Dark Matter: Still Existing (One in a Continuing Series)

Last month we mentioned a paper on the arxiv that made a provocative claim: evidence from the dynamics of stars above the galactic disk indicates that there is essentially no dark matter in the vicinity of the Sun. I am not an expert on galactic dynamics, but nevertheless I and others were immediately skeptical, especially since there is overwhelming evidence for the existence of dark matter from other measurements. Skeptics, of course, happily piled on. But this isn’t an area where one opinion or the other matters very much — better data and better analysis is what matters.

Now we have a better analysis, from people who are experts: Jo Bovy and Scott Tremaine have a paper in which they examine the claim closely. They find it wanting. This was pointed out here in a comment by Ben; Jester and Peter Coles also have useful blog posts up about it.

Short version: the original authors made assumptions about the distribution of velocities of the stars they were looking at, and those assumptions are known to be wrong. Using a better model (i.e., one more compatible with known data), Bovy and Tremaine show that the observations are perfectly consistent with the conventionally-assumed dark matter density. The good news is that they are actually able to use this technique to get a more precise measurement of that density than was previously available. It’s a rare scientific lemon that can’t be turned into at least a little bit of lemonade.

I’m not sure why people get so emotional about dark matter. The original paper here by Bidin et al. was accompanied by a dramatic press release from the European Southern Observatory. I am known as a “dark matter supporter,” but I have no personal investment; I think it would be much cooler if something crazy were going on with gravity. But that’s not what the data indicate. It’s just some new particle we haven’t yet made in the lab, hardly the end of the world.

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62 Responses to Dark Matter: Still Existing (One in a Continuing Series)

  1. David says:

    If people didn’t care, then they wouldn’t study the stuff!

  2. Sean Carroll says:

    You should care about finding out what the answer is. You shouldn’t care too much about which answer is right, or you end up making mistakes.

  3. Moshe says:

    “I’m not sure why people get so emotional about dark matter…”

    I think it’s the word. More generally, it is often presented as something exotic and earth shaking, something the likes of which we’ve never seen before, an extraordinary claim that requires extraordinary evidence. Whereas in reality it is actually quite conservative (in fact, the converse — if all matter coupled to photons — would have been a surprising fact in need of an explanation). I haven’t seen that point emphasized much, that in the grand scheme of things dark matter is likely to be an interesting detail, but ultimately nothing that requires us to dramatically change our conception on how things work on a fundamental level (as opposed to, say, dark energy).

  4. This is an exciting time! And probably one that will see a paradigm change. Either Gravity is wrong at great distances or the standard model is really missing something important. Not only we are not finding a particle in the lab but also we kind of know that whatever dark matter is, it cannot be something we have “seen”. Hope to live to see it…

  5. Scott H. says:

    Another recent good one by Jo Bovy, offering an explanation of where the “missing” satellites went:

    http://arxiv.org/abs/1205.2083

    He appears to be on a bit of a tear with good papers recently.

  6. Ian Liberman says:

    The Institute for Advanced Study in Princeton, NJ, are criticizing the European`s methodology in indicating no dark matter and their corrections suggest a substantial amount around the sun.This is the Bovy and Tremaine study,mentioned in the CV article, which actually illustrates a 20 percent increase dark matter around the sun ,over what was previously thought before the ESO press release. Sean your excellent article in CV puts the whole controversy in perspective.

  7. David says:

    @Sean Carroll Indeed! I was speaking more to the scientists such as yourself and others who continue to study and try to poke holes in the theory, not the ones who just flat-out deny it because of a gut feeling. Being a computer engineer, I couldn’t count the number of times I thought something didn’t feel right but it indeed was, due to the fact that the behavior electrons is generally not intuitive. The same applies for Dark Matter (and other subjects which people choose to blindly deny).

  8. Ben says:

    One shouldnt be confused about the implications of this result. The result of Moni Bidin et al. was much more devastating to MOND-inspired modified gravity theories than to the existence of DM particles: the latter, we could have imagined to be less dense in the solar neighbourhood, while in modified gravity the DM-like effect had to be present. Let’s not get too emotional indeed.

  9. AI says:

    Sean: “You should care about finding out what the answer is. You shouldn’t care too much about which answer is right, or you end up making mistakes.”

    Actually it’s very good that people care which answer is right.

    If you have a group of scientists proposing various ideas it’s much better if each idea has strong proponents and opponents who are passionate about them to the point of being biased then if everyone is lukewarm about all of them. Sure it leads to much drama and fighting but the arguments that ensue are the best way to test said ideas.

