Dark Matter found? Don't break out the champagne just yet.

You may have seen announcements that Dark Matter has been "found". I don't believe there's a publicly available scientific paper on this yet, so the original source for this is two press releases from CERN: One from four days ago and one from today.

First, I want to say what is meant by dark matter being "found" here. It's not evidence that previously-uncertain Dark Matter exists. We already know that Dark Matter exists; the Bullet Cluster observations several years ago was unambiguous confirmation that non-baryonic dark matter exists. We don't know what it is from that, but we know it exists. (Here is a podcast I did three years ago about the evidence for the existence of Dark Matter, including the Bullet Cluster.) So what does it mean to say that these new CERN results may have "found" Dark Matter?

Although we know Dark Matter exists, there remain a huge number of mysteries about it. Many of these can be summarized under: what is it? All we really know is that it's not made out of baryons, that is, protons and neutrons. So, it can't be an excess of dim stars or rogue planets (a model that was once considered a real possibility for our Galaxy's dark matter). Thus far, we've observed it because of the effects of its gravity. We've seen it in comparisons of the structure in the Universe to models of structure growth from early-Universe conditions; in the dynamics of galaxy clusters and galaxies; and through gravitational lensing. It would be nice to observe it in other ways.

To "find" Dark Matter, we'd like to do one (or more) of two or three things. Either, we'd like to see the results of decay products in our atmosphere or in space because of interactions of Dark Matter particles out in space. Or, we'd like to have an actual Dark Matter particle interact with a particle detector we have on Earth (analogous to how we see neutrinos from the Sun). Or, we'd like to actually make some of the stuff in a collider like the LHC at CERN in Switzerland, and see its decay products or signature there.

The current announcement from CERN is potentially of the first type. There is a detector, the "Alpha Magnetic Spectrometer" or AMS, on the International Space Station. This spectrometer is measuring electrons and positrons (the antiparticles of electrons) coming from space— that is, cosmic rays that are electrons and positrons. They see too many positrons for what we'd expect. One possible reason for the excess of positrons is that they are the result of very rare Dark Matter annihilations in our Galactic halo. (Although such annihilations, if they are happening, would be rare, there is so much bloody Dark Matter out there that if it's doing this, it would produce enough excess positrons for us to observe.)

What's really been detected is a positron excess, which is interesting all by itself. Whatever it turns out that this positron excess is coming from, it's going to be at least new astronomy, and potentially also new physics. It may not be as sexy and headline-worthy as "WE FOUND TEH DARK MATTER!!!1!!one!", but it will still be interesting, and will tell us something about nature. What's been seen is consistent with it coming from the Dark Matter halo of our galaxy, but other sources can't be ruled out yet. As more data is collected, the investigators running this experiment will be able to test whether the details of what is seen remain consistent with what would be expected from Dark Matter, versus other possible sources.

20 responses so far

  • Ethan Siegel says:


    The PRL paper from AMS is freely available here: http://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.110.141102

    The paper is actually eminently reasonable and interesting, but to blame the positron excess on dark matter is both premature and not-at-all compelling. Ting should never be allowed to speak without an honesty translator, IMO. The Press Release hype machine is running at full throttle over on his end.

  • rknop says:

    Ah, thanks for the pointer. I've gotten lazy, and assume that any paper will show up on arXiv.org. Since the later press release was today, I also only searched back a day or two, and didn't find a Ting paper.

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  • gearpile says:

    There is no dark matter. The expanding universe can be explained else-wise. There were two big bangs. The first was significantly larger than the second. Most of the mass (or pre-mass) of the first big bang escaped. Mass nearest the origin collapsed back to a new singularity and exploded again, the second big bang. We are part of that second event. No light was generated by the material of the first event for some time. The total mass of the first event was great enough for it to eventually collapse, which is what it is doing now. Our second event “universe” is being drawn out, faster and faster, by the gravity of this encroaching shell. The light from this “Outerverse” has not reached us yet, but it will, and suddenly. Then there will be a new sky, unlike anything we could have imagined. There is no dark matter.

