R(t) is the scale factor (which we write as a(t), with a=1 now). l(t) is the proper distance (which we write as r) and r is the comoving (current)
distance (which we write as ro). These two galaxies are anchored to the grid -- they have no peculiar
velocities. |
As the universe expands, light crosses the
expanding grid (always moving locally at speed c). The redshift is simply R(tobs) / R(tem), the ratio of the scale factors between observing
and emission. |
Figure from OU text "Galaxies and Cosmology"
|