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.
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Dan,
Well, as Sean explained, it’s pretty apparent that merely modifying how gravity drops off with distance is not ever going to explain the Bullet Cluster. The obvious way that you’d have to modify gravity to explain the Bullet Cluster would be to violate the equivalence principle at large distances. That is, despite there being vastly more baryons in the hot cluster gas, somehow you’d have to have most of the gravitational lensing be around the far fewer baryons in the compact objects (mostly stars) in the galaxies. But the Brownstein and Moffat paper doesn’t say anything about that in the abstract, despite the fact that it’d only take a couple of sentences to explain. Nor do they provide any other rough outline of an explanation in the abstract. So why bother to dig through a 28 page paper in detail when we don’t even know where it’s going?
I think Sean’s point is really excellent: especially if you have a theory that is on the fringes, or appears to have been ruled out, you really have to be able to at least provide an outline of a description so that scientists will bother to read further. If you don’t tackle the primary objections straight away, most scientists will just assume you haven’t actually explained anything of value and move on to something that seems more likely to be interesting.
“Dark Matter, Still Existing”
Right -something is there – but what?
Neptune was actually discovered, while Vulcan was not.
So all we have to do now is to actually discover DM in the laboratory, measure its properties and find that they are concordant with astrophysical (Galaxy rotation profiles, cluster dynamics, Bullet Cluster) and cosmological (Large scale structuring, ‘cuspiness’, CMB fluctuations) constraints and then and only then we will really know what we are talking about.
A third possibility to the non-baryonic DM or modified gravity scenarios is that the DM might be baryonic. This might be possible if the expansion history of the universe during the BBN epoch was elongated in time due to the action of an as yet unknown scalar field thus allowing the baryonic density to increase. But in that case the question would be; “Where is all this dark baryonic matter today and why can we not see it?”
Historical Note: although Neptune explained most of the anomalous motion of Uranus there was still a residual that led to the search for Pluto. However Pluto and other plutons have proved to be two OOM too small so that residual still remains.
Just a thought…
Garth
Garth,
How would a scalar field in the early universe not totally mess up big bang nucleosynthesis? And besides, proposing two unknown mechanisms (unknown scalar field and invisible baryonic matter) is quite uninteresting when one unknown mechanism (WIMP) does just fine.
Agreed, one question about WIMP is finding it in the laboratory.
I have no exact reaction rates but the ‘hand waving’ scenario would be the following:
BBN would be adjusted by the action of a scalar field or similar that delayed the expansion rate, so the proportion of neutrons would decrease as they decayed thus requiring the density of baryonic matter to increase to produce the same proportion of helium. Deuterium would be annihilated requiring the present abundance to have been produced at some later stage, such as spallation in shocks around the hypernovae of PopIII stars.
Garth
#52: “…it’s pretty apparent that merely modifying how gravity drops off with distance is not ever going to explain the Bullet Cluster.”
Certainly, I find it pretty believable that if you’re just naively considering modifications to the “1/r^{2}” law of Newtonian gravity you’ll run into situations where tweaking the power 2 to 2 + epsilon(r) or whatever is not going to work. Like the Bullet Cluster. But geometrical theories of gravity of the level of complexity of GR are tensor theories, and they use this thing called a metric. That means, by my estimate, there are still 9 degrees of freedom you have to play with if you are modelling reality as a Lorentzian space.
If you can convince me that you have data that proves that it is impossible to tweak these 10 degrees of freedom to produce a theory in agreement within 5% error of what is observed, then I will be prepared to view this dark matter/dark energy stuff at least as a possibility. But as things stand, I find the “solution” to the problem – the postulation of a large amount of “something else” like WIMPs, nonbaryonic matter, heavy neutrinos!? somewhat bizarre and a bit contrived. Particularly since none of these candidates has ever been observed, even in particle accelerators.
I realize this sounds simplistic, but if space expands and the universe doesn’t, wouldn’t the consequence be external pressure on gravitational systems?
So? Even with tensors, gravity is still locally-defined. Unless your new theory of gravity violates the equivalence principle, then, you’re not going to have the galaxies appear to outweigh the cluster gas that has around ten times as many baryons, if there is no dark matter.
