# Photons vs neutrinos energy ratio

1. Oct 1, 2011

### Calimero

What is the ratio of energy contained in photons vs energy contained in neutrions in the present universe? Also, how is that ratio changing with time? Thanks.

2. Oct 2, 2011

### nicksauce

The ratio is, assuming massless neutrinos,

$$\frac{\rho_{\gamma}}{\rho_{\nu}} = \frac{21}{8}\left(\frac{4}{11}\right)^{4/3}$$

or, approximately, 0.68. The number won't change with time, since both are evolving as free radiation, $\rho\sim a^{-4}$.

3. Oct 3, 2011

### Calimero

Thank you nicksauce.

4. Oct 3, 2011

### Drakkith

Staff Emeritus
nick, how much does that change assuming neutrino's have mass?

5. Oct 4, 2011

### nicksauce

The usual expression is in terms of $\Omega$ is, I think,

$$\Omega=\frac{m_{\nu}}{94h^2eV}$$

I'm not sure what people think the masses are these days.