Monday, 17 March 2014

Curly impressions


Today the BICEP2 experiment announced a significant detection of the primordial B-model in the CMB power spectrum (the excess at lower multipoles in the plot). From that one can infer that the tensor-to-scalar ratio of primordial fluctuations is somewhere between 0.1 and 0.2. Cosmologists are strongly represented in the blogosphere, so for the description of technical aspects of the BICEP results and their impact on the models of inflation better see elsewhere, for example here or here or here.  In this post, let me throw in a few random impressions from a particle physicist.  





  • If this holds up, it's huge, comparable in magnitude to the discovery of the Higgs boson. Probably even more exciting because of the surprise element.  
  • "If this holds up" is the central question now. This sort of  experiments is subject to pesky instrumental effects and systematic effects due to foreground emission.  It's not impossible that BICEP screwed up;  in fact, experts point out some worrying aspects of the data, for example the excess in the BB spectrum at high multipoles. So I would say at this point it's fifty-fifty.  Fortunately, there are many experiments  out there with similar sensitivity (Planck, ACTPole, SPT, POLARBEAR) that should be able to  confirm or refute the claim in the near future. In particular, the release of Planck polarization data this year should straighten many things out. For the sake of this post I'll drop the conditional, but there really should be "If this holds up" in front of every sentence below. 
  • The big thing here is not gravitational waves (we observed them before), and not an evidence for inflation (we've already had a few from the CMB alone:  the temperature isotropy across the sky, the scalar fluctuations, the spectral index). The point is that the amplitude of the primordial tensor modes is directly related to the energy density during inflation. This turns out to be (2*10^16 GeV)^4 -- a whopping energy scale unavailable to particle accelerators in this century. Consequently, the Hubble scale during inflation has also been nailed down, and it's 10^14 GeV. While cosmologist  study the universe when it was being born, particle physicists are getting a glimpse of  physics at a very high-energy scale. 
  • The proximity of the inflation scale to the unification scale in minimal supersymmetry is certainly intriguing. It may be a numerical accident.... but I'm sure that there will soon be models on arXiv  "predicting" this coincidence. 
  • The plot shows the basic parameters of inflation as of today. 
     If you think that the Planck region is not what you remember, that's right, it's not the one usually shown. The red region in this plot is the Planck constraint when the spectral index ns is allowed to run, that is to say, to depend on the distance scale. It is a challenge for  inflationary models to get the required amount of running. In the more likely scenario with small running the BICEP result is in about 3 sigma tension with the Planck constraints.   
  • There's more challenge for inflation model building. In the single-field inflation one can relate how much the inflaton field was displaced during inflation to the value of r and the number of e-folds (the so-called Lyth bound). For r~0.2 one finds that the displacement is larger than the Planck scale. For particle physicists, who generally like effective field theories,  the focus now will be on identifying all exceptions from this rule. But from another (e.g. string-theoretical) point of view this is an opportunity: may be inflation can be our probe of transplanckian physics? In the coming week there will surely  be an arXiv flood on both  of these fronts.  
  • Speaking about model building, Higgs inflation is ruled out, at least in the current version. A robust prediction of Higgs inflation is no tensor modes at an observable level. In other words, we have a new evidence for new physics beyond the Standard Model. 
  • It is worth remembering that the gravitational waves during inflation is the most plausible but not the unique explanation of BICEP results. For example, an early  phase transition or decay of massive particles during inflation may also  generate tensor perturbations. That's another model building direction worth following in the coming weeks.   
  • If you hear a sledgehammer in the corridor of your lab, that may be your local Planck member banging his head on the wall. Yeah,  apart from many noble aspects, science also has this lowly competition side. A billion dollar experiment that misses a Nobel-prize-worth low-hanging fruit... I wouldn't wish  to be in their skin if BICEP is right.  
  • One more thing we learned from the BICEP announcement: mankind can study the universe moments after the bing bang, but setting up an internet connection is a totally different story ;)  
The BICEP2 paper is here, for more material see this page.  

