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I often use "quasi-Keplerian" to refer to the textbook analysis of test-particle motion in a strong field situation, e.g. in a model such as the Schwazschild vacuum solution. This is justified because it turns out that, as was first shown by Einstein himeslf, we can analyze such scenarios using standard perturbation theory. That is, we approximate the motion as a perturbation of Keplerian motion.
More generally, there is a very well-established formalism, Post-Newtonian formalism, for studying quite general scenarios by using the fundamental laws of gtr to obtain a kind of power series representing the scenario as a perturbation from a correspoding Newtonian scenario.
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How do we know that a quasi-keplerian or post-newtonian perturbation theory should apply to galaxies when it was not developed by experimentation on galaxies? Also, since the theory gives totally bunk answers when amounts of dark matter are not assumed, shouldn't we conclude keppler's law in any form just doesn't cut it for describing the motion of galaxies?
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This is one of those situations where sending someone to Wikipedia might be appropriate. Ask again if WP doesn't help.
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It's not that a strobe effect cannot be produced by a spinning beam of radiation. I'm wondering how we know that a spinning beam is causing the strobe effect. There's more than one way to skin a cat.