I’ve never met someone who believed the Earth was flat. I’ve met a few who believed it was six thousand years old, but not many. Occasionally, I run into crackpots who rail against relativity or quantum mechanics, or more recent discoveries like quarks or the Higgs. But for one conclusion of modern physics, the doubters are common. For this one idea, the average person may not insist that the physicists are wrong, but they’ll usually roll their eyes a little bit, ask the occasional “really?”
That idea is dark matter.
For the average person, dark matter doesn’t sound like normal, responsible science. It sounds like cheating. Scientists try to explain the universe, using stars and planets and gravity, and eventually they notice the equations don’t work, so they just introduce some new matter nobody can detect. It’s as if a budget didn’t add up, so the accountant just introduced some “dark expenses” to hide the problem.
Part of what’s going on here is that fundamental physics, unlike other fields, doesn’t have to reduce to something else. An accountant has to explain the world in terms of transfers of money, a chemist in terms of atoms and molecules. A physicist has to explain the world in terms of math, with no more restrictions than that. Whatever the “base level” of another field is, physics can, and must, go deeper.
But that doesn’t explain everything. Physics may have to explain things in terms of math, but we shouldn’t just invent new math whenever we feel like it. Surely, we should prefer explanations in terms of things we know to explanations in terms of things we don’t know. The question then becomes, what justifies the preference? And when do we get to break it?
Imagine you’re camping in your backyard. You’ve brought a pack of jumbo marshmallows. You wake up to find a hole torn in the bag, a few marshmallows strewn on a trail into the bushes, the rest gone. You’re tempted to imagine a new species of ant, with enormous jaws capable of ripping open plastic and hauling the marshmallows away. Then you remember your brother likes marshmallows, and it’s probably his fault.
Now imagine instead you’re camping in the Amazon rainforest. Suddenly, the ant explanation makes sense. You may not have a particular species of ants in mind, but you know the rainforest is full of new species no-one has yet discovered. And you’re pretty sure your brother couldn’t have flown to your campsite in the middle of the night and stolen your marshmallows.
We do have a preference against introducing new types of “stuff”, like new species of ants or new particles. We have that preference because these new types of stuff are unlikely, based on our current knowledge. We don’t expect new species of ants in our backyards, because we think we have a pretty good idea of what kinds of ants exist, and we think a marshmallow-stealing brother is more likely. That preference gets dropped, however, based on the strength of the evidence. If it’s very unlikely our brother stole the marshmallows, and if we’re somewhere our knowledge of ants is weak, then the marshmallow-stealing ants are more likely.
Dark matter is a massive leap. It’s not a massive leap because we can’t see it, but simply because it involves new particles, particles not in our Standard Model of particle physics. (Or, for the MOND-ish fans, new fields not present in Einstein’s theory of general relativity.) It’s hard to justify physics beyond the Standard Model, and our standards for justifying it are in general very high: we need very precise experiments to conclude that the Standard Model is well and truly broken.
For dark matter, we keep those standards. The evidence for some kind of dark matter, that there is something that can’t be explained by just the Standard Model and Einstein’s gravity, is at this point very strong. Far from a vague force that appears everywhere, we can map dark matter’s location, systematically describe its effect on the motion of galaxies to clusters of galaxies to the early history of the universe. We’ve checked if there’s something we’ve left out, if black holes or unseen planets might cover it, and they can’t. It’s still possible we’ve missed something, just like it’s possible your brother flew to the Amazon to steal your marshmallows, but it’s less likely than the alternatives.
Also, much like ants in the rainforest, we don’t know every type of particle. We know there are things we’re missing: new types of neutrinos, or new particles to explain quantum gravity. These don’t have to have anything to do with dark matter, they might be totally unrelated. But they do show that we should expect, sometimes, to run into particles we don’t already know about. We shouldn’t expect that we already know all the particles.
