At the EDGES of MADness: phenomenology of relic millicharged atomic dark matter
Abstract
Millicharged dark matter lives in a corner of parameter space that has been considerably shrinking over the past decades, due to constraints from cosmology, astrophysical systems, and ground-based experiments. A particularly interesting probe is the 21-cm absorption signal at Cosmic Dawn, which attracted interest in light of the EDGES claim: the reported excess absorption can be explained by millicharged dark matter with a fractional electric charge of~10−6 ≲ ε≲ 10−4, but it must be a subpercent of the total dark matter abundance to remain consistent with the cosmic microwave background (CMB). We show that very low fractions of millicharged dark matter naturally arise from atomic dark matter: a dark sector model that replicates the physics of standard atomic hydrogen. Dark ions undergo the process of dark recombination, leaving a relic ionized component that, in the presence of kinetic mixing, is naturally millicharged. We modify the cosmological Boltzmann solver CLASS to incorporate this scenario, and we investigate the parameter space of millicharged atomic dark matter (MAD) that is allowed by CMB, large-scale structure, and supernovae data. We find there are cosmologically viable regions of MAD parameter space that are consistent with laboratory and astrophysical bounds and also overlap with the EDGES-defined window. Although the EDGES result is likely not a Cosmic Dawn signal, MAD leaves distinctive cosmological signatures, accessible to upcoming 21-cm observations.
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BibTeX
@misc{vanzan2026edges,
title = {{At the EDGES of MADness: phenomenology of relic millicharged atomic dark matter}},
author = {Eleonora Vanzan and Kimberly K. Boddy and Ely D. Kovetz},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.40064},
eprint = {2610.40064},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.40064},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - At the EDGES of MADness: phenomenology of relic millicharged atomic dark matter
AU - Eleonora Vanzan
AU - Kimberly K. Boddy
AU - Ely D. Kovetz
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.40064
UR - https://arxiv.org/abs/2610.40064
AB - Millicharged dark matter lives in a corner of parameter space that has been considerably shrinking over the past decades, due to constraints from cosmology, astrophysical systems, and ground-based experiments. A particularly interesting probe is the 21-cm absorption signal at Cosmic Dawn, which attracted interest in light of the EDGES claim: the reported excess absorption can be explained by millicharged dark matter with a fractional electric charge of~10^−6 ≲ ε≲ 10^−4, but it must be a subpercent of the total dark matter abundance to remain consistent with the cosmic microwave background (CMB). We show that very low fractions of millicharged dark matter naturally arise from atomic dark matter: a dark sector model that replicates the physics of standard atomic hydrogen. Dark ions undergo the process of dark recombination, leaving a relic ionized component that, in the presence of kinetic mixing, is naturally millicharged. We modify the cosmological Boltzmann solver CLASS to incorporate this scenario, and we investigate the parameter space of millicharged atomic dark matter (MAD) that is allowed by CMB, large-scale structure, and supernovae data. We find there are cosmologically viable regions of MAD parameter space that are consistent with laboratory and astrophysical bounds and also overlap with the EDGES-defined window. Although the EDGES result is likely not a Cosmic Dawn signal, MAD leaves distinctive cosmological signatures, accessible to upcoming 21-cm observations.
ER -