Photons vs neutrinos energy ratio

[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.

 

[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}.

 

[Calimero]

Thank you nicksauce.

 

[Drakkith]

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

 

[nicksauce]

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

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.