    – gearpile

  • rknop says:

    You can put that one up there with the Earth being on the back of a giant turtle.

  • Tam Hunt says:

    Rob, I find it interesting to read this post in juxtaposition with your dismissive post on plasma cosmology. In the latter post you state essentially that there aren't any major problems with Big Bang Cosmology and that we continue to strengthen this model of reality as new evidence arrives. And yet you admit above that there are huge question marks about dark matter. As far as I can tell, there is almost zero hard evidence for dark matter. Thirty years of experiments trying to find WIMPs or other candidates have all failed or been highly ambiguous at best. Rather, we have anomalous cosmological observations that contradict relativity theory and Newtonian gravity, and dark matter has been postulated to explain these anomalous observations. This is not evidence for relativity theory or Big Bang Cosmology; it is, instead, highlighting the inadequacy of these theories and the need for a serious re-think. But I'd appreciate hearing from you about direct evidence of dark matter.

  • rknop says:

    We don't know what Dark Matter is, but the evidence for its existence is overwhelming.

    Here are some links:

    An Ethan Siegel blog post: http://scienceblogs.com/startswithabang/2010/06/24/convincing-a-young-scientist-t/

    A podcast I did a couple of years ago for 365 Days of Astronomy: http://cosmoquest.org/blog/365daysofastronomy/2010/06/26/june-26th-dark-matter-not-like-the-luminiferous-ether/

    The video of a talk I gave several years ago in Second Life: http://vimeo.com/4559703

    A 2006 NASA Press Release about the Bullet Cluster: http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

  • Tam Hunt says:

    Rob, I would say that your statement raises logical and philosophical problems from the get-go. How do we know dark matter exists if we don't know what it is? If it exists, what exactly are we saying exists?

    Again, all we know is that we have anomalous cosmological observations. Dark matter has been postulated as a potential mechanism to explain these observations. Have we really moved beyond this postulate yet?

  • rknop says:

    Dark Matter is something that interacts gravitationally, but not via the Electromagnetic or Strong interactions. (It may interact via the Weak interaction; we can't rule that out.) It makes up most of the mass of galaxies and galaxy clusters. In galaxies, dark matter is spread out over a larger spatial scale than the stars and gas are. We know that it is not made out of baryons (i.e. protons and neutrons).

    So, we don't know everything about it, but we know something about it. There was a time when humanity didn't understand what light was (we didn't have even a classical theory of electromagnetism, we didn't know what a photon was and certainly didn't have any observations that we understood as being a detection of a single photon), and yet we knew that light existed. It's entrely possible to know that something exists, and know something about it, without knowing everything about it.

    There is a wide range of astronomical observations that support this. It is a misrepresentation to describe these observations as "anomolous cosmological observations". Yeah, there are observations that can't be explained without either dark matter of by modifying our theory of gravity; if that's what you mean by "anomalous", then almost all observations in science that have led us to discover new things can be described as such. Dark Matter, as a theory, has the advantage that it explains all kinds of stuff, whereas modified gravity theories are able to explain some of it, but not all of it. The Bullet Cluster, in partciular, is a pretty classic scientific experiment. The theory, nonbaryonic dark matter, made the prediction that it should be possible to find a place where most of the gravitational mass was not where most of the bayronic mass was. That's exactly what we see in the Bullet Cluster-- the gravitational mass is revealed through gravitational lensing, and the baryonic mass is revealed through the x-rays it emits.

