We are visual animals. What we see informs our perception of the world, so it often helps to make a sketch to help conceptualize difficult material. When first confronted with MOND phenomenology in galaxies that I had been sure were dark matter dominated, I made a sketch to help organize my thoughts. Here is a scan of the original dark matter tree that I drew on a transparency (pre-powerpoint!) in 1995:
At the bottom are the roots of the problem: the astronomical evidence for mass discrepancies. From these grow the trunk, which splits into categories of possible solutions, which in turn branch into ever more specific possibilities. Most of these items were already old news at the time: I was categorizing, not inventing. Indeed, some things have been rebranded over time without changing all that much, with strange nuggets now being known as macros (a generalization to describe dark matter candidates of nuclear density) and asymmetric gravity becoming MOG. The more things change, the more they stay the same.
I’ve used this picture many times in talks, both public and scientific. It helps to focus the mind. I updated it for the 2012 review Benoit Famaey wrote (see our Fig. 1), but I don’t think I really improved on the older version, which Don Lincoln had adapted for the cover illustration of an issue of Physics Teacher (circa 2013), with some embellishment by their graphic artists. That’s pretty good, but I prefer my original.
Though there are no lack of buds on the tree, there have certainly been more ideas for dark matter candidates over the past thirty years, so I went looking to see if someone had attempted a similar exercise to categorize or at least corral all the ideas people have considered. Tim Tait made one such figure, but you have to already be an expert to make any sense of it, it being a sort of Venn diagram of the large conceptual playground that is theoretical particle physics.
There is also this recent figure by Bertone & Tait:
This is nice: well organized and pleasantly symmetric, and making good use of color to distinguish different types of possibilities. One can recognize many of the same names from the original tree like MACHOs and MOND, along with newer, related entities like Macros and TeVeS. Interestingly, WIMPs are not mentioned, despite dominating the history of the field. They are subsumed under supersymmetry, which is now itself just a sub-branch of weak-scale possibilities rather than the grand unified theory of manifest inevitability that it was once considered to be. It is a sign of how far we have come that the number one candidate, the one that remains the focus of dozens of large experiments, doesn’t even come up by name. It is also a sign of how far we have yet to go that it seems preferable to many to invent new dark matter candidates than take seriously alternatives that have had much greater predictive success.
A challenge one faces in doing this exercise is to decide which candidates deserve mention, and which are just specific details that should be grouped under some more major branch. As a practical matter, it is impossible to wedge everything in, nor does every wild idea we’ve ever thought up deserve equal mention: Kaluza-Klein dark matter is not a coequal peer to WIMPs. But how do we be fair about making that call? It may not be possible.
I wanted to see how the new diagram mapped to the old tree, so I chopped it up and grafted each piece onto the appropriate branch of the original tree:
This works pretty well. It looks like the tree has blossomed with more ideas, which it has. There are more possibilities along well-established branches, and entirely new branches that I could only anticipate with question marks that allowed for the possibility of things we had not yet thought up. The tree is getting bushy.
Ultimately, the goal is not to have an ever bushier tree, but rather the opposite: we want to find the right answer. As an experimentalist, one wants to either detect or exclude specific dark matter candidates. As an scientist, I want to apply the wealth of observational knowledge we have accumulated like a chainsaw in the hands of an overzealous gardener to hack off misleading branches until the tree has been pruned down to a single branch, the one (and hopefully only one) correct answer.
As much as I like Bertone & Tait’s hexagonal image, it is very focused on ideas in particle physics. Five of the six branches are various forms of dark matter, while the possibility of modified gravity is grudgingly acknowledged in only one. It is illustrated as a dull grey that is unlike the bright, cheerful colors granted to the various flavors of dark matter candidates. To be sure, there are more ideas for solutions to the mass discrepancy problem from the particle physics than anywhere else, but that doesn’t mean they all deserve equal mention. One looking at this diagram might get the impression that the odds of dark matter:modified gravity are 5:1, which seems at once both biased against the latter and yet considerably more generous than its authors likely intended.
There is no mention at all of the data at the roots of the problem. That is all subsumed in the central DARK MATTER, as if we’re looking down at the top of the tree and recognize that it must have a central trunk, but cannot see its roots. This is indeed an apt depiction of the division between physics and astronomy. Proposed candidates for dark matter have emerged primarily from the particle physics community, which is what the hexagon categorizes. It takes for granted the evidence for dark matter, which is entirely astronomical in nature. This is not a trivial point; I’ve often encountered particle physicists who are mystified that astronomers have the temerity of think they can contribute to the dark matter debate despite 100% (not 90%, nor 99%, nor even 99.9%, but 100%) of the evidence for mass discrepancies stemming from observations of the sky. Apparently, our job was done when we told them we needed something unseen, and we should remain politely quiet while the Big Brains figure it out.
