Does the Density of the universe change over time?
There are 3 main components of the universe. 1) Radiation (light). The density of radiation falls as 1/a^4, where a is the scale factor of the universe. This means that the density of light decreases over time 2) Matter. This is probably what you are asking about. The density of matter falls as 1/a^3. This means that the density of matter also decreases with time, and for exactly the reasons you would think (constant matter, increasing volume). 3) Dark Energy. The density of dark energy is constant with respect to the universal scale factor. Cool, huh? In conclusion, for what you are concerned with, yes.
