Thermal Neutrinos: Detecting Ancient Particles in the Universe?

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Discussion Overview

The discussion revolves around the nature of cosmic neutrinos, particularly whether neutrinos emitted in the early universe can be classified as thermal neutrinos and the implications for their detection. The conversation explores theoretical aspects, including the conditions under which neutrinos may achieve thermal equilibrium and the effects of cosmic expansion on their energy spectrum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question if neutrinos from the early universe could slow down enough to become thermal neutrinos and whether these could be detected.
  • It is proposed that the cosmic neutrino background should be thermal, with a specific mention of the PTOLEMY project aimed at measuring it.
  • Others argue that neutrinos emitted by nuclear processes after the big bang are not thermal, suggesting that even if they slow down in the distant future, their energy spectrum will differ from a thermal one.
  • A participant raises the question of whether cosmic background neutrinos remain in thermal equilibrium through interactions with matter or if they are thermal due to expansion effects.
  • One response asserts that before decoupling, neutrinos were in thermal equilibrium with the surrounding matter, and after decoupling, the expansion of the universe redshifts their spectrum into a thermal form.
  • It is noted that while massless neutrinos can be approximated as thermal, their non-zero masses eventually shift their spectrum away from being thermal.

Areas of Agreement / Disagreement

Participants express differing views on the thermal nature of cosmic neutrinos, with some asserting that they are thermal due to expansion effects while others maintain that their emission processes prevent them from being classified as thermal. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are limitations regarding the assumptions made about neutrino behavior, the dependence on definitions of thermal equilibrium, and the unresolved mathematical steps concerning the transition from relativistic to non-relativistic states.

Cato
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Would neutrinos emitted in the distant and early universe be slowed enough to become thermal neutrinos? Could these be detected?
 
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The cosmic neutrino background should be thermal. PTOLEMY is a plan to measure it.

Neutrinos emitted by nuclear processes after the big bang are not thermal, even if they become slow in the very distant future their energy spectrum will look different.
 
mfb said:
The cosmic neutrino background should be thermal. PTOLEMY is a plan to measure it.

Neutrinos emitted by nuclear processes after the big bang are not thermal, even if they become slow in the very distant future their energy spectrum will look different.

Do the cosmic background neutrinos remain in thermal equilibrium by interacting with matter? Or are they thermal simply because of the effects of expansion?
 
Cato said:
Do the cosmic background neutrinos remain in thermal equilibrium by interacting with matter? Or are they thermal simply because of the effects of expansion?

The latter.

Before decoupling, neutrinos were in thermal equilibrium with the "cosmic soup". It can be shown that after decoupling, expansion of the universe redshifts a thermal spectrum for radiation into a thermal spectrum. Weinberg in his book "Cosmology" demonstrates his on pages 101-102. Weinberg

Thus the photon density has been given a black-body form even after hye photons went out of equilibrium with matter, but with a redshifted temperature.

The above applies to massless neutrinos, which is a good approximation for some of the history of the universe. Their non-zero masses, however, gradually shift their spectrum away from a thermal spectrum. From "Relativistic Cosmology" by Ellis, Maartens, and MacCallum page 304:

The most massive neutrinos become non-relativistic well after radiation matter inequality. We can estimate the non-relativistic redshift by setting the mean energy per neutrino equal to the mass.
 
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Terrific. Thanks very much.
 

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