what that high-energy physics is not, rather than what it is. Until now.

So, granting the context of inflation, the BICEP measurement tells us that inflation occurred around the GUT scale, just two orders of magnitude below the Planck scale. This is on the doorstep of quantum gravity. I will say more about this below.

**Implication 2: exclusion of most of parameter space**

The next reason for caring about this result is that it cuts a terribly impressive swath through the space of inflationary models, leaving a tiny fraction of the parameter space that appeared possible a priori. The energy scale of inflation is an essentially free parameter, and this measurement restricts it to the extreme upper limit of the previously allowable range.

Planck 2013 constraints on \(r\) |

BICEP2 constraints |

**Implication 3: quantization of the gravitational field**

Another reason that this result is significant is that the primordial gravitational waves from inflation are quantum-mechanical in origin. Just as the temperature anisotropies — and the distribution of large-scale structures, including our own galaxy — originate from quantum fluctuations of the inflaton, the tensor perturbations responsible for primordial B-mode polarization are the result of quantum fluctuations of the two polarization modes of the graviton. To belabor the point: the inflationary prediction is derived by promoting the fluctuations of the gravitational field to operators, imposing canonical commutation relations, specifying the vacuum state, and computing the correlation functions. The tensor fluctuations write quantum gravity on the sky.

Does this mean that the BICEP measurement, if confirmed by other experiments, provides direct evidence of quantum gravity? Perhaps, with more work. Within the context of inflation, quantum fluctuations of the gravitational field are indeed the simplest, best-established explanation for this signal. But one should bear in mind that even in inflation (to say nothing of other models), there could be alternative sources for a strong primordial gravitational wave signal, and these alternative sources could well be classical (or at least, not involve vacuum fluctuations of the graviton): see e.g. this paper. We do not presently have a sharp analogue of Bell's inequality through which one could really prove the quantum-mechanical origin of the observed perturbations. Instead, the argument is that our best theory at present for how the perturbations could have arisen is quantum-mechanical, and until a more compelling classical theory is presented, we may take the primordial tensor signal as

*preliminary*evidence for quantum gravity.

So we now have the first piece of

*experimental*evidence that gravity is quantized. But what quantum gravity theory is realized in Nature? This brings us to a final and most fascinating implication of the BICEP measurement.

**Implication 4: symmetry properties of quantum gravity**

The BICEP measurement has one more major implication, with the potential to connect experimental cosmology to string theory. Let us continue to work in the context of inflation; this is a reasonable assumption because all existing alternatives are now under extreme pressure in light of BICEP2.

Here is the key fact, known as the

**Lyth bound:**

The derivation is quite simple, but I will not reproduce it now; you can find a more complete presentation here or here. And here is some fine print:

In an inflationary model producing detectably large primordial gravitational waves, the inflaton field $$$\phi$$$ moves over a distance $$$ \Delta \phi > M_{p}$$$ during inflation.

- When multiple fields evolve, it is the collective excitation that obeys the bound.
- Slightly smaller $$$\Delta\phi$$$ can be achieved by contrived dynamics: make the model live at high energies only while the observed CMB is produced, then very quickly reduce the energy scale. I'd expect this feature to be discernible, in most cases, from the tilt and running of the scalar spectrum.
- The Lyth bound refers to the arc length of the inflaton trajectory. The distance from the start to the end could be much smaller, as in this paper.
- In models with nontrivial kinetic terms, the form of the bound is superficially modified. The bound formulated by Baumann and Green is always stronger than the naive Lyth bound.

This line of thinking would not constrain inflationary models if we were simply free to take the cutoff $$$\Lambda \gg M_p $$$. But we cannot: graviton-graviton scattering violates unitarity around the Planck scale, so general relativity + quantum field theory

*must*change somehow at this scale. There could be new degrees of freedom (e.g., massive string states), and/or the theory could become strongly coupled, but it cannot remain unmodified. So it is extremely implausible that the cutoff scale for an inflationary effective field theory will be $$$\gg M_p $$$. At first sight, this seems to imply that super-Planckian field displacements cannot be described in a consistent effective theory.

