I love telling the stories of Neptune and Vulcan. Not the Roman gods, the planets that were originally hypothesized to explain the mysterious motions of other planets. Neptune was propsed by Urbain Le Verrier in order to account for deviations from the predicted orbit of Uranus. After it was discovered, he tried to repeat the trick, suggesting a new inner planet, Vulcan, to account for the deviations of the orbit of Mercury. It didn’t work the second time; Einstein’s general relativity, not a new celestial body, was the ultimate explanation.
In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.
We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.
But it’s enormously more difficult to invent a theory of modified gravity in which the direction of the gravitational force points toward some place other than where the ordinary matter is. So the way to rule out the modified-gravity hypothesis is to find a system in which the dark matter and ordinary matter are located in separate places. If you see a gravitational force pointing at something other than the ordinary matter, dark matter remains the only reasonable explanation.
And that’s precisely what the Bullet Cluster gives you. Dark matter that has been dynamically separated from the ordinary matter, and indeed you measure the gravitational force (using weak lensing) and find that it points toward the dark matter, not toward the ordinary matter. So, we had an interesting question — dark matter or modified gravity? — and now we know the answer: dark matter. You might also have modified gravity, but one’s interest begins to wane, and we move on to trying to figure out what the dark matter actually is.
But some people don’t want to give up. A recent paper by Brownstein and Moffat claims to fit the Bullet Cluster using modified gravity rather than dark matter. If that were right, and the theory were in some sense reasonable, it would be an interesting and newsworthy result. So, you might think, the job of any self-respecting cosmologist should be to work carefully through this paper (it’s full of equations) and figure out what’s going on. Right?
I’m not going to bother. The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).
And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.
But the data say otherwise. So most of us suck it up and get on with our lives. Don’t get me wrong: I’m happy that some people are continuing to work on a long-shot possibility such as replacing dark matter with modified gravity. But it’s really a long shot at this point. There is a tremendous presumption against it, and you would have to have a correspondingly tremendous theory to get people interested in the possibility. I don’t think it’s worth writing news stories about, in particular: it gives people who don’t have the background to know any better the idea that more or less everything is still up for grabs. But we do learn things and make progress, and at this point it’s completely respectable to say that we’ve learned that dark matter exists. Not what all of us were rooting for, but the universe is notoriously uninterested in adapting its behavior to conform to our wishes.
good analogy
“Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source,”
how a _gravitational_ force. Please do not start fueling a campaign against electromagnetism.
I’m a big fan of magnetism. If someone has a theory of gravity like that, and would like to use it to explain the Bullet Cluster, tell me how it works.
Your central objection to modified gravity seems so obvious that it cries out for an explanation. Have B&M offered even an attempt at squaring their theory with observations of the Bullet Cluster? I find it hard to believe that they wouldn’t defend their position.
I know that you are just baiting here Sean, but I will just point out what I consider to be blindingly obvious. Namely,
(Predicted matter distribution via our current understanding of general relativity) + (“Dark Matter” defect) + (“Dark Energy” defect) = observed matter distribution
Via a simple rearrangement, we can write
Predicted matter distribution via our current understanding of general relativity = Observed matter distribution – (“Dark Matter” defect) – (“Dark Energy” defect)
So, as you said, we have two choices: we might choose to believe that our data is incomplete and there is stuff out there that our instruments cannot detect, which seems to be the camp you fall in, OR, we might choose to believe that our data is more or less complete and our understanding of general relativity is lacking.
I am inclined to believe it more likely that the second case is true, and that there are errors in our current theoretical understanding that do not matter on the scale of our solar system but become glaring at galactic scale. I think that positing the existence of extra matter/energy sources to conform to a cherished theory, particularly when there are errors of ?95%?, seems to be stretching credibility just a little. After all, to throw your words back at you (and I hope you forgive me for it), the universe is notoriously uninterested in adapting its behaviour to conform to our wishes.
^”After all, to throw your words back at you (and I hope you forgive me for it), the universe is notoriously uninterested in adapting its behavior to conform to our wishes.”
strange that I felt the same way reading the last line.
In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.
I love this analogy! And I’d never heard of Vulcan before… it’s a shame how many awesome things get left out of the grade-school science curriculum just because they don’t exist. 🙂
It really doesn’t help that their modified gravity theory is scalar-vector-tensor gravity, which is quite complex, vastly more so than just positing the existence of a new, weakly-interacting particle (something predicted by grand unified theories anyway). And I have to agree with Sean: they’re not bothering to state, in clear terms in their abstract, how they’re explaining the Bullet Cluster. From reading the abstract and skimming the introduction, there is no indication of an explanation that I could see, so I have to wonder why anybody would bother to pore through the full 28-page paper to figure out what in the world it’s saying.
Now that we’re pretty sure dark matter exists, have people given up on MOND theories, in which gravitational theory itself is modified – or is there still some reason to believe in the latter?
Also, what’s the latest on the Pioneer 10 etc. anomalies and their significance?
Finally, what about light itself exerting gravitational effect? It should, because of the mass equivalent of energy. There’s a contradiction if we allow light not to exert gravity. We could convert a lump of matter into light, and then the gravity from the matter would suddenly no longer propagate from that location – which creates problems (I don’t have time right now to explain why, most of you can fill in.) But the problem is, you can’t (?) have a field proceeding from something going at the speed of light, the field theory doesn’t work right (again, for reasons most can fill in.)
Oops, OK I sometimes don’t take enough time, since I see some people didn’t give up on MOND. But I’m still curious about the Pioneer 10 questions.
