We revisit the dark degeneracy that arises from the Einstein equations relating geometry to the total cosmic substratum but not resolving its individual components separately. We establish the explicit conditions for the dark degeneracy in the fluid description of the dark sector. At the background level, this degeneracy can be formally understood in terms of a unified dark sector Equation of State (EoS) that depends both on the dynamical nature of the dark energy (DE) as well as on its interaction with the pressureless dark matter. For linear perturbations, the degeneracy arises for specified DE pressure perturbations (or sound speed, equivalently) and DE anisotropic stress. Specializing to the degeneracy between non-interacting dynamical DE and interacting vacuum DE models, we perform a parameter estimation analysis for a range of dynamical DE parametrizations, where for illustration we explicitly break the degeneracy at the linear level by adopting a luminal sound speed for both scenarios. We conduct this analysis using cosmological background data alone and in combination with Planck measurements of the cosmic microwave background radiation. We find that although the overall phenomenology between the dynamical DE and interacting approaches is similar, there are some intriguing differences. In particular, there is an ambiguity in the strength of constraints on ${\Omega }_{m0}$ and ${\sigma }_8$, which are considerably weakened for interacting vacuum DE, indicating that the dark degeneracy can change the significance of tensions in cosmological parameters inferred from different datasets.

我们重新审视了由爱因斯坦方程式引起的黑暗退化，该方程式将几何学与整个宇宙底层联系在一起，但并未单独解决其各个组成部分。我们在暗区的流体描述中建立了暗退化的显式条件。在背景水平上，可以通过统一的暗区状态方程（EoS）正式理解这种简并性，该方程既取决于暗能量（DE）的动力学性质，也取决于其与无压暗物质的相互作用。对于线性扰动，退化是由于指定的DE压力扰动（或等效的声速）和DE各向异性应力引起的。专门研究非交互动力学DE和交互真空DE模型之间的退化，我们对一系列动态DE参数化执行参数估计分析，为说明起见，我们通过在两种情况下均采用光速来显式地打破线性级上的简并性。我们仅使用宇宙学背景数据并结合宇宙微波本底辐射的普朗克测量来进行此分析。我们发现，尽管动态DE和交互方法之间的总体现象相似，但还是存在一些有趣的差异。特别是，在以下方面的约束强度存在歧义 我们仅使用宇宙学背景数据并结合宇宙微波本底辐射的普朗克测量来进行此分析。我们发现，尽管动态DE和交互方法之间的总体现象相似，但仍存在一些有趣的差异。特别是，在以下方面的约束强度存在歧义 我们仅使用宇宙学背景数据并结合宇宙微波本底辐射的普朗克测量来进行此分析。我们发现，尽管动态DE和交互方法之间的总体现象相似，但仍存在一些有趣的差异。特别是在约束强度上存在歧义${\Omega }_{米0}$ 和 ${\sigma }_8$，这对于与真空DE相互作用产生了相当大的削弱，表明黑暗的简并性可以改变从不同数据集推断出的宇宙学参数中张力的重要性。