49 comments:

Anonymous said...

The relation to the scale of Grand Unification, 2*10^16 GeV, is indeed intriguing. That scale is, unfortunately, not only not available to particle accelerators in this century, but forever, I'd say.

Anonymous said...

"Forever" is very pessimistic.

There are undoubtedly extragalactic astrophysical sources that accelerate protons to 10^25 eV ... we don't see them because of GZK.

One could envision an Earth-orbit 1 AU radius muon collider that could be relatively easy to construct in the next hundred years, especially if condensed matter physics gives cheap access to space via a C-60 space elevator. I don't know what energy scale a 1 AU muon collider to achieve before synchrotron losses overwhelm the system.

Luboš Motl said...

Good to hear your comments, even if they are a bit more cynical than those of Liam McAllister, for example. ;-)

Sesh Nadathur said...

Cheers for the link. I'm not so sure about other experiments being able to confirm the result, unfortunately: a Planck source I spoke to today was not optimistic about their ability to match BICEP on the polarisation front. Also the latest on when Planck polarisation results will be available is only "sometime in 2014".

Also, from what I hear cosmic defects would struggle to produce such a large r.

Cédric Bardot said...

Does it mean we have now some evidence for a second scalar field (not necesseraly fundamental) the inflaton, a big brother to the Higgs? How big (vev, mass) today (not just before Big Bang nucleosynthesis) by the way ? Could the inflaton talk to (interact with) the Higgs boson, could the big one helps the light one to cope with its quadratic divergences ... I stop here to avoid rhyme inflation with infatuation

Jester said...

Yes, the hypothesis of a new scalar field to support inflation is getting more and more plausible. We don't know its properties, but the mass should not be larger than 10^16 GeV. We know nothing about its interactions with the Higgs, and we have no idea how it could help with the hierarchy problem.

Jester said...

Sesh, people say that Planck data should at least help to better model the foregrounds. This is one important source of systematics in BICEP.

Anonymous said...

This might not be the right place to ask, but, does anyone know what the discovery says about the likelihood of direct detection of the primordial gravitational waves? Obviously, the fact that the GWB was proven to exist increases the likelihood of directly observing it, but I was wondering if there is anything more quantitative about how sensitive the gravitational wave detector needs to be, if it is at all feasible.

Jester said...

As far as I understand, we will never detect primordial gravitational waves directly. On the other hand, there is a chance to detect gravitational waves from violent astrophysical events (like black-hole mergers) in the next generation of experiments.

vmarko said...

On one conference back in 2010, I heard a comment that LISA would be able to observe gravitational waves from pre-CMB sources, and at very low frequencies. This would potentially give us another insight into the early cosmology.

But it was an off-the-cuff comment from one of the lecturers, don't hold my word on that one...

HTH, :-)
Marko

Anonymous said...

Pulsar timing arrays will likely detect primordial gravitational waves in the next decade. They will definitely detect them once the Square Kilometer Array comes online in the 2020s, which is orders of magnitude better than what we have now. The SKA will produce an exabyte of data per *day* (!) compared to 10^-3 exabytes the LHC produces in a *year*.

See

US collaboration: http://nanograv.org/
EU: http://www.epta.eu.org/
AU: http://www.atnf.csiro.au/research/pulsar/ppta/

Leo said...

Re: direct detection of primordial GWs. They end of at extremely low frequencies. They start out at a wavelength close to the size of the horizon at the time they were generated, but are then stretched out by expansion. This means they are generically at extremely low frequencies and amplitudes—too low for a LISA-like mission. It's possible they're in the frequency band of PTAs, but at an extremely low amplitude. It would take something special to boost the amplitude enough to see them. Some models of preheating (see e.g. work by Mustafa Amin) can create lots of GWs at the *end* of inflation from the decay of oscillons, and this can boost both the frequency and amplitude.

Ervin Goldfain said...

