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

Staff: Mentor

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.