So, lots of fine tuning, plus additional extra unknown physical mechanisms? Sounds exceedingly unlikely to me. Now, if ever some evidence came to light that ruled out dark matter, then there might be a reason to delve into such theories. But when the vastly simpler explanation of dark matter is available, it seems our time would be better-spent elsewhere.
I’d suggest that a clue to the explanation of the galaxy rotation curve lies in the close relationship between Milgrom’s law and the rate of accelration of the universal expansion, which Lee Smolin says he was so struck by in his book The Trouble with Physics (Ch. 13).
Except that no theorist is likely to take my reasons for this suggestion seriously.
What close relationship between Milgrom’s law and the rate of acceleration of the expansion of the universe?
According to Smolin:
…there seems to be a region in in the interior of the galaxy where Newton’s laws work. Outside this region, things get messy.
The key question is: Where is the special orbit that separates the two regions? We might suppose that it occurs at a particular distance from the centre of the galaxy. This is a natural hypothesis, but it is wrong. Is the dividing line at a certain density? Again, the answer is no. What seems to determine the dividing line, surprisingly, is the rate of the acceleration itself. As one moves further out from the centre of the galaxy, accelerations decrease, and there turns out to be a critical rate that marks the breakdown of Newton’s law of gravity. As long as the acceleration of the star exceeds this critical value, Newton’s law seems to work and the acceleration predicted is the one seen. There is no need to posit any dark matter in these cases. But when the acceleration observed is smaller than the critical value, it no longer agrees with the prediction of Newton’s law.
What is the special acceleration? It has been measured to be 1.2 X 10^-8 centemetres per second per second. This is close to c^2/R [10^8 per second^2], the value of the acceleration produced by the cosmological constant!
The Trouble with Physics p. 210
#58: “So? Even with tensors, gravity is still locally-defined. Unless your new theory of gravity violates the equivalence principle, then, you’re not going to have the galaxies appear to outweigh the cluster gas that has around ten times as many baryons, if there is no dark matter.”
I might make the observation here that electromagnetic effects are relatively negligible in the cluster gas relative to galaxies, and if the final say on relativity includes anything to do with contributions due to the EM field that I’d say this would probably be part of your answer.
Unless I’ve misunderstood you, I’m not sure why you think it is impossible to describe gravity on large scales with a purely local theoretic description. Your point about the equivalence principle is a good one, of course, and well taken – but as far as I can see it is not really established with any mathematical rigor beyond the notion of a working hypothesis. When one has data that fly in the face of a working hypothesis, that were not taken at the scale the working hypothesis was originally tested, you have two options:
(i) You can continue to view the hypothesis as valid, and draw appropriate conclusions (dark matter/dark energy),
or
(ii) One can do a rethink of the fundamentals that led you to consider that hypothesis in the first place.
As far as I understand, good science involves thinking carefully about both options. Clearly (i) could well be right, particularly if it leads you to make predictions that are later confirmed (eg detection of new types of matter, or improvements in existing experimental technique that allow you to discover mass sources you previously missed, or made a hock out of the calculations, or something). However, if you make things so that it is almost impossible to detect these types of matter, or that one has no real clue what this stuff is, even if one views it as a simple solution — how can it possibly be a simple solution if you don’t even know what it is you’re looking for?
Andrew Daw,
Potentially interesting, but given that modified gravity doesn’t appear to explain Milgrom’s Law (due to the Bullet Cluster result), it seems more likely to just be a coincidence. There doesn’t, after all, appear to be any physical significance to the relative acceleration of an object sitting at the future horizon in a de Sitter universe.
Milgrom’s Law is more obvious, as it’s going to be directly related to the enclosed mass.
Tumbledried,
People have been thinking very hard about ways to modify gravity in order to account for the apparent existence of dark matter. And this pursuit was very important, until the Clowe et. al. Bullet Cluster result was published. Working with esoteric theories is absolutely vital to the pursuit of science, but that work needs to be informed by the data if it is to be useful.
Also, if the EM force had something to do with the difference, then that would be a violation of the equivalence principle. Remember that we have some pretty tight constraints on how photons gravitate from solar system experiments.
