# Homework Help: Cosmology thermodynamics

1. May 18, 2012

### dingo_d

1. The problem statement, all variables and given/known data
Find the change in temperature due to conservation of entropy, which happens during reheating when pions ( $\pi^{\pm},\ \pi^0$ ) get out of thermodynamic equilibrium beneath 140 MeV.

2. Relevant equations

3. The attempt at a solution

I tried to compare this to the exiting of neutrinos, but I had no luck with that.

I have the connection of entropy and temperature:

$s(T)=\frac{2\pi^2}{45}g_{eff}^s T^3$

where $g_{eff}^s$ are entropy degrees of freedom given by:

$g_{eff}^s=\sum_{b=1}^{N_b} g_b\left(\frac{T_b}{T}\right)^3+\frac{7}{8}\sum_{f=1}^{N_f}g_f\left(\frac{T_f}{T}\right)^3$

I could find that looking at masses of all SM particles and seeing which are heavier than 140 MeV, and then putting them in. Only u, d, s quarks, electron, muon have lower masses than pions so I would count them out.

But other than that I have no idea how to solve this :\

Any help would be appreciated...