Study Questions When Large Boson Systems Can Be Described Classically
A new preprint argues that having a large number of identical bosons in a quantum state is not sufficient on its own to justify using classical field equations to describe the system. The key factor, the authors find, is how closely the quantum state resembles a coherent state, not merely how large the occupation number is. This has direct implications for ultra-light dark matter models, which routinely rely on classical field approximations.
Physicists commonly assume that when the mean occupation number of a single-particle state is sufficiently large, a classical field description becomes valid — an assumption widely invoked in cosmology, particularly for ultra-light dark matter dynamics. Researchers Gaurav Goswami and collaborators tested this assumption rigorously by applying the criterion that twice the phase uncertainty must be smaller than the magnitude of the mean field value. They found that an arbitrary quantum state with large occupation numbers does not necessarily behave classically, and that additional constraints on the state vector are required to recover classical behavior. The analysis shows it is the proximity of the state to a large-occupation coherent state — not the occupation number alone — that determines whether a classical field description is valid. The paper further examines how much deviation from a coherent state is tolerable before classical behavior breaks down. These findings raise questions about the uncritical use of classical field equations in ultra-light dark matter research, where the quantum state of the field is rarely specified or justified. The preprint, submitted to arXiv on June 8, 2026, spans 9 pages with 3 figures and has not yet undergone peer review.
What's missing
The authors do not appear to provide concrete observational or numerical tests against specific ultra-light dark matter models, leaving open the question of how large the practical deviation from coherent states would need to be to materially affect existing cosmological predictions. The conditions under which realistic dark matter fields would or would not approximate coherent states in a cosmological setting are not fully addressed.
What different sources said
- arXiv astro-phCenter
Identical Bosons, large occupation numbers and classical field description
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