
505Newtonian Gravity and Newtonian Cosmology
© 2010 Taylor & Francis Group, LLC
14.13.1 densiTy ParameTer Ω
Obviously, the critical density sets a natural scale for the density of the universe. So some astrono-
mers prefer to characterize the behavior of the universe in terms of the density parameter Ω, which
is dened by
Ω = ρ/ρ
c
. (14.33)
In general, Ω is a function of time because both ρ and ρ
c
depend on time. The present value of the
density parameter is denoted Ω
0
.
We can express the density parameter Ω in terms of s and H. Let us go back to Equation 14.32:
s
s
Hks
c
2
2
2
8
3
8
3
=−
π
ρ
π
ρΩ
Ω /.
Rearranging gives
Ω− =1
k
sH
. (14.34)
Then, we can ...