The discussion centers on the question of why dark energy does not cause the space within galaxies to expand, despite its role in the overall expansion of the universe. Participants explore various theories and analogies related to cosmology, gravity, and the nature of space, while considering both conceptual and technical aspects of the topic.
Participants do not reach a consensus on the nature of dark energy and its effects on the expansion of space within galaxies. Multiple competing views remain, with some asserting that space does not expand inside galaxies while others maintain that it does, albeit countered by gravity.
Limitations include the dependence on definitions of space and expansion, as well as unresolved questions regarding the initial conditions of the universe and the nature of dark energy.
Oh, yeah, the effect of dark energy on galaxies is most definitely not zero. It's small, but not zero. The effect is even measurable for large galaxy clusters:andrewkirk said:This question has no answer because 'a(t)' is a parameter of the FLRW model, which does not apply within a bound system such as a galaxy. Asking what the FLRW 'a' is inside a galaxy would be like asking what is the radius of an ellipse.
The FLRW model assumes perfect homogeneity of the universe, but a galaxy is by definition a region in which the homogeneity assumption does not apply.
The question we can ask, that does make sense within a galaxy, is:
'What effect do the factors that make the universe's expansion accelerate have within a galaxy?'
The answer to that is most likely reached via analysis of the impact of a nonzero \Lambda on the curvature of spacetime within a galaxy. My facility with tensor equations is regrettably way below the level necessary to answer that question. But it seems reasonable to be confident that the answer is not 'none whatsoever'.