Why the CMBR redshift is so higher than the redshift of the most distant therefore oldest galaxies in the universe? We know that cosmological redshift rises with distance from the object but at distance from 13.4BLY to 13.7 BLY the total redshift rises with great slope. So my question is can the redshift of the CMBR be a combination of cosmological and gravitational redshift which is not the case with the light from the most distant galaxies?
$\begingroup$
$\endgroup$
asked Jan 21, 2022 at 15:07
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
The relationship between redshift and look-back time is highly non-linear!
Here's a plot from https://astronomy.stackexchange.com/a/14836/2531 which goes nearly far enough (the redshift of the CMB is about 1100) but it illustrates the essential point.
We are already at half the age of the universe by $z=1$ and at 13 Gyr ago by $z=10$. The reason for the non-linearity is that is just the way it works for a FLRW cosmology - if you go back to the first second after the big bang then the redshift is $z \sim 10^{10}$. $$ t(z) = \frac{1}{H_0} \int_{z}^{\infty} \frac{dz}{(1+z)[\Omega_M(1+z)^3 + \Omega_k(1+z)^2 + \Omega_{\Lambda}]^{1/2}} $$ where the $\Omega$ values for matter, curvature and dark energy refer to their present values.
$\begingroup$
$\endgroup$
It is simply because the CMBR formed much earlier in the history of the universe than the first galaxies. The universe just had to cool down enough for the CMBR to form, but many other processes had to happen after that before stars and galaxies formed.