If physicists did what the cartoons suggest, it really would be cheating. If we proposed dark matter because our equations didn’t match up, and stopped checking, we’d be no better than an accountant adding “dark money” to a budget. But we didn’t do that. When we argue that dark matter exists, it’s because we’ve actually tried to put together the evidence, because we’ve weighed it against the preference to stick with the Standard Model and found the evidence tips the scales. The instinct to call it cheating is a good instinct, one you should cultivate. But here, it’s an instinct physicists have already taken into account.
4 gravitons 
While almost everyone does surely agree that some new kind of field needs to be introduced (be it matter-like or not), are you ever kept up at night thinking about the truly terrifying nightmare scenario that BSM or extra-GR effects aren’t necessary at all to account for the acceleration discrepancy problem, and something like stellar feedback or averaging backreactions vanishes the problem away?
I know such models are heavily constrained these days, but the idea that the universe could end up being so aggressively, mind-numbingly boring is indeed a horror so great that one can’t help but wake up in the night in a cold sweat thinking about something so terrible.
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To be clear, are you talking about dark energy, dark matter, or both?
I’m not terribly invested in the existence of dark matter personally (since it’s probably not the lightest SUSY particle at this point, it may not be something “important” from a BSM perspective at all, to the extent that “important” means anything in that context), but I get the impression that it’s pretty definite at this point, that it’s really unlikely that it’s just a normal GR effect that’s poorly understood. Nothing is impossible of course, this is science after all.
For dark energy, I did have a guest post a while back arguing precisely that kind of thing, so if anything I’m unusually comfortable with the idea.
But more generally, I don’t think I’d be terribly bothered in a “the universe is boring this sucks” sense if the evidence we had for a dark sector turned out to be explicable within SM+GR. I’d be bothered in a “science really fucked this one up, what else are we screwing up” sense, but I’m not super aesthetically attached to a universe with a dark sector.
What I am super aesthetically attached to is the idea that the masses and couplings of the SM have some deeper explanation. The idea that they might just be inexplicable “brute facts” definitely keeps me up at night.
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Have you ever heard of Modified inertia by a Hubble-scale Casimir effect (MiHsC) or quantised inertia? (https://physicsfromtheedge.blogspot.com/). What do you think about it?
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I hadn’t heard of it. It sounds like the kind of thing that is very difficult to make consistent with existing evidence about the applicability of relativity and quantum mechanics, but I don’t know enough about this specific proposal to know if it’s true there.
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The author uses his theory to explain the galaxy rotations without invoking any dark matter
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On the plus side of understanding dark matter soon, new telescopes should produce a wealth of new data. On the negative side, the field seems currently at an impasse; those advocating a matter like solution have models that seem currently unable to account for much or most of the observed phenomena. However, rightly or wrongly, I get the impression that the alternative of modifying GR is getting very little theoretical support. I guess the riposte to this is that it’s really, really, hard to modify GR. Do you share my pessimism that any reasonable theory is many years away?
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(By the way, a clarification for people who missed it: in my post, I tried to emphasize that I wasn’t meaning to draw a line between the theories usually called dark matter and MOND-like modifications of GR. Both add new fields, both likely trigger the same “this feels like cheating” instinct as a result, and the technical question of which to prefer/whether to mix them in some way is a much more subtle one than the base-level question of “do we have evidence for beyond SM+GR physics here?”)
I don’t think modifying GR is that unsupported by theorists as these things go. People actually got a lot more excited about these things with LIGO, which offers the possibility of testing some of the more extreme modifications of GR in a much more “clean” context than rotation curves and large-scale structure. That said, a lot of these more extreme modifications also risk violating evidence we already have. I get the impression there’s still disagreement in the field about this and I don’t have the expertise to disambiguate it.
Regardless I’m pessimistic, if only because nature just has no particular reason to cooperate with us on this. Dark matter doesn’t have to have any sort of non-gravitational interactions that would let it be detected with satellites or the like, and most of the kinds of astrophysical evidence we can gather about it seem too murky to actually give us enough insight to feel like we understand what’s going on, without big deviations from GR which again nature doesn’t have to cooperate with us on. But I’m very much not in this field, presumably the people who are have good reason to feel optimistic!
