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

By Sean Carroll | May 23, 2012 10:46 am

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.

  • http://machine.devedcomputers.com David

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

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

      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.

  • Moshe

    “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).

  • http://gnomonicablog.com Fernando Curiel

    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…

  • Scott H.

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


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

  • Ian Liberman

    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.

  • http://machine.devedcomputers.com David

    @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).

  • Ben

    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.

  • AI

    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.

  • Brett

    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)

  • http://juanrga.com Juan Ramón González Álvarez

    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.

  • eric gisse

    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?

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    “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?

  • Marten van Dijk

    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.

  • Chris the Canadian


    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.

  • Low Math, Meekly Interacting

    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.

  • Chris the Canadian

    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?

  • Marten van Dijk

    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).

  • Rick

    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. :)

  • ComeOn

    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!

  • julianpenrod

    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!

  • Brett

    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.

  • Low Math, Meekly Interacting

    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.

  • http://www.globosapiens.net/rangutan Rangutan

    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.

  • http://www.globosapiens.net/rangutan Rangutan

    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

  • http://empiricalperspectives.blogspot.com/ James Goetz

    I know that the following is a tangent question and I’ll probably bring it up in forum if left answered here, but here is the question: Has science for the most part eliminated the possibility of antimatter galaxies? Also, could we know if a galaxy in the telescope were an antimatter galaxy?

  • scribbler

    The reason many of us are so dubious about dark matter is that there is no evidence that it exists. We have evidence that there is something wrong with our view of the Cosmos and how gravity functions. If one views dark matter as a “zero” or place holder to quantify that lack of knowledge, then it has a scientific value. If dark matter is used to demarcate exactly how far off our current view of gravity and the universe is off, it has a use.

    If however you seek to proffer properties and distribution of an unknown particle called dark matter, I will ask for proof, as any scientist should.

    There are no direct observations currently available…

    As for the computer models that generate the universe when certain corrections for dark matter are inputted, I asked a man who programmed one such model to replace the dark matter in the calculations with an equal amount of powdered Oreo cookies. Sure enough, his model produced the universe. Are we to then presume that dark matter is made up of powdered Oreo cookies?


  • http://juanrga.com Juan Ramón González Álvarez

    Over the last six decades, vast amounts of data clearly demonstrate discrepancies between the observed dynamics, in large astronomical systems, and the predicted dynamics when either Newtonian gravity or general relativity are applied to the directly observable distribution of mass. The appearance of these discrepancies has two possible explanations: either these systems contain large quantities of a new kind of unseen matter –the Dark Matter– or the gravitational law has to be modified at this scale.

    This dichotomy is not entirely new in the history of physics. Astronomers already attributed to a new kind of unseen matter the discrepancies between the Newtonian predictions for the motion of Mercury and its observed motion. A new planet was supposed to exist orbiting near the Sun. The confidence on the universal validity of Newtonian gravitational theory was so high that to “the people of the late 19th century, Vulcan was real. It was a planet. It had theoretical credibility and had actually been seen. Even textbooks accorded it a chapter.

    The first discovery of Vulcan was announced on 2 January 1860 during a meeting of the Académie des Sciences in Paris. Several rediscoveries and confirmations were done in posterior decades, somehow as discoveries of the hypothetical Dark Matter are announced in our days. All of us know now that Vulcan does not exist and that the motion of Mercury includes gravitational effects which cannot be accounted by Newtonian gravity alone. In a striking parallelism with the Vulcan case, the hypothetical Dark Matter has never been directly detected despite much experimental and observational effort during several decades. The situation has not changed in recent years, with Xenon10 excluding previously unexplored parameter space and Fermi finishing with another null detection of any sign of the existence of the hypothetical Dark Matter. :-)

  • http://coraifeartaigh.wordpress.com cormac

    Re “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;”
    Exactly. My understanding is that nowadays, most top theoreticians often have several groups working on mutually incompatible theories. A question of finding out which way the cookie appears to crumble, rather than prejudging the experimental evidence. Which is why I’ve never really understood Hoyle’s stance on the steady-state theory…

  • http://empiricalperspectives.blogspot.com/ James Goetz

    Oops. I look back and see that I asked a question about “antimatter” while the original post was about “dark matter.” I must have had an extraordinary tired or dyslexic moment. My apologies….

