Of Particular Significance

Will BICEP2 Lose Some of Its Muscle?

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 05/19/2014

A scientific controversy has been brewing concerning the results of BICEP2, the experiment that measured polarized microwaves coming from a patch of the sky, and whose measurement has been widely interpreted as a discovery of gravitational waves, probably from cosmic inflation. (Here’s my post about the discovery, here’s some background so you can understand it more easily. Here are some of my articles about the early universe.)  On the day of the announcement, some elements of the media hailed it as a great discovery without reminding readers of something very important: it’s provisional!

From the very beginning of the BICEP2 story, I’ve been reminding you (here and here) that it is very common for claims of great scientific discoveries to disappear after further scrutiny, and that a declaration of victory by the scientific community comes much more slowly and deliberately than it often does in the press. Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong.  (To its credit, the New York Times article contained some cautionary statements in its prose, and also quoted scientists making cautionary statements.  Other media outlets forgot.)

Doing forefront science is extremely difficult, because it requires near-perfection. A single unfortunate mistake in a very complex experiment can create an effect that appears similar to what the experimenters were looking for, but is a fake. Scientists are all well-aware of this; we’ve all seen examples, some of which took years to diagnose. And so, as with any claim of a big discovery, you should view the BICEP2 result as provisional, until checked thoroughly by outside experts, and until confirmed by other experiments.

What could go wrong with BICEP2?  On purely logical grounds, the BICEP2 result, interpreted as evidence for cosmic inflation, could be problematic if any one of the following four things is true:

1) The experiment itself has a technical problem, and the polarized microwaves they observe actually don’t exist.

2) The polarized microwaves are real, but they aren’t coming from ancient gravitational waves; they are instead coming from dust (very small grains of material) that is distributed around the galaxy between the stars, and that can radiate polarized microwaves.

3) The polarization really is coming from the cosmic microwave background (the leftover glow from the Big Bang), but it is not coming from gravitational waves; instead it comes from some other unknown source.

4) The polarization is really coming from gravitational waves, but these waves are not due to cosmic inflation but to some other source in the early universe.

The current controversy concerns point 2.

Dust in the Sky

One of the challenges for the BICEP2 measurement (and any others like it) is that dust grains in the galaxy, spinning in the presence of magnetic fields, absorbing light, and re-radiating it as somewhat-polarized microwaves, can give a signal that would mimic or obscure a signal of polarized microwaves that are generated by ancient gravitational waves. Consequently, one of the things that the BICEP2 people had to show, to convince first themselves and then others that they were really seeing effects of gravitational waves, was to obtain and present evidence that the effect of polarized dust, in the part of the sky in which they were looking, is too small to affect their measurement. It is this evidence that is now coming into question.

If you had a perfect experiment and all the money and time you could want, it would be straightforward to figure out whether you were seeing polarized microwaves from dust or polarized microwaves caused by gravitational waves.  Here’s how: the amount of polarized light from dust grows rapidly as you look at higher and higher frequency microwaves, peaking at about 400 GHz (GHz = billions [G] of cycles per second [Hz]) while the amount from gravitational waves falls slowly at higher frequencies. So if you could measure the polarization at various frequencies, then by looking at whether your polarization signal does or does not increase rapidly with frequency between 100 and 400 GHz, you could distinguish dust-generated polarization easily from gravitational-wave-generated polarization.

But BICEP2’s high-precision measurements of microwaves have only been made at one frequency (150 GHz — measurements at 100 GHz will also be available in future).  So to assure their polarization signal wasn’t from dust, the BICEP2 experimenters had to obtain information from another source. In their paper, they gave an argument that polarized dust was far too small — at least four and perhaps ten times too small — to affect their measurement. They did this in two types of ways, indicated in the figure below, which is taken from the BICEP2 paper and annotated by me.

Figure from BICEP2's discovery paper, annotated by me.  BICEP2 data is black dots showing a large polarization signal.  This is consistent with the red solid curve, which is what would be expected for a large amount of gravitational waves.  Colored dashed curves near the bottom of the graph show various predictions for the size of the effect from dust.  (Ignore the corresponding solid curves unless you're an expert.)  The dashed curves labeled DDM1 and DDM2 are estimates by BICEP2 based on Planck data, some of which has not been published.
Figure from BICEP2’s discovery paper, annotated by me. BICEP2 data is black dots showing a large polarization signal. This is consistent with the thin red dashed curve, which is what would be expected for a large amount of gravitational waves from cosmic inflation. Colored dashed curves near the bottom of the graph show various predictions for the size of the effect from dust. (Ignore the corresponding solid curves unless you’re an expert.) The dashed curves labeled DDM1 and DDM2 are estimates by BICEP2 based on Planck data, some of which has not been published.  The worry is that these estimates might not properly account for Planck’s data, and that correct estimates might be as large as BICEP2’s signal, or be so uncertain that no inference concerning BICEP2’s signal can yet be drawn.

One argument relied on various people’s models of how dust is distributed around the galaxy. These models all show that the amount of dust-generated polarization should be extremely small: most of the lower curves in the figure are from these models, and they lie very close to zero compared to the BICEP2 data.  But this argument is only slightly reassuring, because these models are really highly-educated guesses.   A claim of such an important discovery shouldn’t rest on that kind of foundation.

