"Starboard" wrote in message
oups.com...
How can an astronomer know the difference between light that's red
shifted due to Doppler, as opposed to light red shifted due to gravity,
Compton effect, etc..., ?

The Compton effect is the change in wavelength _and_ direction of a single
photon as it interacts with matter. An astronomer could not distinguish
between those effects _for a single observed photon_.

But since the Compton scattering does change the direction of the photons,
unless you are willing to say that all of the photons from a distant object
are deflected in exactly the same amount and direction (or how theyare
subsequently re-scattered back to the original direction, and provide a
viable explanation of WHY any of this would happen), then you would not be
able to form an image of the distant object.

Since we can form an image of the distant object, and we can conceive of no
realistic mechanism to make all the photons scatter to remain in a parallel
path - then we can rule out Compton. For the whole group of photons.

From a single vantage point, Doppler (from real motion) and gravity are
indistinguishable. It you had a large enough baseline, you could use
parallax to differentiate, but the baseline would have to be a non-trivial
fraction of the distance. We would not be able to make a measurement
billions of lightyears away from here to distinguish. The gravity red-shift
woud be the same no matter what direction you look at the dense object; the
Doppler would be different based on the actual direction of motion.

However, Doppler due to space expansion would not be distinguishable.

Jim