  • scribbler

    28, there is a third possibility: That the base assumptions about the Big Bang are false and need to be reformed.

    I find it more probable that Gravity and Newton and Einstein are good and usable. The “adjustment”, I think should come in the presumption of force that initiated the expansion of the universe. The presumption is that a huge release of energy cooled to form matter. If that presumption is false, then there is no set standard of how much matter there should be or a preclusion to it being arranged as it is without that addition of any invisible, undetected particle.

    Clearly our initial view of the nature of the origin of the universe is flawed. Sure, patch it up by adding a constant that keeps the works flowing but do not fail by not addressing the real issue: We need a new view…


    To be clear, the “inequity” comes from an assumption that gravity should be overcoming a certain amount of force from the Big Bang. I believe it is that presumed amount of force that is in need of revision and not all of observed physics…

  • Sunny D

    I didn’t read all these comments but here’s my rant:

    Why can’t scientists avoid mistakes like these the first time??? Making wrong assumptions about velocity around the sun is like a surgeon making assumptions on you before going to work. It just creates a nasty mess and takes away other people’s time trying to fix/find your mistake.

    Also, people are eager to turn over dark matter because it is still theoretical, it hasn’t been thoroughly discovered/detected.

  • Bob

    Its funny how astrophysicists say things like “i think it would be much cooler if something crazy was going on with gravity”, while most physicists accept that it is just dark matter. Personally, I don’t see what is so “cool” about the idea that the lagrangian of gravity should be given by some ad hoc jumble of random interactions, including Sean’s favorite 1/R theory. I think it is much more interesting that instead nature is organized into symmetries and interactions whose strength is set by the length scale of interest through the principles of effective field theory.

    In summary, physicists are content with understanding nature and its principles, symmetries, etc, which tell us that gravity looks like GR at large scales, while astrophysicists just want to see random nonsensical garbage in the sky.

  • ad

    Ah, the dark matter/MOND debate. Good times. I think people get all riled up about this because….science is practiced by human beings. Science which attempts to discover objective relationships among phenomena is practiced by human beings who have emotional judgments about what is true/beautiful. And different people have different judgments.

    So, in an attempt to be objective, let me mention some of the points that Low Math, Meekly Interacting brings up.

    1. The CMB data shows that there exists some sort of dark matter. To most cosmologists, this shows that MOND by itself is falsified. End of story.

    2. The regularity seen at the galactic scale strongly disfavors CDM. To MONDians, CDM has been falsified at the galactic scale. See http://arxiv.org/abs/astro-ph/0009074, specially the problems noted in Section 3.

    So, there is some amount of tension and disconnect here. It seems sometimes, that DMists and MONDians are speaking past each other, neither comprehending the valid point the other is making. Concerning 1., MONDians would say okay, there exists some form of HDM (e.g., sterile neutrinos) which would solve the large scale problems of MOND. CDMists would say there exists a much simpler solution. One particle, WIMPS, that solves both issues on the large and small scales.

    But MONDians would then say that CDM fails (see point 2). In order to explain what we see at the galactic scale, CDM would have to be self-interacting. But once you have that, there’s goes the explanation for the Bullet Cluster. Not only that, CDM would probably have to interact with baryons in order to explain what is seen on the galactic level, that when the baryon density is high, DM density is low. When baryon density is low, DM density is high. So then, CDMians could say there are two types of DM particles, one is HDM and the other is a self-interacting CDM in order to explain both scales. This may be the case, but then CDM’s simplicity argument is somewhat spoiled. And if CDM interacts with baryons strongly, wouldn’t we have detected them by now?

    Maybe the issues pointed out in the paper I linked have been resolved (the paper is from 2000 after all), but I haven’t been able to find anything (could be due to my incompetence).

    But this could be (to paraphrase slightly) a “Who the Hell ordered that?!” moment. The data could be telling us that there is more than one type of DM or that something like MOND+HDM is correct. Ugly facts killing beautiful theories once again.

