Fig. 1.

Illustration of the geometry of the problem. At a given time t, a shell of radius R0(t) emits photons towards the observer (along the z-axis). Different regions of different z coordinates (illustrated here with three colored circles) are located at different distances from the observer. Thus, for all the photons that reach the observer at a given time, there is a corresponding geometrical delay τ of their emission time.
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