Danger, Phil Anderson

By Sean Carroll | January 3, 2006 1:48 pm

[Update: Prof. Anderson was kind enough to reply in the comments.]

Another somewhat problematic response to Brockman’s World Question Center is given by Philip Anderson, one of the world’s leading condensed-matter theorists. Like fellow Nobel Laureate Robert Laughlin, Anderson takes a certain pleasure in tweaking the noses of his friends on the high-energy/astrophysics side of the department. We can all use a little tweaking now and then, but must be expected to get tweaked back in return.

Anderson talks about dark matter and dark energy; his piece is short enough that we can go through the whole thing.

Dark Energy might not exist

I hope this idea isn’t too dangerous, by the way, since certain of your favorite bloggers have been quite active in this area. Overall, one gets the impression in the World Question Center that these folks somewhat overestimate how dangerous they are really being.

Let’s try one in cosmology. The universe contains at least 3 and perhaps 4 very different kinds of matter, whose origins probably are physically completely different. There is the Cosmic Background Radiation (CBR) which is photons from the later parts of the Big Bang but is actually the residue of all the kinds of radiation that were in the Bang, like flavored hadrons and mesons which have annihilated and become photons. You can count them and they tell you pretty well how many quanta of radiation there were in the beginning; and observation tells us that they were pretty uniformly distributed, in fact very, and still are.

All true, although the “you can count them” bit is a little confusing — I think the “them” he’s referring to is the photons. The basic idea is that the total number of photons hasn’t changed much since the extremely early universe, which is basically right; it may have increased by a factor of 100 or so during phase transitions when other stuff annihilates into photons, but by cosmological standards that’s not a big change.

Next is radiant matter — protons, mostly, and electrons.

I think by “radiant” he means “not the non-baryonic dark matter.” Neutrons would also count.

There are only a billionth as many of them as quanta of CBR, but as radiation in the Big Bang there were pretty much the same number, so all but one out of a billion combined with an antiparticle and annihilated. Nonetheless they are much heavier than the quanta of CBR, so they have, all told, much more mass, and have some cosmological effect on slowing down the Hubble expansion.

Not even “much” more mass — a factor of 102 or 103, but okay, now we’re nit-picking.

There was an imbalance — but what caused that? That imbalance was generated by some totally independent process, possibly during the very turbulent inflationary era.

Yes; that’s baryogenesis. Maybe it happened during inflation; that’s not a leading candidate, but certainly a plausible one. So far, just a slightly idiosyncratic retelling of the conventional story.

In fact out to a tenth of the Hubble radius, which is as far as we can see, the protons are very non-uniformly distributed, in a fractal hierarchical clustering with things called “Great Walls” and giant near-voids. The conventional idea is that this is all caused by gravitational instability acting on tiny primeval fluctuations, and it barely could be, but in order to justify that you have to have another kind of matter.

Now we’re getting into a bit of trouble. This statement would have been perfectly reasonable, if somewhat alarmist, fifteen or so years ago. These days we know a lot more about the distribution of matter on very large scales, from the microwave background as well as large-scale structure surveys. It’s not a fractal in any interesting sense on very large scales; certainly the density fluctuations on those scales are quite tiny. And when he says “barely could be,” I think he means “fits the data remarkably well.” The Cold Dark Matter model has some issues with the structure of individual galaxies and clusters, but for the overall distribution it’s a fantastic fit.

So you need — and actually see, but indirectly — Dark Matter, which is 30 times as massive, overall, as protons but you can’t see anything but its gravitational effects. No one has much clue as to what it is but it seems to have to be assumed it is hadronic, otherwise why would it be anything as close as a factor 30 to the protons?

That’s just a mistake. “Hadronic” means “made of quarks”; almost nobody thinks the dark matter is hadronic, and in fact it would be extremely difficult to reconcile that idea with primordial nucleosynthesis. The fact that it’s close to the density of protons is certainly interesting, and we don’t know why.

But really, there is no reason at all to suppose its origin was related to the other two, you know only that if it’s massive quanta of any kind it is nowhere near as many as the CBR, and so most of them annihilated in the early stages. Again, we have no excuse for assuming that the imbalance in the Dark Matter was uniformly distributed primevally, even if the protons were, because we don’t know what it is.