Oh, and by the way, after glancing at the Brownstein and Moffat paper again, I noticed that they are not attempting to explain the Bullet Cluster without dark matter. They now are trying to show how using both dark matter and modified gravity makes for a better fit to the data. But given that it’s only a two sigma improvement, and given that their modified gravity scenario adds a number of new unknown variables, one cannot conclude that modified gravity (on galaxy cluster scales) is remotely useful.
Really, the modified gravity people would, in my opinion, be making much better use of their time by attempting to look at potential modifications of gravity that would explain the current acceleration of the universe.
Jason,
Thanks for your response. You make some convincing arguments. I guess I just have an inherent bias against peculiar, trendy naming of unexplained phenomena, which may be impairing my judgement on these matters.
Incidentally, I was not really defending the Brownstein/Moffat paper per se – I was merely attacking this (relatively) new obsession with “dark matter”/”dark energy”. I find these concepts inelegant and unsatisfying for reasons that are perhaps not entirely logical, though you probably by now have the measure of my main counterarguments. I guess my fundamental grounding for this objection is more intuitive than anything else, and one fellow’s intuition is another’s scrapheap, so to speak.
Cheers.
Well, as far as dark energy (or modified gravity to explain the acceleration) is concerned, I have no opinion either way. The data just aren’t good enough yet. From looking at the proposed theories from a slight distance, it feels like the dark gravity people are having a hard time at it, while people suggesting that it might be some novel form of dark energy haven’t had any issues producing a wide range of at least self-consistent models that aren’t experimentally ruled out already. I honestly don’t know whether or not that should tell us anything about which is more likely to be correct.
But given the bullet cluster result, it just seems manifestly unlikely that you can in any way explain that without dark matter. And I also don’t see why it is interesting at all to attempt to show that both dark energy and dark matter are active. My reasoning is this: how do you know that your mass profile of the dark matter is accurate? Most people, when examining dark matter from lensing, tend to assume an isothermal profile. But we know that when a galaxy (or cluster) has recently undergone an interaction, it can take a while for the dark matter to relax into an isothermal profile. And since we know that the Bullet Cluster underwent a recent interaction, I’m going to be highly skeptical of any result that is going to depend upon the details of the mass profile, as any examination of modified gravity must (since weak lensing directly measures the mass profile, and if you are going to attempt to use weak lensing to determine the relationship between the underlying mass and gravity, you have to make some assumptions as to the mass profile..Bernstein and Moffat assumed isothermal, as I expected).
Slight correction: weak lensing does not directly measure the mass profile. It directly measures the gravity profile, from which we can infer the underlying mass profile given gravity. Bernstein and Moffat attempted to assume the underlying profile and from that infer gravity. But this sort of thing is doomed to failure when you assume an isothermal profile when your system recently underwent an interaction.
No matter how convincing the argument, if an investment advisor was trying to sell me on a company, 96% of whose assets I had to take on faith, I’d back out of there with my hand on my wallet.
Dark matter is to explain why the outer perimeter of galaxies spin faster then they should and dark energy is to explain why galaxies appear to be flying away faster then they should. I realize cosmology is at a point where theory says these dark forces are the best explanation, but maybe it is time to sit back and look at the theory.
Matter causes positive curvature of space, so why wouldn’t radiation cause negative curvature of space? A cosmological constant to balance gravity.
The data on redshift looks like it. The additional spin could as well be external pressure, as internal attraction. No need for dark energy, since the expansion of space is balanced by gravity.
Big Bang Theory, Inflation Theory, dark matter, dark energy. What will the next step be?
More on name.
So we have Sean’s picture.
We accept that there is evidence of the universe currently expanding more then it was before?
What currently in our universe would cause this? Are there times when the universe would “speed up” faster then other times?
I don’t quite understand how you can explain rotation curves via MOND without knowing how much dark matter there is in a galaxy? You’d need a different MOND depending on the amount. Or are they just assuming there’s very little?
Rob
Standard candle and expansionary rate of the universe?
Plato:
Was that an answer to my question? If it was I didn’t get it, so please elaborate.
