Cosmology-Application of the friedmann equation

[andyfreesty1e]

Homework Statement

The universe is expanding as shown by a Hubble constant H of 75 Km s^-1 Mpc^-1, but a constant density of matter of 10^-26 kg m^-3 is maintained by the continuous creation of hydrogen atoms as the universe expands. Find the rate of creation of hydrogen atoms per unit volume.
dont really know how to go about this

Homework Equations

for a matter dominated universe, a~t^2/3 and rho~a^-3

The Attempt at a Solution

 

[grey_earl]

So you have ρ(t) given, which decreases with time, but you want to maintain a constant density ρ_0. The creation rate is just d/dt [ ρ_0 - ρ(t) ].