I’m not sure what “no excuse” means. If he means “no data support the assumption,” that’s wrong; the idea that fluctuations are adiabatic (correlated fluctuations in dark matter, photons, and baryons) has been pretty well tested, and agrees with the CMB very well. There is some room for a bit of variation (known as “isocurvature perturbations”), but the limits are pretty constraining. Perhaps a real cosmologist could chime in. If he means “we have no idea why the distributions are correlated,” that’s also false; in the simplest models of inflation, it’s exactly what you would expect, as the energy density from the inflaton decays into everything with some fixed amplitudes. Again, there are ways around it, and we don’t know that inflation is correct, but it’s by no means inexplicable.

Finally, of course there is Dark Energy, that is if there is. On that we can’t even guess if it is quanta at all, but again we note that if it is it probably doesn’t add up in numbers to the CBR.

Well, we actually guess that it is not quanta (i.e., particles) — if it were, the number density of particles would presumably dilute away as the universe expands, decreasing the density of dark energy, which isn’t what we observe. The dark energy is nearly constant in density, which is why most people imagine that it’s vacuum energy or the potential of some very light field, not particle excitations.

The very strange coincidence is that when we add this in there isn’t any total gravitation at all, and the universe as a whole is flat, as it would be, incidentally, if all of the heavy parts were distributed everywhere according to some random, fractal distribution like that of the matter we can see — because on the largest scale, a fractal’s density extrapolates to zero.

This “very strange coincidence” is of course a prediction of inflation, that the universe is spatially flat. The bit about the random fractal distribution manages to somehow be simultaneously wrong and ill-defined. Again, we know what the distribution looks like on large scales, from CMB fluctuations, and it’s incredibly smooth. If it were wildly fluctuating, including on scales much larger than our Hubble radius, then most of the universe would have a large amount of spatial curvature — that’s certainly what we see in the local distribution. Not that it’s very clear what such a distribution would actually look like in general relativity.

That suggestion, implying that Dark Energy might not exist, is considered very dangerously radical.

Well, not so much “radical” as “incorrect.” Anderson doesn’t mention the fact that the universe is accelerating, which is curious, since that’s the best evidence for dark energy. His offhanded proposal that density fluctuations are somehow responsible is similar in spirit to the original proposal of Kolb, Matarrese, Notari, and Riotto, that ultra-large-scale inhomogeneities could mimic the effects of dark energy. Everyone now agrees that this idea doesn’t work, although the authors are trying again with small-scale fluctuations. While that hasn’t been cleanly ruled out, it’s a long shot at best; most folks agree that we either need dark energy, or somehow to modify gravity.

Anderson then goes on to argue against any particular conception of God, on the basis of Bayesian probability theory. I’m not a big God booster, but he probably didn’t run this idea by anyone in the Religious Studies department, any more than he ran his dark-energy ideas by any of the local cosmologists (I understand that Princeton has one or two). I think it’s great when smart people step outside their areas of expertise to make interesting suggestions about other fields (if I didn’t, the blogging thing would be kind of indefensible). But we shouldn’t forget that there are smart people in other parts of the university, and have some respect for their expertise. Or is that another one of those dangerous ideas?

  • anon.

    A small nitpick to an otherwise good post: “hadronic” doesn’t imply different numbers of quarks and antiquarks, “baryonic” does. Anderson only assumes (for some totally unclear reason) that dark matter is hadronic. Since it’s pretty clearly not baryonic, maybe he’s postulating some mysterious stable meson. Or more likely he’s babbling about things he doesn’t understand.

    (As for possibilities not involving dark energy, I’m still hoping to hear someone knowledgeable comment on the recent Mansouri paper.)

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Sorry, I got that exactly backwards; now fixed.

    In return, I’d love to give you a knowledgeable comment on the Mansouri paper, but I don’t have one. Just the overall feeling that I have previously expressed: fluctuations are very small, perturbation theory should be fine, so I’m not going to pay close attention to this stuff unless it’s clear that there’s been a real breakthrough. I could be wrong, but there are an infinite number of ideas out there, and you have to pick the ones you think are promising.