For a categorization of solutions, I suppose it is tolerable if dangerous divorced from the origins of the problem to leave off the evidence. There is another problem with placing DARK MATTER at the center. This is a linguistic problem that raises deep epistemological issues that most scientists working in the field rarely bother to engage with. Words matter; the names we use frame how we think about the problem. By calling it the dark matter problem, we presuppose the answer. A more appropriate term might be mass discrepancy, which was in use for a while by more careful-minded people, but it seems to have fallen into disuse. Dark matter is easier to say and sounds way more cool.
Jacob Bekenstein pointed out that an even better term would be acceleration discrepancy. That’s what we measure, after all. The centripetal acceleration in spiral galaxies exceeds that predicted by the observed distribution of visible matter. Mass is an inference, and a sloppy one at that: dynamical data only constrain the mass enclosed by the last measured point. The total mass of a dark matter halo depends on how far it extends, which we never observe because the darn stuff is invisible. And of course we only infer the existence of dark matter by assuming that the force law is correct. That gravity as taught to us by Einstein and Newton should apply to galaxies seems like a pretty darn good assumption, but it is just that. By calling it the dark matter problem, we make it all about unseen mass and neglect the possibility that the inference might go astray with that first, basic assumption.
So I’ve made a new picture, placing the acceleration discrepancy at the center where it belongs. The astronomical observations that inform the problem are on the vertical axis while the logical possibilities for physics solutions are on the horizontal axis. I’ve been very spare in filling in both: I’m trying to trace the logical possibilities with a minimum of bias and clutter, so I’ve retained some ideas that are pretty well excluded.
For example, on the dark matter side, MACHOs are pretty well excluded at this point, as are most (all?) dark matter candidates composed of Standard Model particles. Normal matter just doesn’t cut it, but I’ve left that sector in as a logical possibility that was considered historically and shouldn’t be forgotten. On the dynamical side, one of the first thoughts is that galaxies are big so perhaps the force law changes at some appropriate scale much large than the solar system. At this juncture, we have excluded all modifications to the force law that are made at a specific length scale.
There are too many lines of observational evidence to do justice to here. I’ve lumped an enormous amount of it into a small number of categorical bins. This is not ideal, but some key points are at least mentioned. I invite the reader to try doing the exercise with pencil and paper. There are serious limits imposed by what you can physically display in a font the eye can read with a complexity limited to that which does not make the head explode. I fear I may already be pushing both.
I have made a split between dynamical and cosmological evidence. These tend to push the interpretation one way or the other, as hinted by the colors. Which way one goes depends entirely on how one weighs rather disparate lines of evidence.
I’ve also placed the things that were known from the outset of the modern dark matter paradigm closer to the center than those that were not. That galaxies and clusters of galaxies needed something more than meets the eye was known, and informed the need for dark matter. That the dynamics of galaxies over a huge range of mass, size, surface brightness, gas fraction, and morphology are organized by a few simple empirical relations was not yet known. The Baryonic Tully-Fisher Relation (BTFR) and the Radial Acceleration Relation (RAR) are critical pieces of evidence that did not inform the construction of the current paradigm, and are not satisfactorily explained by it.
Similarly for cosmology, the non-baryonic cold dark matter paradigm was launched by the observation that the dynamical mass density apparently exceeds that allowed for normal matter by primordial nucleosynthesis. This, together with the need to grow the observed large scale structure from the very smooth initial condition indicated by the cosmic microwave background (CMB), convinced nearly everyone (including myself) that there must be some new form of non-baryonic dark matter particle outside the realm of the Standard Model. Detailed observations of the power spectra of both galaxies and the CMB are important corroborating observations that did not yet exist at the time the idea took hold. We also got our predictions for these things very wrong initially, hence the need to change from Standard CDM to Lambda CDM.
Most of the people I have met who work on dark matter candidates seem to be well informed of cosmological constraints. In contrast, their knowledge of galaxy dynamics often seems to start and end with “rotation curves are flat.” There is quite a lot more to it than that. But, by and large, they stopped listening at “therefore we need dark matter” and were off and running with ideas for what it could be. There is a need to reassess the viability of these ideas in the light of the BTFR and the RAR.
