If universe is expanding with the velocity of light then why the distance between earth and sun is constant? Also why the distance from one Galaxy to another remains constant? And so on...
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$\begingroup$ Possible duplicates: physics.stackexchange.com/q/2110/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Oct 17, 2021 at 19:40
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$\begingroup$ Using Stokes drag law applied for a universe expansion, one can calculate space drag force due to space viscosity, $F_{\rm {d}}=6\pi \mu R~HD$. Last term is Hubble law. Substituting values, gives $6 \pi \cdot 10^9 Pa \,s \cdot 6371 km \cdot 70 km/s/Mpc \cdot 149070000 km \approx 10^{10} ~N$ force with which space tries to pull Earth away from Sun. However Sun gravitationaly attracts Earth with $10^{22} ~N $ force. So universe needs expansion rates $10^{12} \times$ greater to break Earth and Sun apart. Which may happen by some BigRip scenario in undefined future. $\endgroup$– Agnius VasiliauskasCommented Oct 17, 2021 at 20:32
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If you are close enough, the force of gravity can overcome the Hubble Expansion, It is then said that you are "gravitationally bound".
Also why the distance from one Galaxy to another remains constant?
You can't say this at all:
- You have to specify which two galaxies you are talking about. If they are close enough to each other, they are gravitationally bound and not subject to the Hubble Expannsion If they are far enough away from each other, then they are not gravitationally bound a subject to the Hubble Expansion.
- Furthermore, You can't even say that the distances between gravitationally bound galaxies remains the same. The Andromeda galaxy is moving towards the Milky Way.
