The elongation of light waves occurs not because galaxies are moving away from each other, but because light waves are stretching along with the fabric of expanding space (cosmological redshift). Although the Milky Way is held together by gravity, space expansion occurs everywhere, including within the Milky Way. Since space expansion occurs in the Milky Way and light stretches due to space expansion, why don't we observe cosmological redshift within the Milky Way?
$\begingroup$
$\endgroup$
2
-
2$\begingroup$ The cosmological redshift arises because things are receding from us. Within the galaxy, they are not receding. Note that expansion of space is not a physical phenomenon. It's just a coordinate choice. $\endgroup$– StenCommented Oct 27, 2023 at 4:14
-
$\begingroup$ If space would expand within galaxies- they would break apart, i.e. become unstable. Albeit, this may happen in BigRip cosmological scenario some time in the future. $\endgroup$– Agnius VasiliauskasCommented Oct 27, 2023 at 22:05
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
4
space expansion occurs everywhere
It doesn't. There are significant parts of the Universe that is not expanding. The Milky Way is one of them, because the local density of matter within the Milky Way is greater than that of dark energy. Your body is not expanding either, for the same reason.
See this question and also this question on Astronomy SE.
-
$\begingroup$ It seems like there might be a misunderstanding. I understand that gravity holds the Milky Way together; however, the expansion of the universe occurs everywhere. My question revolves around why, despite the expansion of space that leads to the cosmological redshift, we don't observe this redshift within the Milky Way. $\endgroup$ Commented Oct 27, 2023 at 4:17
-
1$\begingroup$ @DinislamMaushov the answer to "why we to not observe a redshift within the milky way is in the answer above " : "because the local density of matter within the Milky Way is greater than that of dark energy" $\endgroup$– anna vCommented Oct 27, 2023 at 4:53
-
$\begingroup$ en.wikipedia.org/wiki/Dark_matter#Observational_evidence $\endgroup$– anna vCommented Oct 27, 2023 at 4:59
-
2$\begingroup$ I've seen physicists like L. Susskind state that, assuming dark energy exists at all scales (even small ones) the net effect would be to add a nigh-insignificant tension to gravitationally bound and matter-bound systems. So the equilibrium spacing between the Earth and Sun is perhaps nanometers greater than it would be without dark energy. And the electron ground state orbital radius of atoms, and chemical bond spacing, some amount of $10^{-xx}$ greater than predicted by Quantum Mech alone (maybe even less than Planck Length and of debatable reality) $\endgroup$– RC_23Commented Oct 27, 2023 at 16:28
$\begingroup$
$\endgroup$
2
On short distance scales, the overall expansion of the universe is too small to detect. This is why, for example, we can't detect it within the bounds of our own solar system or our neighborhood in the galaxy.
answered Oct 27, 2023 at 19:14
-
$\begingroup$ "too small to detect" is misleading. The global expansion of the universe has no effect on dynamics within the solar system or galaxy, even in principle. $\endgroup$– StenCommented Oct 27, 2023 at 21:16
-
$\begingroup$ @Sten, how should I edit? -nn $\endgroup$ Commented Oct 28, 2023 at 5:55