This is fabulous content; thank you. I knew virtually nothing about the evidence for dark matter and now feel like I know a fair amount.
One hangup: I’m missing one or more inferential steps in interpreting the picture of the Bullet Cluster. I understand that the pink is gas and purple is mass, but I am not getting how that relates to dark matter. Since the rest of the piece is very explicitly “if A then B because x, A is true because y, therefore B” in spelling out the thought-steps, could you really break it down for the poets in the back? Maybe after “Explain this with modified gravity.”
I’d be interested in an explainer about the theories of what dark matter might be, if there are major schools of thought. (If it’s just people’s pet theories, no need to bother.)
Yes, pink is gas and purple is mass, but also the gas there makes up the dominant component of the visible mass in the Bullet Cluster, far outweighing the stars.
Also, physicists have come up with a whole lot of possible candidates for dark matter particles. The supersymmetry-based ones took a decent kicking at the LHC, and I’m unsure of the motivations for some of the other ones, but the two that look most promising (to me, others may differ in opinion) are axions and sterile neutrinos, as those were conjectured to plug holes in the Standard Model, so they’ve got a stronger physics motivation than the rest. But again, it might be something no physicist saw coming.
For axions, there’s something in particle physics called the strong CP problem, where there’s no theoretical reason whatsoever why strong-force interactions shouldn’t break CP symmetry. And yet, as far as we can tell, the CP-symmetry-breakingness of the strong-force interaction is precisely zero. Axions were postulated as a way to deal with this, and for certain mass ranges, they would work. They’d be extremely light particles.
And for sterile neutrinos, there’s a weird thing we’ve noticed where all the other quarks and leptons can have left-handed or right-handed chirality, but neutrinos only come in the left-handed form, nobody’s ever found a right-handed neutrino. Also, in the vanilla Standard Model, neutrinos are supposed to be massless. And as it turns out, if you introduce some right-handed neutrinos and do a bit of physics fiddling, something called the seesaw mechanism shows up, which has the two effects of making ordinary neutrinos very light (and they are indeed thousands of times lighter than any other elementary particle with mass), and the right-handed neutrinos very heavy (so it’s hard to make them at a particle accelerator). Also, since the weak interaction (the major way we know neutrinos are a thing) is sensitive to chirality, the right-handed neutrinos don’t really do much of anything besides have gravity and have slight interactions with neutrinos, with are already hard to detect. So that’s another possibility.
Additionally, there’s no reason to assume that all dark matter is just one thing. There could be multiple things going on, as long as most of the things going on don’t self-interact.
Heck, for that matter there could be a small (!) dark sector that DOES self-interact as long as its total mass was within the error bars for baryonic mass inferred from primordial nucleosynthesis.
I think what’s happening is basically that the pink shows where the visible mass is, but the purple shows where the mass should be according to gravitational lensing. Dark matter should pass straight through, and that is what we see according to lensing, even though the pink lags behind because it can collide (since it’s mostly the hot plasma).
At least, I think that’s what’s happening… I myself am really confused and am pretty unconfident in that explanation.
I’m also confused as to what modified gravity predicts, and how bullet clusters disprove it. I guess what we’d see is that modified gravity would alter the gravity around the visible mass, not just make it magically act like it just passed through. Ie, a lot about gravity would have to change for such a drastic difference between the mass as perceived through x-rays and the mass as perceived through gravitational lensing.
This is fabulous content; thank you. I knew virtually nothing about the evidence for dark matter and now feel like I know a fair amount.
One hangup: I’m missing one or more inferential steps in interpreting the picture of the Bullet Cluster. I understand that the pink is gas and purple is mass, but I am not getting how that relates to dark matter. Since the rest of the piece is very explicitly “if A then B because x, A is true because y, therefore B” in spelling out the thought-steps, could you really break it down for the poets in the back? Maybe after “Explain this with modified gravity.”
I’d be interested in an explainer about the theories of what dark matter might be, if there are major schools of thought. (If it’s just people’s pet theories, no need to bother.)
Yes, pink is gas and purple is mass, but also the gas there makes up the dominant component of the visible mass in the Bullet Cluster, far outweighing the stars.
Also, physicists have come up with a whole lot of possible candidates for dark matter particles. The supersymmetry-based ones took a decent kicking at the LHC, and I’m unsure of the motivations for some of the other ones, but the two that look most promising (to me, others may differ in opinion) are axions and sterile neutrinos, as those were conjectured to plug holes in the Standard Model, so they’ve got a stronger physics motivation than the rest. But again, it might be something no physicist saw coming.
For axions, there’s something in particle physics called the strong CP problem, where there’s no theoretical reason whatsoever why strong-force interactions shouldn’t break CP symmetry. And yet, as far as we can tell, the CP-symmetry-breakingness of the strong-force interaction is precisely zero. Axions were postulated as a way to deal with this, and for certain mass ranges, they would work. They’d be extremely light particles.
And for sterile neutrinos, there’s a weird thing we’ve noticed where all the other quarks and leptons can have left-handed or right-handed chirality, but neutrinos only come in the left-handed form, nobody’s ever found a right-handed neutrino. Also, in the vanilla Standard Model, neutrinos are supposed to be massless. And as it turns out, if you introduce some right-handed neutrinos and do a bit of physics fiddling, something called the seesaw mechanism shows up, which has the two effects of making ordinary neutrinos very light (and they are indeed thousands of times lighter than any other elementary particle with mass), and the right-handed neutrinos very heavy (so it’s hard to make them at a particle accelerator). Also, since the weak interaction (the major way we know neutrinos are a thing) is sensitive to chirality, the right-handed neutrinos don’t really do much of anything besides have gravity and have slight interactions with neutrinos, with are already hard to detect. So that’s another possibility.
Additionally, there’s no reason to assume that all dark matter is just one thing. There could be multiple things going on, as long as most of the things going on don’t self-interact.
Heck, for that matter there could be a small (!) dark sector that DOES self-interact as long as its total mass was within the error bars for baryonic mass inferred from primordial nucleosynthesis.
I think what’s happening is basically that the pink shows where the visible mass is, but the purple shows where the mass should be according to gravitational lensing. Dark matter should pass straight through, and that is what we see according to lensing, even though the pink lags behind because it can collide (since it’s mostly the hot plasma).
At least, I think that’s what’s happening… I myself am really confused and am pretty unconfident in that explanation.
I’m also confused as to what modified gravity predicts, and how bullet clusters disprove it. I guess what we’d see is that modified gravity would alter the gravity around the visible mass, not just make it magically act like it just passed through. Ie, a lot about gravity would have to change for such a drastic difference between the mass as perceived through x-rays and the mass as perceived through gravitational lensing.
Here’s a link that seems to confirm what I wrote: https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf