Astrophysical ambulance-chasers everywhere got a bit excited this week, and why wouldn’t they? Here are some of the headlines we read:
- Findings Raise New Questions About Dark Matter (redOrbit)
- Dark matter theory challenged by gassy galaxies result (BBC)
- More Evidence Against Dark Matter? (Science NOW)
Wow. More evidence against dark matter? I didn’t know about the original evidence.
Sadly (and I mean that — see below) there is no evidence against dark matter here. These items were sparked by a paper and a press release from Maryland astronomer Stacy McGaugh, with the rather more modest titles “A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies” and “Gas rich galaxies confirm prediction of modified gravity theory,” respectively.
I’m the first person to defend journalists against unfair attacks, and we all know that headlines are usually not written by the people who write the actual articles. But we can legitimately point fingers at a flawed system at work here: these articles are a tiny but very clear example of what is wrong wrong wrong about our current model for informing the public about science.
McGaugh’s new paper doesn’t give any evidence at all against dark matter. What it does is to claim that an alternative theory — MOND, which replaces dark matter with a modification of Newtonian dynamics — provides a good fit to a certain class of gas-rich galaxies. That’s an interesting result! Just not the result the headlines would have you believe.
It’s obvious what happens here. Nobody would read an article entitled “Gas rich galaxies confirm prediction of modified gravity theory” — or at least, most editors doubtless feel, fewer people would be interested in that than in evidence that went directly against dark matter. So let’s just spice up the story a bit by highlighting the most dramatic possible conclusion we can imagine drawing, and burying the caveats until the end. Net result: a few more people read the articles than otherwise would have, while many more people just read the headlines and are left with less understanding of modern cosmology than they started with. Scientists and journalists together have a responsibility to do a better job than this at making things clear, not just making things sound exciting.
But let me take this opportunity to lay out the problems with MOND. It’s a very clever idea, to start. In galaxies, dark matter seems to become important only when the force of gravity is not very strong. So maybe Newton’s famous inverse-square law, which tells us how the force of gravity falls off as a function of distance, needs to be modified when gravity is very weak. Miraculously, this simple idea does a really good job at accounting for the dynamics of galaxies, including — as this new result confirms — types of galaxies that weren’t yet observed back in 1983 when Mordehai Milgrom proposed the idea. Whether or not MOND is “true” as a replacement for dark matter, its phenomenological success at accounting for features of galaxies needs to be explained by whatever theory is true.
Which is an important point, because MOND is not true. That’s not an absolute statement; among its other shortcomings, MOND is not completely well-defined, so there’s a surprising amount of wriggle room available in fitting a variety of different observations. But to the vast majority of cosmologists, we have long since passed the point where MOND should be given up as a fundamental replacement for dark matter — it was a good idea that didn’t work. It happens sometimes. That’s not to say that gravity isn’t somehow modified in cosmology — you can always have very subtle effects that have yet to be discovered, and that’s a possibility well worth considering. But dark matter is real; any modification is on top of it, not instead of it.
Let’s look at the record:
- MOND is ugly. Actually, that’s very generous. More accurately, MOND is not a theory; it’s only a phenomenological rule that’s supposed to apply in a limited regime. The question is, what is the more general theory? Jacob Bekenstein, in an heroic bit of theorizing, came up with his Tensor-Vector-Scalar (TeVeS) theory, which hopefully reduces to MOND in the appropriate limits. Here is the action for general relativity:
And here is the action for TeVeS:
Don’t worry about what it all means; the point is that the theory underlying MOND isn’t really simple at all, it’s an ungodly concatenation of random fields interacting in highly-specific but seemingly arbitrary ways. That doesn’t mean it’s not true, but the theory certainly doesn’t win any points for elegance. - MOND doesn’t fit clusters. Long ago, rotation curves of galaxies were the strongest evidence in favor of dark matter. Very long ago. We know better now, and a mature theory has a lot more hoops it needs to jump through. The nice thing about MOND is that, despite the ugliness above, when you get down to making predictions for large astrophysical objects, there really isn’t any wriggle room: you fit the data or you don’t. It works for galaxies, but when it comes to clusters — you don’t. Not close. Proponents of MOND understand this, of course, and they’ve come up with a clever workaround. It’s called “dark matter.” That’s right — even MOND’s biggest supporters admit that you need dark matter to explain galaxies. Let’s just emphasize that for those who find all this text kind of tedious:
Even with MOND, you still need dark matter.
