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arXiv 1707.03417astro-ph.CO
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对“WIMPs的动力学解耦：解析表达式”一文的评论

Comment on "Kinetic decoupling of WIMPs: Analytic expressions"

Isaac Raj Waldstein, Adrienne L. Erickcek

备注：
4 pages, 1 figure; comment on arXiv: 1501.02233
发表信息：
Phys. Rev. D 95, 088301 (2017)
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摘要：

Visinelli和Gondolo（2015，以下简称VG 15）在一般宇宙学模型中推导出了暗物质温度演化的解析表达式。然后，他们计算了暗物质动力学退耦温度$T_{\mathrm{kd}}$，并将他们的结果与Gelmini和Gondolo（2008，以下简称GG 08）在早期物质主导时代（EMDE）对$T_{\mathrm{kd}}$的计算进行了比较，EMDE发生在宇宙被大爆炸核合成之前的衰减振荡标量场或半稳定的大质量粒子主导的时候。VG 15发现暗物质在EMDE中的膨胀温度低于辐射主导时代，而GG 08发现暗物质在EMDE中的膨胀温度高于辐射主导时代。VG 15将这种差异归因于匹配常数的存在，该常数确保了暗物质温度在从EMDE过渡到随后的辐射主导时代期间是连续的，并得出结论，GG 08的结果是不正确的。我们表明，差异是由于这样一个事实，即VG 15比较$T_\mathrm{kd}$在EMDE的辐射主导的宇宙，将导致在相同的暗物质温度在后期的去耦温度。由于在EMDE期间退耦使暗物质比在辐射支配期间退耦时更冷，因此这个温度比标准热历史中的$T_\mathrm{kd}$高得多，而标准热历史中的$T_\mathrm{kd}$确实比EMDE中的$T_{\mathrm{kd}}$低，如GG 08所述。

英文摘要：

Visinelli and Gondolo (2015, hereafter VG15) derived analytic expressions for the evolution of the dark matter temperature in a generic cosmological model. They then calculated the dark matter kinetic decoupling temperature $T_{\mathrm{kd}}$ and compared their results to the Gelmini and Gondolo (2008, hereafter GG08) calculation of $T_{\mathrm{kd}}$ in an early matter-dominated era (EMDE), which occurs when the Universe is dominated by either a decaying oscillating scalar field or a semistable massive particle before Big Bang nucleosynthesis. VG15 found that dark matter decouples at a lower temperature in an EMDE than it would in a radiation-dominated era, while GG08 found that dark matter decouples at a higher temperature in an EMDE than it would in a radiation-dominated era. VG15 attributed this discrepancy to the presence of a matching constant that ensures that the dark matter temperature is continuous during the transition from the EMDE to the subsequent radiation-dominated era and concluded that the GG08 result is incorrect. We show that the disparity is due to the fact that VG15 compared $T_\mathrm{kd}$ in an EMDE to the decoupling temperature in a radiation-dominated universe that would result in the same dark matter temperature at late times. Since decoupling during an EMDE leaves the dark matter colder than it would be if it decoupled during radiation domination, this temperature is much higher than $T_\mathrm{kd}$ in a standard thermal history, which is indeed lower than $T_{\mathrm{kd}}$ in an EMDE, as stated by GG08.

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