We’re visible animals. What we see informs our notion of the world, so it usually helps to make a sketch to assist conceptualize troublesome materials. When first confronted with MOND phenomenology in galaxies that I had been certain have been darkish matter dominated, I made a sketch to assist arrange my ideas. Here’s a scan of the unique darkish matter tree that I drew on a transparency (pre-powerpoint!) in 1995:
On the backside are the roots of the issue: the astronomical proof for mass discrepancies. From these develop the trunk, which splits into classes of attainable options, which in flip department into ever extra particular potentialities. Most of these things have been already previous information on the time: I used to be categorizing, not inventing. Certainly, some issues have been rebranded over time with out altering all that a lot, with unusual nuggets now being referred to as macros (a generalization to explain darkish matter candidates of nuclear density) and uneven gravity changing into MOG. The extra issues change, the extra they keep the identical.
I’ve used this image many instances in talks, each public and scientific. It helps to focus the thoughts. I up to date it for the 2012 evaluation Benoit Famaey wrote (see our Fig. 1), however I don’t assume I actually improved on the older model, which Don Lincoln had tailored for the quilt illustration of a problem of Physics Instructor (circa 2013), with some embellishment by their graphic artists. That’s fairly good, however I desire my unique.
Although there are not any lack of buds on the tree, there have actually been extra concepts for darkish matter candidates over the previous thirty years, so I went trying to see if somebody had tried an identical train to categorize or no less than corral all of the concepts individuals have thought-about. Tim Tait made one such determine, however you need to already be an knowledgeable to make any sense of it, it being a form of Venn diagram of the big conceptual playground that’s theoretical particle physics.
There’s additionally this latest determine by Bertone & Tait:
That is good: nicely organized and pleasantly symmetric, and making good use of shade to tell apart various kinds of potentialities. One can acknowledge most of the identical names from the unique tree like MACHOs and MOND, together with newer, associated entities like Macros and TeVeS. Curiously, WIMPs usually are not talked about, regardless of dominating the historical past of the sphere. They’re subsumed underneath supersymmetry, which is now itself only a sub-branch of weak-scale potentialities relatively than the grand unified idea of manifest inevitability that it was as soon as thought-about to be. It’s a signal of how far now we have come that the primary candidate, the one that continues to be the main focus of dozens of huge experiments, doesn’t even come up by title. It is usually an indication of how far now we have but to go that it appears preferable to many to invent new darkish matter candidates than take significantly alternate options which have had a lot larger predictive success.
A problem one faces in doing this train is to resolve which candidates deserve point out, and that are simply particular particulars that needs to be grouped underneath some extra main department. As a sensible matter, it’s unattainable to wedge the whole lot in, nor does each wild concept we’ve ever thought up deserve equal point out: Kaluza-Klein darkish matter is just not a coequal peer to WIMPs. However how can we be truthful about making that decision? It will not be attainable.
I wished to see how the brand new diagram mapped to the previous tree, so I chopped it up and grafted each bit onto the suitable department of the unique tree:
This works fairly nicely. It appears just like the tree has blossomed with extra concepts, which it has. There are extra potentialities alongside well-established branches, and fully new branches that I might solely anticipate with query marks that allowed for the potential of issues we had not but thought up. The tree is getting bushy.
In the end, the purpose is to not have an ever bushier tree, however relatively the alternative: we wish to discover the best reply. As an experimentalist, one needs to both detect or exclude particular darkish matter candidates. As an scientist, I wish to apply the wealth of observational data now we have gathered like a chainsaw within the palms of an overzealous gardener to hack off deceptive branches till the tree has been pruned all the way down to a single department, the one (and hopefully just one) right reply.
As a lot as I like Bertone & Tait’s hexagonal picture, it is vitally targeted on concepts in particle physics. 5 of the six branches are numerous types of darkish matter, whereas the potential of modified gravity is grudgingly acknowledged in just one. It’s illustrated as a uninteresting gray that’s in contrast to the brilliant, cheerful colours granted to the varied flavors of darkish matter candidates. To make sure, there are extra concepts for options to the mass discrepancy downside from the particle physics than anyplace else, however that doesn’t imply all of them deserve equal point out. One this diagram would possibly get the impression that the chances of darkish matter:modified gravity are 5:1, which appears directly each biased towards the latter and but significantly extra beneficiant than its authors seemingly meant.
There isn’t any point out at all the information on the roots of the issue. That’s all subsumed within the central DARK MATTER, which is certainly an apt depiction of the division between physics and astronomy. Proposed candidates for darkish matter have emerged primarily from the particle physics neighborhood, which is what the hexagon categorizes. It takes without any consideration the proof for darkish matter, which is totally astronomical in nature. This isn’t a trivial level; I’ve usually encountered particle physicists who’re mystified that astronomers had the temerity of assume they’ll contribute to the darkish matter debate regardless of 100% (not 99%, not 99.9%; 100%) of the proof for mass discrepancies stemming from observations of the sky. Apparently, our job was finished once we advised them we would have liked one thing unseen, and we must always stay politely quiet whereas the Massive Brains determine it out.
For a categorization of options, I suppose it’s tolerable if harmful to depart off the proof. There’s one other downside with putting DARK MATTER on the heart. It is a linguistic downside. Phrases matter; the names we use body how we take into consideration the issue. By calling it the darkish matter downside, we presuppose the reply. A extra applicable time period is likely to be mass discrepancy, which was in use for some time by extra careful-minded individuals, but it surely appears to have fallen into disuse. Darkish matter is simpler to say and sounds far more cool.
Jacob Bekenstein identified that a good higher time period can be acceleration discrepancy. That’s what we measure, in spite of everything. The centripetal acceleration in spiral galaxies exceeds that predicted by the noticed distribution of seen matter. Mass is an inference, and a sloppy one at that: dynamical information solely constrain the mass enclosed by the final measured level. The overall mass of a darkish matter halo will depend on how far it extends, which we by no means observe as a result of the darn stuff is invisible. And naturally we solely infer the existence of darkish matter by assuming that the pressure regulation is right. That gravity as taught to us by Einstein and Newton ought to apply to galaxies looks like a fairly darn good assumption, however it’s simply that. By calling it the darkish matter downside, we make all of it about unseen mass and neglect the chance that the inference would possibly go astray with that first, fundamental assumption.
So I’ve made a brand new image, putting the acceleration discrepancy on the heart the place it belongs. The astronomical observations that inform the issue are on the vertical axis whereas the logical potentialities for physics options are on the horizontal axis. I’ve been very spare in filling in each: I’m attempting to hint the logical potentialities with a minimal of bias and litter, so I’ve retained some concepts which are fairly nicely excluded.
For instance, on the darkish matter aspect, MACHOs are fairly nicely excluded at this level, as are most (all?) darkish matter candidates composed of Normal Mannequin particles. Regular matter simply doesn’t minimize it, however I’ve left that sector in as a logical risk that was thought-about traditionally and shouldn’t be forgotten. On the dynamical aspect, one of many first ideas is that galaxies are massive so maybe the pressure regulation adjustments at some applicable scale a lot giant than the photo voltaic system. At this juncture, now we have excluded all modifications to the pressure regulation which are made at a selected size scale.
There are too many strains of observational proof to do justice to right here. I’ve lumped an unlimited quantity of it right into a small variety of categorical bins. This isn’t preferrred, however some key factors are no less than talked about. I invite the reader to attempt doing the train with pencil and paper. There are critical limits imposed by what you may bodily show in a font the attention can learn with a complexity restricted to that which doesn’t make the top explode. I concern I’ll already be pushing each.
I’ve made a break up between dynamical and cosmological proof. These are likely to push the interpretation by some means, as hinted by the colours. Which method one goes relies upon totally on how one weighs relatively disparate strains of proof.
I’ve additionally positioned the issues that have been recognized from the outset of the trendy darkish matter paradigm nearer to the middle than people who weren’t. That galaxies and clusters of galaxies wanted one thing greater than meets the attention was recognized, and knowledgeable the necessity for darkish matter. That the dynamics of galaxies over an enormous vary of mass, measurement, floor brightness, gasoline fraction, and morphology are organized by just a few easy empirical relations was not but recognized. The Baryonic Tully-Fisher Relation (BTFR) and the Radial Acceleration Relation (RAR) are crucial items of proof that didn’t inform the development of the present paradigm, and usually are not satisfactorily defined by it.
Equally for cosmology, the non-baryonic chilly darkish matter paradigm was launched by the commentary that the dynamical mass density apparently exceeds that allowed for regular matter by primordial nucleosynthesis. This, along with the necessity to develop the noticed giant scale construction from the very easy preliminary situation indicated by the cosmic microwave background (CMB), satisfied practically everybody (together with myself) that there should be some new type of non-baryonic darkish matter particle outdoors the realm of the Normal Mannequin. Detailed observations of the facility spectra of each galaxies and the CMB are necessary corroborating observations that didn’t but exist on the time the thought took maintain. We additionally obtained our predictions for this stuff very mistaken initially, therefore the necessity to change from Normal CDM to Lambda CDM.
The general public I’ve met who work on darkish matter candidates appear to be nicely knowledgeable of cosmological constraints. In distinction, their data of galaxy dynamics usually appears to start out and finish with “rotation curves are flat.” There’s fairly much more to it than that. However, by and huge, they stopped listening at “subsequently we want darkish matter” and have been off and operating with concepts for what it could possibly be. There’s a must reassess the viability of those concepts within the gentle of the BTFR and the RAR.
Individuals who work on galaxy dynamics are involved with the apparent connections between dynamics and the noticed stars and are inclined to be suspicious of the cosmological inference requiring non-baryonic darkish matter. Over time, I’ve repeatedly been approached by eminent dynamicists who’ve associated in hushed tones, much less the cosmologists overhear, that the darkish matter should be baryonic. I can perceive their reticence, since I used to be, initially, a kind of individuals who they didn’t wish to have overhear. Baryonic darkish mater was loopy – we want extra mass than is allowed by massive bang nucleosynthesis! I often avoided elevating this subject, as I’ve loads of causes to sympathize, and attempt to be a sympathetic ear even after I don’t. I did convey it up in an prolonged dialog with Vera Rubin as soon as, who scoffed that the theorists have been too intelligent by half. She reckoned that if she might exhibit that Ωm = 1 in baryons at some point, that they’d have by some means mounted nucleosynthesis by the subsequent. Her angle was well-grounded in expertise.
