Comoving means the reference frame follows the particle itself (as opposed to us, the source, etc). Space is expanding behind the photon in addition to in front of it and it has to travel a long time to reach us. This makes us get different answers if we ask “how far is it between the object that emitted this photon and us today (proper distance)” vs asking “how long/far has the photon traveled(comoving distance)”. For photons the comoving distance (ly) is identical to time of flight (yr).
I belive astronomy’s use of “comoving” is slightly different from the special relativity meaning and astronomers ignore the time dialation and length contraction effects. So the frame follows the particle for the purposes of expansion but is still in our/the observers frame as far as time, length, velocity etc.
Comoving means the reference frame follows the particle itself (as opposed to us, the source, etc). Space is expanding behind the photon in addition to in front of it and it has to travel a long time to reach us. This makes us get different answers if we ask “how far is it between the object that emitted this photon and us today (proper distance)” vs asking “how long/far has the photon traveled(comoving distance)”. For photons the comoving distance (ly) is identical to time of flight (yr).
I belive astronomy’s use of “comoving” is slightly different from the special relativity meaning and astronomers ignore the time dialation and length contraction effects. So the frame follows the particle for the purposes of expansion but is still in our/the observers frame as far as time, length, velocity etc.