cragar said:
When they talk about the redshift of the photon, they say it is because space itself is expanding and it is not a Doppler shift. So the photon actually loses energy on its way to earth, Is this similar to Gravitational redshift? It seems weird that the photon would get shifted just because space is expanding.
Pedagogically it's a contentious issue how that should be explained. People have long arguments and get doctrinaire about what you should tell students.
It can be analyzed and pictured in different ways.
Doppler analysis does not work if you just consider the recession speed at the moment of emission and at the moment of arrival. I think that is the message Lineweaver is getting across. t behaves as if the wavelength was gradually extending all during the travel. Because in the end the wavelength is expanded by the same factor that distances have expanded.
But you can set up a long series reference frames and a long series of small doppler shifts and do the analysis and get the right answer. So some people like to understand it that way.
Others find that too long and involved and they prefer to think of the wavetrain being elongated as the distance between here and there increases---by the same z+1 factor.
It is "AS IF" the wavetrain were being stretched by expanding space---but that is just a pictorial way to describe it. Space is not a stretchy material.
I think if one were to write out Maxwell's equation for EM wave propagation in a geometry with expanding distances it would work out
as if the waves were being stretched while they were in flight. But anyway people argue about how this should be explained.
It is a smart question to ask, but I'm embarrassed to say I don't have a satisfactory answer.
I don't like the explanation with the long series of little doppler shifts, a long chain of imagined intermediate observers. It is too elaborate for my taste. I like the stretch picture but I accept that space is not a material substance, so it is just a picture.
