Dark Energy: is it `just' Einstein's Cosmological Constant Lambda?
  The Cosmological Constant Lambda, 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 Lambda 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 over the past 6 billion years, a discovery recognised
by the 2011 Nobel Prize in Physics. 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 Lambda-CDM standard model, which has survived many observational
tests over about 30 years. However, there are currently indications of
inconsistencies (`tensions' ) within Lambda-CDM 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 Lambda-CDM further and to estimate the
cosmological parameters to sub-percent level. If Lambda-CDM will remain the
standard model, then the ball is back in the theoreticians' court, to explain
the physical meaning of Lambda. Is Lambda 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?