  • Shantanu

    Sean , could you comment on how sanguine you are about SUSY/extra-dimensions etc, given the LHC results.

  • scribbler

    34, or CDM is really powdered Oreos after all!!! 😉

  • Bob

    I can’t believe there are still people working on MOND. Wow, how many times must a theory be killed before people work on something else? Are there people still working on the platonic solids theory of the solar system too? Well actually, the platonic solids theory, although garbage, is still much more beautiful than the MOND theory.

  • Obo

    I can’t believe there are still people working on CDM. Wow, how many times must a theory be killed before people work on something else? Are there people still working on the platonic solids theory of the solar system too? Well actually, the platonic solids theory, although garbage, is still much more beautiful than the CDM theory.

  • Bob

    wow Obo…only problem with your comment is that CDM has not been killed; all clean observations, such as CMB, growth of structure, lensing, rotation curves, bullet cluster, etc, clearly support it. While MOND has been falsified numerous times.

    Well perhaps, you aren’t interested in evidence. Maybe you just like writing silly comments in blogs..

  • Aeronin

    “It is the glory of God to conceal a matter; to search out a matter is the glory of kings.”
    Proverbs 25:2 NIV

  • david

    CDM will be supported by galatic rotation curves when someone proves that the DM around a galaxy must arrange itself in such a way as to make those rotation curves appear to follow MOND. If there’s no reason for the DM to arrange itself in this way, then a prediction of CDM is that we should find plenty of galaxies whose rotation curves don’t follow MOND.

  • Obo

    wow Bob…only problem with your comment is that MOND has not been killed; all clean observations, such as galaxy rotation curve shapes, surface brightness, galaxy rotation curve fits, fitted M*/L, Tully-Fisher Relation, Galaxy Disk Stability, tidal dwarfs, dwarf Spheroidals, Faber-Jackson relation, Bullet Cluster velocity (bulk & collisional), Strong Gravitational Lensing, Big bang nucleosynthesis, early structure, Background Radiation, CMB (first/second acoustic peak & early re-ionization), etc, clearly support it. While CDM has been falsified numerous times.

    Well perhaps, you aren’t interested in evidence. Maybe you just like writing silly comments in blogs..

  • Bob

    Haha Obo, its funny how most of your examples were not clean measurements at all, most of your conclusions are wrong, and the ONLY clean ones, BBN and CMB, certainly support CDM and kill MOND. If you are arguing against that, then you have no clue what you are talking about. Not even the craziest of all MOND supporters use BBN and CMB to support their claims, they recognize that these are major problems. Wow, you just have no clue what you are talking about.

  • Obo

    Haha Bob, its funny how most of your examples were not clean measurements at all, most of your conclusions are wrong, and the ONLY clean ones certainly support MOND and kill CMD… It is pretty evident you aren’t interested in evidence, only in trolling in others’ blogs.

    Wow, you just have no clue what you are talking about. MOND is alive and well! A forthcoming review titled *Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions* will be published in LRR http://relativity.livingreviews.org/Articles/upcoming.html . Click on the section Experimental Foundations of Gravitation (Subject Editor: Clifford Will) or click on the toggle-all link on the same page

  • Bob

    Well u brought up BBN, CMB, and bullet cluster, among other things, and I pointed out that all people, even MOND supporters, admit that this is a problem for their theory. But you had no response for this. So you arn’t really even a MOND supporter, probably just a laymen who doesn’t understand these observations or their consequences.
    You can copy and paste my entries all you want. Everyone who reads this can tell that when someone does that it shows they have lost the debate, and can’t defend their position. Others are just laughing at you as you do this. You are unable to form your own ideas and use reasoning, logic, and evidence to support your position when you just parrot someone else.

