Hi All,
My first post here.Background: PhD in math, theoretical physics minor as an undergrad, part-time work at Uni, part time in telecommunications industry.
This is probably common knowledge to all the folks here, but since the point is never discussed in popular astronomy books (my main source) I want to ask about it now given that today's APOD gives it some context.
In all those astronomy books intended for the public the redshift is always explained as a Doppler shift, i.e. coming from relative motion. Then the books go on to describe Hubble's efforts that eventually lead to a method of sizing up the universe and estimating the distances to the galaxies et cetera. Now on a course on general relativity (it was 20 years back, so you will find me *very* rusty) it was described to me how a photon climbing up a strong gravitational field also loses energy and is thus (by Planck's law) redshifted. This made sense at the time (some named principle was involved), but the details escape me.
So I just wonder, how do the folks analyzing the data like in today's image tell one redshift from another? A distant stellar object, in order to be detectable in these parts, has to be mighty big, both luminous (in some frequency band at least) and massive. So how can they tell which fraction of the redshift is caused by gravity, and which fraction is "true Doppler shift"?
I guess some estimates and statistical methods are used, but such estimates would then add a potential error term to Hubble's constant, the age of the universe et cetera, right? I do realize that in popular books some "lies to children" are often necessary
![Smile :-)](./images/smilies/icon_smile.gif)
Cheers,
Jyrki