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  • Open Access

Probing the state of ultralight dark matter via its density fluctuations

Saarik Kalia*

  • *Contact author: kalias@umn.edu
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Phys. Rev. D 113, 043534 – Published 23 February, 2026

DOI: https://doi.org/10.1103/jyq8-3lz1

Abstract

Dark matter (DM) candidates with very small masses, and correspondingly large number densities, have gained significant interest in recent years. These DM candidates are typically said to behave “classically.” More specifically, they are often assumed to reside in an ensemble of coherent states. One notable exception to this scenario is when isocurvature fluctuations of the DM are produced during inflation (or more generally by any Bogoliubov transformation). In such contexts, the ultralight DM instead resides in a squeezed state. In this work, we demonstrate that these two scenarios can be distinguished via the statistics of the DM density fluctuations, such as the matter power spectrum and bispectrum. This provides a probe of the DM state which persists in the limit of large particle number and does not rely on any nongravitational interactions of the DM. Importantly, the statistics of these two states differ when the modes of the squeezed state are all in-phase, as is the case at the end of inflation. Later cosmological dynamics may affect this configuration. Our work motivates future numerical studies of how cosmological dynamics may impact the initial squeezed state and the statistics of its density fluctuations.

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Phys. Rev. D 113, 043534– Published 23 February, 2026
  • Received 21 May 2025
  • Accepted 30 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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