Adam,

Do you agree with McAllister's statement that this finding (if proven true) may be the first experimental evidence for Quantum Gravity?

Anupam Mazumdar said...

Hi, Jester,
One comment, Lyth bound can be avoided if you have varying \epsilon parameter as shown in http://arxiv.org/pdf/1306.4496.pdf, and in http://arxiv.org/pdf/1305.6398.pdf
High scale inflection-point models. So it is not completely correct to infer that we had a Super-Planckian VEV inflation.

Lyth bound holds if \epsilon changes monotonically, not otherwise - see the discussion:http://arxiv.org/pdf/1110.5389.pdf
Lyth bound will only cover the Super-Planckian VEVs, and not sub-Planckian models.

Anupam Mazumdar said...

Hi, Ervin
McAllister's Claim were first shown by Ashoorioon, Dev an Mazumdar in the following paper: http://arxiv.org/pdf/1211.4678.pdf
Indeed McAllister is correct and after this Ashoorioon et al paper many others followed with some interest such as Krauss+Wilczek paper.

Ervin Goldfain said...

Anupam,

So, in your interpretation and based on the references you cited, are these results unambiguous evidence for Quantum Gravity? Or do we need independent validation to support this extraordinary claim?

Anonymous said...

@Anonymous comment #2: On an astronomical scale, synchrotron radiation kills every circular collider. With twice the LHC magnetic field strength, we get ~1 TeV of cms energy per km of circumference. 2pi AU just gives 10^12 GeV - we would need 1 light year to get 10^16 GeV. But then even a proton would lose 10^31 GeV per turn to synchrotron radiation. This corresponds to 700kN or roughly the force a Boeing 747 uses to lift off.

Anonymous said...

If this holds, this is also pretty bad news for asymptotic safety scenarios in gravity.

Jester said...

the day is so crazy i can't keep up with the comments.... Thanks for the links Anupam, I only know the very basics of it, need to read more. Ervin, I don't have an opinion yet, I need to understand better his claim. Likewise, i have no idea what asymptotic safety has to say about r...

Marni Dee Sheppeard said...

Wow, soooooooo cool (if true). What the LCDM people will say is obvious. What is far more interesting is what the twistor people will think. Penrose won't know what to do about this, I suspect. It suggests a 'new kind' of supersymmetry, because the twistor SM demands this. I hope it turns out to be right. Like you say, just as big as the Higgs boson.

Chris said...

Jester and Leo, my understanding is that future direct detection experiments such as BBO, DECIGO, and Ultimate-DECIGO are predicted to be able to detect gravitational waves for a quadratic inflation model. A while back I was a coauthor on a paper where we forecasted how the direct detection experiments would improve constraints on inflation models
http://arxiv.org/abs/0912.3683
Roughly speaking we found Planck(+Planck polarization) with BBO could constrain r to about 10%. Which admittedly is not much better than what BICEP is doing. But as the direct detection is at a much smaller length scales it can break degeneracies between inflation models and different thermal histories.

Anonymous said...

Jester, In the comments of your previous post (Flexing biceps) you had mentioned: "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."

While in the current post's comments you have said about the inflaton "We don't know its properties, but the mass should not be larger than 10^16 GeV."

Which of the above two is correct and why?

Thanks.

Jester said...

I think both :) The BICEP2 measurement did not nail the inflaton mass - it depends on a model. But I think the mass larger than 10^16 GeV would be inconsistent with the measured V(inflaton)~[10^16 GeV]^4 plus the slow-roll conditions. Maybe there's a loophole for the latter argument but I'm not aware of one.

Anonymous said...

Thanks Jester for your clarification. So you seem to be saying that the VEV of the inflaton field is at around or more than the Planck mass, while the mass of the inflaton field is probably at the GUT scale.

Anupam Mazumdar said...

Hi, Ervin,
The large B-mode signal is unambiguous result of the fact that we cannot treat gravity classically, even at the linearized level it has to be quantised. This is sooooo amazing -- can't sleep over the results of BICEP at all.