Some people try to claim that the necessary dark matter could be neutrinos rather than some brand-new particle, and that’s supposed to be morally superior somehow. But there’s no two ways around the conclusion that dark matter is real.
- MOND doesn’t even fit all galaxies. For almost twenty years now we’ve known that MOND fails for a certain type of galaxies known as “dwarf spheroidals.” These are small (thus the name) and hard to observe, so MONDians have come up with various schemes to explain away particular galaxies. That might even be okay — nobody said fitting the data would always be easy, even in the correct theory — except that it’s precisely this kind of extra work that is being scoffed at in the case of dark matter in these recent news items.
- Gravity doesn’t always point in the direction of where the ordinary matter is. This is the lesson of the famous Bullet Cluster (and related observations). The evidence from gravitational lensing is absolutely unambiguous: to fit the data, you need to do better than just modifying the strength of Newtonian gravity. Once again, people try to wriggle out of this in TeVeS and other MONDian approaches. However, the way they do it is by imagining that other fields have energy, which warps spacetime, and therefore a gravitational field. We have a useful phrase to describe new fields whose energy warps spacetime: “dark matter.” MOND-like theories don’t replace dark matter so much as they make it much more complicated.
- MOND doesn’t fit the cosmic microwave background. Saving my favorite for last. One of the coolest things about the temperature anisotropies in the cosmic microwave background is that they are sensitive to the existence of dark matter. In the early universe, dark matter just collapses under the pull of gravity, while ordinary matter also feels pressure, and therefore oscillates. As a result, the two components are out of phase in the even-numbered peaks in the CMB spectrum. In English: dark matter pushes up the first and third peak in the graph below, while suppressing the second and fourth peak. That would be extremely hard to mimic in a theory without dark matter; indeed, this was predicted before the third peak was precisely measured. But now it has been. And…
See that dotted line? That’s the theory with dark matter, fitting all the data. See the solid line? That’s the MOND (really TeVeS) prediction, definitively inconsistent with the data. Can some clever theorist tweak things so that there’s a MOND version that actually fits? Probably. Or we could just accept what the data are telling us.
Having said all that, I’m glad that some people are still thinking about MOND-like approaches. You can still learn interesting things about galaxies, even if you’re not discovering a new law of nature. And dark matter, to be honest, isn’t established with 100% certainty; it’s really more like 99.9% certainty, and you never know.
What’s less admirable is people (mostly outside the professional community, but not all) hanging onto a theory because they want to believe it, no matter what new information comes along. Personally, I think it would be much cooler if gravity were modified, compared to the idea that it’s just some dumb new particle out there. I’ve put some thought into the prospect myself, which helped lead to some productive research ideas. But ultimately the universe doesn’t care what I prefer. Dark matter is real — gravity could also be modified, but there’s no reasonable doubt about the dark matter. So let’s try to figure it out.
Shantanu, your first link is broken.
Bud,
Dark matter is no different than ordinary matter, except that it is non-baryonic and doesn’t interact with light. There is absolutely nothing about it which is in conflict with GR or even basic physics. A dark matter particle can annihilate with it’s anti-particle to produce radiation as ordinary matter does.
Quick question: I think I recall seeing somewhere that TeVeS amounts to another dark matter theory, and a poor one at that. I.e., these fields have quanta, and they must behave somewhat like DM (little or no interaction with normal matter except pertaining to gravitation) or be in even worse trouble than they are. I.e. all you get is extra Dark Stuff that makes some curves fit better while failing to fit even more. Is this correct? If so, TeVeS seems rather too costly to justify, i.e., it violates Occam’s Razor rather blatantly even before failing certain observational tests (which it does), and so is highly unlikely to prove to be a fruitful area of research.
Wikipedia: “As important as dark matter is believed to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive.”
Eric #27: “Dark matter is no different than ordinary matter, except that it is non-baryonic and doesn’t interact with light. There is absolutely nothing about it which is in conflict with GR or even basic physics. A dark matter particle can annihilate with it’s anti-particle to produce radiation as ordinary matter does.”
Eric, you seem to know quite a lot about a phenomenon whose nature eludes concrete understanding. This appears to be a common tendency in public discussion. Why do people continually speak as if their hypotheses are known fact? Even the author of the Wikipedia article cannot bring himself (or herself) to state clearly that there is no understanding of the “dark matter” phenomenon.
Reading Wikipedia, it appears that modern cosmology now accepts cosmic inflation as a fact. The measurement uncertainty principle provides theory with room to hide natural events. Talk about a shell game! Meanwhile ignoring topological defects.
