On my lecturer's notes on Dark Matter I am told:
"It is customary to define densities normalised by the time dependent volume $V (t) = a(t)^3$. The reason for this is that, in the absence of number changing processes, the comoving number density remains constant with time evolution."
I don't understand what is meant by a number changing process.
The image is given as an example of the preservation of the comoving number density.
Initially I thought a number changing process was one in which the number of the particles changed as the process evolved but I believe the image is trying to emphasise that the number of particles is kept constant.