  • anon.

    OK, thanks. I understand that perturbation theory seems valid, but then as far as I can tell he claims to have an exact solution to Einstein’s equation with no spurious thin shells of energy at the junction. It looks like he has a valid solution, but I guess there are a few possibilities: maybe the solution is mistaken, or his identification of variables with observable quantities is mistaken, or data just don’t support the degree of inhomogeneity one needs for his arguments to work. I suppose I’ll just wait for someone else to check these things, unless I find myself with an unexpected abundance of free time (I say while commenting on a blog).

  • Elliot

    All I have to say was there were a number of more interesting “dangerous ideas” and corresponding articles posed on this site than Philip Anderson’s.

    Go check it out for yourself.


  • http://eskesthai.blogspot.com/2005/12/wave-function-and-summing-over.html Plato

    I certainly hope one can appreciate my Hillbilly Humour and let loose a bit? :)

    An “artistic philosophy on strings” that might issue from my neck of the woods:)


  • Kea


    If you are so willing to criticise the ideas of Kolb et al. (I’m not saying they’re not wrong) perhaps you should actually read the Mansouri paper.

  • Chris W.

    By the way, in this passage,

    There are a number of ways of making a formal probability theory which incorporate Ockham’s razor, the principle that one must not multiply hypotheses unnecessarily. Two are called Bayesian probability theory, and Minimum Entropy. …

    I believe Anderson meant to say Maximum Entropy, in reference to Maximum Entropy methods.

  • http://jenniferhead.cfa.harvard.edu Jennifer

    Sean, great post, Anderson was either dishonest or lazy in my opinion. Too bad.

    Kea, Sean did read the Kolb et al. paper, I think he mentioned it on the blog. The Mansouri paper just came out, Kolb et al.’s previous work which Sean criticized has nothing to do with this new paper. So it is perfectly acceptable to criticize the first without reading the second (kinda like saying how can you criticize coca-cola when you haven’t tried pepsi??).

    And as far as I can see from a very quick perusal of the Mansouri paper, he is explaining the Type Ia SN evidence for acceleration (correctly or not, I don’t know) but not touching CMB, x-ray, or other compelling evidence for it.

  • Paul Valletta

    Counting photons:


    has to consider the photons source?

    Which is I guess the fact that the Big-Bang has to be considered an “Anhilation” event?

    What event was the ‘Big-Bang’, has a baring on what photon count you can do, if there were particles of matter, emitting and absorbing “photons”, then one has to know the total number of Proton/Anti-Proton number.

    Andersons handwaving into the expectation of “another kind of matter”, is not that much wayward, according to:


    has proposed with:http://arxiv.org/abs/hep-ph/9911386
    and :


    that there is a high probability that ‘Quark-Condensate’ is the “missing” form of matter?

  • Rasi

    Being a condensed matter theorist myself, I feel ashamed that lot of condensed matter physicists always look at high energy and cosmology as rival fields and try to disparage it someway or the other (To be honest, I do it quite a few times too:-).
    Infact, in a recent international conference I attended, the introductory talk was about why condensed matter doesn’t capture the public imagination as much as topics like black holes and string theory. We were struggling to agree on whether there are any general goals at all in condensed matter that unite all researchers working in this area.
    What is missed is that the sheer variety of experimentally accessible systems is really one of the most attractive features of condensed matter physics and makes it distinct. No doubt there are lot of unifying principles in this area, but the very unpredictability of seemingly well-understood systems was the reason why amazing phenomena like high Tc superconductivity and fractional quantum hall effect could be discovered.

    Moroever, seeing the amount of cross-fertilization between condensed matter and high energy today, it is very unfortunate that lot of leading physicists still adopt such a narrow attitude.

