We know - or theorize - that a spinning black hole drags spacetime in its surroundings, producing measurable effects on the accretion disk (the minimum stable orbit radius). What I cannot grasp is why this isn't the case for a rotating galaxy. Why the mismatch in classical rotational velocity predictions isn't explained from the spacetime dragging of the total mass of the galaxy at a given radius value? Why do we need dark matter to adjust for the "strange" measurements?
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$\begingroup$ OP seems to be mixing relativistic and non-relativistic effects. $\endgroup$– Qmechanic ♦Commented Jan 15, 2019 at 6:10
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$\begingroup$ Related: physics.stackexchange.com/q/362097 $\endgroup$– safesphereCommented Jan 15, 2019 at 7:24
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The spacetime dragging is only significant very close to the event horizon of a spinning black hole i.e. within a few Schwarzschild radii. While in principle the effect exists for a rotating galaxy in practice it is so small as to be completely undetectable.
answered Jan 15, 2019 at 8:27