Thinking about the expansion of space, it is commonly explained that gravitationally bound objects (like galaxies and galaxy clusters) will not be affected by the expansion of space (dark energy). I personally don't get why this would be, I would think galaxies, and things orbiting one another (e.g. solar systems) would be impacted. If space doubled, the orbit distance should double. I do get that things bound together gravitationally (perhaps a second definition / originally meant definition of "gravitationally bound") (e.g. planets, stars, asteroids, etc...) would not get ripped apart as space gradually doubles. They would just pull themselves back together, and resist the very weak force of the expansion of space. Is my understanding correct that things held together by gravity (like stars and planets) will hold together as space continues to expand, but things in orbit (e.g. stars orbiting within the galaxy, and planets orbiting stars) will be moved further out with the expansion of space? It seems this could be fairly easily verified or refuted by seeing if ancient galaxies (those really far away) are denser than closer galaxies. If they are denser, then this is strong support for the idea, and if not, then it largely refutes it.

I have a feeling this might be why quasars stopped happening at some point, and are now just observable in great distances (where we are seeing them in the past). My understanding is that a quasar is the supermassive black hole at the center of a galaxy feeding on large amounts of material. Back to my main question though, does the expansion of space thin out galaxies / solar systems?


Expansion of space does affect orbits, but the effect is really small.

Rate of space expansion is currently about 2cm/s per light year. Sun orbits Milky Way at about 230 km/s, and space expansion contributes about 0,5km/s. It is going to be difficult to measure other variables, such as stellar masses, with such detail.

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    $\begingroup$ It's not as simple as this. $\endgroup$ – Rob Jeffries Aug 9 '19 at 18:20

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