David,
Well, we do know that neutrinos make up some fraction of the dark matter, and I believe that these particles are non-baryonic and do not interact with light. We know that dark matter doesn’t interact with light because otherwise it would be possible to see it. We know that it is non-baryonic from Big Bang Nucleosynthesis and from WMAP. Thus, any non-baryonic matter that doesn’t interact with light is by definition dark matter. The real question is, what type of particle makes up the bulk of the dark matter? Of course, it is not impossible that topological defects such as cosmic strings could make up a portion of the dark matter, however I believe this is possibility has been strongly constrained.
I meant figures 2 & 4 of
http://arxiv.org/abs/0806.2585
#30 Eric — We don’t know that the observed phenomenon is due to matter particles at all. All that is “known” is what is observed, and what is observed seems to be anomalous motion of what is observed! Big Bang nucleosynthesis is not “known” — it is postulated to explain observations. The Big Bang is not “known” — it is postulated to explain observations, and “constrained” so that it agrees with the observations. What is “known” is what is observed. There does not appear to be any model that explains all that is observed. So there is no theory here, only a number of hypotheses, which as far as I can figure out are used to “justify” each other. These stem from Friedmann; the basic assumption is that all the energy we now observe was at one time in a point of infinite density and virtually infinitely hot. In order to let the universe out of that GR straitjacket, the measurement uncertainty principle is invoked. Quantum mechanics transfers measurement uncertainty onto nature itself. I believe that is a fundamental mistake.
I know zero about string theory, the topological defects I referred to are those which ought to occur after inflation, when the forces “freeze out” during symmetry breaking. There is no reason why the electromagnetic interaction for example should have the same strength throughout the universe when various regions are not in causal contact. This is not me, this is Penrose (as far as I understand what he says). But it makes sense. But it appears to be ignored.
So I do not believe you can state as a species of fact that there are particles of matter which you describe as “dark matter”. Promoting the assumption of “fact” or “known entity” is not right.
David,
It was never stated that dark matter is a fact, only that it is a theory which is accepted by most cosmologist as the most likely explanation for the anomalous motion of galaxies. I should also state that there is other evidence for the existence of dark matter apart from the motion of galaxies. Some of this evidence comes from microlensing and some of it comes from the acoustic oscillations in the cosmic microwave background. It is true that the precise properties of the dark matter are not presently know, however there are many good candidates.
As for your statement about the “GR straightjacket”, I simply do not understand what you are talking about. Also, the cosmic strings I referred to have nothing to do with string theory. They are in fact examples of the topological defects that you mentioned and have been extensively studied.
#33 Eric — The “GR straitjacket” is described this way by Wikipedia: “According to general relativity, the initial state of the universe, at the beginning of the Big Bang, was a singularity.” How did the universe escape from its infinitely curved spacetime? The modern assumption is that the first moments of the universe are smeared out by the measurement uncertainty principle. Given a head start, Friedmann takes over. He gave a solution to the Einstein field equations (I only read this; I don’t know how) in the form of “metric expansion”: the universal “space” expands. Can you explain how “space” expands? The whole of modern physics seems to assume that the space-time continuum and the energy in the space-time continuum are two separate entities. But according to Einstein, spacetime is merely a structural quality of the gravitational field. So there is an ambiguity: spacetime and energy are somehow integrally connected in the “spacetime-energy complex”, which can be expressed as “space tells matter how to move, and matter tells space how to curve”. And yet, according to Friedmann, space has this independent ability to expand. It doesn’t make any sense. A more likely (and simpler) initial condition would be expanding space. But that would require rewriting the past hundred years of “theory”. Not much profit for institutionalized Science there. But I have sneaking suspicion it will have to be done eventually.
If you can explain away the ambiguity, please do. I have not come across any sensible explanation for this yet.
What we got here is a failure to communicate. Have we detected non-baryonic dark matter particles in the lab? No. Until that happens, we can’t be sure that the stuff exists.
The rest all boils down to what Landau said: “Cosmologists are often wrong, but never in doubt.”
Did it ever occur to anyone that we have space gravity and the big bang all wrong! If the universe is 93 billion lights years in diameter we are going to need a lot of dark matter. This obsession is preventing science from moving forward and making the kinds of advances needed to survive in a tiny corner of the cosmos. With everyone looking for dark matter I sure we will miss the real story of how we got here.
thank you Sean!