So the really strong argument BICEP2 presented was based on reinterpreting some data from the Planck experiment.  This data had only partly been published; some of it had only been made public in graphical form, in plots that were presented during talks at conferences.  It mainly involves polarization from dust, measured by Planck across the sky, mainly at 353 GHz (where dust has the biggest effect).   The BICEP people said to themselves: suppose that the polarization Planck sees comes entirely from dust; well, we know how the polarization effect depends on frequency, so we can infer how much polarization from dust we would find at the BICEP2 frequency. This argument showed that the effect of dust in the region of the sky they were looking at was far too small for them to worry about, even if they viewed it very conservatively; there’s no way it could explain the signal that they see.  More precisely, of the two estimates I’ve marked with an arrow, even the worst case scenario, DDM2 (the dashed blue curve), was far below the BICEP2 data.  So there was no way, in the view of the BICEP2 team, that their signal could be coming from dust.

But some experts believe that there’s something problematic with the way BICEP2 did this, and that when done right, it is possible that the dust polarization will be a far bigger effect than BICEP2 estimated.  [Raphael Flauger is one person with serious concerns: here is a talk that he gave this past week, but I’m afraid that only experts have a hope of understanding it, and I myself have a number of crucial unanswered questions about his presentation.]  I emphasize that I cannot confirm their belief is correct, as I cannot yet construct a completely coherent story from public information — so I’m not going to speculate about it. I advise you to emulate the universe, and be patient; the truth will come out eventually, almost certainly within a year.

What Planck Didn’t Say

Another part of this story is that a couple of weeks ago Planck put out a paper (not published yet, but submitted for publication in a journal) showing their measurements of polarized dust across the sky… or rather, across much of the sky.  They’ve done measurements at various frequencies and can, as I described already, figure out how much polarization is coming from dust. You would think this measurement would have settled the issue; either BICEP’s argument agrees with or doesn’t agree with what Planck now says. But the whole point here is that BICEP2 makes more precise polarization measurements than Planck does. And in their recent paper, Planck only presents measurements in parts of the sky where they have a strong enough signal that they are confident that they know what they’re seeing, at high precision. Unfortunately, the region where BICEP2 is looking — a region where the amount of dust is very low, which is exactly why BICEP2 wants to do measurements there — is a region where Planck can’t see a strong enough signal to be sure how much polarized dust is present.

So it may well be that currently no one — not Planck, not BICEP2, not anyone — really knows for sure how much polarized dust there is in the BICEP2 region of the sky. [Or if the Planck folks already basically know, one can imagine, given how much rides on the answer, that they’re not telling until they’re absolutely sure.] Whether the real effect of dust is as small as BICEP2 claimed, or whether it is as big as their signal — whether their huge discovery of gravitational waves stands, or whether they’ve merely discovered that dust in the galaxy is far more polarized than expected — is simply not something that we can know for sure yet.

But all of this confusion probably does add some uncertainty — both the colloquial uncertainty (did BICEP2 really make a big discovery?) and the technical uncertainty (how statistically significant is BICEP2’s observation?). It’s up to BICEP2 to go back through their arguments and determine how much change, if any, they have to make… and whether, after any appropriate adjustments, their discovery has become notably more tentative than it was before.

Science, Inc.

What you are getting a glimpse of, if you are following this story, is the scientific process in action. In physics — I can’t speak for other sciences, and I know there are some where it is not true — the assumption by the experts is that every claim of a scientific result, especially a major discovery, is wrong until proven right.  Every result, especially one of particular significance, is poked and prodded, scrutinized and questioned, and subject to a battery of stress tests. Of course the scientists doing the measurement do this first, as best they can, knowing that it’s better to discover mistakes in private than in public. Then their colleagues do the same, checking the details of the measurement, repeating it (more or less), and trying to do even better measurements of the same effect. Anywhere along the way during this process, an experiment can fail to pass muster. But those that do pass muster — such as the recent discovery of a Higgs particle, the discovery that the universe’s expansion is accelerating, and the indirect discovery of gravitational waves back in the 1980s — will be around for the ages.

BICEP2’s result, along with its widespread interpretation as evidence of inflation, may yet survive the gauntlet of tests to which it is being subjected. But it’s far too soon to tell.

Share via:


93 Responses

  1. Matt, thanks a million for the pigback to Astroblogs. It’s in my native language, and i am finally getting an idea to what the Tensor to Scalar ratio is.

    Michel Beekveld

  2. I’m generally not in favor of doing science by press release, but it does have some advantages. I just learned that my Dad, who is 92, blind, and never went to to college, has a (very general) idea of what BICEP2 is looking for and that it may have come to grief over galactic dust. While he’s not exactly a typical Oklahoman, it still suggests that BICEP2 has generated an extraordinary degree of public interest in some rather abstract physics.

  3. Re- Groucho quip: The Sufi philosopher Nasreddin’s (spelling varies) (11th-13th Century CE) story about his donkey comes to mind in describing science. He didn’t want his neighbor to borrow it, so he hid it from sight before the neighbor came over. Just as he claimed it wasn’t there, it bayed. The neighbor called him on it, and he replied: “Who are you going to believe, me (your mulah) or a donkey?” Authority vs evidence, Persian version.
    I mostly agree with Page; scraping of un-peer reviewed ‘data’ is at best a “note in added proof” and belongs in the Supplemental Materials. Including it in the main body of a manuscript should cause automatic ‘return to sender”.
    I’m too old to resent, but not too old to remember, that you should take every opportunity to dance. Celebrations of milestones are a good thing, and there is no doubt that the public announcement was a milestone. It would be awful if, after the announcement, the team went on with business as usual. Why would anybody suggest that, as some seem to be??

