2.2. The Baryon Density

In the expanding Universe, the number densities of all particles decrease with time, so that the magnitude of the baryon density (or that of any other particle) has no meaning without also specifying when it is measured. To quantify the universal abundance of baryons, it is best to compare n_B to the CBR photon density n. The ratio, n_B / n is very small, so that it is convenient to define a quantity of order unity,

(7)

where _B is the ratio (at present) of the baryon density to the critical density and h is the present value of the Hubble parameter in units of 100 km s^-1 Mpc^-1 (_{B B} h²).