  • Tam Hunt says:

    Rob, Tim Eastman, an associate of mine who is a consultant for NASA and an expert in plasma astrophysics, writes this of the so-called proof of dark matter resulting from the bullet cluster:

    "The underlying paper by Clowe et al. ignores the possible contributions of low- to medium energy plasmas (only "hot plasmas" are mentioned in the paper), and they also make no mention of the possible contributions of brown dwarf stars and several other possible contributors to the matter "budget" that they are working with (even though the paper reads as being very proper, complete and inclusive). Further, in spite of numerous limitations and approximations made in the paper, they then claim "proof" (the word is even in the paper's title), which to me is pure rhetoric because the word "proof" is mostly just appropriate in logic or mathematics and, in science and being notably less rigorous, may possibly be applicable to certain proofs in mathematical physics or lab studies where some particular facts are established in ways that are not model dependent. However, such conditions contrast greatly with this case where almost every parameter (even the associated redshift z) is model dependent and such contingent observational data are being related with a complex model - the very use of the word "proof" in this radically inappropriate way makes me very skeptical of this paper (not including the limitations noted above). Unfortunately, this kind of sloppiness goes on all the time in the mainstream literature and rarely does anyone complain, apparently because such papers yield the "correct" conclusions [illustrating the pervasive influence of cognitive bias, the most significant component of which is confirmation bias]."

    While I am not versed enough in the nuances of dark matter theory or plasma astrophysics to weigh in definitively, I know enough to realize that dark matter and dark energy are generally "patches" for relativity theory and Big Bang Cosmology. I interviewed Bob Kirshner on his book, The Extravagant Universe, and while I liked his book a lot and liked Kirshner a lot, the more I learn about current cosmology the more I think the more accurate title would be "The Highly Implausible Universe." There are just too many epicycles going on to make it convincing for me. I think there's a far simple and more predictive alternative out there.


    Lee Smolin, author of the new book, Time Reborn, agrees with me on the broad points:


    Doesn't it seem implausible that we'd have almost zero hard evidence of WIMPS at this late stage in the game of testing various candidates for dark matter?

  • rknop says:

    OK, first of all, a little bit of googling indicates that Tim Eastman is a plasma cosmology proponent. That means that he's an extremely unreliable source when it comes to anything having to do with cosmology, and anything he says should be approached with care and suspicion. Plasma cosmology is woo (it's no longer even reasonable enough to warrant the term "alternative hypothesis") that has been broadly and widely debunked, and anybody still clinging to it is clinging to it out of either honest ignorance, or willful ignorance. I would put it in the same category as N-rays, creationism, or "alternatives" to continental drift. If you want to learn more about why plasma cosmology (and the whole "electric universe" business) in general is unsupported rhetoric, and not science at all, I recommend Tom Bridgman's blog Dealing With Creationism in Astronomy. In particular, he has a summary page, Channges for Electric Universe 'Theorists'.

    As for the substance of his comment: most of it is a semantic argument that, like so many semantic arguments, is completely irrelevant. Effectively, he's complaining that the authors use the word "proof" in a way that it would not be used in formal logic. Has Dark Matter been formally proved the way that Fermat's Last Theorm has? No. Neither has biological evolution. Yet, from the point of view of modern science, we know that biological evolution happened, it's not scientifically reasonable to insist that there is any doubt to it, and so in a more colloquial sense of the word, it is reasonable to say that evolution has been proved. Likewise, the Bullet Cluster proves the existence of Dark Matter.

    His other objections are reasonable-sounding objections that aren't important. It's a common rhetorical technique to bring up things that haven't been addressed that are irrelevant, but that aren't obviously irrelevant, so that those who don't know the field migiht think they're reasonable objections. Climate change deniers do this sort of thing all the time. Specifically with regard to brown dwarfs, compact objects like neutron stars, or low-luminosity stars, there are lots there, but they've gone along with the galaxies. It's true that when we observe galaxies, the light we're seeing is mostly from the high luminosity stars-- high-mass stars, and giant stars-- and that most of the stellar mass isn't what's luminous. So, just given that, it's possible that low-mass or compact stars are spread much more uniformly than the luminous stars, and indeed, if you go back a couple of decades, that was a viable alternative to WIMPs for the makeup of the Dark Matter. However, there are two problems with that. First, nearly two decades ago, compact stars were ruled out as a significant component of our own Galaxy's dark matter by observations such as were made by the MACHO Project. (Here's a paper.)

