# Measuring Gravitational Redshift due to Galaxies without GR

1. May 9, 2015

### quantumfoam

Hi guys.

How do astrophysicists measure the redshift of electromagnetic waves from galaxies due to gravity without the use of General Relativity? If I can be more specific, how do astrophysicists know that the gravitational redshift of light emitted from some part of a galaxy or galaxy cluster is small relative to kinematic redshifts (if these light emitting components of a galaxy or galaxy cluster are moving away from us of course) without using General Relativity to prove that such a redshift is small? For example, when creating the rotation curves for galaxies, it is often claimed that the redshifts measured are significantly due to kinematic effects rather than due to gravitational redshifts. How do astrophysicists know this without using General Relativity to show that this is true?

2. May 10, 2015

### Orodruin

Staff Emeritus
Well, to start with, it does not matter for the rotational curves as you are looking at differences of redshift rather than absolute values.

You can also estimate the amount of redshift by estimating the mass.

3. May 10, 2015

### quantumfoam

I'm sorry. I don't think I understand how it doesn't matter for rotational curves. Could you please explain it a little more?

4. May 10, 2015

### Bandersnatch

When you measure rotation, you look at red and blue-shifted lines in the galactic (or stellar) spectrum spread symmetrically around the expected line position. It'll produce a symmetrical spread of certain width, corresponding to the difference in velocities between the limb rotating towards you (blue-shifted) and the one rotating away (red-shifted). It doesn't matter where exactly the whole thing is in the spectrum (i.e., how shifted by gravity), since it's the width that gives you rotation data, and it doesn't change.

5. May 10, 2015

### quantumfoam

Thank you very much!