  4. Losing muscle, or usual dirty politics?

    This is turning into a witch-hunt as Multiverse does kill Genesis, no doubt. Linde said it himself.

    Now we have a guy from the papist US stronghold NJ, using a slide to “beat” BICEP2 over their using — a slide?! Hilarious!

    But it does buy time, enabling them to work out a new fable each week, and slow down everything to the point of inconclusiveness (same trick they applied to seismology aka “Jesuit science” so that “hell below might exist”; in cosmology it’s so that “heavens above might exist”).

    Short-term goal seems to be to delay BICEP recognition until after this year’s Nobels, so they have another year to destroy BICEP by shredding it into pieces of inconclusiveness.

    Fable by fable — a Genesis!

    Recall Samuel Morse’s warnings against papists: http://en.wikipedia.org/wiki/Samuel_Morse#Slavery.2C_anti-Catholic_and_anti-immigration_efforts (note how craftily they put those warnings under the section on his support of slavery, obviously to minimize the effect of his more than justified warning against popes).

    1. It’s not so much Flauger et al use a slide to slay a slide – the same slide – as BICEP2 hasn’t published yet, so all we’ve got so far is the open and closing stanzas of the Hunting of the Snark, a critique that ‘it looks like nonsense’, the author responding ‘there’s more to come showing otherwise’, and … waiting.

    2. Please don’t bother the rest of us with these unjustified rantings. They’re not useful or intelligent. In a science such as physics, the truth will emerge, independent of the politics. Don’t confuse the short-term political up-and-down with the search for the underlying, reproducible facts of nature.

  5. That’s okay. Everyone has been, is or will be confused at some point. You of course are referencing Einstein’s parenthesis non close parenthesis blunder. An error of omission is no error at all. I don’t want to burst your bubble but you didn’t explain that very well. I can see that two objects moving in opposite directions at the speed of light would increase the distance between them greater than the speed of light. Inflation is, forgive me, an expansion on Einstein. I’m pretty sure he was unfamiliar with the technical term “whoosh.”

    I agree to disagree. The Universe was built and is ruled by physics, the world in which we live by philosophy. We physically test those philosophies all the time whether we realize that or not. Experimentation requires a control group for valid results. This is a common problem of the two.

  6. I had the opportunity to be the first to comment on this post and now I’m really glad I didn’t. Reading other’s comments first allows one to say that which has thus far remained unsaid.
    BICEP2’s findings, if accurate, are exciting. Confirmation of the Inflation theory or inflaton force, the repulsive force of gravity would be monumental.The possibility that Einstein was wrong and something can and did move faster than the speed of light was heretofore thought of as science fiction. One can only hope the theory is observationally verifiable by another facility. Theories like inflation, string theory, dark matter/energy though expressed flawlessly with mathematical precision, until confirmed by experimentation and/or observation will remain philosophies awaiting the next best guess. There is no law of relativity, so feel free to try and break it.

    1. Taken literally you are right that there is no “law of relativity”, as this phrase is not used in physics to my knowledge. There are Einstein’s theories of special and general relativity, named after the special and general principles of relativity they are built upon. Inflation does not violate these theories, as you can read here: http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/

      I disagree with your equalization of (hypothetical) theories and philosophies. It is exactly the precise mathematical formulation and consistency of these theories that sets them apart from mere “philosophies”. Due to their quantitative formulation you can (at least in principle) test physical theories experimentally. You cannot do that with philosophies.

    2. You’re very confused. It is Einstein’s theory that predicts the possibility of faster-than-light expansion of space. [I explained this in my inflation article.] BICEP2, if correct, would be stunning *CONFIRMATION* of Einstein’s theory of relativity, not a contradiction of it.

  7. Reblogged this on thecuriousastronomer and commented:
    Another excellent discussion of the skepticism over the BICEP2 results. As Matt correctly states in the penultimate paragraph, what we are seeing here is the scientific process in action. A discovery is always treated with a certain amount of disbelief by the scientific community, and the more spectacular the discovery the more this is the case. BICEP2’s result is now being analysed, scrutinised and picked apart by cosmologists around the World, and it will not be treated as correct until other experiments confirm the result, and if they don’t or if a flaw is found in the BICEP2’s analysis, then the result will be treated as a false detection.

  8. Excellent, sobering article Matt. And i could understand it.
    I do have a lot of trouble getting my head around the parameter “Tensor to Scalar ratio”. Is there a (relative) simple explanation on this?

    Michel Beekveld

    1. This is just the ratio of B to E modes (‘Interesting’ to ‘boring’ stuff); if it were 0 there would be no B modes, no gravitational waves the larger it is the larger the amplitude of said waves and the higher the energy scale of inflation.

      Unless IU’ve totally misunderstood something.

  9. We could add other possible points, such as:
    5) The polarized microwaves are real, but they are not ancient waves and they are not coming from dust; they are polarized by the lattice structure of the Higgs vacuum.

    1. No, we could not add such points, if we are scientists. We could add such points if we are highly unscientific speculators. You can easily calculate that this makes no sense.