    The second problem also addresses low-energy plasmas (although I'm always suspicious when a plasma cosmologist says anything about the importance of plasmas in cosmology, and you should be too). Observations of the ratios of the light elements in the Universe indicate that the total mass in baryonic matter is at most several percent of the mass that's observed in galaxy clusters. Most of our Galaxy, and most of the mass in the Universe, can't be made up of protons and neutrons.

    Both of these observations were well-established by the time that the Bullet Cluster paper was written, so there was no need for those folks to address them directly.

    One final thing: Dark Matter and Dark Energy are two very different matters. Dark Matter exists; the evidence is overwhelming. The current indications by those who work hard on the matter indicate that it's likely that Dark Energy is "stuff" (although the stuff is assuredly exotic, and may well be Vacuum Energy, which some would object calling "stuff"). However, it may well be that Dark Energy isn't stuff at all, but a pointer to where our fundamental theories or our cosmological models are breaking down, an indication that we've found the edges of where they apply. That's still a possibility with regard to Dark Energy. Dark Matter is much less mysterious.

  • Tam Hunt says:

    Rob, Tim Eastman is a highly circumspect proponent of plasma cosmology, as he describes in my recent interview with him:


    From what I've read of your critiques of plasma cosmology, it seems to be essentially "the mainstream view is generally adequate so any wholesale alternative is a priori wrong." I don't find this at all convincing b/c it seems to me that the mainstream view, as I mentioned above, is highly implausible. There is a parade of incredibles in the mainstream view, from dark matter to dark energy to black holes to inflation (particularly incredible). Epicycles through and through.

    But you still haven't addressed my basic objection to your arguments: you assert that dark matter "exists" even if we don't know all the details. Yet we don't know even the basics about dark matter. All we know, with any certainty at all, is that there is anomalous evidence re galactic rotation, etc. There are an infinite number of theories that could explain this evidence (literally) and yet dark matter, you assert, is the best explanation even though 30 years or experiments have failed to find any convincing evidence of the alleged matter.

    We all have our biases, and I'll admit I come to cosmology at this point in my life highly skeptical of relativity theory, based on my basic rejection of the spatialization of time as a wrongheaded approach. But I try to be objective and I find the dark matter hypothesis at this point unconvincing.

    I'd urge you to read Lerner's book, The Big Bang Never Happened, and give it serious consideration. I'd appreciate your considered critiques of his book. I'm not an advocate of plasma cosmology, per se, but I certainly am open to a new approach because the Highly Implausible Universe approach of mainstream cosmology ain't cutting it for me.

  • rknop says:

    Everybody who's wrong thinks he's Galileo.

    You say you come at this skeptical of relativity theory, because you don't like the idea of the spatialization of time. What that says is that you're coming at this from an uninformed position. Relativity is an extremely well-tested theory, that has stood up to every experimental test we can throw at it. Whether or not you like how it sounds is irrelevant.

    Also, you're not listening. We have found convincing evidence for the existence of Dark Matter-- over and over and over again, through mulitple different lines of reasoning and investigation.

    You're misrepresenting my positon. It's not just that the mainstream view is adequate-- the mainstream view is robust and has survived experimental tests. What's more, the plasma cosmology view is worse than inadequate; it makes predictions that are wrong, and doesn't even make anything like precise predictions in many places. So, the mainstream view is robust and well-supported, and the alternative you're proposing is weak and unsubstantiated.

    There's way more than galactic rotation to Dark Matter. Go read all the links that I gave you before.

    In the mean time, there's not much point in continuing this conversation. You're not raising any objections other than your own personal aesthetic objection to Dark Matter, and, no matter how many times you say that you find it implausible, that doesn't really matter much against the weight of scientific evidence. However, somehow I doubt that the weight of scientific evidence is going to convince you, and I know I won't be able to convince you in this comment thread. So, take your denial somewhere else, please. And, if you're going to be honest about it, go and read eveyrthing out there that debunks Lehrner's book, and that debuks all the kinds of objections to Dark Matter that you keep bringing up.