There is a critical loophole! The light field $$$\phi$$$ might have suppressed couplings to the heavy degrees of freedom, if the ultraviolet theory respects an approximate shift symmetry\[

\phi \to \phi + {\rm const}.

\] In a Wilsonian effective field theory, such a shift symmetry is a totally legitimate possibility, and is not spoiled by loops of the light fields.

Now, in the case of an inflationary model producing detectable $$$r$$$ (a.k.a. a 'large-field model'), the shift symmetry must protect the inflaton $$$\phi$$$ over distances $$$\gtrsim M_p$$$, so the corresponding cutoff scale $$$\Lambda$$$ (i.e. the scale up to which $$$\phi$$$ is sequestered from interacting with massive degrees of freedom) is at least the Planck scale.

Here is the summary:

Because the cutoff scale must be at or beyond the Planck scale, to assume such a shift symmetry is to make an assumption about global symmetries in the ultraviolet completion of gravity. In this sense, every inflationary model producing detectable gravitational waves rests on assumptions (or, sometimes, knowledge) about quantum gravity.An inflationary model producing detectably large primordial gravitational waves can be natural in the Wilsonian sense if an approximate shift symmetry protects the inflaton $$$\phi$$$ from coupling to massive degrees of freedom at the Planck scale.

Several approaches to the Lyth bound in large-field inflation are evident in the literature:

- Reject/ignore naturalness arguments, write down desired potential energy function.
- Agree that there is an issue, write down shift symmetry in low-energy theory, propose that quantum gravity will not spoil this symmetry.
- Work in an actual quantum gravity theory in order to see whether suitable symmetries arise.

The shadow hanging over approach (2) is that very general arguments in quantum gravity suggest that all (exact continuous internal) global symmetries are broken at some level in quantum gravity. (At least, in a quantum gravity theory with reasonable black hole thermodynamics. This need not be string theory.) So I personally find it hard to accept as an axiom that any desired low-energy global symmetry — one protecting Planck-scale displacements! — can be UV-completed without problem. Rather, the conservative position is that obstructions could arise, and one needs to work directly in a quantum gravity theory in order to establish the existence of the necessary symmetries.

An influential early work of type (2) is Natural Inflation, by Freese, Frieman, and Olinto. The idea is that the inflaton can be a pseudo-Nambu-Goldstone boson (axion) with a large axion decay constant, $$$ f \gg M_p $$$. As a low-energy model, this works beautifully. The question is whether this effective theory admits an ultraviolet completion (including gravity), in other words whether quantum gravity respects the axion shift symmetry to an adequate degree.

In string theory, achieving $$$ f \gg M_p $$$ is not straightforward. In all computable regions of parameter space explored to date, the decay constants of individual axions are parametrically small. But on closer examination, achieving $$$ f \gg M_p $$$ for a single axion is not actually necessary in order to achieve large-field axion inflation in string theory.

Two sorts of mechanisms present themselves: let the inflaton be a combination of multiple axions, or let the inflaton involve multiple circuits of a single axion circle. The former idea may work with alignment of two axions (Kim, Nilles, Peloso 2004), or collective excitation of many axions, a.k.a. 'N-flation' (Dimopoulos, Kachru, McGreevy, Wacker 2005). The latter idea is known as axion monodromy inflation (Silverstein and Westphal 2008; L.M., Silverstein and Westphal 2008; see Eva Silverstein's guest post).

Equipped with these basic symmetry mechanisms, one can ask whether complete and consistent models exist, and what their predictions are. Let me focus on axion monodromy inflation realized on NS5-branes in a compactification of type IIB string theory, as described here and here. This model is in my view the most computable of the bunch, and can serve as a benchmark within the space of more general monodromy models. Like all sufficiently honest models of inflation in string theory, the constructions are technically challenging, with a lot of moving parts, but in my own (biased) view the ingredients are plausible ones.

