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enter image description here

$$\dot z\equiv\frac{\mathrm d z}{\mathrm d t_{\text{obs}}}(t_0)=(1+z)H_0-H(z)$$

The picture and equation above are quoted from Liske et al. (2008).

According to the equation, the redshift of the cosmic microwave background radiation is expected to decrease at this point in time.

However, I think the redshift that will be measured in the future will increase.

Doesn't this equation apply to the cosmic background radiation?

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I think you cannot apply this equation to the cosmic microwave background and indeed, the redshift of the CMB is increasing with time.

The difference is that the photons we receive from the CMB will always come from a fixed epoch in the universe (the epoch of recombination).

In contrast, the photons that we receive from a distant galaxy were emitted at an epoch that depends on the redshift of the galaxy and this will change with time. In other words, we can watch galaxies getting older. At high redshifts, as a galaxy ages it will experience a deceleration in the universal expansion, as we see it, and thus its redshift decreases. At later times and lower redshifts, the expansion accelerates and the redshift increases.

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  • $\begingroup$ thanks. Your answer is always helpful. In the case of cosmic background radiation, we always observe fixed points of view, so we have to think differently than in the case of galaxies. $\endgroup$ Jul 3, 2021 at 10:48
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    $\begingroup$ Just to point out that this is theoretical speculation, based on present cosmological models. Nobody has ever observationalyl measured the change of redshift of the CMB or galaxies with time (and given the time scales required to do this, nobody probably ever will). $\endgroup$
    – Thomas
    Jul 3, 2021 at 11:11
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    $\begingroup$ @Thomas indeed redshift drift is a prediction that depends on the cosmological parameters. However, contrary to your statement, there are plans to attempt to measure it. $\endgroup$
    – ProfRob
    Jul 3, 2021 at 11:47
  • $\begingroup$ Yes, results from SKA might be available in 10 or 20 years according to arxiv.org/abs/1907.04495 $\endgroup$ Jul 3, 2021 at 12:45
  • $\begingroup$ "At high redshifts, as a galaxy ages it will experience a deceleration in the universal expansion, as we see it, and thus its redshift decreases." Can you please elaborate on this? Isn't the expansion accelerating? $\endgroup$ Jul 3, 2021 at 17:39

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