# Cosmological vs. Relativistic Doppler Shift

1. Mar 22, 2015

### thrush

Does anybody know a good read/primer that compares the observed cosmological redshift values to predictions made by the relativistic (gravitational) redshift?

Thank you!

2. Mar 22, 2015

### Staff: Mentor

The observed cosmological redshift is not viewed as a "gravitational redshift"; the latter concept applies to light emitted from an isolated gravitating body, which the universe is not.

3. Mar 23, 2015

### martinbn

Are you asking for comparision of what is observed and what is predicted by the theory?

4. Apr 6, 2015

### thrush

Sorry I missed the response emails...

The observed cosmological redshift is not viewed as a "gravitational redshift"; the latter concept applies to light emitted from an isolated gravitating body, which the universe is not.

Sounds right, thank you.

Are you asking for comparision of what is observed and what is predicted by the theory?

Thank you, yes. What I am looking for is a primer on the comparison between the estimated spectral line redshift from say, a star:

$\frac{\lambda_{r}}{\lambda_{e}}=\frac{\sqrt{1-\frac{2GM}{R_{r}C^{2}}}}{\sqrt{1-\frac{2GM}{R_{e}C^{2}}}}$

and that which is actually measured here on Earth, and how the two differ. It's actually for a citation, so almost anything will do.

THANKS!

Last edited: Apr 6, 2015
5. Apr 6, 2015

### Staff: Mentor

Just to confirm, this is not "cosmological redshift". This is ordinary gravitational redshift, and it has been measured in spectral lines from the Sun and from white dwarfs, and the results are in agreement with the GR prediction. See, for example, here:

http://www.einstein-online.info/spotlights/redshift_white_dwarfs

This article notes that many sources quote gravitational redshifts as a speed, which can cause confusion: this does not mean that the gravitational redshift is due to the Doppler effect. Nor does it mean that gravitational redshift is the same as cosmological redshift, which is also often quoted as a recession speed. Cosmological redshift is due to the expansion of the universe, and the formula for it is quite different from the one you give.