# Does Hubble's constant apply to galaxies that are blue-shifted/ moving towards us? Another question

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?

• I could in principle, but that is not the universe we observe. If we were in a collapsing universe it might well apply. Oct 7, 2020 at 22:13
• 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)
– Luke
Oct 8, 2020 at 3:21

• 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. Oct 12, 2020 at 9:23