Evolution and thermalization of dark matter axions in the condensed regime

Abstract

We discuss the possibility that dark matter axions form a Bose-Einstein condensate (BEC) due to the gravitational self interactions. The formation of BEC occurs in the condensed regime, where the transition rate between different momentum states is large compared to the energy exchanged in the transition. The time evolution of the quantum state occupation number of axions in the condensed regime is derived based on the in-in formalism. We recover the expression for the thermalization rate due to self interaction of the axion field, which was obtained in the other literature. It is also found that the leading order contributions for interactions between axions and other species vanish, which implies that the axion BEC does not give any significant modifications on standard cosmological parameters.

Attractive versus repulsive interactions in the Bose-Einstein condensation dynamics of relativistic field theories

“…which corresponds to a so-called Q ball [46]. 16 This is in stark contrast to our usual choice of initial conditions (2.4) without an initial charge and more similar to the nonrelativistic model with a conserved particle number.…”

“…[7][8][9]) and would leave distinct imprints in direct detection experiments [10][11][12]. While many arguments have focused on the formation rate of a condensate [13][14][15][16][17][18][19], a qualitatively even more important point is the attractive nature of the relevant interactions [20,21] that tends to favor localized structures instead of a spatially constant condensate [21]. Inspired by this, it is one of our main aims to study the impact of attractive interactions and to delineate the differences to the repulsive case.…”

Gravitational self-interactions of a degenerate quantum scalar field

“…[8,[10][11][12] that cold dark matter axions thermalize, as a result of their gravitational self-interactions, on time scales shorter than the age of the Universe after the photon temperature has dropped to approximately one keV. When they thermalize, all the conditions for their Bose-Einstein condensation are satisfied, and it is natural to assume that this is indeed what happens.…”

Stimulated emission of dark matter axion from condensed matter excitations

“…In contrast to the standard WIMPs which are usually fermions, axions obey the Bose-Einstein statistics. In order to consistently explain the dark matter density in terms of the axion, a large number density of the bosonic axions is required in our galaxy, and the possibility of the Bose-Einstein condensation of dark matter axions due to such a large density has been discussed [15][16][17][18][19].…”

“…Therefore, if the dark matter axions are in thermal equilibrium in our galaxy, the axion field is naturally expected to be in a BE condensate phase at the extreme low temperature T a ( T c ). The possibility of the thermalization of the dark matter axions by the small self-interaction and the gravitational interaction has been recently discussed in the literatures [15][16][17][18][19], and some consequences from the resulting BE condensation have been explored. In this paper, we assume that the dark matter axions do thermalize and form the BE condensate in our galaxy.…”