  • Ben

    Bob, I guess insulting all the respectable scientists interested into the MOND phenomenology is not really an appropriate way for non-trolls to comment on blogs. Not even the craziest of all pure LambdaCDM supporters would dare to say that there isnt at least a bit of an interesting phenomenology summarized by MOND, whatever the explanation for it, which is the reason people are indeed still working on it. I fully understood that you didnt understand that, so it’s up to you to make an effort or not to try to understand it, but in any case you’ll have to live with it, because Nature doesnt care about what you think. And like it or not, it chose to make galaxies look MONDian, for whatever reason, maybe just to annoy you (of course I agree that it didnt make the CMB look so, and that is actually the difference between you and me, well probably not the only difference, hehe)

  • Bob

    Ben, you missed the point entirely.
    In 42 Obo claimed, among many other things, that CMB, BBN, and bullet cluster proves MOND and kills CDM. I pointed out that this is wrong. It simply is wrong. Nature doesn’t care about people’s emotions and desires to work on random things. When the data is in, that is all that matters. CMB, etc, hurts MOND, but supports CDM. Obo claimed the opposite. There is no point in arguing this. I am surprised that I am the only one here pointing out this error, while you and others are wholeheartedly supporting this false claim.

    This isn’t religion, where you can just make up anything, this is science. Learn what it is before you speak next time.

  • Ben

    Bob, I was referring to your fist intrusion into the debate: “I can’t believe there are still people working on MOND.” …. Concerning the rest, what did I write at the end of my comment? Oh, just that the CMB is not MONDian… Actually I can add that it supports the existence of some form of DM. And then? Why are people working on MOND? Well, because galaxies look MONDian. Why? Nobody knows, apart perhaps you? Since you look so smart, you should immediately submit to Nature your explanation of why this is so. I look forward to reading this exciting work. Best regards.

  • Bob

    Ben, the rotation curves of galaxies is due to this thing called dark matter. Perhaps you’ve heard of it.

  • Obo

    Bob, neither BBN/CMB nor the Bullet cluster killed MOND.

    MOND can explain lots of phenomena that cannot be explained by CDM. You were given many examples, still you decided to ignore all of them. As stated above, MOND can explain some aspects of the Bullet cluster where CDM gives the wrong result, and vice verse. This is all published in the literature. Still you are among those who believe that the Bullet cluster confirmed CDM and killed MOND, which is just untrue (and unfair).

    Even if you are not aware of the specialised literature (evidently you are not), there are known press releases by MOND experts explaining why the Bullet cluster did not kill MOND.

    Moreover, you have been given a link to a forthcoming review to be published in the experimental section of the journal Living Review of Relativity (the section is head by well-respected general relativist C. Will). That review summarizes all the phenomenology and predictions that support MOND.

    You may believe that the recognized general relativist is “just a laymen who doesn’t understand these observations or their consequences” and you can insult to Ben as well, but you are the only here who do not understand…

    You can continue ignoring the facts and you can continue your childish attacks but MOND will continue to work because Nature do not care about what you post on blogs.

  • Brett
  • Ben

    @Bob: perhaps, but the question is why it conspires with baryons to conform to the MOND law in galaxies. If you dont get that basic point, I cannot do anything more for you. I’m trying not to be emotional here, just point out the basic facts. That’s why, when an obvious mistake is made, as in the case of the Moni Bidin et al. result, I have no problem at all pointing it out (I was actually the first to point it out here, in the comment linked to by Sean hereabove). But when looking at galaxy data, it’s also a fact that they look MONDian. I have no preconceived conception about why this is so, I just think it is an interesting scientific question to address.

  • Bob

    @Ben, Simulations starting from the primordial universe, seem to give a good description of galaxies with CDM. That is a great success of CDM and is further strong evidence that it is right.

    You seem so fascinated by saying galaxies “look MONDian”. First of all, what does it mean to look MONDian? MOND was constructed to give the galaxy rotation curve. So of course it looks MONDian by construction. If the galaxy rotation curves had been different, MOND would have just chosen the corresponding alteration of Newton’s law to get it. The only way it can be interesting is if it simultaneously describes completely different things, on a totally different length scale, namely cosmology. And CMB, large scale structure, etc, kill it. Or if it could describe completely different physical systems, such as bullet cluster, and MOND fails again. So it works for galaxies by construction, so that is just circular. And essentially fails in places where it can be predictive.

