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Does Hubble's constant apply to galaxies that are moving towards Earth with a velocity and does Hubble's constant also able to different systems such as binary stars, stars in another galaxies or black holes?

Also, why is there such as large uncertainty in the Hubble's constant?

It is because: for further away galaxies, the expansion rate is not constant due to dark matter and so galaxies' velocities vary. Also when measuring the much further away galaxies, the parallax angle must become so small that the absolute uncertainty for the parallax angle must be large, causing the distance measured from the galaxy to Earth to have a large uncertainty?

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  • $\begingroup$ I could in principle, but that is not the universe we observe. If we were in a collapsing universe it might well apply. $\endgroup$ Oct 7, 2020 at 22:13
  • $\begingroup$ I believe Hubble's constant (i.e. more distant galaxies are more red-shifted) is mainly used at very far distances, where the red-shift from the expansion of the universe is much greater than individual motions of galaxies. Some of those distant galaxies may be blue-shifted relative to the baseline Hubble red-shift, but nothing that far away has a net blue shift. (I'm not a cosmologist though; happy for corrections on any of the above) $\endgroup$
    – Luke
    Oct 8, 2020 at 3:21

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The only galaxies that we observe to be blue-shifted are some galaxies in the Local Group such as Andromeda. These galaxies are relatively close neighbours to the Milky Way, so although they are still subject to the Hubble flow they also have an individual radial speed towards us that is greater than the expansion of space.

Galaxies which are further away will also have their own individual speeds, but the Hubble speed at their distance is greater than these individual speeds, so all distant galaxies appear red-shifted. Nevertheless, their individual speeds will cause a distribution of red-shifts around the theoretical Hubble red-shift. This is one factor which makes it difficult to determine the Hubble constant with high precision.

Other sources of uncertainty are the difficulty in precisely measuring the actual distance to a far galaxy, and the fact that the Hubble "constant" is not actually constant at all, but has changed over lifetime of the universe.

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  • $\begingroup$ There are several dozen galaxies in the Virgo Cluster with blueshifts,. This is because it is a massive cluster within which the galaxies are orbiting with large velocities. So even though the mean redshift of the cluster is slightly over $1000$ km/s, some of the galaxies happen to be moving in directions towards us along our line of sight with $V > 1000$ km/s, and so we see a net blueshift. $\endgroup$ Oct 12, 2020 at 9:23

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