Lepton Number and Expansion of the Universe
Cheng Tao Yang, Jeremiah Birrell, Johann Rafelski

TL;DR
This paper explains the non-integer effective number of neutrinos as a consequence of lepton asymmetry in the early universe, proposing a new ratio between baryon and lepton numbers that resolves existing observational tensions.
Contribution
It introduces a novel interpretation of $N^{ ext{eff}}_ u$ as arising from lepton asymmetry, challenging the standard assumption of negligible lepton number.
Findings
Reconciles Planck-CMB and H0 measurements with lepton asymmetry.
Proposes a new ratio between baryon and lepton numbers consistent with observations.
Predicts a non-integer $N^{ ext{eff}}_ u$ greater than 3.
Abstract
We show that the non-integer effective number of neutrinos can be understood as an effect of lepton asymmetry in the early Universe carried by the Dirac neutrino cosmic background. We show that (CMB only) and (CMB and ) require a ratio between baryon number and lepton number to be . These values are close to the baryon-to-photon ratio . Thus instead of the usual and , we propose to use and as another natural choice, which resolves the tension between Planck-CMB and and leads to a non-integer value of .
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Taxonomy
TopicsCosmology and Gravitation Theories · Particle physics theoretical and experimental studies · Black Holes and Theoretical Physics