For a simple one parameter mass model, indeed it fixes the mass, but more involved potentials, no -mass cannot be constrained.

But guys - this is a major breakthrough, nature has given us - Planck mass, 10^{16} GeV, Electroweak scale and the scale of CC. everything can be built on these scales to understand the broad features of nature...

vmarko said...

Folks, I fail to understand the comments regarding quantum gravity. AFAIK, the nonzero tensor mode says there are gravitational waves, which is an effect that can be described by classical GR (and we knew about the existence of those for a long time now).

So where is anything "quantum" here? I would expect to see quantum gravity effects on the Planck scale (10^19 GeV), while the discussed inflaton scale is 10^16 GeV (a thousand times smaller). Am I missing something here?

Best, :-)
Marko

Alex said...

@vmarko,

but where do they come from

Jay Brown said...

Quote by Marni Dee Sheppeard: "Penrose won't know what to do about this, I suspect."

Penrose's theory actually predicts CMB distortions by gravitational waves, only here they originate in the previous cosmic aeon (massive black hole collisions). But from what I remember he, Gurzadyan and others only looked at large scale structures in the CMB, not at the B-modes. It'll be interesting to see if these new observations can be explained by CCC.

Anonymous said...

What are the implications if these results from the perspective of multiverse ?

Anonymous said...

Jester,

Just a few weeks ago you were despondent and depressed and look at the gift nature gave you! 2014 turns out to be awesome...
I think the lesson here is you should never stop blogging
-TB

Jester said...

Large tensor mode is clearly anthropic. Without it we would die of boredom.

Jester said...

I also don't understand the fuzz about `quantum gravity'. True, these are quantum fluctuations of the metric propagating from inflation to the last scattering surface . And yes, it's amazing that we can observe quantum effects happening in the early universe. But gravity and the scalar field mix during inflation, and previously we have seen quantum fluctuations of the scalar mode. So it's little surprise that all parts of a single system behave in the same quantum way.

Also, the energy density during inflation is well below the Planck scale. So we're in the regime where Einstein gravity is a good effective theory, and we don't see what people usually call `quantum gravity', that is the gravitational physics above the Planck scale.

Jester said...

Anon 08:22, actually in simplest models with a quadratic inflaton potential the inflaton mass will be of the order of the Hubble scale during inflation: m~Sqrt[V]/M_Planck~10^14 GeV.
Right Anupam, i had single field in mind for the sake of this argument. But I guess even multi-field inflation will happen along a direction in the field space where the mass is much smaller than Planck? Or is there a counterexample?

senanindya said...

After the BICEP2 results, we now know that ns=0.96 and r=0.2.

From what I understand, this fits extremely well with the basic chaotic inflation model given by V(Φ)=λΦ4.

We also know that amplitude of density fluctuations is ≈10−5 and Energy scale of inflation is around 1016 GeV.

My question: Given also this information, can we now make an educated guess for how many e-foldings happened during inflation ? Or at least a theoretical lower (upper ?) bound ?

PS: I am only referring to inflation of the patch of space that now contains our observable universe, not inflation of Universe as a whole.

Jester said...

We had known before that we need at least around 60 e-folds to explain the uniformity of the CMB. I don't think anything changed in this respect after BICEP2.

senanindya said...

Ok, some context for the question. I've seen several claims by Linde that inflation resulted in around 10^12 e-foldings. He had an article at Scientific American based on this number - I dont know if its a lower or upper bound.
I recall vaguely that this follows from assuming a quadratic potential and using the fact that density fluctuations have amplitude 10^ -5
Was wondering what happens if you repeat the calculation for quartic potentials, because I dont remember the details

Jester said...

That's metaphysics. All we care for and all we can probe is the last ~60 e-folds. If you wait another 10 billion years you'll see the e-fold no.61 ;)

Marni Dee Sheppeard said...

Of course there is no QG in the vanilla gravitational wave idea. QG should show up quantitatively in new constraints on LCDM, CCC etc., starting with the GUT scale.

