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.
Ohwilleke, what you are presuming in your previous statement about avoiding new particles is suspect. Science is never about perfection. One takes all the observational facts together and uses the preponderance of the evidence to come to conclusions. For example, you imply neutrinos as dark matter is good because no new particle. However, three facts go against it:
1. There are not enough of them in the universe to account for DM.
2. They aren’t really dark, just dim.
3. Why are they so heavily distributed at the periphery of of galaxies.
What you are saying goes against the preponderance of the evidence. It also ignores that all the facts above taken together, but using neutrinos as the DM, might imply a new kind of force. Do you even want to consider that if you hold occam’s razor dear? I wouldn’t. I would much rather except that there just might be new particles we are seeing bound together by one of the four forces we know. Scientists have tried to use the weak force and used it to speculate on the idea of wimps. But so far no luck detecting them and there sure isn’t any good reason why most all the wimps would be distributed near the perimeter of galaxies. That contradicts all known physics and also would imply a new force.
What about the strong force? The strong force really is a fairly good fit for how galaxies behave at the perimeter where they are tightly confined. Perhaps the new particle IS the galaxy, but instead of gluons confining quarks one has the cold expanded version of it, i.e. Cosmic strings confining baryonic matter. So far they haven’t eliminated this possibility. One just needs to scale the temperature and the corresponding cross-sectional distance of space together. This would involve a varying Newtonian constant G that scales with the temperature and volume of the universe.
Hrm. Well, I certainly didn’t mean to equate TeVeS with MOND, but my understanding was that it was a much-improved theory that reduced to old-school MOND, and so was more worthy of attention. I don’t know enough about the other MOND-derived or inspired variants to comment much, but it seems like the field is plagued with models that fit certain galactic rotation curves nicely, yet inevitably wind up in contradiction with some other fact about the universe on a different scale. Meanwhile, the worst we can say about lambda-CDM is that, as of yet, it cannot explain everything, and we have not observed DM particles directly. That said, is LDM in actual contradiction with much of anything? Wanting for help, maybe, but is there robust data that threatens it in any way?
Sincere question: Could some of these galactic rotation observations go the way of the Pioneer anomaly? Is there a general consensus that they do in fact constitute a real problem, or is there legitimate reason to harbor some doubt that a problem exists, despite what we think we see?
Good article, Sean. Thanx.
To the question “Could some of these galactic rotation observations go the way of the Pioneer anomaly?”, I would personnally say that there is a huge difference between one single controversial observation (the Pioneer anomaly), and more than a hundred rotation curves of galaxies (nearby-enough to measure their rotation curve accurately) agreeing with the mond scaling relation (that is, pretty much all of them). In the same vein, some people would like to compare the mond phenomenology with a sort of galactic Titius-Bode law, except that again, the latter was applicable only to some inner planets of the lonely Solar System. That is, in my opinion, not exactly comparable with dozens of *independent* galaxy rotation curves. Indeed, wherever possible there must be independent confirmation of the facts, and that’s precisely what we have here. To make the fine-tuning problem clear, in LambdaCDM, the respective distribution of baryons and CDM should depend on the individual history of each galaxy (number of mergers, interactions with the environment, gas accretion, etc.), and yet all of them seem to harbor the same scaling relation (the mond scaling relation) at redshift zero. I am surely ready to admit that this does *not* imply that the only solution is an extreme one such as modified gravity, and that something we dont fully understand yet could be happening in the process of galaxy formation in the context of LambdaCDM. But what I find damaging is to ignore such observational evidence because we dont fully understand it yet. And until an explanation has been found (and/or until CDM has been detected directly), I think it is also dangerous to call for not exploring alternative explanations (and yes we agree, these should involve the existence of dark matter of some kind, be it HDM in the form of sterile neutrinos + modified gravity, or mass-varying axions inducing modified gravity effects, or dipolar dark matter, or the “twin matter” of Milgrom’s bimetric theory, or whatever…), even though I also agree that these often end up having usually their own theoretical or phenomenological problems. But we know that they have such problems because people are working on them!
“Phillip, in what way is the RSS feed acting up?”
I have a handful of RSS feeds, mostly for blogs. Occasionally, Firefox says “Live bookmark failed to load” instead of displaying the most recent topics. Usually, it’s just a short-lived problem and usually occurs when there are problems with the site itself. However, in the last few days (much longer than the typical outage), the problem at Cosmic Variance has remained, while the blog itself is directly accessible.
Since my other RSS feeds work, including the one for Bad Astronomy (also a Discover blog), I’m assuming that the problem is not at my end and is probably a problem with the Cosmic Variance RSS feed itself.
On a related topic, I miss an extra RSS feed for comments (both here and at Bad Astronomy); since there are relatively many comments, an extra feed would be nice, otherwise one tends to miss new comments, especially on other topics. Many blogs, such as those of Telescoper and Ted Bunn, have extra RSS feeds for comments (as well as for new blog posts, of course—two separate feeds).