This specific model (linear inflation from axion monodromy) predicted the following:\[

r \approx 0.07

\] and \[

n_s \approx 0.975.

\] So if the BICEP2 constraints are taken at face value, this model is under significant pressure, because it predicts too

*low*an amplitude of tensors! Time will tell: because the BICEP2 measurements are also in apparent tension with Planck, some parameter adjustments may be necessary (for example, allowing running of the scalar power spectrum). I am not prepared to use the BICEP2 results to differentiate among large field models without a bit more analysis, but — assuming the measurement is real — we can certainly use it to exclude everything

*except*for large field models.

The model also predicts the existence of periodic modulations of the spectrum and bispectrum, but with model-dependent amplitude. The amplitude can easily be small enough to make detection of these modulations impossible.

The modulations illustrate how confronting the real structure of quantum gravity — approach (3) above — can lead to novel predictions, as compared to purely low-energy reasoning. The limitations on axion decay constants make a straightforward realization of 'traditional' natural inflation seem difficult in string theory, but one can construct close cousins of natural inflation, with slightly different signatures. Moreover, by comparing experimental constraints on the tensor and scalar perturbations to the predictions of models of inflation in string theory, we can learn which of the symmetry principles allowed by string theory might be realized in our universe.

**Further implications for string theory**

What does the BICEP2 measurement tell us about string theory itself, beyond the fact that quantum gravity is essential for interpreting the data?

BICEP2 certainly does not tell us that string theory is correct, or that it is incorrect. It does not tell us which string theory is (or is not) realized in Nature. The inflationary models described above are arguably best understood in type II string theory, but that is an artifact of our present time and our present theoretical tools.

We do learn one model-independent thing about string theory: because the inflationary Hubble scale is so large,\[

H \approx 10^{14}\GeV ,

\] we can exclude a wide range of models in which quantum fluctuations at this scale would destabilize the compactification. In particular, if the Kaluza-Klein mass is below $$$H$$$, the same fluctuations that give rise to the scalar and tensor perturbations of the CMB would give rise to perturbations of the extra dimensions. When fluctuations of this sort are large, a four-dimensional description ceases to make sense, because the whole compactification is dynamical. By this logic we can exclude models with very low Kaluza-Klein scales, i.e. models of large extra dimensions. (Perhaps there is a model-building trick that can make large compactification robust against quantum fluctuations during inflation, but I'm not aware of a compelling idea.)

**Closing thoughts**

The BICEP result, if correct, is a spectacular and historic discovery. In terms of impact on fundamental physics, particularly as a tool for testing ideas about quantum gravity, the detection of primordial gravitational waves is completely unprecedented. Inflation evidently occurred just two orders of magnitude below the Planck scale, and we have now seen the quantum fluctuations of the graviton. For those who want to understand how the universe began, and also for those who want to understand quantum gravity, it just doesn't get any better than this.

In fact, it all seems far too good to be true. And perhaps it is: check back after another experimental team is able to check the BICEP findings, and then we can really break out the champagne.

By Liam McAllister, March 17th, 2014

## snail feedback (39) :

Thanks a lot, Liam, for this extremely nice text on this exciting day! Let's hope that a loose cable won't be found somewhere and the excitement won't turn into disgust. ;-)

Wow.

Nice.......that was quick:)

Thank you for creating this post.

Good Job MarkusM....is there a time stamp on that video?

Lubos, I must congratulate you on a very well written post. ---- Shyamoli Chaudhuri Plassmann.

Thanks a lot but I am not the author of this text! My only contribution - and I am proud about it, too ;-) - is that I picked a great writer and (easily) convinced him to do this service for us.

I added "By Liam McAllister" at the end, too, to avoid confusions. ;-)

Thanks for a very lucid post .