  • Ben

    Bob, I’m sorry but you’re wrong. The challenges for CDM on galaxy scales are so numerous that it wouldnt be possible to list them all here. It is also a common misconception to say that MOND was constructed to fit rotation curves. It was indeed constructed to give flat RCs, but it actually also predicted the shape of RCs in all galaxies prior to the discovery and measurement of RCs of plenty of different types, such as low surface brightness ones, or tidal dwarf galaxies. These are true headaches for CDM and prefectly fit by MOND. But your misconception is a common one, one of people who are mostly interested in the large scale structure where it is indeed legitimate to say that MOND fails, and who are relatively ignorant of galactic dynamics, where it is absolutely illegitimate to say that MOND fits “by construction” (and even more illegitimate to say that CDM explains everything). In the DM language, what we now need to understand is the anti-correlation between the surface density of baryons and that of DM, naturally predicted by MOND (in MOND it is the “dark matter effect”, but phenomenologically, that is the same thing), with a transition baryonic surface density of the order of 1.5 kg/m^2, above which no effective dark matter effect is present, and below which it appears, in exactly the right proportion as to give the asymptotic velocity predicted by MOND. What keeps MOND alive as a possibly interesting paradigm is that it is up to now the only paradigm explaining this anti-correlation naturally, until we can explain that from feedback and interactions between DM and baryons. The problem is that this same anti-correlation holds in tidal dwarf galaxies, where CDM cannot be present and where the mass discrepancy is supposed to be due to baryonic DM, in the form of e.g. molecular H2. In MOND, both DM-effects would have the same cause (the boosted gravity), but in CDM, two different types of DM in different objects would have to interact with visible baryons to lead to the same unexplained anti-correlation. This actually looks *very* difficult to explain. Of course, there are not that many RCs of tidal dwarf galaxies available, so it would be extremely interesting to have more. Either they still conform to MOND and they become the biggest possible headache for CDM, or we find some that dont conform to MOND, and they kill MOND once and for all, because it would then fail on the scale where it works. On the other scales, you are totally right that it doesnt work. I’m not at all denying the evidence against MOND on galaxy cluster scales or at the CMB level, quite the contrary. But, assuming the MOND paradigm to have some validity on galaxy scales (which it at least has as a phenomebnological law, but not necessarily as modified gravity), this would imply either the presence of additional non-baryonic dark matter that wouldnt condense on galaxy scales (not very elegant, but not ruled out either by clusters or the CMB), or the fact that the new fields involved into the MOND phenomenology on galaxy scales stop interacting with baryons in the way they do there, and start behaving like normal CDM on these scales. Of course, you will probably then say something like “Occam’s razor: I prefer the more mundane explanation that there is plain simple CDM everywhere, and that this will explain the galaxy phenomenology one way or the other once we simulate the baryonic physics better, improve our resolution, etc.”. Perhaps, but all you have to realize is that this will not be easy, especially if you have to explain all “normal” galaxies as well as tidal dwarfs at the same time. There Occam’s razor’s odds are not too promising for CDM. But I would be happy if that’s the solution in the end. Let’s just say that, even in that case, there will always be some credit to Milgrom for having predicted this galactic phenomenology, even if it is just to summarize how DM behaves on these scales. Sorry for having been a bit long but this is an important and serious topic, not one that deserves anathemas or violent unfounded emotional reactions. Facts, just facts, and also knowing all aspects of the problems, are necessary to judge. I’m indeed “fascinated” by the fact that galaxies are MONDian, while you seem to imply that this is trivial. If it were, believe me, no serious scientist would work on MOND anymore indeed, no editor of serious journals would accept such publications anymore, and no big review journals would invite people to write reviews on the topic…

  • Eric Habegger

    @ Ben
    ” Facts, just facts, and also knowing all aspects of the problems, are necessary to judge. I’m indeed “fascinated” by the fact that galaxies are MONDian, while you seem to imply that this is trivial. If it were, believe me, no serious scientist would work on MOND anymore indeed, no editor of serious journals would accept such publications anymore, and no big review journals would invite people to write reviews on the topic…”

    You seem to be ignoring the fact that scientists doing this work are all still human. Ideas, just like objects have inertia and people, even scientists, have been known to hold onto outmoded ideas long after they have they should have relinquished them. This is not a statement about the logicality of MOND. It is a statement about your argument above because science doesn’t live in a perfect world. You can never say that something must be good science otherwise people would not be working on it. People do that all the time and it would seem to me it should be obvious to everyone that is true.

