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://www.youtube.com/watch?v=5cIBD22RhUE
@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.
@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.
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…
@ 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.
@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.
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
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.
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….
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.
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.
Dark matter can do anything you want. Because it is dark, you don’t know anything about it. It is like a ghost.
http://www.geocities.jp/imyfujita/galaxy/galaxy01.html