I agree with the sentiment of this post 100%. I just wish I knew how to persuade the editors of popular science magazines to start thinking this way. There seems to be little enthusiasm left in most publications for explaining in any depth what we already know fairly well; the only thing that rates coverage now is the drama of theory X slogging it out against theory Y, and nobody really cares whether theory Y is a significant contender or has just sprouted some new epicycles and free parameters since the last fight.
A recent New Scientist was trumpeting the proposal that gamma ray bursts are light that has travelled back in time around naked singularities. I guess mere colliding neutron stars and imploding hypermassive stars are so boring now that it’s only by biting the heads off chickens with time travel that you can get readers to pay attention.
“the idea that more or less everything is still up for grabs”
We only know about 4% of the Universe, which is ordinary matter, and I don’t think there are good dark matter candidates yet, let alone dark energy “theories.” I am not sure who you think have grabbed anything, but I think he/she didn’t grab much.
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I took up the question of the bullet cluster with Moffat about a year ago, and he admitted that his fit required there to be a coincidental alignment of what we see. Specifically, the centers of the overall matter distribution (which are inferred from lensing) and the centers of the distribution of optically visible galaxies (which have moved more or less ballistically) are very closely lined up. (There are two sets of centers, one for each of the clusters in the collision.) The optical and lensing centers don’t line up exactly, but their misalignment is exactly what you would expect from the fact that the interacting x-ray gas makes up a nontrivial part of the total mass. In Brownstein’s and Moffat’s model, the fact that these the centers of the galaxy distributions and the lensing centers are aligned is purely coincidence, which to my mind suggests strongly against their model.
I seem to recall a SciAm article from some years back proposing that “dark matter” is actually matter on another brane (or a fold of our brane), which interacts only gravitationally because only gravitons make it off the brane to influence matter elsewhere.
I have no idea what the current status of this model might be, except to note that I never hear about it in the context of the “dark matter vs. modified gravity” rumble. Perhaps for theoretical reasons it’s already been exluded, or maybe it’s just terribly unpopular. But if it still lives on in the work of some theorist or theorists out there, what does the Bullet Cluster observation have to say about such “brane world” models?
I’ve heard about the branes-leaking-gravity idea as well and find it to be fascinating. Does anyone have any more info on it?
I’m not sure I get the objection to positing modified gravity; it doesn’t seem absurd and I suppose (from the post) that it works out mathematically?
Neil B.,
Actually, general relativity does provide a very specific prediction as to the gravitational field emitted by photons.
In fact, early in the universe, the gravitational effect of radiation was the dominant effect. But since radiation dilutes faster than matter (1/a^4 vs. 1/a^3), rather early on matter came to dominate, significantly before the cosmic microwave background was emitted. Currently the photon density of the universe is around one part in ten thousand to one part in one hundred thousand of the total energy density.
As for the Pioneer anomaly, I don’t know. Never heard of the issue.
People (scientists, not just crackpots) are still working on the Pioneer anomaly — at least, a former student of mine is a postdoc in one such group, and I assume there must be more.
The last time I looked at this issue it seemed overwhelmingly likely though that there’s some explanation that just involves unmodelled properties of the spacecraft, like anisotropic thermal emission.
Dick,
The Bullet Cluster aside, I would like to say that complexity of modified gravity theory (as compared to dark matter postulate) is not a very convincing reason to ignore it. Sean’s example of Vulcan vs GR is an example.
You know this reminds me of that ancient story of people who believed that the Sun was the center. First they thought the Earth was the center, but eventually data piled up and forced people to believe that Jupiter had moons, and forcing those moons to orbit Earth would make things implausible as they had to force it to orbit in a weird and complex path around the Earth, rather than the simpler method: don’t let it orbit Earth in the first place. I guess the modified gravity-ists are trying to do the same thing – go for a much more complicated method rather than a simpler dark-matter method. Did I do this analogy right?
Is it possible that dark matter (which by now it’s pretty obvious does exist) is a stable “atom” cooked up within impending supernovae and comprising an iron nucleus with neutrinos orbiting in place of electrons?
If the neutrinos were tightly bound, almost skimming the nucleus instead of fluffed out like electrons, the structure would be invisible to all but possibly the highest photon frequencies and unlikely to interact significantly with normal matter even at its most dense.
This would explain why dark matter tends to clump round and in galaxies – it is formed within them, and why it is diffuse – it is flung out at high speeds in supernova explosions.
This hypothesis might be tested in a couple of ways: firstly by attempting to spot slight irregularities (dispersion or absorption?) in gamma sources viewed through a supposed clump of dark matter; also by seeing if galactic rotation speed band anomalies (explained by dark matter) are less for very distant and hence early galaxies, in which one assumes supernovae have had less time to produce this exotic exhaust!
John R Ramsden (#22):
Neutrinos don’t bind to nuclei; they either sail right through them, or, with some phenomenally small probability, induce beta decay.
With or without neutrinos around, iron nuclei would have the same charge as ever, and would have to be bound to, or associated in a plasma with, electrons.
Also, I thought there were aspects of the CMB suggestive of dark matter in the early universe (i.e. long before galaxies were formed) influencing structure formation.
Thanks Greg, yes shortly after posting I remembered neutrinos were neutral (DOH!) so it would have to be some other particle, negatively charged of course and stable (at least when bound in the conjectural atom – as for neutrons, that need not be the same as stable in isolation).
But if as you say dark matter was around before galaxies even formed that would blow the idea out of the water anyway.
Wouldn’t dark matter particles be rather easy to find if they carried charge?
Alex