Neutrinos are expected to freestream (i.e., not interact with anything) since they decouple in the early Universe at a temperature $T\sim 2\mathrm{MeV}$. However, there are many relevant particle physics scenarios that can make neutrinos interact at $T<2\mathrm{MeV}$. In this work, we take a global perspective and aim to identify the temperature range in which neutrinos can interact given current cosmological observations. We consider a generic set of rates parametrizing neutrino interactions and by performing a full $Planck$ cosmic microwave background (CMB) analysis we find that neutrinos cannot interact significantly for redshifts $2000\lesssim z\lesssim {10}^5$, which we refer to as the freestreaming window. We also derive a redshift dependent upper bound on a suitably defined interaction rate ${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)$, finding ${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)/H(z)\lesssim 1-10$ within the freestreaming window. We show that these results are largely model independent under some broad assumptions, and contextualize them in terms of neutrino decays, neutrino self-interactions, neutrino annihilations, and Majoron models. We provide examples of how to use our model independent approach to obtain bounds in specific scenarios, and demonstrate agreement with existing results. We also investigate the reach of upcoming cosmological data finding that CMB Stage-IV experiments can improve the bound on ${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)/H(z)$ by up to a factor 10. Moreover, we comment on large-scale structure observations, finding that the ongoing DESI survey has the potential to probe uncharted regions of parameter space of interacting neutrinos. Finally, we point out a peculiar scenario that has so far not been considered, and for which relatively large interactions around recombination are still allowed by $Planck$ data due to some degeneracy with $n_s$, $A_s$, and $H_0$. This scenario can be fully tested with CMB-S4.

中文翻译：

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宇宙学中中微子相互作用的全局观：普朗克所见的自由流窗口

预计中微子会自由流动（即不与任何东西相互作用），因为它们在早期宇宙中的一定温度下解耦$吨～2\mathrm{电子伏特}$. 然而，有许多相关的粒子物理场景可以使中微子在$吨<2\mathrm{电子伏特}$. 在这项工作中，我们从全球视角出发，旨在根据当前的宇宙学观测确定中微子可以相互作用的温度范围。我们考虑了一组将中微子相互作用参数化的通用速率，并通过执行完整的$磷l\mathrm{一个}nCķ$宇宙微波背景 (CMB) 分析，我们发现中微子不能与红移发生显着相互作用$2000\lesssim z\lesssim {10}^5$，我们称之为自由流窗口。我们还在适当定义的相互作用率上推导出了一个依赖于红移的上限${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)$, 发现${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)/H(z)\lesssim 1-10$在免费流媒体窗口内。我们表明，在一些广泛的假设下，这些结果在很大程度上与模型无关，并根据中微子衰变、中微子自相互作用、中微子湮灭和马约龙模型对它们进行背景化。我们提供了如何使用我们的模型独立方法在特定场景中获得界限的示例，并证明与现有结果的一致性。我们还调查了即将到来的宇宙学数据的范围，发现 CMB Stage-IV 实验可以提高对${\mathrm{\Gamma }}_{\mathrm{nfs}}(z)/H(z)$高达 10 倍。此外，我们评论了大规模结构观测，发现正在进行的 DESI 调查有可能探索相互作用中微子参数空间的未知区域。最后，我们指出了一个迄今为止尚未考虑的特殊情况，并且仍然允许围绕重组的相对较大的相互作用$磷l\mathrm{一个}nCķ$由于一些退化的数据$n_s$,${\mathrm{一个}}_s$， 和$H_0$. 这个场景可以用 CMB-S4 进行全面测试。