md65536 Posted September 25, 2022 Share Posted September 25, 2022 (edited) On 9/23/2022 at 12:59 PM, MigL said: The fact is, the universe is only expanding at certain scales; below those scales, galaxies may actually be coming together, such as our galaxy and Andromeda, because the gravitational coupling exceeds the 'dark energy' of the expansion. IOW, expansion is distance ( scale ) dependent. I was about to reply to this saying basically that I think that space is expanding at all scales, but on smaller scales (even between Milky Way and Andromeda), gravity overwhelms expansion so that the galaxies are "moving through" space relative to each other faster than expansion can separate them. A google search shows only results that disagree with my view, and seem to suggest that if two things (in an otherwise empty universe, say) aren't separating, then I guess there's no meaningful way to say that that space is expanding? If two more-distant galaxies are separating at an increasing rate due to expansion, people use phrases like, "gravity has no effect at those distances" due to expansion, which is also not what I thought. Say for example you have 2 masses in an empty universe where their gravitational attraction exactly balances expansion of space between them, so they remain at a fixed distance. Would you say, space is expanding between them, but gravity accelerates them through that space at a rate that keeps them at the same distance? Maybe even there is a measurement that shows that gravity still applies and that expansion is also happening. Or would you say there is no expansion of space and no gravitational effect between the 2 masses in this system? It's nonsense to say the masses are moving through space or accelerating, because those only make sense relative to something else, and they're not moving relative to anything. There is no way to distinguish expansion and gravity, because any effect (like redshift) of expansion that would otherwise cause the 2 masses to separate, would be exactly cancelled out by an opposite effect of gravity that would otherwise cause the 2 masses to converge, and so no such effects are measurable. Is either correct? Could you also say that "gravitational effect" and "expansion" are just emergent effects of the metric, and are not fundamental and separate parts of the metric or the universe? It is the metric that has these two masses relatively stationary, and gravitational attraction and expansion are simply zero for these 2 masses? If so, then I think you could model or label the system so there is both a gravitational effect and expansion of space between the 2 masses, but you would only do that if you had a reason to separate them, otherwise it is simply an unnecessary complication. It seems, I shouldn't think of expansion as some intrinsic process that's happening throughout the universe, but is just a measurement that is a consequence of our universe's particular metric tensor? Edited September 25, 2022 by md65536 Link to comment Share on other sites More sharing options...
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