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There is some excitement and optimism within the modified gravity community, with the somewhat-recent discovery of RelMOND/AeST (https://arxiv.org/abs/2007.00082), which has all the usual benefits of MOND plus a perfect fit to the CMB power spectrum.
But there is also extreme pessimism, as some astronomers are of the opinion that the physicists in the particle dark matter community are at least a decade behind on the relevant astronomical literature, and as a result most of their arguments against modified GR (“but the Bullet Cluster!”) were completely and conclusively disproven years ago, yet modified gravity remains unpopular only because the DM community blindly accepts old and flawed arguments that their PI’s gave them as grad students.
So modified gravity may very well be totally the wrong approach in the end, but in the situation where it does happen to be correct, sociological forces will prevent any real progress for many years to come. Couple that with the universe’s total refusal to give any kind of break to the experimenters on the particle DM side (have we hit the neutrino floor yet for WIMPS?), and there are many reasons to be pessimistic from any angle, unfortunately. Be you a fan of particle DM or MOND, there seems to be little relief on the horizon.
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Regarding the Bullet Cluster, there’s a lot of debunked arguments floating around on the particle side certainly, but I get the impression it also gets presented poorly on the MOND side. I don’t know your level of expertise in these things, but I remember after reading Sabine Hossenfelder’s take on the Bullet Cluster that I looked up Sean Carroll’s and realized the situation wasn’t as clean as she was presenting it either.
Sociologically things can certainly get stuck, but fundamentally I’m not sure one should see particle DM as a relevant stumbling-block for MOND researchers, because the two rely on almost completely different experimental equipment and theorist populations. Fundamentally, if someone gets to the point where they can pin down some beyond-GR Horndeski parameter or the like in a model-independent way, then I don’t think it matters if particle DM people keep searching for axions, you’d have an un-ignorable result.
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I had posted this comment earlier but it doesn’t seem to show up. Will try again.
Hello, I came across your post from reddit and wanted to engage with you on the subject. I’m a layperson who has a great interest in astrophysics and cosmology and have always nurtured a few questions regarding Dark Matter and Dark Energy. I’m assuming that people like me are the intended audience of your post, (surely, proper scientists don’t think of the Dark Matter hypothesis as a “cop-out”) and so would like to pose a few questions here:
As I understand it, the existence of Dark Matter is inferred from it’s gravitational effect, whilst there’s no “visible mass” to account for the same. One thing that’s bugged me is how accurate do we believe our measurements of mass of objects to be? Today, there’s no consensus on the Hubble constant (dubbed as the crisis in cosmology) – estimated using the CMBR vs. that estimated using supernovae as standard candles. I believe, the error is assumed to be on the Supernovae method which is also based on the Cosmic Distance Ladder. I believe the Cosmic Distance Ladder also plays a role in the estimation of masses of objects in deep space (Distance & apparent brightness leads to luminosity which in turn leads to mass estimate). And the Cosmic Distance Ladder could have compounding errors starting from the parallelax methodology. So the question here is, why can’t the additional gravity be attributed to errors in mass estimation?
Another question I’ve had is that, interstellar space (within a galaxy) while appears to be mostly barren might still hold stray / rogue objects – planes, clouds, even black holes. I believe the last few years have seen discovery of more and more stray / rogue black holes that are closer to earth. Now, unlike a stellar system, the mass of a galaxy doesn’t concentrate at the central SMBH. So these stray / rogue objects could have a significant impact on the overall mass of a Galaxy. So do we need “Dark Matter” to explain how galaxies are held together? Perhaps we have errors on our estimation of galaxy masses because we haven’t seen all the mass?
The last question is in regards to the Standard Model, according to which Leptons and Bosons are almost massless. But we also know for a fact that an electron is 0.0005 AMU in mass. It is plausible that we don’t have precise instruments to accurately measure the mass of some of these particles and we might find out their accurate mass sometime in the future. The same particles also emanate from different stellar and stellar-like objects (neutrinos, photons, gluons). Is it conceivable that the vast amount of particles emanating from these objects could account for the missing mass in galaxies?
PS. Sorry about the long post. Also, apologies in advance if some these questions are non-sensical ./ dumb or based on downright wrong understanding. I’m just a curious person who’s looking for some answers.