Anonymous said...

"But I think the mass larger than 10^16 GeV would be inconsistent with the measured V(inflaton)~[10^16 GeV]^4 plus the slow-roll conditions. Maybe there's a loophole for the latter argument but I'm not aware of one."

No need to pick up a model. The energy scale we observed V~Mp^2H^2, solve for H ~ 10^14GeV. This is the energy scale of the 'experiment' at which the tensor and scalar exit the horizon. (This changes a bit if cs \neq 1. However, from today we learned it cannot be too far from 1, unless we truly buy this running...)

Incidentally, Mp^2\dot H is also a very important scale ("\dot\phi^2") and that one is much closer to V^(1/4) for such "large" \eps ~ \dot H/H^2 (again with cs ~ 1).

Marty said...

For example, an early phase transition or decay of massive particles during inflation may also generate tensor perturbations.

Is there a decent reference that discusses tensor modes generated by the decay of massive particles during inflation?

Jester said...

I learned this from 1109.0542.

Marty said...

Thanks, Jester! After grabbing the paper, I saw that Sean Carroll mentioned that paper. There are a couple of related papers in the comments section of that posting.

Anonymous said...

With regards to the decay of massive particles producing gravitational waves (1109.0542), this claim was further investigated and shown to very unlikely (1209.3848).

Particle production during inflation corresponds to a Bogoliubov transformation of the field mode function with occupation number equal to n_k=|beta_k|^2.

Anonymous said...

So interesting that this results constrains CDM axions which are now in severe tension with data and had their parameter space massively reduced.

There is also interesting ongoing work on the stability of our vacuum, and the implication of a UV completion.

andrew said...

"After the BICEP2 results, we now know that . . . r=0.2."

No. This is not what we know.

What we know that BICEP2 reported that r=0.2+0.07/-0.05 and that Planck has combined its data to date and prior results to report that r<0.11 at the two sigma level and is not inconsistent with zero (which is basically r=0.01 +0.05/-0.01).

The best fit to all available data is roughly r=0.10 to r=0.11, which is in roughly two sigma tension with both the Planck data and the BICEP2 data. But, we will need more data points before we can comfortably reconcile those estimates. It is entirely possible that one of these two data points is flawed.

Fortunately, we have the Planck polarization data coming later this year, and several other experiments also trying to measure r so we shouldn't have to wait too long to have a better estimate of the scalar-tensor ratio than we do today.

Robert L. Oldershaw said...

For those who have maintained a semblance of scientific objectivity in the rapture over the BICEP2 results, please check out this well-informed warning from Dr. Peter Coles, a theoretical cosmologist who knows the details.

http://telescoper.wordpress.com/2014/03/19/time-for-a-cosmological-reality-check/

Hold your bets, my friends!

Robert L. Oldershaw
http://www3.amherst.edu/~rloldershaw

Marni Dee Sheppeard said...

Gee, that didn' take long. Arxiv paper on Higgs-inflaton plus sterile neutrino DM, to explain BICEP2:
http://arxiv.org/abs/1403.4132

kashyap vasavada said...

Now it seems that the concept of inflaton field is getting a strong support. Just as a crazy speculation (!): is it possible that after inflation is over, the inflaton field has a remnant CC or Lambda which remains constant for most of the history of our universe? Or is this completely absurd? If that is true it would be very economical. One would not need two independent fields for repulsive expansion. Both would contribute to expansion, in one case very rapid exponential expansion and in the other case lot slower but nevertheless an accelerated expansion.

Marni Dee Sheppeard said...

After reviewing the Planck "Cosmological Parameters" paper, I see why BICEP2 put in the varying A_L analysis. This seems to be the key tension between BICEP and Planck, meaning that LCDM has trouble reconciling them but A_L around 1.2 seems to work (with r then around 1.7). Does this support CCC instead? That's a tricky question, because CCC does not do a 'full early universe' scenario.