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I know that it is not supposed to be expanding but space is space and it is expanding everywhere else outside it so why not inside it too, and gravity just holds everything back.

I suppose if it is, it would be insignificant, on a cosmic scale, for what is 3 MPcs, out of 14.2K, but then again our instruments are so precise now, but then again there are all the other Doppler and gravitational shifts.

The kinematic/velocity time dilation is caused by the motion thru space, or it moving thru U. It's the same, thing. So the out-flowing space would cause time to run a bit slower in our outer regions. And, if space flows out from our local group, wouldn't it also flow out from a black hole!?

And I mean/t the redshift caused by the expansion of space and not the Doppler shift caused by the peculiar velocities, of course.

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    $\begingroup$ I guess that the correct answer is yes there is a cosmological redshif within the local group of galaxies, but the various motions of the galaxies in the local group involve far greater redshift or blueshift that masks the cosmological redshift. Trying to detect the cosmological redshift in the local group would be like trying to detect the breathing of someone running straight at you. $\endgroup$ Jan 31 at 3:17
  • $\begingroup$ @MA.Golding: Ye's, but then again the COBE, SCP and High-z SST was also very hard. $\endgroup$ Feb 1 at 15:06
  • $\begingroup$ The large scale motion is simply the net sum of all the small scale motions. Otherwise, you get tears in the fabric of space. $\endgroup$
    – eshaya
    Feb 1 at 16:05

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No. The cosmological redshift is due to recessional motion. There is no additional redshift due to "expansion of space" (which is not a physical phenomenon).

The only reasons "expansion of space" is often emphasized as the source of the redshift are:

  1. It's the most convenient way to calculate the cosmological redshift for cosmologically distant objects. Over cosmological distances, recession speeds are not uniquely defined, and it is tricky to construct the speed that correctly corresponds to the cosmological redshift. (By "cosmological distances" I mean comparable to the Hubble scale, so billions of light years or more.)

  2. Often, the author holds the misconceived notion that "expansion of space" is physical. That's a common misconception that has even made its way into published scientific articles.

A useful reference on this topic: https://arxiv.org/abs/0808.1081

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