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.
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?
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?
😉
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. 🙂
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…
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….
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…
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.
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.
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.
Sean , could you comment on how sanguine you are about SUSY/extra-dimensions etc, given the LHC results.
Thanks
34, or CDM is really powdered Oreos after all!!! 😉
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.
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.
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..
“It is the glory of God to conceal a matter; to search out a matter is the glory of kings.”
Proverbs 25:2 NIV
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.
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..
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.
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
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.
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)
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.
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.
Ben, the rotation curves of galaxies is due to this thing called dark matter. Perhaps you’ve heard of it.
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.