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As a general principle I would advise a grain of salt whenever a physicist claims to have solved an outstanding astrophysical problem. Verlinde is a great phys
by antognini 10y ago
As a general principle I would advise a grain of salt whenever a physicist claims to have solved an outstanding astrophysical problem. Verlinde is a great physicist --- I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. But someone with a background in pure physics often lacks the background in astronomy to know whether their ideas will work. (I remember hearing a visiting physicist present his ideas about stars producing axions, and having an astronomy professor point out that if he was right, Cepheid variables would not pulsate at the observed frequencies.)
The article mentions the Bullet Cluster as one observation that MOND has difficulty explaining. Another observation that I'm not sure how MOND would explain is the dramatic difference in dynamics between a globular cluster and a dwarf spheroidal galaxy. Globular clusters are observed to have virtually no dark matter, whereas some dwarf spheroidal galaxies have somewhere between 1000 to 10,000 times as much dark matter as ordinary matter [1]. These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies.
I'll admit that I have not looked very carefully at Verlinde's paper. But the evidence for dark matter is much more than galaxy rotation curves these days. A credible alternative to WIMPs has to explain more than that.
All that aside, from a theoretical perspective the work is very interesting because it provides a much-needed framework to understand why something like MOND could exist. But I don't think that any of the direct detection experiments should pack it in quite yet.
[1]: https://arxiv.org/abs/1510.03856 https://arxiv.org/abs/1510.03856
[2]: Well, sort of. Galaxy formation is still a big open problem, and globular cluster formation is probably even less well understood. But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter.
- bokononon 10y ago> I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. I remember reading this too. Did anything ever become of this theory?
- m_mueller 10y ago> Globular clusters are observed to have virtually no dark matter That's a fascinating angle, first time I've read this. > But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter. Could you expand a bit how WIMPs can explain that? Do dwarf spheroidals have a strong central black hole while globular clusters haven't? Is the galactic black hole needed to capture WIMPs from cosmic background, sort of like gas giants capture lots of moons?
- antognini 10y agoDwarf spheroidals would form in the process of galaxy/structure formation. In this process, it's really dark matter that drives growth, and ordinary matter is just along for the ride. In the early universe the dark matter is diffusely spread throughout the universe, with slight overdensities in various places. These then collapse under their own gravity and form sheets, then filaments, then finally fragment into different clusters. This would produce dark matter haloes of a variety of sizes, some of which would be dwarf spheroidals. I should note that there are problems with this picture --- the predicted distribution of dark matter halo masses does not match the observed distribution. In particular, standard galaxy formation theory predicts far more dwarf spheroidals than are actually observed. In the case of globular clusters, the cluster would form independently of dark matter. Just as in an open cluster in the galaxy, you would perhaps have some sort of a gaseous cloud of ordinary matter (mostly hydrogen), which would collapse, fragment, and form stars. There are lots of open questions about how globular clusters form, though, so this is just a very vague sketch.
- m_mueller 10y agoThanks!
- raverbashing 10y agoYou make good points, just don't call it MOND, because it's not "THE MOND" Maybe we'll only have the answers when JWST is launched
- privong 10y ago> These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies. I think the order of magnitude size difference would be important in the possible origin differences in a MOND-ian universe. If globular clusters formed in a higher-pressure environment (within a high-z galaxy or merger) they could be more compact and stay in the Newtonian regime. And if dwarf spheroidals formed in lower-pressure environments that might be the origin of larger sizes, putting them in the MOND acceleration regime. But, that's just speculation. And to emphasize your statement that "the evidence for dark matter is much more than galaxy rotation curves these days", even MOND-ian models require some form of "dark" matter to explain cluster dynamics.