  • Dissident

    Needless to say, if everyone just keeps ignoring new ideas until they are proclaimed to be a real breakthrough, this will never happen. Somebody’s got to do the reading before they can do the proclaiming…

  • http://ofteninerror.blogspot.com/ Urbano

    Well, just to “kick the dead dog”…

    Nonetheless they are much heavier than the quanta of CBR, so they have, all told, much more mass, and have some cosmological effect on slowing down the Hubble expansion

    I don´t see exactly what is the “weight” of the quanta of CBR (ok, during all the thing he was referring to “density” instead…), but in any case radiation also contributes to slow down the Hubble expansion…

  • Shantanu

    Sean ,many years ago people talked about a “baryonic dark matter problem” in which
    the amount of baryon density inferred from BBN was about a factor of 10 or so larger
    than that of visible stars , and if I recall right the leading candidate for this baryonic dark matter
    was ordinary “diffuse hot gas” (which presumably is just hydrogen). Does the baryonic dark matter problem still persist, or was it satisfactorily resolved years ago, or was this problem never there in the first place and just blown out of proportion?

    Kea, you should ask about the Mansouri paper on cosmocoffee.info

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Shantanu, we are still do not have a perfect inventory of where the baryonic matter is — there was a paper on this just a couple of days ago. But we have a pretty good idea, with most of the baryons in clusters residing in hot intergalactic gas. And we are relatively confident that we know how many baryons there are, since the baryon density derived from fitting the CMB anisotropies is in excellent agreement with the result from Big-Bang nucleosynthesis.

  • http://www.kaleberg.com Kaleberg

    But, “dark matter does not exist” is so elegant. It’s almost like “the ether does not exist”, except we’re all wrapped up in dark matter now, so we can’t even imagine how to get anything like our universe without something like dark matter. Of course, dark matter is nowhere as near as close to phlogiston as the ether was after the Michelson-Morley experiment.

    Still, it might be worth going on record as saying “dark matter does not exist”. This could be an easy way to be considered prophetic.

  • Tom Renbarger

    I got a bit of a kick out of seeing Paul Steinhardt and Lee Smolin basically stating one of their pet ideas as the most dangerous. Maybe that’s a little unfair, but that’s what struck me when I read their pieces.

  • phil anderson

    Hey, I think you are taking too seriously something meant to be purely provocative. Of course I’m not a real “expert”. But I don’t see how my points are all silly.

    Nitpicking about verbiage-“-radiant”, perhaps I should have used “radiating”.
    “hadronic”, I wanted a word for made of heavy particles such as the supersymmetric equivalent of hadrons–whatever it is, it has to have some mechanism like ordinary matter to keep it from decaying away; and my point that it is odd that it’s also very few quanta is valid and puzzling even to the local experts–and to you.

    I may be missing something but I know of no survey of the radiating component, as opposed to CBR, that assures its smoothness. There are a lot of theoretical arguments (Sachs-wolf, is it, for instance?) but nothing direct.
    And the history of cosmology is that holes in theoretical arguments appear with most new data.

    Maybe I’m wrong., I’m not a relativist; but my points were A] that noone has done cosmology on a seriously inhomogeneous universe;B] that the flat universe is just not decelerating, it isn’t really accelerating; C] there’s a bit of the “phlogiston fallacy” here, one thinks if one can name Dark Energy or the Inflaton one knows something about it. And yes, inflation predicts flatness, and I even conditionally accept inflation, but how does the crucial piece Dark Energy follow from inflation?–don’t kid me, you have no idea.

    The remark about my probability exercise amount to saying that I should consult Mobil-Exxon before saying anything about global warming. That paragraph should have been left off if you wanted to look cool and impartial.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Phil, thanks for responding good-humoredly; I’ll try to respond. And yes, I understand the desire to be provocative, but there are a lot of less-informed people out there who think that cosmologists are just inventing dark matter and dark energy for no good reason, so we become defensive.

    Nitpicking verbiage — fine, I agree.

    Smoothness of the baryonic component — well, I think there are several empirical reasons to think that it’s smooth on large scales. First and foremost is of course the CMB. Yes what we measure are the photons, but they are certainly affected by the matter distribution. If the latter were notably inhomogeneous, it would definitely perturb the CMB temperature at a level greater than 1 part in 100,000. But also, direct galaxy surveys have been getting better and better, and the number-density fluctuations of galaxies are unmistakably less on large scales than on small ones. In fact, they fit extremely well to a model in which all the primordial fluctuations were that famous 1 part in 100,000 (see e.g. astro-ph/0601168). There have been astrophysicists who have argued for a fractal distribution, and I remember a “debate” in Princeton around 1992, with Marc Davis defending the conventional view — suffice it to say that the fractal viewpoint didn’t gain any new adherents that day.

