# 10. Models¶

## 10.1. Homogeneous and Isotropic Neutrino Gas¶

$$\vec P_\omega$$ can be time-independent.

Regarding the discussion in Pictures, we know immediately that $$\vec H$$ is aligned or anti-aligned with $$\vec P_\omega$$ since

$\vec H \times \vec P_\omega = 0,$

as $$\vec P_\omega$$ is time-independent.

Anyway the equation we need to solve is then

$\vec a$

### 10.1.1. What to expect?¶

Without solving the equation, we know that

### 10.1.2. Solving Eqns¶

What is special in this model?