Prospects of Detecting Cosmological Domain Walls in the Cosmic Microwave Background
Abstract
Topological defects are a generic prediction of field theories with spontaneously broken symmetries, and domain walls, the two-dimensional defects left by a broken discrete symmetry, are among the simplest. We study the imprint of dark, purely gravitationally interacting domain walls on the Cosmic Microwave Background (CMB), in the regime where a low symmetry-breaking scale η allows the walls to evade the Zel'dovich overclosure bound while remaining cosmologically relevant. The relativistic repulsion of matter by a planar wall carves out an evacuated void bounded by an overdense shell, and we compute the resulting photon deflection field analytically for walls both aligned with and arbitrarily inclined to the line of sight. The deflection stamps coherent, strongly non-Gaussian distortions onto the temperature sky, with a characteristic two-peak structure set by the relativistic displacement scale and an inclination dependence that vanishes for face-on walls. Because these signatures are localised and anisotropic, they are poorly captured by two-point statistics; we therefore analyse them with a curvelet/wavelet decomposition matched to the wall's geometry, and with simulation-based inference, training a Neural Posterior Estimator (NPE) to recover the scattering redshift z, energy scale η, and inclination θ, and a Neural Ratio Estimator (NRE) to evaluate the likelihood-ratio test statistic for limit-setting. Casting these methods as sensitivity forecasts for a South Pole Telescope-like (SPT) survey, we find that the curvelet and wavelet statistical thresholds populate a common η∼ 10−100 MeV band over the accessible redshift range. The neural-ratio sensitivity curve, which exploits the full map morphology, reaches a factor 1.3-1.9 lower in η at z ≳ 20, or ∼ 2−7 in the wall tension, landing essentially on the per-resolution-element noise floor.
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BibTeX
@misc{gurbuz2026prospects,
title = {{Prospects of Detecting Cosmological Domain Walls in the Cosmic Microwave Background}},
author = {Berkin Gürbüz and Robert Brandenberger and Yitian Sun},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31761},
eprint = {2610.31761},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31761},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Prospects of Detecting Cosmological Domain Walls in the Cosmic Microwave Background
AU - Berkin Gürbüz
AU - Robert Brandenberger
AU - Yitian Sun
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31761
UR - https://arxiv.org/abs/2610.31761
AB - Topological defects are a generic prediction of field theories with spontaneously broken symmetries, and domain walls, the two-dimensional defects left by a broken discrete symmetry, are among the simplest. We study the imprint of dark, purely gravitationally interacting domain walls on the Cosmic Microwave Background (CMB), in the regime where a low symmetry-breaking scale η allows the walls to evade the Zel'dovich overclosure bound while remaining cosmologically relevant. The relativistic repulsion of matter by a planar wall carves out an evacuated void bounded by an overdense shell, and we compute the resulting photon deflection field analytically for walls both aligned with and arbitrarily inclined to the line of sight. The deflection stamps coherent, strongly non-Gaussian distortions onto the temperature sky, with a characteristic two-peak structure set by the relativistic displacement scale and an inclination dependence that vanishes for face-on walls. Because these signatures are localised and anisotropic, they are poorly captured by two-point statistics; we therefore analyse them with a curvelet/wavelet decomposition matched to the wall's geometry, and with simulation-based inference, training a Neural Posterior Estimator (NPE) to recover the scattering redshift z, energy scale η, and inclination θ, and a Neural Ratio Estimator (NRE) to evaluate the likelihood-ratio test statistic for limit-setting. Casting these methods as sensitivity forecasts for a South Pole Telescope-like (SPT) survey, we find that the curvelet and wavelet statistical thresholds populate a common η∼ 10−100 MeV band over the accessible redshift range. The neural-ratio sensitivity curve, which exploits the full map morphology, reaches a factor 1.3-1.9 lower in η at z ≳ 20, or ∼ 2−7 in the wall tension, landing essentially on the per-resolution-element noise floor.
ER -