  10. Brett says:

    I agree with Fernando at #4; whatever it is, it’s an exciting time to be alive. That’s enough for me. But to entertain myself; I still hope that it is a modification of gravity because I’m personally banking on it. I really do find it curious that the formation of a galaxy is directly related to the size of its’ black hole, something we are a very small part of. It would seem as though the way in which we experience gravity is only a glimpse at gravity as a whole. Not necessarily MOND, but a modification of some sort. It’s exciting either way.

    It’s so true what Sean says though; the dumbest people I’ve ever met in my life have been obsessed with being right as a result of some sort of delusional ego problem. The smartest people I’ve ever met are always open to being incorrect or having someone point out something they may have missed; and as a result, they usually have all the answers before those answers are required. Reminds me of the greatest contributors to physics.

    Didn’t Bovy and Tremain also discover that there’s actually 20% more dark matter than previously thought? (in that same paper)

  11. This is what really fascinates me about the DM paradigm. It is permanently maintained in a safe state. When the original preprint appeared. A DM supporter wrote in this same blog:

    But it might mean that the distribution in the Milky Way was very different from the kinds of models we like to use, for example by being much lumpier.

    Now that it seems that the original preprint was in mistake, the above ‘explanation’ is ignored. If the work was confirmed, someone would submit a preprint appealing to some hypothesis for explaining the discrepancy.

    It does not matter if your favourite DM model predicts A or B, it can be always modified/adapted/changed once the data (A or B) is known. Of course I am not the first who notices the perennial safe state of the DM paradigm.

  12. eric gisse says:

    Juan, it warms my heart to see that you still take great offense at how science progresses.

    Out of curiosity, do you still maintain GR has no Newtonian limit or did you finally realize how silly you were being?

  13. “And probably one that will see a paradigm change. Either Gravity is wrong at great distances or the standard model is really missing something important.”

    Where’s the paradigm change? First, the assumption that everything must shine is bizarre; dark matter as such shouldn’t be a surprise. One day we will discover something we didn’t know before. Interesting, but no paradigm change. Did European zoology have a paradigm change when it learned about gorillas?

  14. Marten van Dijk says:

    Dark Matter does not exist, Scarlet Matter does.

    On Scarlet Matter:

    They seek it here, they seek it there,
    those physicists seek it everywhere.
    Is it in heaven?, is it in hell?,
    that demmed elusive particle.

  15. Chris the Canadian says:

    Again,

    Give me all the ‘papers’ and mathematical equations you want … where is it? The stuff itself? If there is such vast quantities of it out there why can’t we even find a single molecule of it? I may be ignorant of the math, but my biggest problem is that this theory of dark matter is being presented as a truth without any physical evidence of it’s existence. It’s like a theologin trying to prove the existence of a bible story. They believe the story happened so they can turn the evidence to ‘prove’ the existence of the story.

    I refuse to believe dark matter exists because I don’t see any tangible real proof of its existence, just a bunch of theories and equations ands papers from people telling us to ‘trust them, it’s there.’ Sounds a bit religious to me.

  16. Low Math, Meekly Interacting says:

    I think maintaining a tentative stance is entirely justified in the absence of direct observation, but there’s nothing religious at all about the dark matter hypothesis. It’s not without problems, but it’s almost certainly the LEAST contrived explanation for what is actually observed, and it has a better record of fitting and predicting cosmic observations than any alternative thus far. Presently there is no way whatsoever to use GR and arrive at what we see without some other component making up the bulk of matter or modifying gravity. Unless one is comfortable with the wild inconsistencies believing only in baryonic matter requires (which would make you quite unreasonable, even for a positivist), I don’t understand the hostility.

  17. Chris the Canadian says:

    It’s not hostility. I’m merely saying that we are being asked to take it on ‘faith’ that dark matter exists, when there isn’t any actual tangible evidence of its existence. Could the original theory be flawed and the math and science we are using to try and explain the original theory is also flawed? Is that not a possibility?

    I’m not a religious zealot and I believe in the sciences. The existence of dark matter and dark energy are theories. That is all they are. I find that science websites and papers on the topic act as though they are facts and proven to exist when they actually haven’t. That is why I get distraught when I read an article on a website like Discover about dark matter really existing. We still don’t know, so why can’t everyone admit that they still don’t know instead of claiming the truth one way or the other?

  18. Marten van Dijk says:

    On the one hand I read that data implicate that it’s just some particle that we haven’t yet made in the lab (hardly the end of the world) and on the other hand one shouldn’t care too much about whether the answer is right.

    It is just maybe yes, maybe no, maybe rain, maybe snow, if you ask me (I am not hostile , I am a decent guy who doesn’t visit brothels and tea parties).

