Plateau and Dip: The Music of Cosmic Strings ✦

Original: "Gravitational Wave Imprints of a High-Quality Axion and the Origin of Flavor Hierarchies"

· K. S. Babu, Sai Charan Chandrasekar, Sudip Jana, Sudip Manna

arXiv:2606.05320v1 · 2026-06-03 · CC BY 4.0 · ⏱ 1 min · HEP Phenomenology Cosmology High Energy HEP Experiment

A gravitational-wave signal with a flat plateau and a sharp dip — like a note frozen in time — could be the first evidence of a common origin for axion dark matter and the fermion mass hierarchy.

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Gravitational waves are not just ripples — they are the score of the early Universe. It may conceal a dip: a sudden silence caused by the rebirth of cosmic strings at the QCD epoch. Just as a violin string, snapping, silences the orchestra for a moment, so the flavor string, losing its interaction with the axion, changes its radiation pattern. The LISA detector may well catch this pause from a time when the Universe was only a few microseconds old — and then we will learn where particle masses come from and what dark matter is.

🎯 An axion string can decay into several unit strings, which annihilate, avoiding the dominance of domain walls. The characteristic dip in the spectrum is not just a 'cosmic thermometer,' but an imprint of the exact moment when the Universe became transparent to axions; it fixes the temperature of the QCD transition with an accuracy unattainable by any terrestrial experiment.

N_{\text{DW}} = q_X w_S - q_S w_X = 1

The string carries exactly one domain wall, preventing the Universe overclosure problem.

f_{\text{cut}} \approx 8.67 \times 10^{-3}\,\text{Hz} \left(\frac{T_{\text{QCD}}}{\text{GeV}}\right) \left(\frac{10^{-11}}{G\mu_0(v_X)}\right)^{1/2}

The cutoff frequency corresponding to the epoch when purely gauge strings form at the moment of the QCD transition. A kind of 'note' by which we pinpoint the birth of a new type of string.

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Plateau and Dip: The Music of Cosmic Strings ✦