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I've got two problems with this. First: if theory and observation don't match up, that can be due to lacking theory or lacking observations. Both should be in

by Beldin 4y ago

I've got two problems with this. First: if theory and observation don't match up, that can be due to lacking theory or lacking observations. Both should be investigated. Second: all candidates for dark matter i know posit an enormous number of places where this dark matter is. It is not that "dark matter solves this problem". It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass. So not only is this theory supposing that most material in the universe is invisible to us, but its properties other than mass are unknown to us except it allows us to fill in the blanks exactly as needed. E.g, we're not expecting a lot of dark matter black holes, as the hypothesised stuff doesn't even interact with itself and so cannot slow down. Basically, "dark matter" is fairy dust that physicists sprinkle over everything till it works. And you know what: maybe that is the way to go. Probably it plays at least some part in answering these questions. But until we have a clearer picture, we may as well call it "magical fairy dust". That at least reminds us how much we're fudging.

pfortuny 4y ago
Until it is experimentally tested, DM is essentially equivalent to the ether. A useful unexplained tool which to date seems to work. Until another Michelson-Morley experiment comes (and MOND might have something to say, or at least they seem to be testing DM experimentally), yes: as untestable as string theory is.
DiogenesKynikos 4y ago
> It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass It's actually an extremely simple initial distribution of dark matter in the early universe. You can specify the entire distribution with one number: a spectral index. The thing is, we already know that the universe had that distribution of energy density anyways, because of the Cosmic Microwave Background. If you take that very simple distribution and let the laws of physics run for 13 billion years, you get the required present-day distribution of dark matter. That distribution can be verified in several ways, including gravitational lensing, the rotation curves of galaxies, the motions of galaxies inside clusters, and the temperatures of hot gas inside galaxy clusters. The fact that such a simple set of initial conditions can explain pretty much everything we can see on scales ranging from stars to the entire visible universe is why cold dark matter is overwhelmingly the favored paradigm.

by Beldin 4y ago

pfortuny 4y ago

Until it is experimentally tested, DM is essentially equivalent to the ether. A useful unexplained tool which to date seems to work. Until another Michelson-Morley experiment comes (and MOND might have something to say, or at least they seem to be testing DM experimentally), yes: as untestable as string theory is.

DiogenesKynikos 4y ago

> It is a particular distributing of mass that solves the problem - typically, a highly specific distribution of mass It's actually an extremely simple initial distribution of dark matter in the early universe. You can specify the entire distribution with one number: a spectral index. The thing is, we already know that the universe had that distribution of energy density anyways, because of the Cosmic Microwave Background. If you take that very simple distribution and let the laws of physics run for 13 billion years, you get the required present-day distribution of dark matter. That distribution can be verified in several ways, including gravitational lensing, the rotation curves of galaxies, the motions of galaxies inside clusters, and the temperatures of hot gas inside galaxy clusters. The fact that such a simple set of initial conditions can explain pretty much everything we can see on scales ranging from stars to the entire visible universe is why cold dark matter is overwhelmingly the favored paradigm.