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Looks like your previous comment got caught by the spam filter. I’m guessing you accidentally did the same comment twice in quick succession, so it assumed it was coming from a bot.
While there definitely may be issues with the distance ladder (though it’s not the only possible explanation for the Hubble constant discrepancy, it may well be a sign of a physical change from the early to later universe), it’s not the sort of thing that could explain dark matter. That’s because dark matter measurements involve matter that’s distributed differently, not merely with a different absolute mass than expected. So for example, all the matter in a galaxy is approximately the same distance away from us with respect to the distance ladder, the issue is that the galaxy rotates in a way that suggests that the matter isn’t concentrated in the center like the visible matter is, but instead spread out. Similar things apply to gravitational lensing measurements and the like.
In regard to missing normal matter (small black holes, rogue planets), there are ways of estimating that. One can look at the patterns of lensing and see if it looks like it’s coming from compact sources (“microlensing”), or look for when small objects block the light of stars. As best people can tell by these estimates there just aren’t enough of these objects to explain the effect. This wikipedia page has more information.
Finally, for SM particles we know the mass of most of them pretty well from collider experiments. The one exception are neutrinos, but we have a limit on how massive they can be based on the fact that we see them at roughly the same time as light from distant supernovae, so they can’t travel much less than the speed of light. They also don’t typically have the right properties to be dark matter (they move too fast, basically, to have the right distribution in space). For photons and gluons we have theorems that they have to be massless, and regardless they’d contribute to the calculation in a different way. (Gluons in particular are coupled by the strong force and so won’t be flying around in space any more than individual quarks can.)
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I believe Jerome’s “nightmare scenario” might well come to pass, where SM+GR–correctly applied–is able to explain the gravitational phenomena now attributed to dark matter and dark energy by a vast majority of physicists. It may well be a tale of 3 and 4 gravitons creating field self-interactions…
Alexandre Deur, an QCD physicist at Jefferson Labs, has published approximately ten refereed articles describing applications of his theory, which posits that these dark universe phenomena can be quantitatively described by (post-Newtonian) gravitational self-interaction terms in GR. Although conceptually useful, quantum gravity is not essential here. He describes how in classical GR in low-field scenarios the Einstein-Hilbert Lagrangian density can be expanded in terms of powers of the field; the lowest order term describes Newtonian gravity, and the next order terms describe gravitational field self-interactions, which can be non-negligible. This is a standard expansion described for instance by Salam in 1974 and by A. Zee in his book about QFT.
Even though galactic velocities are typically non-relativistic, these next order terms will become important when Sqrt(GM/L) becomes large enough, which is generally the case at galactic scales and larger. For further information about Deur’s work on gravity, here is a link:
http://dispatchesfromturtleisland.blogspot.com/p/deurs-work-on-gravity-and-related.html
Deur recently submitted an article to Arxiv where he claims that his approach can explain the current “Hubble Tension” discrepancy.
Deur’s work deserves wider recognition and closer scrutiny by experts in gravity and in astronomy. With the new, more accurate results of the JWST coming in, it may be possible to conclusively distinguish between Deur’s approach vs MOND-like theories vs dark universe models in the next few years…
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I’ve heard a little bit about Deur’s work before, there’s a frequent commenter who’s a fan of his. Knowing very little about how it’s set up, I’m skeptical, mostly because it seems to hinge on the idea that literally nobody who sees evidence of DM is including any post-Newtonian corrections or doing any numerical GR. That seems very unlikely to me, but I don’t know, maybe literally every DM measurement is Newtonian!
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Hello,So how does the dark matter work?It attracts normal matter ( and I would assume itself) only from large distances, but when it comes close all of the sudden looses it’s attraction ?If there were dark matter which attracts other matter, we would have constant collisions, clumping of dark/ dark-normal matter, maybe couple of “very black” holes ( made out entirely of black matter) and every other combination of different matter held together by gravity.I think our understanding of gravity needs to be completely overhauled. The explanation of your ants with plama cutters cutting your plastic bags and stealing your marshmallows is really more plausible.Cheers Kris
Sent from Yahoo Mail on Android
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So the thing that’s bugging you here is genuinely counter-intuitive. It’s that gravity, by itself, doesn’t guarantee that matter clumps up.