  • rknop says:

    If you're interested in a critique of Lerhner's book (which was pretty trivial to find in Google), here's a decent one:


    Some of what Lehrner asserts has been shown to be wrong in the 22 years since the book came out. However, a lot of it was already known to be wrong even back then. It's sad to me that this book gets so much attention from people who have aesthetic problems with the Big Bang. It gives them something to latch on to that sounds reasonable and plausible, but ultimately it's just wrong, makes incorrect statements about the mainstream, and is not supported by observation.

    Just because you have aesthetic problems with a theory or think that it sounds implausible doesn't mean that theory is wrong. Heck, Einstein had serious aesthetic problems with Quantum Mechanics, and he's one of the finest scientific minds of the 20th century -- and, yet, Quantum Mechanics has been shown to be right again and again, and many of the specific things Einstein objected to have stood up.

  • Tam Hunt says:

    Rob, I'm not an advocate of plasma cosmology. Rather, I'm an advocate of re-thinking the mainstream view and I've been examining plasma cosmology as a candidate for solutions for what ails the mainstream view (see my "Alternative Cosmologies" interviews at my column; I've done three so far, looking at process physics, quasi-steady state cosmology, and plasma cosmology).

    Similarly, Tim Eastman is very cautious about being labeled a plasma cosmology proponent. Where I think there is some merit to plasma cosmology, as Eastman makes clear in his interview, is that we shouldn't ignore the role of EM/plasmas at the cosmological scale.

    My concerns with the mainstream view are more than aesthetic. I agree that aesthetics are in fact very important in science, but my concerns about relativity theory, for example, go beyond aesthetics. Let's not forget that dark matter and dark energy have been proposed as solutions exactly because general relativity has failed to match observations. There are many alternatives to special relativity that don't require spatialization of time. And there are various alternatives to general relativity that lead to different conclusions about the nature of time (shape dynamics, for example). As Lee Smolin writes in his new book, Time Reborn, it was probably a big mistake to spatialize time, which results in time being an illusion.

    My feeling is that we will likely, over the next few decades, engage in a large re-think of the bases of modern physics, including both general relativity and quantum physics.

    However, we're not going to resolve those issues here today. What I would like to do here, if you're willing, is engage in further back and forth on two or three specific issues where plasma cosmology differs from Big Bang Cosmology, and see if we can't get a bit more definitive answers. I'll let you pick the issues and I'd enjoy it greatly if we could, as an exercise in comparative physics and philosophy of science, see if we can't get to the bottom of at least two or three specific issues. Are you game?

  • rknop says:

    Actually, no, I'm not game. I have much better things to do with my time, and other things I'd like to get posted on this blog.

    Plasma Cosmology and the Big Bang are nowhere near being in the same ballpark when it comes to making predictions that are borne up by observation and explaining the empirical data we have. Engaging in a discussion about how the two of them explain the data would be similar to engaging in a discussion about how Intelligent Design and Evolution explain the data that supports Biological Evolution. It's well-tread and well-worn ground, but debaters are able to come up with ill-grounded but plausible-sounding objections (such as you keep doing with "the spatialization of time") that are utlimately meaningless.

    I've already pointed you to an excellent takedown of Plasma Cosmology on Tom Bridgeman's site. If you're interested in learning why Plasma Cosmology isn't actually a real theory that is at all a viable alternative to the Big Bang, go there and read that. It's not worth my time to try to replicate it here in an ill-founded debate for one person.

    As for Relativity -- I'm not familiar with Lee Smolin's arguments. However, every test we've thrown at Relativity, Relativity has stood up to. It's going to take something EXTREMELY compelling to replace it nowadays. What's more, I doubt that "the spatialization of time" is going to go away. Relativity shows that Newton's gravity is "wrong", yet we still use Newton's Gravity all the time, and for a lot of things, it's more useful than GR. Why? Because it's an easier theory to deal with, and except in extreme cases or cases requiring a lot of precision, it's good enough. Likewise, Relativity has been so precisely tested that anything that replaces it is going to have to make all the same predictions that it does, including things like time dilation.

