非标准热历史中暗物质的准解耦态
Quasidecoupled state for dark matter in nonstandard thermal histories
摘要：
当动量转移率等于膨胀率时，暗物质就会脱离与标准模型粒子的动力学平衡。在辐射主导的宇宙中，这基本上与暗物质从等离子体中动力学地扩散同时发生。暗物质也可能在早期物质主导时代（EMDE）中脱离与标准模型粒子的动力学平衡，这发生在宇宙的能量含量由衰减振荡的标量场或大爆炸核合成之前的半稳定大质量粒子主导时。到目前为止，人们一直认为EMDE期间的动力学解耦与辐射主导时代的方式相似。我们表明，这是不是这样的。通过研究暗物质温度的演化，我们建立了一个准解耦的暗物质的EMDE状态，在此状态下，暗物质温度比等离子体温度冷却得快，但比暗物质完全解耦时的冷却慢.暗物质不会完全退耦，直到EMDE结束，宇宙成为辐射主导。我们还扩展了准去耦的标准，以其他非标准的热历史，并考虑准去耦如何影响自由流长度的暗物质。
英文摘要：
Dark matter drops out of kinetic equilibrium with standard model particles when the momentum-transfer rate equals the expansion rate. In a radiation-dominated universe, this occurs at essentially the same time as dark matter kinetically decouples from the plasma. Dark matter may also fall out of kinetic equilibrium with standard model particles during an early matter-dominated era (EMDE), which occurs when the energy content of the Universe is dominated by either a decaying oscillating scalar field or a semistable massive particle before big bang nucleosynthesis. Until now, it has been assumed that kinetic decoupling during an EMDE happens similarly to the way it does in a radiation-dominated era. We show that this is not the case. By studying the evolution of the dark matter temperature, we establish a quasidecoupled state for dark matter in an EMDE, during which the dark matter temperature cools faster than the plasma temperature but slower than it would cool if the dark matter were fully decoupled. The dark matter does not fully decouple until the EMDE ends and the Universe becomes radiation dominated. We also extend the criteria for quasidecoupling to other nonstandard thermal histories and consider how quasidecoupling affects the free-streaming length of dark matter.
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