Transcript: Normally astronomers talk about the brightness or luminosity of galaxies. However, galaxies are not point sources. Their light is spread out or diffuse. Another measure of a galaxy brightness is its surface brightness or its flux per unit area. As measured on a fixed size of detector the surface brightness of galaxies in the local universe is independent of distance. As the distance to the galaxy increases the flux or apparent brightness goes down as the distance squared. But the area of the galaxy covered by the detector increases as the distance squared, and the two factors cancel out. But in cosmological scales surface brightness is not constant. It reduces as one plus z to the fourth power. One plus redshift to the fourth power is a substantial number. At a redshift of one it’s two to the power four, or a factor of sixteen. This diminishing surface brightness of galaxies is what makes high redshift galaxies so difficult to detect, but it also becomes a test of the standard cosmology because only in the model of cosmological redshifts caused by the expansion of the space does surface brightness decline with this particular relationship.