In some ways I'd go even further. Claims that MOND successfully explains small-scale structures like galaxies have yet to be actually demonstrated because there aren't any MOND simulations employing full baryonic physics. And that ain't trivial; they might well have all the same problems that current simulations do, or worse, or just completely different problems altogether. With the caveat that the code to do so now exists, and we should expect results in the not too distant future, there's not really much evidence that MOND actually does solve all the so-called problems with dark matter. And when you look at them in detail, virtually all of these aren't nearly as serious for dark matter as MOND supporters like to claim (e.g. http://astrorhysy.blogspot.com/2015/08/seven-very-good-reasons-to-be-little.html).
I'm not sure I'd go so far as to say that dark matter is a virtual certainty; I hold my own assessment at roughly 85% in favour of it existing. There are plenty of weird objects out there that make little or no sense, but they are may or may not say anything at all about dark matter. I view dark matter as a thoroughly sensible evidence-backed assumption and no other theory comes close. It's certainly a good - in fact the best - default option to assume when explaining observations, and any counter-evidence has to be pretty darn strong at this point. Doesn't mean we can't investigate alternatives, though after 30-odd years of trying to make it work, MOND looks like a dead end to me. Maybe someone will eventually come up with a relativistic version that will solve everything, but at this point I'd bet in favour of discovering a plausible dark matter particle/substance before that happens. Which does not, of course, mean that a spectacular physics revolution won't happen, just not in the way the MONDers would like.
That's my informed opinion, at any rate.
Originally shared by Ethan Siegel
“On the scales of groups of galaxies, individual galaxy clusters, colliding galaxy clusters, the cosmic web, and the leftover radiation from the Big Bang, MOND’s predictions fail to match reality, whereas dark matter succeeds spectacularly. It’s possible, and perhaps even likely, that someday we will understand enough about dark matter to understand why and how the MOND phenomenon on the scales of individual galaxies arises. But when you look at the full suite of evidence, dark matter is practically a scientific certainty. It’s only if you ignore all of modern cosmology that the modified gravity alternative looks viable. Selectively ignoring the robust evidence that contradicts you may win you a debate in the eyes of the general public. But in the scientific realm, the evidence has already decided the matter, and 5/6ths of it is dark.”
Look at a galaxy, watch it rotate, and be extremely careful. Look at how the rotation speeds vary as you move farther and farther away from the center. And then look at all the matter you can see: gas, dust, stars, black holes, neutron stars, plasma, etc. Does the matter describe the galactic motions you see? No, no it doesn’t. Immediately, there are probably two potential solutions that pop into your head:
1.) Either there’s some additional, invisible, unseen mass, like dark matter, or,
2.) The law of gravity that you’re using is wrong.
Both of these seem like reasonable options, and if all you had was individual galaxies, they both would be.
But we have an entire Universe. And when you look at the full suite of evidence available, one of these options is no option at all.
https://www.forbes.com/sites/startswithabang/2018/07/26/theres-a-debate-raging-over-whether-dark-matter-is-real-but-one-side-is-cheating/
I kinda look at the MOND / dark matter debate as I would implement a stub method. I know the guts of the method will require much more implementation than I can afford to give it immediately, but I need the goddamn thing to return a value right away so I stub in some approximation and leave myself a nastygram in the method comments.
ReplyDeleteThe problem with Dark Matter, seen from where I sit, is our present day inability to measure it. But hey, we've have troubles with neutrinos, too. Wolfgang Pauli just stubbed it in as a proposed explanation for how beta decay works. Eventually, Fermi writes a paper on it, integrating the world of Dirac and Pauli - it just goes on getting weirder and weirder....
Of course Dark Matter is an open question because it hasn't been detected directly. But like Pauli's postulation of neutrino , the equation works. The M in MOND stands for Miraculous.
Rhys Taylor By chance I recently read the Scientific American article they reference, and something that caught my eye therein was the claim that dark matter must act as a pressure-less gas (I think that was the phrase). That didn't make sense to me -- seems like almost an oxymoron.
ReplyDeleteThe Scientific American article is locked behind a paywall, but other of Sabine's blog posts have noted that there are field additions to GR that behave like a superfluid and exactly mimic dark matter without actually being matter. Ethan Siegel manages to pooh-pooh the whole thing without explicitly addressing that point, which makes me wonder who's actually guilty of overstatement ("cheating") here.
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