  10. If you look for something hard enough and long enough then you will eventually find it.

    Even if it’s not there.

    1. A couple of counterexamples: 1) The luminiferous aether (the stuff postulated back then as the medium carrying light waves) was looked for very hard and not found. 2) A method for turning X (chemically) into gold was looked for extremely hard and for a very long time and it was not found.

      1. A counter-counterexample:
        Remote sensing by satellites, has shown the mean global troposphere temperatures have been virtually flat for nearly 18 years.

        Yet that has not prevented legions of gov’t funded investigators, and their political supporters, from insisting temperatures really _are_ increasing, even when the data is in stark disagreement.

        It’s a variation on the old Groucho quip, “Who are you going to believe, me or your lying eyes?”

        1. I wonder what you mean with “virtually flat”. I just had a look at the temperature graphs in the IPCC report and there is nothing I would call flat: http://www.climatechange2013.org/images/figures/WGI_AR5_Fig2-24.jpg

          Instead there are large short-term variations and at first sight it seems to me (layman) that you cannot tell much by looking at 18 years. As far as I see the experts also do not make claims for such short periods (from section “[…] it is virtually certain that globally the troposphere has warmed and the stratosphere has cooled since the mid-20th century. Despite unanimous agreement on the sign of the trends, substantial disagreement exists among available estimates as to the rate of temperature changes,”

          The statements about temperature changes are quite differentiated (see http://www.ipcc.ch/report/ar5/wg1/ ).

        1. So far, yes, or better almost 🙂 If nature would cater to the wishes of physicists, they would have been found already. There are many other examples of things physicists are looking hard for that have not (yet?) shown up, like supersymmetric partners of particles, magnetic monopoles or proton decay. These are not in the same category as the aether, however, as they have not been excluded by experiments either, so far.

          1. I have no objection against people looking for unicorns in a scientifically sound way – as long as they do not withdraw funding from physics searches which are much better motivated. 🙂 But that’s a topic for a zoology blog.

          2. On the contrary science has a higher probability of making a unicorn via genetic engineering. A magnetic monopole on the other hand is less probable even with a superkamikazi collider ( kamikazi in the sense that it would destroy the earth in the process of finding the monopole)

          3. The aether is detected every time a double slit experiment is performed; it’s what waves.

            ‘Offset between dark matter and ordinary matter: evidence from a sample of 38 lensing clusters of galaxies’

            “Our data strongly support the idea that the gravitational potential in clusters is mainly due to a non-baryonic fluid, and any exotic field in gravitational theory must resemble that of CDM fields very closely.”

            The offset is due to the galaxy clusters moving through the aether. The analogy is a submarine moving through the water. You are under water. Two miles away from you are many lights. Moving between you and the lights one mile away is a submarine. The submarine displaces the water. The state of displacement of the water causes the center of the lensing of the light propagating through the water to be offset from the center of the submarine itself. The offset between the center of the lensing of the light propagating through the water displaced by the submarine and the center of the submarine itself is going to remain the same as the submarine moves through the water. The submarine continually displaces different regions of the water. The state of the water connected to and neighboring the submarine remains the same as the submarine moves through the water even though it is not the same water the submarine continually displaces. This is what is occurring as the galaxy clusters move through and displace the aether.

            ‘The Milky Way’s dark matter halo appears to be lopsided’

            “The emerging picture of the asymmetric dark matter halo is supported by the \Lambda CDM halos formed in the cosmological N-body simulation.”

            The Milky Way’s ‘dark matter halo’ is lopsided due to the matter in the Milky Way moving through and displacing the aether.

  11. Apropos to mention a sentence from Richard Feynman’s 1974 commencement speech at Caltech, titled “Cargo Cult Science”:

    “The first principle is that you must not fool yourself — and you are the easiest person to fool”.

    1. Having listened via video to several leaders from the BICEP experiments, for hours now, they certainly are an EARNEST bunch. I can see why there’s so much cheer-leading going on (not least from them; they seem particularly prone to waxing mesmeric over their shared history at Caltech-JPL-Harvard. Heh, maybe I would too.).

      They’ve got a 5-sigma-confidence level of having caught … SOMETHING … by the tail.

  12. I would vote for:
    3) The polarization really is coming from the cosmic microwave background (the leftover glow from the Big Bang), but it is not coming from gravitational waves; instead it comes from some other unknown source.

    Because it opens a fully new (more logic) understanding of the bigbang inflation, as the fractal alike plitting of the big bang black hole ( Lyman Alpha structured dark matter).

    1. I could be attracted to reading such papers if my confidence wasn’t pre-shrunk by combination of simple spelling errors in common words, opaque syntax, the implication of an assembly process that brings to mind time spent in junkyards and thrift stores, and a persistent sense of Corey-esque bafflegab. These are not criticisms of the ideas in your paper, which I did not read because the prospect brought back childhood memories of broken-down cheap arcades in ghost towns and inadequately inspected rides in dodgy carnivals, which I could be off-base in feeling is in any way your fault.

  13. BICEP2 stated from the beginning that it could distinguish its signal from dust at the level of 2.5 sigma at best (2.2-2.3 sigma were stated at the Caltech workshop). So I’m surprised, how many people didn’t register this.
    This was an announcement of preliminary results, from this figure.
    *If* dust polarization is understood correctly, *then* more data will, either confirm the signal, or wash it out. *If* there is uncertainty over how dust polarization appears, *then* this must be, first understood, before new polarization maps can be correctly interrpreted. I’d recommend watching the videos of the Caltech workshop, in particular M. Zaldarriaga’s presentation, http://www.ustream.tv/recorded/47637248 and the questions session afterwards.

    1. Thanks. I think one should be more precise. BICEP2 was only able to distinguish signal from dust *using their own data* (in particular, data from BICEP1) at 2.2-2.3 standard deviations. But BICEP2 also had a separate argument: the DDM1 and DDM2 models. So it’s not as simple as saying that it was only 2.3 standard deviation separation. Once those models are not believable, then it becomes more problematic.

      Up to now I have not been able to confirm some of what Zaldarriaga says.

      1. “I have not been able to confirm some of what Zaldarriaga says”

        May I suggest that the “some” to which you refer includes those parts where Zaldarriaga depended on Flauger’s report on ‘replicating’ BICEP2’s “scraping” data from the Planck slide?

        I got the impression Zaldarriaga obsequiousness was multi-directional, also politely distancing himself from his colleague Flauger’s work with an ‘on the one hand, I trust him’ versus an ‘on the other hand, I don’t entirely understand what he’s done here’.

      2. From reading the paper, the DDM1 and DDM2 (Data Driven Model 1 and Data Driven Model 2) are based on what Planck had published up to 2013. What’s interesting is that, nonetheless, the resolution of synchrotron or dust couldn’t be better than 2.2 respectively 2.3 standard deviations. Apparently, there’s a lot about how interstellar dust responds to the proagation of electromagnetic waves that is not understood to the precision that would be necessary to reduce this uncertainty.

  14. One question: if dust can polarize the microwave background radiation, does it matter when the radiation went through the dust? Could it happen from condensation dust early after the “big bang”, or, for that matter, at any subsequent time?

    1. The dust isn’t polarizing the cosmic background radiation; it is emitting polarized microwaves that could be mistaken, if bright enough, for polarized cosmic background microwaves. There was not dust early after the big bang. There are effects from dust in distant galaxies, somewhat after the first galaxies form. But the dominant effect is believed to be from dust within our galaxy. Warning: I’m not an expert on interstellar dust.

      1. Are you up to explaining why this contingency wouldn’t be ruled out by the distinction between b-mode versus e-mode polarization? Because, way-amateur way-hack that I am, I’d assumed from the beginning of this fooferah that the only source of b-mode was gravitation originating from before the Hot Big Bang.

        And does any of this implicate the Synchrotron?

        1. Experts tell me that there’s evidence from WMAP that synchrotron is tiny and a red herring.

          There are many sources of E-mode polarization, but all of my discussion has been about B-mode. Fake B-modes can be generated by detector effects mis-measuring E-modes. B-modes in the CMB can be generated by gravitational lensing of CMB E-modes. And polarized light from synchrotron radiation or from polarized dust can include B-modes as well as E-modes. I think this is the main list of issues.

          1. Thanks, Professor Matt!

            Now I’m feeling a bit more confident that this is all headed towards determining the level of confidence in the BICEP2 results, and that Planck actually thinks it might – MIGHT – have something important to say on that.

      2. Sorry, “polarize” was a bad choice of verb. What I meant was, “lead to polarized electromagnetic radiation”. Are we sure there was no dust early after the big bang? (With some latitude as to the meaning of “early”.) As I understand it, after then big bang, there was hydrogen, helium and a small amount of lithium. I am not sure exactly how much lithium was formed, and I am not sure what observational evidence there is, but all lithium would automatically form lithium hydride, which has a melting point of 680 degrees C, and a molecule has an electric moment of about 2/3 that of sodium chloride. Accordingly, I would expect that to coalesce into dust if the concentrations were sufficient. In this context, too, there is a problem in forming the first stars. Hydrogen and helium are very poor radiators of heat, hence it should be difficult for the initial stars to get rid of accretional energy, especially while they are still small in the initial stages, when the temperatures are relatively cool and gravitational fields small. Something had to radiate energy, and lithium hydride would do that, so perhaps the fact that stars did form may indicate the presence of sufficient LiH?

        1. We are extremely confident that there was no dust, as it is considered now shortly after the big bang. The temperature was too high for a long time, firstly for atoms to be stable then for molecules to form. Lithium tends to form LiH+\LiHE+ ions which tend not to clump too well.

          Secondly the concentration of lithium was far, far too small; in order for grains to form there needs to be a low temperature, sufficient pressure and enough time for the atoms to meet. Once the temperature dropped low enough the atoms were in too diffuse by orders of magnitude. Incidentally the first generation of stars were very unique in both their formation and evolution because of the lack of heavy elements.

          But the bigger issue is that the dust does not alter the signal we’re looking for; microwaves may pass through it perfectly unpolarized. What matters is that dust is ‘hot’ and thus glows in microwaves. The dust present *now* in the way of the signal we want to measure adds an additional signal that, if large enough, will mask the existing signal and make us think we’re seeing something we’re not. When the dust formed doesn’t matter, whether it’s there now is the issue.

          1. I agree the ions mentioned form when hot, but the system still has to cool, and surely there comes a point where ions are neutralized and there is insufficient energy left to pump up Rydberg emissions. How does it cool from there? Why do we not get an ever expanding system of hydrogen and helium atoms at more or less constant temperature? (In the absence of dust or heavy polarizable atoms, and in the absence of sufficient pressure, two hydrogen atoms colliding generally cannot get rid of their vibrational energy and any molecule then flies apart.) Similarly, yes, the early stars had no heavy elements, so how did they accrete? How was the kinetic energy removed, especially at the beginning of accretion? You say the amount of lithium was trivial, but do we have observational evidence for the amount, as opposed to modelling outputs? If you accept that cooling involved the emission of radiation, and if there were strong and large magnetic fields present and highly polarized molecules, why could there not be polarized emissions? Maybe there can’t be, and this is a little outside my experience, but I can’t see the harm in asking the question.

            The issue I am raising is not that this is necessarily the solution, but rather can we be absolutely sure that given the expected situation as the Universe cooled that there is no alternative way the radiation could have been so polarized? The point about a theory, from my point of view, is that it must account for more than is put into it, and the true theory will properly account for all observations that are relevant to it. Support for an unusual theory, like cosmic inflation, needs to be questioned very deeply.

        2. You raise some very good questions. It is always nice to see someone who has ‘done their research’ so to speak.

          Ions are actually a remarkably common form of matter in the cosmos; only in cool and dense matter do they tend to neutralize. In gas clouds it is much more difficult, especially if those clouds contain only H, He and Li. Li is the most easily ionized atom present and as such is easily converted to Li+\LiH+ by collision with other ions.

          This is why the paucity of lithium comes into play; if there are enough LiH molecules they can start to cluster and form grains. However this requires a considerable concentration or duration and the tendency is for the ionized lithium to remain dispersed. I am not aware of any figures dealing with LiH specifically but vapor deposition often deals with concentrations bottoming out at parts per thousand.

          In regards to the collapse of gas clouds this is an interesting subject in itself. Matter initially expands nonuniformly (some volumes being denser than others.) but over time and under the influence of dark matter and early black holes this has been shown (in models) to lead to the formation of volumes where matter self gravitates. (If you are interested one of the most detailed models produced to date has some interesting details here: http://www.nature.com/nature/journal/v509/n7499/full/nature13316.html ) These volumes as you note tend to be quite large and so first generation stars tended to be of a massive size, far larger than any in our current universe can be. They also have very interesting internal dynamics. These volumes condense by providing sufficient pressure of gas to allow said gas to coo, a positive feedback loop.

          The amount of lithium produced in the big bang is easy to calculate… if you know the ratio of radiation to baryons at the time of nucleosynthesis.This means there are a range of possible values but commonly accepted values give us about the right amount that we observe by studying the spectra of ancient stars. (This direct observation is a powerful indicator of how much lithium is present in an object.) The main indicator for the conditions at the time is deuterium abundance; being only barely stable and easily destroyed its abundance is extremely sensitive to the conditions of nucleosynthesis. (And no known process can produce significant amounts of deuterium post-BB.)

          I say ‘about’ since there is in fact a “cosmological lithium discrepancy”. The discrepancy is a factor of 2.4―4.3 *below* the theoretically predicted value and is considered a problem for the original models. Since He-4 and He-3 abundances are in line with predictions many favor the idea that first generation stars (which we observe to calculate lithium abundance at the time of their formation.) have mixed lithium into their cores and destroyed it, becoming depleted. (http://www.nature.com/nature/journal/v442/n7103/abs/nature05011.html ) The problem we thus face is the universe has even less lithium in it than the trace we predict.

          Your question on polarization is apt. Currently there is no other (non speculative) process that we know of that can polarize the light. This is the issue raised by Professor Strassler in point 3; we cannot be sure that there isn’t such a process, but to the best of our current knowledge there isn’t. This remains an open area of debate and only time will tell, but I am hopeful.

          1. Thanks for the full response and the links. Yes, the deuterium abundance must constrain Li abundance, and that would appear to put paid to that speculation. On the other hand, one learns nothing if one does not ask. Again, thanks for the answer.

  15. “Unfortunately, the region where BICEP2 is looking — a region where the amount of dust is very low, which is exactly why BICEP2 wants to do measurements there — is a region where Planck can’t see a strong enough signal to be sure how much polarized dust is present.”

    Shouldn’t they have an upper bound though? BICEP doesn’t need to know how much dust there is, just that there’s not too much.

  16. I think it is very unfair to talk about the failings of the press without mentioning the failings of the BICEP people themselves. It wasn’t the press who called the press conference. It wasn’t the press who put up a youtube video of Andrei Linde drinking champagne. [Is that a phone ringing in the background? A call from Stockholm?]. It wasn’t the press who suggested that scraping data from a conference slide is a good way to do science. And so on. If the BICEP people had behaved more sensibly, then these doubts, which, as you say, are perfectly normal science, would not be “news”.

    1. I can see your point of view, but I don’t agree. The BICEP people made their choice as to how to make their announcement; they’re confident in what they did, so they chose to celebrate it. If they’re wrong, they’ll pay the price, and they know it. And as scientists, they know very well that nothing is firm until their result is confirmed by someone else. (As for that video of Linde — they can hardly have known it would go viral.)

      By contrast, the press has a responsibility to report responsibly; they have nothing to lose if the result is wrong, or modified, so why not remind the non-expert to be cautious?