    Given all that, if I had to bet, I would bet that Lee Smolin is playing the iconoclast (a role he's played before, and somethign I can apprecaite), and has taken it too far. I don't know this, not having read his book, but if I had to bet, that's the way I'd bet. And, to be honoest, there are other physics books I want to take the time to read, and it's going to take more than the word of somebody who believes that Plasma Cosmology and the Big Bang are anywhere near on the same footing to convince me the book is worth the time to read.

  • rknop says:

    (It's also possible that you're not accuratly representing with Smolin is arguing, I should note.)

  • Tam Hunt says:

    Rob, here's my interview with Smolin: http://www.independent.com/news/2013/apr/17/time-reborn/

    I highly recommend his book and I think you'll enjoy it.

    Too bad you're not interested in a deeper debate - I fear modern science is missing this approach far too often. But I understand you have many demands on your time, as do I (I'm a lawyer by day, avid reader by night).

    Re relativity theory, it has of course not withstood all the tests thrown at it. Dark matter and dark energy were proposed exactly relativity theory failed to explain the observed phenomena.

    Anyway, I've appreciate the dialogue and look forward to reading more of your blog posts.

  • Tam Hunt says:

    I missed a 'because' between 'exactly' and 'relativity' above.

  • rknop says:

    I don't expect to convince you, but just in case somebody else comes by and reads this thread, I want to make sure to correct it.

    Dark Matter and Dark Energy fit entirely within relativity theory; you are simply wrong to state otherwise. Indeed, the most natural form of Dark Energy is the Cosmological Constant, which Einstein introduced into GR before he learned that the Universe wasn't static, but was expanding (at which point he threw it out). However, it does fit right in. (There are more natural ways to do it now, although one could argue about whether it's more natural to put it in the source term or just see that mathematically, the theory allows for a constant of integration.)

    Again, Dark Matter fits entirely with relativity. What does not fit is the notion that the lumionus mass is all of the gravitational mass. To say that Dark Matter is a contradiction of relativity, you have to start with the position that the only matter that's out there is similar to the stuff that we're made of; at which point, yeah, there's a problem. But, if you start instead with the position that this theory that is extremely well confirmed in the lab, in the solar system, and in a wide variety of astronomical observations, is right, then you come to the conclusion that most of the mass is not made out of the same stuff that we're made out of. And, indeed, why [i]should[/i] we expect most of the mass of the Unvierse to be the same stuff as us? To expect that is to make the same mistake that humanity made before the Copernican era, which is to assume that what we see and where we are is the natural center of how the Universe is.

    So, which do you assume? You can make the anthrocentric assumption that the matter that's out there is the same as us; or, you can make the assumption that this theory that has stood up to a huge variety of other tests is still valid out in the Bullet Cluster. The latter is a much smaller scientific leap. As such, it's incorrect to say that Dark Matter is an observation inconsistent with Relativity theory. If you want to say that Relativity is so wildly wrong in the Bullet Cluster so that Dark Matter doesn't exist, then it's incumbent on you to show that you have a theory that at least has a hope of explaining all the other observations and experiments that are consistent with relativity. (Not to mention all of the other evidence for Dark Matter.)

    In any event, what you're proposing is not a deeper debate. Even I don't know enough to really have a deep debate about the frontiers of Relativity theory and where it's being tested and pushed. (I know some people who do, though.) And you certainly don't know enough. So, what we would have would be you throwing up some reference to somebody who said something, and me trying to explain why it's wrong, and you not believing me because you've got a mental block that there must be something wrong with the truth of Dark Matter just because you don't like it. This is not a deeper debate, it's just internet messageboard rambling. It would do nothing to progress science.

    The Unvierse doesn't exist in the way that we like and that seems right to us; it is what it is.