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arXiv · 2509.03595

Tracing the Neutrino-Induced Phase Shift in the 21-cm Spectrum

Gabriele Montefalcone·Hector Afonso G. Cruz·Julian B. Munoz·Ely D. Kovetz·Marc Kamionkowski
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Abstract

We study the phase shift that free-streaming neutrinos imprint on the 21-cm power spectrum during cosmic dawn, computing for the first time its effect on both density- and velocity-induced acoustic oscillations. Neutrinos are known to generate a characteristic phase shift in the acoustic oscillations of the photon-baryon plasma before recombination, a signature already detected in the cosmic microwave background (CMB) as well as the spectrum of baryon acoustic oscillations (BAOs) extracted from galaxy surveys. We show that in the 21-cm signal this phase shift is distinct from that observed in the CMB and BAO spectra, exhibiting a characteristic mode and redshift dependence arising from the additional contribution of the so-called velocity acoustic oscillations (VAOs), sourced by the baryon-dark matter relative velocities. Our results establish the phase of acoustic oscillations in the 21-cm spectrum as a promising new avenue for probing free-streaming light relics at cosmic dawn, complementary to existing CMB and BAO measurements.

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Gabriele Montefalcone, Hector Afonso G. Cruz, Julian B. Munoz, Ely D. Kovetz, Marc Kamionkowski. 2025-09-03. Tracing the Neutrino-Induced Phase Shift in the 21-cm Spectrum. https://arxiv.org/abs/2509.03595

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