      As for scraping data from a conference slide — that is what they said they did. I am not sure that is the full story. Someday we will know.

      1. While the excitement over the initial BICEP2 announcement is quite easy to understand, what certainly does not help is to see influential theorists rushing to point out how the announcement validates some of the most popular mainstream theories (Silverstein on axion monodromy, Brandenberger on string gas cosmology, Smolin and McAllister on the first experimental evidence for Quantum Gravity and so on). Putting the cart in front of the horse is never a good idea…

          1. I hardly think the inadequacy of the dust modeling is these theorists’ fault. They included the ‘if confirmed’ in their comments by and large. Also, regarding Natural Inflation vs monodromy, the date of discovery of a model does not have anything to do with how complete or compelling it is. The idea that the first measurement of B modes in the regime that could well include a primordial inflation component does not warrant further investigation of the relevant theory is surprising to hear from you.

        1. @Anonymous

          “I hardly think the inadequacy of the dust modeling is these theorists’ fault. They included the ‘if confirmed’ in their comments by and large.”

          This is not the point of my comment. Rushing to promote any unproven theory on tentative claims goes against the very spirit of scientific research which rests on healthy skepticism and a good dose of patience.

          1. Firstly, the claims made by the experimentalists were not expressed tentatively, they announced a major discovery. Still, most people included the caveats appropriately. Secondly, further research into some of the theory you mention is well motivated by the detection of B modes with some possibility of containing a part that is primordial. The theorists who considered sources for such signals years ago have every reason to return to that, and to explain it to others who have not worked on it before; there has been considerable interest in the community. This is active research, not lack of patience. Sitting around being skeptical does not add much given that the next steps are experimental tests by other instruments sensitive to foregrounds. I think this may reflect misplaced frustration with the experimental overclaim, but don’t blame the theorists for this one.

          2. @ Anonymous

            You are certainly entitled to you opinion, but I simply don’t buy your arguments. If you jump the gun on unconfirmed results, chances are that you will end up being viewed in a negative light in the long run. As extraordinary claims require extraordinary evidence, it is ill advised to prematurely seek vindication on preliminary data.

            Let’s agree to disagree.

  17. I am sure it was right for the results to be published, and hopefully point 1 can be put away. But even if we get around point 2, there remain points 3 and 4, and I feel they may be a bigger challenge. The results are interesting, in my view, but more for stimulating further work than for reaching a definitive conclusion.

  18. Professor Strassler,

    Forgive me for being obvious, but I just have to say I loved the shameless plug…
    “Every result, especially one of particular significance…”


  19. Bob, while your comment is somewhat off-topic, and I am not a climatologist, I am enough of a scientist to know that assessing the IPCC report and the BICEP2 announcement is not an apples-to-apples comparison. The BICEP2 announcement was a PRELIMINARY announcement made by a SINGLE experimental collaboration about a quantity that they were claiming to measure for the FIRST time. Therefore as Prof. Strassler has so clearly explained, it must be taken as provisional until vetted by other experts and confirmed by other experiments. The IPCC report is on almost the complete other end of the spectrum on the journey from “provisional” to “accepted science”. It is the work of hundreds of scientists from dozens (or more) of research groups, summarizing the already scrutinized and replicated findings of numerous independent experiments.

    1. I agree. There is no comparison, Bob, between these two things. And I would advise against doing science by word substitution; you have to understand what you’re saying. You could have done the same word substitution with the discovery of the sun, but you would be making a serious mistake.

      1. I think the BICEP2 controversy is quite a good analogy to the AGW argument. With AGW there’s a measured signal of temperature increase which we’re attributing to a very specific influence – industrial humans, even though temperature variations have been occuring over billions of years for all kinds of random reasons.

        Is the AGW “signal” just from “dust” too?

        Perhaps not, but I still think it’s an ok analogy


          1. Highly unscientific? You are the loudest blogger in urging caution over the BICEP2 results (“if if if”….) but you are saying the climate argument is so simple that is is scientifically settled? That’s incredible – we need data points over the next few decades to settle the climate debate – we can’t do any experiment to get these data points otherwise than just wait – until then we are restricted to mathematical models on computers – and just like in the case of BICEP2 these models may have vulnerabilities. In the case of cosmological polarisation we are lucky that loads more data points with be available soon, from Planck and other experiments.

            I’m quite sympathetic to the argument that in the case of climate change we have a lot more to lose, but we also have a lot to lose if a great big asteroid hits us and no one suggests devoting a large percentage of the econonomic output to defending against that.

            With quite good probability, a very cheap energy source will be discovered by science long before climate change can cause major issues – and in that case we will just turn on a global air-conditioner and create the climate to our choosing rather than the primitive situation we currently have where it’s all a bit random, with perhaps a slight warming effect due to us.

            1. I have urged considerable caution over the question of the *cause* of climate change. My own view is that *climate change* itself is now firmly established by the data.

  20. “… it is very common for claims of great scientific discoveries to disappear after further scrutiny, and that a declaration of victory by the scientific community comes much more slowly and deliberately than it often does in the press. Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong.

