The discussion centers on the relationship between dark energy and the expansion of space within galaxies. Participants assert that while dark energy causes the universe to expand, its effects are negligible within galaxies due to the overwhelming influence of gravitational forces. The analogy of "pennies on a balloon" is frequently referenced to illustrate that while space expands, the internal forces binding galaxies together prevent them from expanding significantly. The consensus is that galaxies remain stable in an expanding universe, with only minimal expansion occurring within them.
PREREQUISITESAstronomers, cosmologists, and physics students interested in the dynamics of cosmic expansion and the effects of dark energy on galaxy formation and stability.
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'.