Speaker: Ken'ichi Saikawa,

Title: ``Evolution of dark matter axions in the condensed regime",

Axion is one of the best motivated candidates of dark matter of the universe. It is produced non-thermally in the early universe and described as a coherently oscillating scalar field. Recently, it was pointed out that gravitational interactions can thermalize the system of dark matter axions, which leads to the formation of Bose-Einstein condensate (BEC) of them. This phenomenon predicts some new observational signatures, which can be used to distinguish axions from other particle dark matter candidates. The formation of BEC is argued on the ground that the gravitational interaction rate of the system exceeds the expansion rate in the condensed regime, where the transition rate between different momentum states is large compared to the energy exchanged in the transition. Since the evolution of the system in the condensed regime is not throughly understood, it is important to investigate the microscopic physics occurring in that regime in order to confirm the formation of BEC and its observational consequences.

In this work, we studied the evolution and interaction of coherently oscillating axions in detail, by developing the formalism to calculate the evolution of quantum occupation number of the axion field. Taking a coherent state as a quantum state of the coherent oscillation of axions, we have evaluated the interaction rate of them, which suggests that it exceeds the Hubble parameter around 1-10 keV. The general relativistic effects were taken into account by the use of the effective field theory technique, and it was shown that the interaction between the sub-horizon modes and the super-horizon modes (almost homogeneous condensations) also rapidly occurs. On the other hand, we found that the leading order contributions for interactions between axions and other species vanish, which implies that axions do not give any significant modifications on standard cosmological parameters.

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Last-modified: 2014-01-05 (日) 19:32:32 (2017d)