    Doing forefront science is extremely difficult, because it requires near-perfection. A single unfortunate mistake in a very complex experiment can create an effect that appears similar to what the experimenters were looking for, but is a fake. Scientists are all well-aware of this; we’ve all seen examples, some of which took years to diagnose. And so, as with any claim of a big discovery, you should view the IPCC assessment report as provisional, until checked thoroughly by outside experts, and until confirmed by other experiments.”

    ^ what I wish some responsible “scientist” would have written.

  21. Maybe they had a bad electrical connection! However it seems too many are hoping to discredit the results before all the evidence is in.

    1. Maybe some are hoping that, but my impression is that most physicists are skeptical of the BICEP2 results and interpretation even though (or because) they wholeheartedly wish for them to be true. Scientists are in love with truth, but its not an easy relationship and they are afraid to be fooled 🙂

  22. I like your statement:
    “Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong.”

    It well complements another statement you made during a talk radio broadcast with Sean Carroll:
    “The way I like to think about it, or like to pose it when addressing this issue with people who raise questions about the scientific process, is that one needs to understand that scientists in general do not trust scientists, they trust science. There is a distinction”

  23. My hunch is that the Planck collaboration did not see the BICEP2 results or that the confidence level in their results is too low to shout eureka!

    1. I gather Planck wasn’t aimed at the BICEP target. Also: how could Planck ‘see’ results they couldn’t anticipate existing?

      The way I understand Flauger et al is this: BICEP2 ADMITTED scraping data from a single data summary slide from Planck, using that date to extrapolate to ‘no significant dust’ in the part of the sky at which BICEP2 aimed; so, consider whether it’s SCIENTIFICALLY RELIABLE to assume that from the Planck slide, given maybe it’s so and maybe not, but either way it’s ASSUMPTION, not actual measurement; and consider that the MARGINS of reasonable error in an assumption like that, given the inverse square law, are pretty darned thin: even a 5 to 8% range of overall average dust interference per where Planck DID measure is enough to take the BICEP2 finding significantly below 5 sigma confidence – the level generally required for acknowledgement of scientific discovery.

      There’s also what Planck’s being doing, and what Planck has said about BICEP2.

      Planck’s been mapping the FULL SKY. If in doing that, Planck’s data shows intersteller dust concentrations sufficient to answer the concerns of Flauger et about assumptions from extrapolation versus actual measurement, then that still might reduce the confidence in BICEP2’s findings below 5 sigma – BUT … consider that when the BICEP2 results first came out, the line from Planck was “we’ll have more to say on this subject”. THAT could mean Planck thinks it MAY been able to measure dust concentration in the area of BICEP2’s target, to some level that currently they’re still working out their level of confidence over.

      Finally, consider what if whatever level of confidence Planck expresses (and that holds up) gains BICEP2 some major cred JUST SHORT of sigma 5: THEN how much more easily will BICEP be able to obtain funding for round 3? That outcome alone would, to most, justify the hype – as happened with the expressions of confidence that those two big experiments at CERN would find the Higgs particle.

  24. Matt: I think the real issue with BICEP2 has been the straight-to-media hype and Nobel-mongering. That and the “Multiverse proved!” reportage has gotten some backs up. And then there’s the big issue of a weak signal from 10ˉ³³ seconds can survive 380,000 years of “trillion-degree maelstrom”.

    1. I’m not drawn to the ‘Nobel-mongering’ critique that you and many others have posted about on this site and elsewhere, in part because the Nobel process, post-WWII at least, is a lot harer to game & typically takes decades (The papers that led to the Nobels so far awarded for the Higgs mechanism were originally published in 1964; the false impression of the process being bound up in some burst of enthusiasm came from the barely-1-year time frame between CERN’s big press conference on measuring the mass of the particle associated with the mechanism working; regardless, the real time lag for the honorees was close to half a century.).

      If there is some ‘independent’ prize being mongered here, a more likely candidate, objectively, is those 3 million dollar babies passed out by Milner & Zuckerberg, or even better that all that jazz from BICEP2 was aimed towards getting funds for the next run in the BICEP series (‘BICEP3: Confirmation or Deflation?’). But the thing is, BICEP comes out of a group of highly respected members of the community of astronomy community; I’d think we’d need a helluva lot more than what’s been seen so far before tossing off wild speculations of fame-blinded hubris.

    2. You should (re-)read Matt’s primers on the discovery, but to quickly summarize: The actual signal being measured is polarization of the CMB and originated ~380k years after the Hot Big Bang and so never encountered the “maelstrom”. The primordial gravity waves that are the hypothesized cause of the signal would not much care about the “maelstrom”. Also, when they were generated they were much stronger, and only became weak as the universe expanded which also caused the temperature to rapidly drop; it wasn’t a trillion degrees for 380k years.

  25. Hey Matt, Im glad to see you’re analysis of the situation, and reproducing that BICEP graph is quite helpful.

    Sorry for going somewhat off topic, but I just wanted to say Im very much looking forward to the rest of the ‘QFT, String Theory and Predictions’ series, and Im also extremely interested to read what you have to say about the baryons-as-branes relationship, which you mentioned you may write about someday.

    So that will be just my small nudge of encouragement. 🙂

Leave a Reply


Buy The Book

A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC


It’s always fun and interesting when a measurement of an important quantity shows a hint of something unexpected. If yesterday’s results from DESI (the Dark

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/05/2024

I hope that a number of you will be able to see the total solar eclipse next Monday, April 8th. I have written about my

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/01/2024