People who work on galaxy dynamics are concerned with the obvious connections between dynamics and the observed stars and are inclined to be suspicious of the cosmological inference requiring non-baryonic dark matter. Over the years, I have repeatedly been approached by eminent dynamicists who have related in hushed tones, less the cosmologists overhear, that the dark matter must be baryonic. I can understand their reticence, since I was, originally, one of those people who they didn’t want to have overhear. Baryonic dark mater was crazy – we need more mass than is allowed by big bang nucleosynthesis! I usually refrained from raising this issue, as I have plenty of reasons to sympathize, and try to be a sympathetic ear even when I don’t. I did bring it up in an extended conversation with Vera Rubin once, who scoffed that the theorists were too clever by half. She reckoned that if she could demonstrate that Ωm = 1 in baryons one day, that they would have somehow fixed nucleosynthesis by the next. Her attitude was well-grounded in experience.
A common attitude among advocates of non-baryonic dark matter is that the power spectrum of the CMB requires its existence. Fits to the data require a non-baryonic component at something like 100 sigma. That’s pretty significant evidence.
The problem with this attitude is that it assumes General Relativity (GR). That’s the theory in which the fits are made. There is, indeed, no doubt that the existence of cold dark matter is required in order to make the fits in the context of GR: it does not work without it. To take this as proof of the existence of cold dark mater is entirely circular logic. Indeed, that we have to invent dark matter as a tooth fairy to save GR might be interpreted as evidence against it, or at least as an indication that there might exist a still more general theory.
Nevertheless, I do have sympathy for the attitude that any idea that is going to work has to explain all the data – including both dynamical and cosmological evidence. Where one has to be careful is to assume that the explanation we currently have is unique – so unique that no other theory could ever conceivably explain it. By that logic, MOND is the only theory that uniquely predicted both the BTFR and the RAR. So if we’re being even-handed, cold dark matter is ruled out by the dynamical relations identified after its invention at least as much as its competitors are excluded by the detailed, later measurement of the power spectrum of the CMB.
If we believe all the data, and hold all theories to the same high standard, none survive. Not a single one. A common approach seems to be to hold one’s favorite theory to a lower standard. I will not dignify that with a repudiation. The challenge with data both astronomical and cosmological, is figuring out what to believe. It has gotten better, but you can’t rely on every measurement being right, or – harder to bear in mind – actually measure what you want it to measure. Do the orbits of gas clouds in spiral galaxies trace the geodesics of test particles in perfectly circular motion? Does the assumption of hydrostatic equilibrium in the intracluster medium (ICM) of clusters of galaxies provide the same tracer of the gravitational potential as dynamics? There is an annoying offset in the acceleration scale measured by the two distinct methods. Is that real, or some systematic? It seems to be real, but it is also suspicious for appearing exactly where the change in method occurs.
One will go mad trying to track down every conceivable systematic. Trust me, I’ve done the experiment. So an exercise I like to do is to ask what theory minimizes the amount of data I have to ignore. I spent several years reviewing all the data in order to do this exercise when I first got interested in this problem. To my surprise, it was MOND that did best by this measure, not dark matter. To this date, clusters of galaxies remain the most problematic for MOND in having a discrepant acceleration scale – a real problem that we would not hesitate to sweep under the rug if dark matter suffered it. For example, the offset the EAGLE simulation requires to [sort of] match the RAR is almost exactly the same amplitude as what MOND needs to match clusters. Rather than considering this to be a problem, they apply the required offset and call it natural to have missed by this much.
Most of the things we call evidence for dark matter are really evidence for the acceleration discrepancy. A mental hang up I had when I first came to the problem was that there’s so much evidence for dark matter. That is a misstatement stemming from the linguistic bias I noted earlier. There’s so much evidence for the acceleration discrepancy. I still see professionals struggle with this, often citing results as being contradictory to MOND that actually support it. They seem not to have bothered to check, as I have, and are content to repeat what they heard someone else assert. I sometimes wonder if the most lasting contribution to science made by the dark matter paradigm is as one giant Asch conformity experiment.
If we repeat today the exercise of minimizing the amount of data we have to disbelieve, the theory that fares best is the Aether Scalar Tensor (AeST) theory of Skordis & Zlosnik. It contains MOND in the appropriate limit while also providing an excellent fit to the power spectrum of galaxies and the CMB (see also the updated plots in their paper). Hybrid models struggle to do both while the traditional approach of simply adding mass in new particles does not provide a satisfactory explanation of the MOND phenomenology. They can be excluded unless we indulge in the special pleading that invokes feedback or other ad hoc auxiliary hypotheses. Similarly, more elaborate ideas like self-interacting dark matter were dead on arrival for providing a mechanism to solve the wrong problem: the cores inferred in dark matter halos are merely a symptom of the more general MONDian phenomenology; the proposed solution addresses the underlying disease about as much as a band-aid helps an amputation.
Does that mean AeST is the correct theory? Only in the sense that MOND was the best theory when I first did this exercise in the previous century. The needle has swung back and forth since then, so it might swing again. But I do hope that it is a step in a better direction.