Abstract
We study the Bose condensation of scalar dark matter in the presence of both gravitational and self-interactions. Axions and other scalar dark matter in gravitationally bound miniclusters or dark matter halos are expected to condense into Bose-Einstein condensates called Bose stars. This process has been shown to occur through attractive self-interactions of the axion-like particles or through the field's self gravitation. We show that in the high-occupancy regime of scalar dark matter, the Boltzmann collision integral does not describe either gravitaitonal or self-interactions, and derive kinetic equations valid for these interactions. We use this formalism to compute relaxation times for the Bose-Einstein condensation, and find that condensation into Bose stars could occur within the lifetime of the universe. The self-interactions reduce the condensation time only when they are very strong.
Highlights
The composition of dark matter is one of the most longstanding problems in cosmology
We refer to any gravitationally bound structure of axion or ultralight scalar dark matter, formed preinflation or postinflation, as an axion minicluster. While these structures have different masses, sizes, and observational signatures, they all consist of scalar dark matter with the potential to form solitonic cores such as Bose stars, where the scalar field is in its ground state
We differentiate the equation of motion for the Wigner function in Eq (19), replacing Utot with λjψj2 in that equation, since we have already treated the gravitational interactions in the previous section
Summary
The composition of dark matter is one of the most longstanding problems in cosmology. While there are several proposed solutions to these problems [5,6,7,8,9], an attractive proposal considers the quantum properties of the dark matter particles In this case, the largescale predictions remain the same as in ΛCDM, but on scales less than the de Broglie wavelength the predictions change. Experiments into the neutron electron dipole moment can constrain models of axion physics [25] These proposed candidates have in common that they thermalize to form compact, gravitationally bound solitons that can be described as Bose-Einstein condensation [26,27,28,29,30,31,32,33]. IV we discuss the relation between these timescales and their implications on the relevance of gravitational and selfinteractions in the thermalization of axion stars
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