Entropy Dilution Mechanism and Dark Matter Isocurvature
Abstract
We explore dark matter isocurvature perturbations in cosmological scenarios with entropy dilution resulting from a late decaying massive state that temporarily dominates the universe before big-bang nucleosynthesis. The dilution mechanism features a non-standard scaling window of the radiation energy density, which allows for a new way to derive the entropy dilution factor. On super-horizon scales, the same scaling window also governs the evolution of isocurvature, and we present an analytic approach to solving the perturbation equations. We characterize various regimes where dark matter isocurvature survives and is subject to constraints from the cosmic microwave background observations. For cosmologically viable scenarios, we find that dark matter is dominantly produced from its dilutor decay, and the isocurvature is suppressed by the entropy dilution factor to the 4/3 power. Our work demonstrates the complementarity between isocurvature and collisionless damping of dark matter as powerful probes of the entropy dilution mechanism.
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BibTeX
@misc{nemevsek2026entropy,
title = {{Entropy Dilution Mechanism and Dark Matter Isocurvature}},
author = {Miha Nemevšek and Yue Zhang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.20918},
eprint = {2609.20918},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.20918},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Entropy Dilution Mechanism and Dark Matter Isocurvature
AU - Miha Nemevšek
AU - Yue Zhang
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.20918
UR - https://arxiv.org/abs/2609.20918
AB - We explore dark matter isocurvature perturbations in cosmological scenarios with entropy dilution resulting from a late decaying massive state that temporarily dominates the universe before big-bang nucleosynthesis. The dilution mechanism features a non-standard scaling window of the radiation energy density, which allows for a new way to derive the entropy dilution factor. On super-horizon scales, the same scaling window also governs the evolution of isocurvature, and we present an analytic approach to solving the perturbation equations. We characterize various regimes where dark matter isocurvature survives and is subject to constraints from the cosmic microwave background observations. For cosmologically viable scenarios, we find that dark matter is dominantly produced from its dilutor decay, and the isocurvature is suppressed by the entropy dilution factor to the 4/3 power. Our work demonstrates the complementarity between isocurvature and collisionless damping of dark matter as powerful probes of the entropy dilution mechanism.
ER -