The Standard Dust Models (Λ=0)
Have a Newtonian Analog:
Replace x by comoving value r using the scale factor R,
x = Rr, and express density in terms of its value at the
present epoch ρ = ρ0R-3, then:
Which matches eq. 7.1 for dust and lambda=0.
Multiplying by the derivative of R and integrating gives
us essentially eq. 7.2