John Merryman,
Dark matter explains much more than just galaxy rotation curves. If it was just galaxy rotation curves, you’d have a point, and we’d have no reason to prefer dark matter over modified gravity, or vice versa. The first major difficulty for modified gravity, however, came in with the CMB. Basically, it is quite difficult with modified gravity theories to explain the oscillations on the CMB, oscillations which are trivially explained by positing the existence of dark matter. Finding a modified gravity theory that works in both situations is a challenge, but with dark matter it “just works”. This made the modified gravity scenario seem rather contrived.
But now that we’ve got the Clowe et. al. Bullet Cluster result, modified gravity to fully explain dark matter’s effects just seems downright ludicrous, because the mass of the galaxy clusters is nowhere near where the baryons are.
The fundamental problem with your hand waving radiation explanation is that we understand how photons gravitate, and they don’t produce the effect you’re talking about. Photons, just like normal matter, produce positive curvature in overdense regions. The primary differences are that they don’t form structure, and their energy density is far too low during the current epoch to have any significant gravitational effect.
Plato,
The most reasonable explanation yet proposed for explaining the expansion without dark energy or modified gravity was to try to examine how the non-linearities of structure formation affect the apparent expansion. The idea here is that as structure forms, your overdense regions become smaller, while your underdense regions become larger. Thus the slightly larger expansion rate of the underdense regions makes the measured expansion rate appear to accelerate. As long as the structure formation is linear (as was the case in the early universe), these effects perfectly cancel and you see no additional acceleration. But at later times, as the structure formation goes nonlinear, the cancellation is no longer perfect, and you get some apparent acceleration. This has, so far, been investigated in spherically-symmetric inhomogeneous spacetimes (the only inhomogeneous spacetimes that have been solved), and the effect has been demonstrated. It remains, however, to show that the effect is large enough to explain the observed acceleration. Here is a (very) recent paper on the subject:
http://www.arxiv.org/abs/0710.5505
Rob,
Well, I presume it comes from varying the free parameters in the modified gravity theory.
Plato,
If the “picture” of our universe met the observations, why do we need to keep adding preposterous patchs to explain all the observations that don’t met our picture?
Inflation Theory? If space itself is expanding, why doesn’t the speed of light increase proportionally? Otherwise it is measuring a standard distance. The Doppler effect isn’t about expanding space, it’s about increasing distance in stable space. The train moves, the track does not stretch. The speed of light is the track. If it is increasing distance though, not expanding space, that would mean we are at the center of the universe.
Dark energy? Galaxies are being flung apart at an increasing rate, based on the redshft of a few photons of light, but these photons are not otherwise disturbed by this 70% of the total energy? Of course it can’t be an optical effect of light filling out enormous volumes of space, because we know light can’t be redshifted by anything other then recessional velocity. If we can propose spending enormous amount of time and money looking for this invisible energy, wouldn’t it also make some sense to spend some exploring the possibility that there might be some other explanation for redshift. Might it just be evidence of a cosmological constant that distorts the path of light, as gravity distorts the path of light? Rather then pulling it into a well, it stretches it out over a hill. If Omega=1, then the expansion is balanced by the contraction of gravity. Where is the additional expansion for the entire universe to expand?
Sometimes, when you just can’t fit everything into the frame you are using, it means it’s time to get a new frame, not play peek-a-boo with the evidence.
Jason,
I’m not arguing for modified gravity. As for my handwaving; we understand how photons gravitate, but is that the same as how lightwaves radiate? Radiation and gravitation would seem to be opposite sides of some larger cycle of expansion and contraction, so when we define light in terms of a particle, it conforms to the contraction side of the equation, but redshifting is a function of the expansion side of the equation. Particle or wave, but not both at the same time.
Sorry that I’ve been traveling and haven’t had time to join in the conversation. But Jason has basically covered everything I would have said.
And just to emphasize what Richard said above: dark matter isn’t just a stopgap or a fudge factor. It’s an extremely predictive hypothesis that fits a wide variety of data. The two big arguments against modified gravity are: (1) it’s hard to come up with a modification that changes the direction of the gravitational force in the weak-field regime without introducing new independently-propagating degrees of freedom [“dark matter”], and (2) the DM idea works so well that any new theory has to magically reproduce its predictions in case after case.
After all, there could actually be a planet Vulcan, too small and dim for us to detect. But not many people are expending resources chasing down that hypothesis.