# Hubble Parameter

1. Dec 10, 2013

### HawkEye5220

1. The problem statement, all variables and given/known data
For a κ=0 universe with no cosmological constant, show that H(z)=H0(1+z)3/2

2. Relevant equations
Friedmann equation: H2=$\frac{8*\pi*g}{3c^2}$-$\frac{κc^2}{r^2}$*$\frac{1}{a(t)^2}$

3. The attempt at a solution
I know that R(z)=R0/(1+z) but I do not know where this comes from. Following this, I should be able to take a density ρ(z)=ρ(now)*(1+z)^3 and input it into the Friedman equation but I am not sure how to proceed

2. Dec 12, 2013

### cepheid

Staff Emeritus
The Friedmann equation should be $H^2 = \frac{8\pi G}{3}\rho$, where rho is the total mass density of the universe (in this case considered to be entirely due to matter). You can ignore the second term with kappa entirely, since it's zero (flat universe). You are correct that you can write $\rho = \rho_0 (1+z)^3$ for ordinary matter. The trick now is figuring out how to rewrite the $(8\pi G)/3$ pre-factor in terms of something else. Hint: what is the expression for the critical density?