Abstract
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?
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