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Analysis of Bose-Einstein condensation times for self-interacting scalar dark matter

We investigate the condensation time of self-interacting axion-like particles in a gravitational well, extending the prior work [arXiv:2007.07438] which showed that the Wigner formalism is a good analytic approach to describe a condensing scalar field. In the present work, we use this formalism to affirm that $ϕ^4$ self-interactions will take longer than necessary to support the time scales associated with structure formation, making gravity a necessary part of the process to bring axion dark matter into a solitonic form. Here we show that when the axions' virial velocity is taken into account, the time scale associated with self-interactions will scale as $λ^2$. This is consistent with recent numerical estimates, and it confirms that the Wigner formalism described in prior work~\cite{Relax} is a helpful analytic framework to check computational work for potential numerical artifacts.