“The RSS does not validate and is thus unreadable for a variety of aggregators”.
Right. And replacing “cosmicvariance” with “badastronomy” in the URL above indicates that the latter is a valid feed, so it seems to be a problem with the Cosmic Variance feed itself. I started having problems a few days ago.
“Even with MOND, you still need dark matter.”
Some people see this as an argument against MOND: it was invented to solve a specific problem, it failed when the bigger picture was taken into account, so it must be wrong. This is how Einstein viewed the Cosmological Constant. However, the Cosmological Constant very probably exists and is probably the most interesting scientific issue of the 20th century (or 21st, depending on when one was convinced, but of course we don’t know what is yet to come in this century). Hubble’s discovery of the expansion of the universe was a great observational discovery, but, at least in retrospect, it is theoretically trivial (i.e. one would expect the universe to either be expanding or contracting, and missing this obvious point is what led Einstein to his “biggest blunder” statement (though as far as I know the only source for this is an anecdote in a book by Gamow)).
Thus, merely the fact that MOND cannot explain all observations which otherwise require dark matter does not, per se, disqualify it as a theory. Maybe the universe is more complicated.
Not that long ago, there were people who weren’t as unprejudiced as those who said “we measure the cosmological parameters, whatever they turn out to be” but a bit more open than the “the Einstein-de Sitter model is correct, let’s move on” school. A common argument was: lambda is non-zero and the universe is flat OR lambda is zero and Omega (Omega_matter of course) is not 1—but not both. The universe could of course be so simple, but a) it might not be and b) there is no a priori reason why we cannot have a universe with a cosmological constant which is not spatially flat.
I think one of the most interesting things in the next, say, 10 years will be whether a significant deviation from spatial flatness is observed or, if not, what constraints are placed on any deviation.
One needs to keep an open mind. Someone once said to me, well, we have this data, and constrain the cosmological constant to a value less than x. With this new, better data set we will have in the future, how tight will the constraints be? My reply was that it depends on what the value of the cosmological constant actually is, whereas my partner in conversation was assuming it was zero. What actually happened, of course, is that we now have both a lower limit to the cosmological constant and an upper limit and it is non-zero.
Sean, Ben, etc.
I very much appreciate your responses and the overall discussion. I do find some of the arguments for MOND and MONDian ideas at least partially persuasive, and I’ve actually learned a couple new things in this discussion.
“For example, I agree about the third peak of the CMB power spectrum. …. That the third peak is high simply means ΛCDM survives this test, not that MOND fails it. ”
I can’t make sense of this statement at all. What does it take for a theory to fail, if a poor fit to data couldn’t do it ?!
Also I want to point out that the third, fourth, fifth peaks are now measured and they look nothing like the solid curve (ACBAR, QUAD, ACT, and soon SPT).
Are you guys familiar with Dr. John W. Moffat’s work on Gravity Theory?
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=moffat+reinventing+gravity?
It’s been around since 2008, and as far as I have heard it has yet to be shown in error.
@Low Math, Meekly interacting: you’re very welcome; @TRM: if you mean this paper: http://arxiv.org/abs/gr-qc/0506021 , yes I know it, and it is just plain wrong. It all relies on the arbitrary choice of one constant of integration (see eq. 53). This constant is arbitrarily chosen to reproduce the MOND phenomenology, but that has nothing to do with that so-called “MOG theory” itself. This integration constant K depends explicitely on the mass M, so each object has a different constant of integration depending on its mass, but the theory does not tell why, yet that constant is the most important ingredient of the “theory”. Well, in my opinion, that’s called cheating, and makes a big difference between the serious MONDian approaches I was mentioning hereabove, and this “theory”, much closer to “crackpotry”
Note finally that it is this same pseudo-theory (sometimes called MOG, sometimes STVG) that has subsequently been applied by Moffat to the bullet cluster in http://arxiv.org/abs/astro-ph/0702146. So there: nothing to worry about for dark matter, definitely.
Problems with the feed should now be fixed. It was just a cut-and-paste issue from the LIGO post.
Pingback: Dark matter matters | Os Anões de Tycho Brahe
To stoke the apoplexy of some of the more amusingly fist-shaking dark matter haterz, here’s another example of an inspiring true-life timeline similar to the Neptune/Vulcan example Sean linked above:
In 1934 Walter Baade and Fritz Zwicky proposed the existence of the [invisible!] neutron star, only a year after the discovery of the neutron by Sir James Chadwick… [Thirty-one years later,] in 1965, Antony Hewish and Samuel Okoye discovered “an unusual source of high radio brightness temperature in the Crab Nebula”. This source turned out to be the Crab Nebula neutron star that resulted from the great supernova of 1054… In 1974, Antony Hewish was awarded the Nobel Prize in Physics “for his decisive role in the discovery of pulsars” without Jocelyn Bell who shared in the discovery.