A typical angle amongst advocates of non-baryonic darkish matter is that the facility spectrum of the CMB requires its existence. Matches to the information require a non-baryonic element at one thing like 100 sigma. That’s fairly vital proof.
The issue with this angle is that it assumes Common Relativity (GR). That’s the speculation through which the matches are made. There’s, certainly, little doubt that the existence of chilly darkish matter is required with a purpose to make the matches within the context of GR: it doesn’t work with out it. To take this as proof of the existence of chilly darkish mater is totally round logic. Certainly, that now we have to invent darkish matter as a tooth fairy to avoid wasting GR is likely to be interpreted as proof towards it, or no less than as a sign that there would possibly exist a nonetheless extra basic idea.
However, I do have sympathy for the angle that any concept that’s going to work has to elucidate all the information – together with each dynamical and cosmological proof. The place one must be cautious is to imagine that the reason we presently have is exclusive – so distinctive that no different idea might ever conceivably clarify it. By that logic, MOND is the one idea that uniquely predicted each the BTFR and the RAR. So if we’re being even-handed, chilly darkish matter is dominated out by the dynamical relations recognized after its invention no less than as a lot as its rivals are excluded by the detailed, later measurement of the facility spectrum of the CMB.
If we consider all the information, and maintain all theories to the identical excessive normal, none survive. Not a single one. A typical method appears to be to carry one’s favourite idea to a decrease normal. I can’t dignify that with a repudiation. The problem with information each astronomical and cosmological, is determining what to consider. It has gotten higher, however you may’t depend on each measurement being proper, or – more durable to remember – truly measure what you need it to measure. Do the orbits of gasoline clouds in spiral galaxies hint the geodesics of take a look at particles in completely round movement? Does the idea of hydrostatic equilibrium within the intracluster medium (ICM) of clusters of galaxies present the identical tracer of the gravitational potential as dynamics? There’s an annoying offset within the acceleration scale measured by the 2 distinct strategies. Is that actual, or some systematic? It appears to be actual, however additionally it is suspicious for showing precisely the place the change in methodology happens.
One will go mad attempting to trace down each conceivable systematic. Belief me, I’ve finished the experiment. So an train I love to do is to ask what idea minimizes the quantity of information I’ve to disregard. I spent a number of years reviewing all the information with a purpose to do that train after I first obtained on this downside. To my shock, it was MOND that did greatest by this measure, not darkish matter. To this date, clusters of galaxies stay essentially the most problematic for MOND in having a discrepant acceleration scale – an actual downside that we might not hesitate to comb underneath the rug if darkish matter suffered it. For instance, the offset the EAGLE simulation requires to [sort of] match the RAR is sort of precisely the identical amplitude as what MOND must match clusters. Moderately than contemplating this to be an issue, they apply the required offset and name it pure to have missed by this a lot.
Many of the issues we name proof for darkish matter are actually proof for the acceleration discrepancy. A psychological grasp up I had after I first got here to the issue was that there’s a lot proof for darkish matter. That could be a misstatement stemming from the linguistic bias I famous earlier. There’s a lot proof for the acceleration discrepancy. I nonetheless see professionals wrestle with this, usually citing outcomes as being contradictory to MOND that truly help it. They appear to not have bothered to test, as I’ve, and are content material to repeat what they heard another person assert. I generally surprise if essentially the most lasting contribution to science made by the darkish matter paradigm is as one big Asch conformity experiment.
If we repeat right now the train of minimizing the quantity of information now we have to disbelieve, the speculation that fares greatest is the Aether Scalar Tensor (AeST) idea of Skordis & Zlosnik. It reproduces the noticed MOND phenomenology (minus clusters) whereas additionally offering a superb match to the facility spectrum of galaxies and the CMB (see additionally the up to date plots of their paper). In distinction, the one merchandise darkish matter aspect that explains galaxy dynamics is the MOND-like group of concepts that accomplish that by intent, and even these wrestle. Merely including mass in new particles doesn’t naturally do that. They are often excluded except we indulge within the particular pleading that invokes suggestions of different ad hoc auxiliary hypotheses. Equally, extra elaborate concepts like self-interacting darkish matter have been useless on arrival for offering a mechanism to unravel the mistaken downside: the cores inferred in darkish matter halos are merely a symptom of the extra basic MONDian phenomenology; the proposed resolution addresses the underlying illness about as a lot as a band-aid helps a decapitation.
Does that imply AeST is the right idea? Solely within the sense that MOND was the most effective idea after I first did this train within the earlier century. The needle has swung backwards and forwards since then, so it would swing once more. However I do hope that it’s a step in a greater route.