Thanx, PlatoHagel.

Can't see one.

Found it at 17:02 UTC here:

http://news.stanford.edu/news/2014/march/physics-cosmic-inflation-031714.html

with only 160 thumbs up.

Just some competition: Matt Reece is also an expert and he just wrote this text:

http://users.physics.harvard.edu/~mreece/inflation.pdf

A coupl of quick questions:

Why Primordial Gravitational Waves imply inflation (smoking gun)? What the alternative theories have to say about them? (e.g. Neil Turoks’ theory?)

How these findings help us select the correct theory of QG? For example what are the implications for Loop Quantum Cosmology and the big bounce model?

Congratulations, Liam Mc Allister, at the beginning of the blog I thought you were the writer, and the end confused me! Well done, and good choice, Lubos!

Note that the BICEP data shows evidence for a blue tilt of the spectrum of gravitational waves. Such a tilt would be inconsistent with the predictions of inflation, but it would agree with the predictions of "String Gas Cosmology", an alternative early universe scenario based on fundamental principles of superstring theory. See arXiv:1105.3581 for details and references.

Robert Brandenberger

Thank you Liam for this very clear presentation -- and thanks, Lubos for arranging for this guest post.

Thanks for your provoking comment, Prof Brandenberger. I've sent you an offer to write a guest blog about the post-BICEP status of string gas cosmology.

After reading this comment and checking the preprint just a little bit, I remain a bit skeptical, maybe because I am too conservative. Isn't string gas cosmology also suffering from the Kaluza-Klein instabilities at the required high scale that Liam mentioned in general? Or is the current research of string gas cosmology assuming near-stringy self-dual radii that are also close to this apparent Hubble scale?

Wonderfully written, thanks. Motl, take notes on how to write clear articles for laymen.

I wasn't excited like that when the Higgs-Boson was found. Somehow it didn't feel like much of a discovery, just a confirmation of what we already knew.

This, however, feels new and exciting, and I am sure plenty of people will feel motivated to delve into cosmology-related research topics if the discovery is confirmed.

Liam McAllister and Lubos: Thanks for posting this nice article. I take it that both of you are convinced that there is no alternative model than gravitational waves to produce polarization in B-mode or is it too much to say at this point?

Thanks for this very nice article :-)

And I like these great news so much that I feel like having to take a big holiday in order to properly enjoy it ...

... reading up everything I always wanted to know about the details of inflation models, etc ... :-D

Cheers !

Liam, thanks for the truly excellent overview, it helped me enormously with "the big picture".

@markusm thanks for video, very touching.

There is another interesting implication of this discovery -- the Standard Model vacuum is unstable -- http://arxiv.org/abs/arXiv:1301.2846

Interesting, Archil. There is also a paper

http://arxiv.org/abs/hep-ph/0402187

that says that tensor modes exclude (minimal only?) SUGRA because of some excessive production of gravitinos etc. at this high scale. It would be interesting to have someone independent review all these things.

The torch is carried.

Hi MarkusM,

Mon, Mar 17, 2014 at 8:59 AM found it in my mail box Stanford upload.

Thanks again.

This exchange shows the process of Science at work! What will come of it?

It's nice to be a spectator!

I scanned through the pdf of the paper. The B signal plot shown above is clearer than the one in the paper, their fig.3 .

The data analysis is extremely convoluted, it would take forever to dig into it . It seems they have painstakingly checked for systematics.

If something is off it will not be a cable, rather some overlooked input or overweighted input ( I am talking of functions entering the analysis train) might generate the B pattern, which by the way is much weaker than the E pattern. Before any celebrations it is imperative that another experiment with DIFFERENTLY programmed analytical tools should confirm it, because I suspect that if there might be a glitch it will be in the analysis programs which are many and convoluted.

Thanks for linking to my notes. My expertise on this topic is smaller than Liam's by orders of magnitude. But hopefully for that reason, my notes will be readable for other people like me who know a reasonable amount of effective field theory but only a little about inflation.

