Photons vs neutrinos energy ratio

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Discussion Overview

The discussion centers on the energy ratio between photons and neutrinos in the present universe, including how this ratio may change over time. It involves theoretical considerations and assumptions regarding the mass of neutrinos.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant proposes a formula for the energy ratio of photons to neutrinos, assuming massless neutrinos, resulting in an approximate value of 0.68.
  • Another participant questions how the energy ratio would change if neutrinos have mass, indicating uncertainty about the current understanding of neutrino masses.
  • A further inquiry is made regarding the expression of the energy ratio in terms of the density parameter Omega, suggesting a lack of consensus on the values of neutrino masses.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of neutrino mass on the energy ratio, and multiple competing views regarding neutrino mass and its effects are present.

Contextual Notes

Limitations include assumptions about neutrino mass and the dependence of the energy ratio on these assumptions, as well as unresolved questions regarding the current estimates of neutrino masses.

Calimero
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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.
 
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The ratio is, assuming massless neutrinos,

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

or, approximately, 0.68. The number won't change with time, since both are evolving as free radiation, [itex]\rho\sim a^{-4}[/itex].
 
Thank you nicksauce.
 
nick, how much does that change assuming neutrino's have mass?
 
Drakkith said:
nick, how much does that change assuming neutrino's have mass?

The usual expression is in terms of [itex]\Omega[/itex] is, I think,

[tex]\Omega=\frac{m_{\nu}}{94h^2eV}[/tex]

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

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