@Low Math, Meekly Interacting: your message is a clear example of the total misunderstanding I was talking about hereabove. You think that MOND is a synonym of TeVeS. Because TeVeS has problems, your throw away what the *DATA* are telling us about the successes of MOND, calling it “highly unlikely to prove to be a fruitful area of research”. Yes, TeVeS has problems, especially if you want to quantize it, as its Hamiltonian is generally not bounded from below (no, the fact that the fields can behave as dark matter is not the problem), but the whole point is that there are many other theories of MOND than TeVeS. To name a few, generalized Einstein-Aether (http://arxiv.org/abs/astro-ph/0607411), bimetric MOND (http://arxiv.org/abs/0912.0790), non-minimally coupled scalar field (http://arxiv.org/abs/0811.3143), dipolar dark matter (http://arxiv.org/abs/0804.3518), and others. Many of them have their own problems too, but that does not make them “highly unlikely to be fruitful”. And as I was stating above, most of them do recognize the existence of some form of dark matter (some form being important here, it all depends on what one means exactly by “dark matter”). LambdaCDM has problems too, mostly fine-tuning problems linked to the success of MOND itself: that doesnt make LambdaCDM “highly unlikely to be fruitful”! Someone was asking about what these fine-tuning problems were. Well, Stacy wrote an excellent paper about that back in 2005: http://arxiv.org/abs/astro-ph/0509305. So, let’s remain honest and admit what the galactic-scale data are telling us, and let’s admit that direct detection experiments haven’t proven yet the existence of non-baryonic cold dark matter particles, as Stacy righteously points out. In that respect, I can’t wait for the next results of XENON100: exciting times ahead.
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Off-topic, but: is anyone else having problems with the RSS feed?
Mass and curvature of space are complementary. That is, mass creates curvature and if space can have an intrinsic curvature of its own it will mimic matter i.e. dark matter. This is the crux of my argument for the existence of the so called dark matter. The emperical MOND formula is a direct consequence of the intrinsic curvature due to the existence of a cosmic size bounded 4th space dimension and I derive a modified MOND formula out of that concept. We could call that as the Dark Space, because it is as hard to detect as Dark Matter. But this at least legitmizes MOND as a viable alternative and get some respect. The modified mond formula might rectify some of the current shortfalls in MOND. I do not have the resources to further study the concept. Thank you.
David, you are taking an interpretation of a calculation as more fundamental than the calculation. This is completely backward. Also, theorists don’t extrapolate from the big bang, they extrapolate from today, and conclude ‘hot dense phase’. The evidence is overwhelmingly consistent with a hot dense phase. And yes, at a certain point, our knowledge of the universe breaks down, and we need new physics. No one claims to know this, and there are several people actively working on it.
What’s your issue?
Phillip, in what way is the RSS feed acting up?
“Have we detected non-baryonic dark matter particles in the lab?”
well, yes. neutrinos for example.
@chris: don’t fuss so, what was meant was “non-baryonic COLD dark matter particles”, and everyone understood it. Anything more interesting and/or constructive to say? For instance, any idea how to explain the successful MOND phenomenology on galactic scales within the LambdaCDM framework?
The RSS does not validate and is thus unreadable for a variety of aggregators:
http://feedvalidator.org/check.cgi?url=http://blogs.discovermagazine.com/cosmicvariance/feed
Valatan #42 wrote,
“David, you are taking an interpretation of a calculation as more fundamental than the calculation. This is completely backward.”
I would appreciate it if you point me to my error. I’m not sure what you are referring to.
“Also, theorists don’t extrapolate from the big bang, they extrapolate from today, and conclude ‘hot dense phase’. The evidence is overwhelmingly consistent with a hot dense phase.”
I understand ‘running the film backward’. That includes the assumption that all the ‘mass-energy’ we now see was at some past time ‘compressed’ into a gravitational singularity. I think I understand the assumption: energy can neither be created nor destroyed. Right? Except it isn’t ‘right’, and all that mass-energy hasn’t necessarily always existed. And what is evident, i.e. observed, is light emitted from a hydrogen plasma at approx. 3000 degrees K. Right? The hot dense phase is an assumption based on the previous assumption. And the model is built to satisfy the requirements of the assumptions in the context of the evidence. So of course it is ‘consistent’. That doesn’t make it right.
“And yes, at a certain point, our knowledge of the universe breaks down, and we need new physics. No one claims to know this, and there are several people actively working on it.”