  • Ben

    @Eric: There seems to be a confusion in your mind between “being right” and being “good science”. Surely, you can never say that something must be right because otherwise people would not be working on it. Surely, scientists can also do bad science. But, most importantly, good science can also be done with ideas that will turn out to be wrong in the end! What makes them good science is the fact that they are mathematically coherent, that there is a body of empirical evidence apparently supporting them, that they can be predictive in a given range of validity, and that there is no current empirical evidence falsifying them in that range of validity. But once they are proven wrong by empirical evidence, they slowly but surely loose momentum. Not the case of MOND, at least not yet (compare it to, let’s say, the static Universe idea). The point is that MOND still works on galaxy scales and that it is actually *predictive* there, where CDM is currently not. It’s a common thing in physics to have laws working only within a given range of validity. That does not make them “wrong” per se: it would seem to me that this point should be obvious to everyone. To come back to the sociology, I’m simply saying that there is a scientific reason, one with a purely *empiricist* motivation, for the gain of momentum of MOND in the community: it’s not just something appearing out of twisted human minds desperately wanting a wrong idea to become right. If the rich and intriguing galactic phenomenology predicted by MOND would be explained away with normal CDM in the future, and if a class of galaxies not obeying MOND were found, a large number of these scientists would simply turn away from MOND. It would actually be insulting to them to insinuate otherwise. Of course, a few would surely hold onto the idea, but it wouldnt have the momentum it still has now as an underdog to the main CDM paradigm: it would join the static Universe idea in the club of really outdated ideas. By the way, speaking about good science which can be done with wrong ideas, it could be the case of LambdaCDM too: it is perfectly conceivable that good science that will turn out to be wrong in the end is currently made by the vast majority of the community. That is actually not a problem, as long as there are good scientific reasons to work on it.

  • Ben

    To end up this whole discussion, I would recommend to read the following short and wise review by Jim Peebles, notably all that is said about MOND there, both negative and positive, or the wise concluding remarks: http://arxiv.org/abs/1203.6334

  • scribbler

    56 says: “What makes them good science is the fact that they are mathematically coherent, that there is a body of empirical evidence apparently supporting them, that they can be predictive in a given range of validity, and that there is no current empirical evidence falsifying them in that range of validity.”

    That is why I would much rather that idea be referred to as “The dark coefficient” or something like it rather than “Cold Dark Matter”. 😉

    Again, as a means to realign our failing understanding of the Universe, the “amount” of matter needed to make things jive again is all that you have listed above. However, the SPECULATION about an invisible particle and its properties is basically useless, scientifically speaking.

  • Everett

    Does DM play any role(s) with the lines of force from the poles of a magnet? Does it have an effect or is it affected in any way?

    Also, is DM consummed by black holes?

    I don’t have any degree here, so say it in a simple mans’ terms. Just curious….

  • Johannes from Philippines

    If Dark Matter exist as particles and they interact through gravity, we must be able to observe a body of Dark Matter clumped together because of gravitational attraction while keeping in mind that they do not interact physically as observed in colliding galaxies. Also, our beliefs or preferences cannot be considered since this is science.

    with blackholes, we use a similar argument in particle physics yet we believe those particles are there.

    in my opinion, the things that we study are still “unknown to us”. We use mathematics to understand these things and to describe them, we actually do not know how an electron looks like, but we use mathematical ideas to easily describe these theories.

  • Marshall Eubanks

    Ben @8 – Yes indeed. The original results were inconvenient for CDM, but pretty devastating for MOND/TeVeS. These new results, by restoring the status quo ante, put the MOND ship back upright.

    We are entering an era where direct tests of TeVeS will be possible, and in a decade or two should know whether these ideas truly reflect new physics.

  • http://www.geocities.jp/imyfujita/galaxy/galaxy01.html Iori Fujita

    Dark matter can do anything you want. Because it is dark, you don’t know anything about it. It is like a ghost.


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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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