Pingback: Por qué la teoría MOND requiere la existencia de la materia oscura « Francis (th)E mule Science's News
Interesting and partially sad discussion here. Once upon a time not too long ago everyone knew for a fact that there was phlogiston and an aether . . .
Peeles & Nusser quite nicely show that LCDM doesn’t really work to make the Local Volume (they find one needs to introduce extra forces or some modified gravity): “Nearby galaxies as pointers to a better theory of cosmic evolution” (2010, Nature):
http://adsabs.harvard.edu/abs/2010Natur.465..565P
Independently, Kroupa et al. quite nicely show that LCDM doesn’t really work to make the Local Group (they find one needs to introduce extra forces or some modified gravity): “Local-Group tests of dark-matter concordance cosmology. Towards a new paradigm for structure formation” (2010, A&A): http://adsabs.harvard.edu/abs/2010A%26A…523A..32K
And finally, there is a science blog with some interesting contents as to overall issues with the current cosmological scenario: The Dark Matter Crisis:
http://www.scilogs.eu/en/blog/the-dark-matter-crisis
So in view of this, Prof. Stacy McGaugh’s work is quite brilliantly pointing into a very specific direction we should be taking very seriously indeed.
Dark-Matter advocates are likely to ridicule all of this, of course.
@Sean: Thanks for a clear comparison.
@67. AThinkingScientist
No need to ridicule. Of course dark matter is not the whole thruth(tm), but the evidence seems to me to support dark matter over mond at this time.
#67 AThinkingScientist —
Hallelujah! Cheers, Applause, Hugs & Kisses, you made my day, week, month….
Yes, the RSS feed is working again now, thanks!
Pingback: MOND versus Donkere Materie | Astroblogs
Approximately 90% of the baryonic mass in galactic clusters is gas. Based on dispersion velocity, MOND predicts an additional ~1 gas mass of “unseen matter” in clusters of galaxies. If the phase space of the cluster is filled (up to the measured radius and max velocity seen in the cluster) with a Fermi-Dirac distribution of neutrinos (times 12=3 gen x part/antipart x 2 spin states) with masses of ~1 eV each, the total mass these neutrinos is coincidentally ~ 1 gas mass. KATRIN claims it will measure the electron neutrino mass down to .2 eV. By 2013 KATRIN (big tritium beta decay end point experiment) may have its first results.
Case 1: KATRIN says neutrino mass <.2 eV. Then the MOND relation does not extend up to clusters of galaxies. Exotic dark matter is needed. LambaCDM is looking good.
Case 2: KATRIN measures a neutrino mass of 1 eV (last beta decay experiment said <2.2 eV 90%CL). The MOND relation now extends up to clusters of galaxies (including explaining the non-interacting yet lensing mass in the Bullet Cluster). More over, neutrinos would now account for Omega~.18 in the matter+dark energy sum to 1, now leaving no room exotic dark matter in the sum. This massive neutrino would slow down structure formation (the present increased cooling argument against a massive neutrino)….which fortunately the increased attraction of the MOND relation at large distances would speed up.
Exciting! Experiment will decide…even if the direct dark matter detection measurements continue to see nothing, the KATRIN result could strengthen the need for exotic DM in galactic clusters by ruling out MOND there, or a massive neutrino could rule out exotic DM in the Omega sum ! …waiting
Pingback: Noticiando uma controvérsia | Os Anões de Tycho Brahe
Gary,
I am excited about KATRIN also. I was not aware of the impact of the results from Katrin on the credibility of MOND; I am more interested in the neutrinos themselves. Hopefully, projects such as CNGS, Daya Bay, KATRIN, T2K, NOvA, and several others can tell us more about them.
Have we detected non-baryonic dark matter particles in the lab? No. Until that happens, we can’t be sure that the stuff exists.
True. This was also argued for the non-existence of anti-particles and the entire neutrino family. See this humorous anecdote by David Griffiths (1987):
Perhaps it’s a similar deal with whatever dark matter is. All’s it seems to want to do is interact with gravity. Neutrinos were first theorized because beta decay appeared to completely violate conservation of energy laws. Only then was it looked for. DM is probly going to be a hell of a lot harder to find in a lab, though. At least with neutrinos, we have a giant source of them 93 million miles away shooting them at us constantly, plus beta decay in nuclear reactors, where they were first discovered empirically. With DM there are no “intense” sources nearby to give us a nice thick flux to see if they’ll maybe bounce of something even once in a quadrillion times. So it is a problem.
At least with neutrinos, they’re tangible. If you have a piece of uraninite, you can hold a neutrino source right in your hand. DM, on the other hand, whew …