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Beer-monster
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Homework Statement
Neutrinos are massless spin-1/2 particles (ignore their tiny finite masses).
There are 6 types of neutrinos (3 flavours of neutrinos and 3 of anti-neutrinos),
and each has just one possible polarization state. In the early universe neutrinos
and antineutrinos were in thermal equilibrium with zero chemical potential,
and filled the universe as a kind of background radiation. Derive a formula for
the total energy density (energy per unit volume) of the neutrino-antineutrino
background radiation when it was at temperature T . Leave your answer as a
constant times a definite integral over a dimensionless variable. Don’t forget to
determine the value of this constant.
Homework Equations
Fermi-Dirac Distribution with zero chemical potential.
[tex] n_{r}(\epsilon) = \frac{1}{e^{\beta \epsilon_{r}}+1} [/tex]
The Attempt at a Solution
Just a quick issues about statistical mechanis in general. If I had a gas of one sort of neutrino at a temperature T each particle would occupy a different one of the ppssible states and the distribution of this would be given by the Fermi-Dirac Distribution shown above.
A gas of a different neutrino flavour should (I think) have a slightly different array of possible states and (though still described by FD stats) the distribution over these states should would be different than the one above.
However, if I mix these gases or 6 of them I'm not sure how I should combine them. Does it count as one distribution function with the energy being a sum of the energies of the components
[tex]\epsilon= \epsilon_{1} + \epsilon_{2} + ...[/tex] etc.
Or am I just thinking about the whole thing wrong.
Thanks for your help.