"hallowed principle of naturalness"

It's nice to see that people still believe in naturalness and use it as a guiding principle to derive important conclusions. Others have prematurely lost faith after LHC...

Strictly speaking OT, but tangentially related.

Actually it's a little brainteaser for you, Luboš. But it won't exactly tax you. :)

Al beeb's Newsnight had a piece on BICEP2 last night. It can be seen via their iPlayer here:

http://www.bbc.co.uk/iplayer/episode/b03ymfqt/newsnight-17032014

I think access to their stuff is restricted, so if you're accessing it from outside the UK you might have to fake your location.

Anyway, the piece lasts about 12 minutes starting at about 1:15 minutes in, but there's a brief 10-second intro just before that starting at about 0:30 which should be enough for you to get the drift. Winky dink! :)

The rest of the content itself is of little interest as it's just the usual MSM crap. What's interesting is a wholly different aspect which I'm sure you'll have twigged by now — mainly because you can't fucking miss it, so obvious is the game being played here! Well, it's al beeb — what else?

For those of you still in doubt you only need to watch the bit in the studio for a few moments starting at about 4:50. If you haven't got it by then, then check your pulse to see if you still have one.

OK, the brainteaser here is in two parts: (a) What stands out like sore thumb (nil points for this part); and (b) what are the chances? An order of magnitude will do.

They can't shovel this shit out fast enough. You're forced to pay for it too if you just so much as own a television set. Yep, that's shit-hole Britain today.

Hi John, is the video the same thing as this?

https://www.youtube.com/watch?v=um9TUFXkSsE

I don't have energy to learn how to setup proxies now again... Sorry.

Haha. Yes, that's the one! (The iPlayer version I mentioned was the whole program so the timings I gave previously are displaced and the 10-sec intro is missing, but that's the whole thing.)

You got it in one! And with serious consideration of the sample space. :)

But then what's new!?

I temporarily forgot you were at Hahvahd and had had tera-doses of this horrendous peecee stuff so you're pretty immune to it! My apologies for that. Doh!

I'm not though. I want to see the BBC, and all who sail in her, utterly destroyed.

"

Otherwise I thought it was a nicely done video, given the speed they needed."OK, I appreciate your ability to remain objective. Let's split our difference? Would you settle for the immediate break-up of the BBC, all it's assets sold off to the highest bidder and the ruination of everyone connected with it after they have completed a couple of months in the torture chamber? The big concession here being that they are allowed to live, just — as destitutes. Deal? :)

Dear John, I got used to much worse things.

It's not just some women chosen to be somewhere for their concave genitals and nothing else.

Many more convoluted things were mundane. For example, in the admission committee, someone forgot about the affirmative action and the secretaries actually sent the "accepted" letters to future physics grad students according to a meritocratic choice.

You know what happened? Everyone had to accept a mistake - the percentage of women was just 10% or something like that - and they just ordered to secretaries to send the "disacceptance" letters. One guy from Canada who had letters as the "best student in Canada for 15 years and so on" was one of the a posteriori fired ones whom I will never forget. These people are completely shameless and their behavior is completely normal.

Or the faculty meetings with Summers and the whining bitches. Summers would normally be a self-confident person with a spin and character, if not arrogance. But those sluts just totally decomposed him. Hours of emotional rants how terrible it is for him to be a man, and hours of his endless apologies and climbing into the sluts' assholes.

Maybe I shouldn't have attended the faculty meetings etc. to see what's really going on, how the balances and checks are set. Ignorance is sometimes bliss. At any rate, since these Spring 2005 meetings, I was absolutely sure that I never wanted to spend another contract in an environment that has a risk of operating in a similar way.

Lubos,

You should turn out a memoir of your Harvard experiences. As a play on a book title notoriously connected to Norman Mailer, you could call it: “In the Belly of the Liberal Hegemonic Beast”. Done with humor, I would bet on an international bestseller (at least among conservatives).

