- #36
PeterDonis
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RUTA said:Correct.
Ok. One thing about that example that makes it inappropriate for modeling something like galaxy rotation curves is that the "interior" FRW spacetime region cannot be stationary, whereas to model something like a galaxy, you would need an "interior" spacetime region that was stationary. This would also be true for modeling an individual star in a galaxy, but a stationary model for a star is easy: a spherically symmetric blob of matter in hydrostatic equilibrium with a constant surface area. A galaxy is not a continuous distribution of matter, although a really rough approximation could perhaps model it as such; but a better model would be a system of objects orbiting their common center of mass under their mutual gravity. I don't know how much models of that sort have been constructed in the literature; the only one that I can bring to mind at the moment is the one in a paper by Einstein in the 1930s where he was trying to prove that black holes were impossible (of course he didn't use the term "black hole" since it hadn't been invented yet) by showing that no such stationary system of mutually orbiting objects could have an "areal radius" smaller than ##3 G M / c^2##, where ##M## is the externally measured mass of the system. A galaxy of course has a much larger "areal radius" so that wouldn't be an issue for such a model.