Hi everyone,(adsbygoogle = window.adsbygoogle || []).push({});

Can anyone point me into the right direction on how to compute the deceleration parameter (##q_0##) for a radiation dominated universe?

I know that q_{0}is given by

##q_0=-\frac{\ddot{a}(t_0)}{a(t_0) H^2 (t_0)} = - \frac{\ddot{a}(t_0) a(t_0)}{\dot{a}^2(t_0)}=\frac{\Omega_0}{2}##

So for a radiation dominated universe, we have ##\omega = + \frac{1}{3}##. And ρ~1/a^{4}, and a~t^{1/2}. I'm not sure how to relate this to that equation.

My notes say ##\Omega _{rad} = 8.2 \times 10^-5## but I'm not sure how this was calculated. How do I compute Ω_{0}=ρ/ρ_{crit}in this case? I know that ρ_{crit}=3H^{2}/8πG, but what is the density ρ?

Using that value I get the following but I'm not sure if it's correct:

##q_0 = \frac{\Omega_0}{2} = \frac{8.2 \times 10^-5}{2} = 4.1 \times 10^{-5}##.

I couldn't find any info on this computation online. So any help is greatly appreciated.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Deceleration Parameter Computation

**Physics Forums | Science Articles, Homework Help, Discussion**