Let me thank you here for all these wonderful posts about BIPED2 and ask a (likely stupid) question. Is the reason the multipole numbers are so high, l = 80, because wavelengths are so tiny at these early energies that the lower order multipoles are essentially transparent to such waves? I guess in a geometric sense, like tiny wavelets on a river not noticing, on average, the pillars of a bridge.

"

I was absolutely sure that I never wanted to spend another contract in an environment that has a risk of operating in a similar way."Deo volentemay you never have to, nor anyone else! May He strike our enemies down with great vengeance.That stuff is the death of civilisation. The only consolation is that the barbarians at the gates will make a beeline for them first when they break through.

However, the barbarians will get killed too, in the rush.

Then we start again.

Liam, others. I suggest you go ahead and take the day and read the results paper. It's actually quite readable. It will probably answer 90% of your questions regarding the believability of the data, observations, and analysis. For example, I see below comments on the repeatability of the result. I agree that we look forward to other experiments confirming this. However, the paper presents the cross correlation against not one but two separate experiments. See Fig 9 and associated text. Although they belong to similar groups, the experiments are quite different and have different systematics associated to them. And I can at least confirm that the BICEP1 and BICEP2/Keck analyses are different ( though algorithmically similar). But again, take a few hours and read the paper, then feel free to raise questions. Note that a very good Live discussion of the results is going on Facebook. It's called "Live discussion of BICEP press conference"

Denis

Dear Dr Barkats, congratulations to your team's groundbreaking result!

I am pretty sure that Liam in particular will read your paper(s) rather carefully.

Dear Denis,

Congratulations to you and to the rest of the BICEP2 team on this historic and tremendously exciting measurement!

Many of us started reading your detection paper the very second that the repository was no longer password-protected, and have continued to pore over it ever since. Reading it is not an issue.

As you will see from my post, after reading your analysis I did not have any specific concerns to raise. What I did have, and still have, despite the thoroughness of your analysis, are the two obvious general concerns: instrumental systematics, and foregrounds. Clearly your team has gone to great lengths to control/minimize both, and I don't imagine that I personally will be able to contribute anything to that discussion.

Nevertheless, I stand by my position that the scientific community should be waiting (in extreme excitement!) for independent experimental confirmation of this discovery. An argument against the possibility of instrumental artifacts (or foreground contamination) from within one experimental team --- even a comprehensive argument that seems flawless --- is simply not as convincing as confirmation by an independent measurement. The implications of your measurement are unprecedented, and this makes confirmation even more imperative.

As you have no doubt seen, most of the cosmologists worldwide who are weighing in on this result have taken a broadly similar tone: be excited, but verify.

I personally will be very interested to see whether subsequent experiments lead to a reduced central value for r.

Liam

Dear John, a writer in the Daily Mail made the very same comments about the program with the two women as we did - and he or she instantly became a target of witch hunts, see:

http://www.theguardian.com/media/2014/mar/21/daily-mail-accused-of-insulting-top-female-scientists

Not a safe world for honest people/.

I also join in the congratulations and should say, as I agree that a second experiment is vital , that I read the paper and will read it again . I appreciate the enormous effort described therein in checking systematics .

Having spent over 40 years of my professional life in writing and running prgrams for experiments for High Energy particle physics, I will add the loophole of programming glitches in complicated computer programming. That is why independent analysis is also important and it is the reason why the LHC has two very expensive equivalent experiments, CMS and ATLAS running in parallel.

For example, human error in typing is about 5%, a number I know from the days we used scanning girls for bubble chamber pictures and passed the information on cards to be fed to the computer ( early 1970). We had a separate card punch machine to catch the errors of the first go and reduce the error.

Let me give you another example: In ALEPH (LEP) we published a Higgs with 4 lovely events, it was at 114 GeV, striking events, still there. The other three experiments did not find any, and that was that.

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