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arXiv 1111.1157astro-ph.COcond-mat.stat-mechhep-ph
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宇宙轴子热化

Cosmic axion thermalization

O. Erken, P. Sikivie, H. Tam, Q. Yang

摘要:

轴子与其他冷暗物质候选者的不同之处在于它们形成简并的玻色气体。结果表明,当光子温度达到约500 eV时,它们巨大的量子简并度和大的关联长度导致冷暗物质轴子通过引力自相互作用热化。当它们热化时,轴子形成玻色-爱因斯坦凝聚体。它们的热化发生在一种状态中,这里称为“凝聚状态”,在这种状态下,玻尔兹曼方程是无效的,因为粒子的能量色散小于它们的相互作用速率。我们推导出了凝聚态粒子热化率的解析表达式,并通过一个玩具模型的数值模拟来检验这些表达式的有效性。我们根据轴子玻色-爱因斯坦凝聚重新讨论轴子宇宙学。它表明,轴子是无法区分从普通的冷暗物质在所有尺度的观测兴趣,除了当他们热化或再热化。即将落入银河系势阱的轴子的再热化使它们获得净整体旋转,因为它们进入最低能量状态,与它们通过潮汐扭矩获得的总角动量一致。这种现象解释了在星系晕中暗物质的焦散环的出现。我们发现,光子可以达到与轴子的热接触,并探讨这种可能性的宇宙学参数的测量的影响。

英文摘要:

Axions differ from the other cold dark matter candidates in that they form a degenerate Bose gas. It is shown that their huge quantum degeneracy and large correlation length cause cold dark matter axions to thermalize through gravitational self-interactions when the photon temperature reaches approximately 500 eV. When they thermalize, the axions form a Bose-Einstein condensate. Their thermalization occurs in a regime, herein called the `condensed regime', where the Boltzmann equation is not valid because the energy dispersion of the particles is smaller than their interaction rate. We derive analytical expressions for the thermalization rate of particles in the condensed regime, and check the validity of these expressions by numerical simulation of a toy model. We revisit axion cosmology in light of axion Bose-Einstein condensation. It is shown that axions are indistinguishable from ordinary cold dark matter on all scales of observational interest, except when they thermalize or rethermalize. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation as they go to the lowest energy state consistent with the total angular momentum they acquired by tidal torquing. This phenomenon explains the occurrence of caustic rings of dark matter in galactic halos. We find that photons may reach thermal contact with axions and investigate the implications of this possibility for the measurements of cosmological parameters.

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