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As stated above...

How can the the physics of early cosmology (articles about which are where I usually come across mentions of such) be affected by an 'effective' number of neutrino types, rather than an actual?

Are some flavours of neutrinos more effective than others? At what, exactly? Inflating the early universe?

EDIT: I still do not understand how the number of different, oscillating, species of massive neutrinos affects the early evolution of the universe, rather than just the total number of all neutrinos (regardless of species and regardless of whether they oscillate), or total mass, or whatever......

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I'm no expert, but had the same question, and believe it works like this.

https://academic.oup.com/mnras/article/406/3/1805/977724 "The number of neutrino species participating in weak interactions is known to be three to high precision [electron neutrino, mu neutrino, tau neutrino]. However, the possibility remains that additional ‘sterile’ species exist."

One of several standard model predictions is Neff = 3.046. "This [non-integer result] is not because there is a non-integer number of neutrino species but simply comes from the definition of Neff." https://moam.info/cosmic-dark-radiation-and-neutrinos_5b886566097c474e418b46a2.html

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    $\begingroup$ The extra $0.046$ comes from neutrino interactions and QED corrections, I think. By the way, not just neutrinos can lift the number $3.046$, but also ultra-light particles, e.g. axions from large-volum string compactifications. And indeed, this is really due to the definition as in the link of Kosm. $\endgroup$
    – psm
    Commented Mar 18, 2022 at 17:12

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