Transcript: An important test of the big bang model is some sort of verification of the reality of cosmic expansion. It’s possible that redshift may be non-cosmological in origin caused by some other phenomenon or property of light. The best test of the cosmological expansion, the reality of expanding space, is the prediction that the surface brightness of an object goes as one plus the redshift to the fourth power. Surface brightness is defined as the flux per unit area of an extended object like a galaxy. This means that in the big bang model galaxies should dim according to one plus z to the fourth power. Two of the factors of one plus z come from the area effect. Another factor of one plus z comes from time dilation in the arrival rate of photons from distant regions, and the fourth factor of one plus z is the redshift itself. It’s a difficult measurement once again because galaxies do not have sharp edges and are not very good standard candles in their properties. However, in the past decade astronomers have indeed confirmed that light from distant objects diminishes according to one plus z to the fourth and that the cosmic expansion is real.