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arXiv · 2607.15335

Primordial tensor mode from the neutrino sector

Shohei Saga·Shuichiro Yokoyama
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Abstract

We show that a regular solution for primordial tensor fluctuations can arise from the higher-order multipoles of the collisionless neutrino distribution after neutrino decoupling. Focusing on the leading case with the neutrino octupole mode (tensor-octupole mode), we derive the initial conditions for the Einstein-Boltzmann system and calculate angular power spectra for the cosmic microwave background (CMB) anisotropies. Compared with the standard gravitational-wave mode, the tensor-octupole mode has a weaker large-scale metric response. It therefore gives a suppressed reionization bump and a different oscillation phase in the tensor CMB power spectra, providing a way to separate it from the standard tensor mode.

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Shohei Saga, Shuichiro Yokoyama. 2026-07-16. Primordial tensor mode from the neutrino sector. https://arxiv.org/abs/2607.15335

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