# How and how much does is the cosmological red shift affected?

1. Aug 15, 2013

### LuckyNate

When we look out into deep space we see a red shift in the light from distant sources because they are more or less all receding from us. How is the red shift distorted by passing through our local galactic gravitational well, before hitting us down here on the ground?(more red/more blue/no change) How acute is the impact of this effect?

Please say it so it's easy for me to read, I'm not a mathematician.

2. Aug 15, 2013

### Staff: Mentor

More blue for the gravitational potential, and depending on the direction for our motion through the galaxy.
The same effect size is present at the source, if it is within a galaxy.

To estimate the effect sizes:
The motion relative to the galaxy leads to a linear frequency shift. Typical velocities are 200km/s, so the relative shift is $\frac{200 km/s}{300,000km/s} = \frac{1}{1500}$. We know our own velocity relative to our galaxy, this can be taken into account. The motion of the source depends on the type of source.

Gravitational effects are quadratic in the velocity. Typical escape velocities are 300km/s, so the relative shift is $\frac{(300 km/s)^2}{(300,000km/s)^2} = \frac{1}{1,000,000}$. This corresponds to a really tiny velocity difference (<1km/s).

This gets different if you observe matter very close to a black hole. There, gravitational redshift (together with the motion of the objects) is important.

3. Aug 15, 2013

### Mordred

To go along with mfb's reply this thread has an article covering the various forms of redshifts and causes.

Its written in a easy to understand manner just scroll down to the Redshift and expansion article