Timeline for Why does space expansion not expand matter?
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| Sep 29, 2019 at 3:57 | comment | added | benrg | @Ben Crowell Your second comment is also incorrect. At large scales there is a tension which I think is proportional to $\ddot a/a$ (not $(d/dt)(\ddot a/a)$). Again this is just the ordinary gravitational influence of the matter (+ dark energy) actually present; it's not an "extra" effect. At small scales there's a tension proportional to Λ (dark energy being present at all scales), plus some pressure from self-gravity, plus tidal effects of nearby matter, and that's it. GR is a theory of gravity, not of fixed spacetime backgrounds. | |
| Sep 29, 2019 at 3:50 | comment | added | benrg | @Ben Crowell That paper (Cooperstock, Faraoni, and Vollick, astro-ph/9803097v1) is wrong. A vacuum with FLRW geometry violates the Einstein field equation. By assuming FLRW geometry at solar-system scales they implicitly assume a uniform FLRW matter distribution at that scale. The effect they calculate is the local gravitational influence of that matter. You can even use Newtonian gravity and get the same answer. But the matter isn't actually there, so the effect doesn't actually exist. | |
| Nov 17, 2018 at 17:06 | comment | added | Andrew Steane | @BenCrowell I believe your answer misses the main point. The statement "the solar system does expand ... but the effect is small" seems to suggest that we can apply the cosmological expansion without adjustment to things like solar systems. This, I believe, is wrong (see my answer to this question). This is not to say there is no influence at all on the solar system, but the influence is in competition with the Sun's gravity and the latter dominates. | |
| Jun 6, 2017 at 23:31 | comment | added | user4552 | However the accelerating expansion of the universe can exert a small "constant" negative force between the electrons and nucleus and make the atom very very slightly bigger than it would have been in a non-accelerating expanding universe. This is also wrong. The strain on a bound system is proportional to $(d/dt)(\ddot{a}/a)$, where $a(t)$ is the cosmological scale factor. This quantity is not constant in realistic models, and can be nonzero even if the cosmological constant is zero. Also, it vanishes identically in a cosmology that consists only of dark energy (=cosmological constant). | |
| Jun 6, 2017 at 23:28 | comment | added | user4552 | If the question is interpreted as why don't atoms and other bound systems expand the answer is that the general expansion of space cannot do continuous work against the electromagnetic force that holds an atom together or any other force that holds a bound system together. This is wrong. For example, the solar system does expand due to cosmological expansion, but the effect is undetectably small. See Cooperstock, Faraoni, and Vollick, "The influence of the cosmological expansion on local systems," arxiv.org/abs/astro-ph/9803097v1 | |
| Sep 16, 2012 at 22:17 | history | answered | FrankH | CC BY-SA 3.0 |