暗物质要真正“温”起来需要满足什么条件？
What would it take for dark matter to be literally warm?
中文总结 AI 辅助
本文探讨暗物质要达到基准温暗物质的字面“温”条件需满足的三种可能性，指出这些条件依赖巧合，主张转向更优方法从海量数据中提取信息。
中文摘要 AI 辅助
温暗物质（Warm dark matter, WDM）在过去数十年里一直是约束小尺度结构的重要基准。本文研究暗物质要达到该基准所假设的字面意义上的“温”需要满足什么条件，即暗物质是在相对论性状态下退耦的热遗迹。要满足当前对WDM质量的约束（接近~10 keV量级），需要以下三种可能性之一：第一种是退耦时刻的热浴中存在~10^4个相对论性自由度，这远超出标准模型或其合理扩展所能提供的范围；第二种是存在一段早期物质主导时期，其熵注入稀释了遗迹密度，但在该场景中，扰动的演化所经历的膨胀历史与推导WDM传递函数时假设的辐射主导历史不同；第三种是暗 sector 从未与标准模型发生热接触，仅通过非对称 reheating 形成更冷的状态。所有这些可能性都依赖于极强的巧合：与WDM无关的物理恰好提供了WDM宇宙学所假设的精确初始条件。同时，WDM常作为具有自洽热历史模型的代理，这类模型通常会在小尺度上抑制结构形成，且其抑制方式在定量和定性上均与WDM不同。因此，本文主张转向更具表现力的参数化方法和基于模拟的方法，以便从即将到来的海量数据中提取更有用的信息。
英文摘要
Warm dark matter (WDM) has served as a valuable benchmark for constraining small-scale structure in the past decades. In this note, I examine what it would take for dark matter to be warm in the literal sense assumed by that benchmark, i.e., a thermal relic that decoupled while relativistic. Satisfying current constraints on the WDM mass, which approach the $\sim$10 keV scale, requires one of three possibilities. One possibility is that there were $\sim 10^4$ relativistic degrees of freedom in the thermal bath at the time of decoupling, which is far beyond what is available in the Standard Model or plausible extensions of it. An alternative is that there was a period of early matter domination whose entropy injection diluted the relic density. However, in this scenario, the perturbations would have evolved through an expansion history that was different from the radiation-dominated one assumed in deriving WDM transfer functions. The third possibility is a dark sector that was never in thermal contact with the Standard Model and was simply born colder via asymmetric reheating. All of these possibilities rely on strong coincidences, where physics that has nothing to do with WDM happens to provide the exact initial conditions assumed in a WDM cosmology. Meanwhile, WDM is often used as a proxy for models with self-consistent thermal histories that generically predict the suppression of structure formation on small scales in a way that is both quantitatively and qualitatively different from WDM. I therefore advocate for a transition to more expressive parameterizations and simulation-based methods in order to extract more useful information from the wealth of upcoming data.