Certainly if some layperson, unconvinced by Creation According to Tribal Myth, were to spend a few years studying the detail of the various models, he or she would come across the caveat in the fine print: “this model is speculative and should not be taken for literal truth”. Meanwhile the unsuspecting masses are enthralled by how the universe arose out of a Big Bang, elegantly promoted as well understood truth, with just a few loose ends to clear up, requiring a few billion-dollars’ worth of high energy collisions whose debris can be sorted for clues. I would suggest rather than seeking “new physics” (does that mean new superparticles, new dimensions, or what?) you take a look at the existing physics. The point at which our ‘knowledge’ of the universe breaks down is the foundational point of the universe as we ‘know’ it.
“What’s your issue?”
Apart from the ‘issues’ above, that issue of “space” remains unexplained.
I think at this point it might be useful to recall Sagan’s Baloney Detection Kit, (especially the fifth point below):
# Wherever possible there must be independent confirmation of the facts
# Encourage substantive debate on the evidence by knowledgeable proponents of all points of view.
# Arguments from authority carry little weight (in science there are no “authorities”).
# Spin more than one hypothesis – don’t simply run with the first idea that caught your fancy.
# Try not to get overly attached to a hypothesis just because it’s yours.
# Quantify, wherever possible.
# If there is a chain of argument every link in the chain must work.
# “Occam’s razor” – if there are two hypothesis that explain the data equally well choose the simpler.
# Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by some unambiguous test). In other words, it is testable? Can others duplicate the experiment and get the same result?
There is, of course, not one dark matter model out there. There are several competing versions. Many of the earlier models were not too specific on the question of why dark matter is distributed in the manner observed and had many “moving parts” that could be adjusted to fit the data.
What would be very helpful to see, but I haven’t seen, is an analysis that shows how lamba CDM produces results that are very precisely approximated by the MOND formula that has far fewer moving parts at the galactic scale, and what about lamba CDM causes that relationship to break down at the galactic cluster level. MOND’s phenomenological success means that this must necessarily be possible in any accurate dark matter theory.
MOND’s accuracy in approximating results like rotation curves and galaxy color for a very broad range of galaxies (it fails at the galactic cluster level, not as mentioned in the original post, the galaxy level), are certainly right up there with the approximations of classical physics like Newtonian gravity, Maxwell’s equations, classical optics and classical thermodynamics. Why?
Also, does it matter that dark matter cosmologies were constructed with the assumption that there is far less ordinary matter than there actually is (as a December 2010 study published in Nature discovered that we had underestimated the amount of ordinary matter by a factor of three by assuming that elliptical galaxies have similar amounts of hard to observe stars to spiral galaxies)?
So, the dark matter model used to make some of these predictions may have made accurate predictions for the wrong reason (a surprisingly common problem in all sciences since theories are crafted to match data), even though a dark matter model of some kind is probably correct.
“Some people try to claim that the necessary dark matter could be neutrinos rather than some brand-new particle, and that’s supposed to be morally superior somehow.”
Describing cosmology without having to invent a particle which has never been observed clearly is superior to describing cosmology in a manner that requires one to invest a particle which has never been observed, all other things being equal. When both dark matter and MOND require new physics, neither is strongly favored by Occam’s Razor. So the distinction between neutrinos and other kinds of dark matter, if it could be established, would be very pertinent.
Occam’s Razor proponents often go on to argue that SUSY and String Theory make undiscovered particles well motivated even if not actually found, but the parallel argument would be that there isn’t a good quantum gravity theory out there and that we know that general relativity needs to be tweaked to accomodate quantum gravity, so MOND like phenomea are also well motivated by the proposition that quantum gravity would be expected to modify GR at some scale with some phenomenological results. See e.g. M. Reuter, H. Weyer, “Do we Observe Quantum Gravity Effects at Galactic Scales?”
The argument from beauty likewise has little to recommend it (Newton’s theory of gravity is an absolute ten on the beauty scale as was the proton-neutron-electron model of the atom that saw each of those as fundamental), particularly when one compares the quite ugly equations that tell you how dark matter halos are distributed that are required to do what MOND and TeVeS do. The fact that a TeVeS is even possible mathematically is pretty remarkable.
McGaugh’s observation on tidal effects in dwarf galaxies is likewise on point, as is the point that the cosmic microwave background issue could simply be the result of receiving somewhat less theoretical attention, since it has flowed from phenomenology to a greater extent.
The Bullet Cluster is the one piece of evidence that does seem to clearly favor dark matter over MOND. Fair enough. There is a good piece of evidence favoring one theory over another, although to play the devil’s advocate — if clusters don’t work MOND because of massive ordinary neutrinos providing dark matter, it is harder to be sure that those massive neutrinos aren’t making the Bullet Cluter act as it does.