Think about the solar system as an example. Each planet’s orbit is pretty stable. The planets are all falling in towards the sun, but because of their velocities they never actually fall in, they keep orbiting around.
Gravity, left alone, is like this. Some trajectories will lead to things colliding and clumping together, but most won’t: objects orbit each other, or fly past each other.
Now, plenty of things do clump: stars are clumps of gas, planets are clumps of dust. But those clumps happen not just through gravity, but through E&M as well. Gas particles collide with other gas particles, releasing heat and slowing each other down. Their orbits get smaller and smaller, and eventually they clump into stars and planets. E&M is a crucial part of normal matter having the structure it does in the universe.
That doesn’t mean that you can’t get dark matter black holes or the like, but they’re much rarer. It also acts as a constraint, not merely on dark matter’s interaction with E&M, but also on possible interactions between dark matter particles: they have to have the right properties to mostly cause those diffuse halos, not to make it mostly clump up into stars.
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I was taught long ago that gravity parted company with the other three basic forces–electromagnetic and strong and weak nuclear forces–in the first picoseconds after the big bang, and that there seems to be less gravity in the universe than there should be, even though in some places there is more than there aughta. Remember several years ago, National Geographic ran a cover article on the possibility of multiple universes occupying the “same” space but different dimensions? My first thought–no, really–was, “could dark matter be a gravitational bleed-over from alternate universes?”
Does dark matter seem to have more effect upon some galaxies than others? Maybe there are more galaxies clustered together there, a dimension or two apart (I can’t even imagine how that might work), sharing their gravity, than in other places. I’ve heard Neil deGrasse Tyson say this, more or less, too. Wish I could remember which episode of which program. Anybody remember?
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There are a couple different ideas you might be confusing here.
One speculative idea, proposed by Lisa Randall and Raman Sundrum, is that our 3d (plus time) universe might be a kind of “membrane” in a bigger 4d (plus time) universe, in the same way you can have two 2d sheets of paper separated in our 3d universe. If so, there could be other 3d membranes. The idea then is that gravity might be weak on our membrane but stronger on another, and “spreads out” across the gap in between, the 4d “bulk space”. You could imagine people using gravity to send messages between the two membranes, if there were people on both to do that.
That might be what Neil deGrasse Tyson was talking about. It doesn’t work to explain dark matter though. One way to think about it is that dark matter seems to be distributed differently than ordinary matter, it doesn’t “clump” in the same way, so that if the gravitational effect came from another membrane it would still have to be dark matter in that other membrane, not ordinary matter. Another is that the effect is much too large: there’s a lot more dark matter than ordinary matter, and the effect Randall and Sundrum’s model predicts would be much much smaller.
(A different idea is that gravity used to be unified with the other forces, or that the other forces used to be unified with each other. I talk about both ideas here.)
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I read somewhere a long time ago that in order to prove that supergravity is renormalizeable you have to take it out to 7 loops. I don’t think it is there yet, and you and your field are working on it. However, I don’t know why 7 loops would prove this. Would you explain this in a future blog post?
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Heh, I’ve actually blogged about it a bunch already! Here and here. Let me know if you have more questions after reading those though!
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thanks
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Do you know what implications modified GR may have for quantum gravity?
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Most quantum gravity proposals end up modifying GR in some way or another, both in the sense that they go beyond GR but also in the sense that they imply deviations from GR that would in principle (perhaps only at very high energies/sensitivities) be measurable. So if people observed such deviations, that would potentially narrow down which proposals for quantum gravity are viable.
Now, I suspect you’re asking more specifically about the type of modified GR used to produce MOND/explain dark matter phenomena. In that case, there probably are various papers arguing that you can get something like MOND out of various quantum gravity theories, though I don’t know of anything specific off the top of my head. It’s definitely not in principle incompatible, but it’s probably also the kind of thing where many different proposals could be compatible with many different observations.
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