
 Distance Measures in Cosmology 
  The unifying aspect is that all distance measures somehow measure the separation between events on radial null trajectories, ie, trajectories of photons which terminate at the observer. 
  For small v / c, or small distance d, in the expanding Universe, the velocity is linearly proportional to the distance (and all the distance measures, eg, angular diameter distance, luminosity distance, etc, converge) 
  The comoving volume element and its integral are both used frequently in predicting number counts or luminosity densities. 
 www.astro.ufl.edu /~guzman/ast7939/projects/project01.html (1088 words) 