    It’s true that nobody has done much work on very inhomogeneous universes, just because the data are so very inconsistent with them. Sorry to be repetitive, but the CMB can’t be that smooth if the matter distribution isn’t! (Infrared surveys of galaxy positions on the sky are also extremely smooth, although it’s not as precise as the CMB.)

    As an empirical matter, I think the universe really is accelerating; see e.g. astro-ph/0512586. That’s not a conclusion that relies on theory.

    As far as explaining what the dark energy is, I certainly won’t kid you, I have no idea! (Likewise inflation.) I’m extremely interested in alternatives, including modified gravity and back-reaction of perturbations, and open-minded about different candidates for dark energy itself. But large-scale fluctuations just won’t do it; people have tried and failed, see astro-ph/0503582.

    Concerning the probability exercise, sorry if I seemed flippant. But I don’t buy your analogy. My point wasn’t that your conclusion was wrong (since I agree with it), just that arguments of this type have been batted to death in the theological literature. I would welcome more input into those conversations from scientists, but out of respect for other scholars I think we should try to make that input be as well-informed as possible.

  • Dissident

    Sean, what exactly is it about astro-ph/0512586 that makes you “think the universe really is accelerating”? That paper takes the supernova “gold set” assembled by Riess et.al., ASSUMES that the dimming is caused by cosmic acceleration and proceeds to extract the expansion history based on that assumption.

    Regarding astro-ph/0503582, it’s true enough that it makes a good case against superhorizon fluctuations a la Kolb et.al. being the cause of the purported acceleration, but that’s only one of the mechanisms based on inhomogeneity which have been proposed recently. Kolb et.al. have since moved on to another contender,


    Brandenberger has his own version,


    and Mansouri claims to be able to do it non-perturbatively even:


    There’s much more out there, some of it discussed at Cosmocoffee:


  • http://eskesthai.blogspot.com/2005/10/microstate-blackhole-production.html Plato

    I think it was more of describing the process from the beginning. But if you do that, then where is it? A condense matter theorist might like to think of the Quark Gluon plasma as a method of determining the very basis form which to begin?

    Shall we just talk about the beginning of this universe, while others like to speak about the “whole process”?

    So you have to have some idea here, and condensed matter theorist would have fell short on trying to explain what is driving the expansion.

    There had to be a “negative valuation geoemtrically enhanced” to have recognized curvatures parameters as a possibility?

    All things would have to have issue from a supersymmetrical point of view? Even from that state, what was it’s predecessor?

  • http://electrogravity.blogspot.com/ Science

    ‘the flat universe is just not decelerating, it isn’t really accelerating’ – Phil Anderson

    ‘As far as explaining what the dark energy is, I certainly won’t kid you, I have no idea! (Likewise inflation.) I’m extremely interested in alternatives, including modified gravity and back-reaction of perturbations, and open-minded about different candidates for dark energy itself.’ – Sean

    Look, Phil Anderson’s comment is EXACTLY the correct prediction made via the October 1996 issue of Electronics World, which was confirmed experimentally two years later by Perlmutter’s observations.

    The lack of deceleration is because the expansion causes general relativity: http://feynman137.tripod.com/#h

    This existing paradigm tries to take general relativity (as based on local observations, including Newtonian gravity as a limit) to the universe, and force it to fit.

    The reality is that gravity and contraction (general relativity) are predicted accurately from the big bang dynamics in a quantum field theory and spacetime context. There is nothing innovative here, it’s old ideas which have been ignored.

    As Anderson says, the universe is ‘just not decelerating, it isn’t really accelerating’, and that’s due to the fact that the gravity is a proved effect surrounding expansion:


    This isn’t wrong, it’s been carefully checked by peer-reviewers and published over 10 years. This brings up Sean’s point about being interested in this stuff. It’s suppressed, despite correct predictions of force strengths, because it doesn’t push string theory. Hence it was even removed from arXiv after a few seconds (without being read). There is no ‘new principle’, just the existing well-known physical facts applied properly.

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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