  19. Rick says:

    Chris the Canadian,

    Agreed. I refuse to believe in black holes because I don’t see any tangible proof of their existence, just a bunch of theories and equations and papers from people telling us to trust them, they’re there. I may be ignorant of the math, but my biggest problem is that the theory of black holes is being presented as a truth without any physical evidence of their existence. Just as with dark matter, black holes are invisible and all you have are some theories like quantum mechanics and general relativity, and some actual observations that are purportedly explained by their existence, but that’s all. You’ll have to do a lot better than that before I take either of these crackpot ideas seriously. 🙂

  20. ComeOn says:

    Yes, I am in full agreement with Rick(19) and the Canadian (17). Furthermore, since I have never seen

    1) quarks
    2) evolution
    3) the pope
    4) my great grandfather

    I see no reason to believe any of these things exist, or have ever existed. All I’ve seen are a bunch of actual observations that are purportedly explained by their existence, but that’s all. You’ll have to do a lot better than that before I take any of these crackpot ideas seriously!

  21. julianpenrod says:

    The fact is that many if not most who stand by the claims of dark matter don’t seem to even have read the paper claiming it exists! Leave aside such things as that they use the term “rotate” to mean motion of stars around the galactic center, as opposed to using “revolve”. You can also put aside the evident assumption that all stars move circularly around the galactic center, not elliptically. The fundamental “problem” is that the conclusions being attacked supposedly assumed stars above and below the galactic disk have the same velocity irrespective of distance from the galactic core, whereas the “corrected” paper says only stars in the disk have that quality. But, if the galaxy is in a massive halo of dark matter, even stars above and below the main disk should revolve by this augmented gravity! So they should move the same as the stars in the disk!

  22. Brett says:

    I partially agree with the Canadian. It’s not just that we can’t observe it as with black holes, or else it wouldn’t be such a good argument to read. It’s that the possibility of Dark Matter being a new particle or a modification of General Relativity are both pretty equal ideas because they have equal evidence supporting them. It would be great if it was the formation of densely packed WIMPs because that’s something easier to prove. If it’s a modification of gravity…how the hell do you prove that? There are less popular ideas though. It could be a result of the delay in observation because of the speed of light; which could support the idea of extra dimensions causing distortion in our ability to observe at such distances. I don’t think that’s likely, but given the amount of evidence available, it’s equally as valid. There’s really no point in people getting so angry about something that has very little evidence, aside from the entertainment value. Is Dark Matter the boundary of a/the Higgs field on a galactic scale? Sounds smart, but it’s meaningless pseudoscience.

    In the next of this series, I would enjoy having the following sentence dissected.

    “It’s just some new particle we haven’t yet made in the lab, hardly the end of the world.”

    Enlighten me about the evidence for it being a new particle. Feel free to enlighten others as well.

  23. Low Math, Meekly Interacting says:

    I’m not the expert here, but the evidence for DM being composed of one or more new particles is certainly circumstantial. That said, it’s extremely good circumstantial evidence, in that predictions based on the hypothesis that DM is a cold WIMP fit the preponderance of observations extremely well. When it comes to the current best observations of the CMB, the agreement with observation is rather spectacular. While there are troubling puzzles still to be solved (like the Abell 520 cluster), shooting holes in, say, TeVeS with data like the latest Sloan Digital Sky Survey results seems almost trivial by comparison. I’m not 100% convinced yet either, but I am really mystified by the hostility some people seem to harbor against the notion of dark matter. Like there’s another idea that’s even remotely as successful? Like it’s even all that weird? I mean, we already have neutrinos, which are essentially very lightweight WIMPs. There’s no shortage of possibilities with decent plausibility, be it the LSP, axions, massive right-handed neutrinos, some combination of those. What’s the problem? You’ve got a testable hypothesis, considerable observational agreement and new data coming in all the time, candidate particles arising naturally in extensions of the SM (some variety of which almost no one thinks isn’t necessary for a great number of reasons). Seems like marvelous, if frustratingly difficult, scientific progress to me.

  24. Rangutan says:

    WTF! Is is obvious that ANY kind of matter near our Sun is well snapped up into our gravitational field or absorbed (combined) with comets (our vacuum cleaners) in the solar system.

  25. Rangutan says:

    WTF! Is is obvious that ANY kind of matter near our Sun is well snapped up into our gravitational field or absorbed (combined) with comets (our vacuum cleaners) in the solar system.

    Anyone confused as to what dark matter is? It is simply matter that does not radiate any, or much, energy because it is not massive enough (possibly only dust and gas) to generate heat or any other radiation and it is too far from hot bodies to reflect very much. RRG2009