GW150914 Redshift: Cosmological & Gravitational

[Leo.Ki]

The cosmological redshift of GW150914 was estimated, but what about its gravitational redshift due to it having been produced so close to the merging black holes? I am expecting varying redshifts depending on where in the vicinity of the phenomenon each stretch of wave was created, and when during the evolution of the phenomenon, resulting in a complex redshift pattern. Thoughts?

 

[bcrowell]

A gravitational redshift can only be defined in a stationary spacetime. We have three distance regimes here: (1) the near field, (2) far field, but small distances compared to cosmological distance scales, and (3) cosmological distances. Only if you look at things from the perspective of distances in the intermediate range do you see gravitational waves on a stationary background. I would assume that since the people doing the numerical relativity simulations were competent, they got all the effects right. Most likely they sampled the results of their calculation in region 2, and then used standard cosmological Doppler shifts to extrapolate from 2 to 3.

I doubt that your idea of conceptualizing the transition from region 1 to region 2 as a varying redshift will work. We simply don't have a way to define such a gravitational redshift so that it extends into region 1, where there is no stationary background.

 

[Leo.Ki]

Thank you so much, Ben. I think I see clearer now. The waves being very probably calculated as part of the whole very dynamic gravitational environment, what I described naively as the gravitational redshift would already be included in the package and would not be something that can be clipped out of the equations.