The Cosmological Constant Λ, a concept introduced by Einstein in 1917, has been with us ever since in different variants and incarnations, including the broader concept of ‘Dark Energy’. Current observations are consistent with a value of Λ corresponding to about present-epoch 70% of the critical density of the Universe. This is causing the speeding up (acceleration) of the expansion of the Universe. Coupled with the flatness of the Universe and the amount of 30% matter (5% baryonic and 25% ‘Cold Dark Matter’), this forms the so-called ‘ΛCDM standard model’. However, there are currently indications of inconsistencies (‘tensions’ ) within ΛCDM on different values of the Hubble Constant and the clumpiness factor. Also, time variation of Dark Energy and slight deviations from General Relativity are not ruled out yet. Several grand projects are underway to test ΛCDM further. If ΛCDM will remain the standard model, then the ball is back in the theoreticians' court, to explain the physical meaning of Λ. Is Λ an alteration to the geometry of the Universe, or the energy of the vacuum? Or maybe it is something different, that manifests a yet unknown higher-level theory?

中文翻译：

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暗能量：它“只是”爱因斯坦的宇宙常数 Λ 吗？

宇宙常数 Λ 是爱因斯坦于 1917 年提出的一个概念，从那时起就以不同的变体和化身出现在我们面前，包括更广泛的“暗能量”概念。当前的观测结果与 Λ 的值一致，该值对应于宇宙临界密度的大约当前时期的 70%。这导致了宇宙膨胀的加速（加速）。再加上宇宙的平坦度和 30% 的物质（5% 的重子和 25% 的“冷暗物质”），这就形成了所谓的“ΛCDM 标准模型”。然而，目前有迹象表明 ΛCDM 内哈勃常数和结块因子的不同值存在不一致（“张力”）。此外，不排除暗能量的时间变化和与广义相对论的轻微偏差。几个大型项目正在进行中，以进一步测试 ΛCDM。如果 ΛCDM 仍然是标准模型，那么球又回到了理论家的法庭，来解释 Λ 的物理意义。Λ 是宇宙几何结构的改变，还是真空能量的改变？或者，也许是不同的东西，体现了一个未知的更高层次的理论？