[PatrickHenry]

Are there any forces in interstellar space that would cause a decrease in the momentum and hence frequency of photons, leading to an observed red shift caused by something other than recessional velocity?

There's a gravitational redshift. Light leaving a very massive body will have predictably stretched wavelengths, and thus it will appear to be shifted into the red end of the spectrum. It's a well-known phenomenon, and taken into consideration by astronomers. I don't think this affects momentum, however.

If I'm wrong, I invite RA to clean up behind me.

[richtig_faust]

I don't think this affects momentum, however.


I was thinking of the exact scenario you mentioned, I believe the phase velocity would have to change in order for this to occur - and according to general relativity I do believe this is the case.