100 years of the cosmological constant Lambda in Einstein's equations: Is Lambda really constant in light of observations?

In this colloquium, I will review some historical aspects of the cosmological constant term in Einstein's equations Lambda, which is one of the fundamental ingredients of LambdaCDM, the standard model of cosmology. Such term is usually associated to the energy density of the cosmic vacuum, and is assumed to be strictly constant. I will assess if this assumption is still tenable in the light of the modern cosmological data, in particular the possibility that the cosmic vacuum is slightly dynamical, i.e. time-dependent. Upon quantitatively comparing the performance of specific realizations of this option against the traditional LambdaCDM model, I will conclude that vacuum dynamics can provide a substantially better description of the overall cosmological observations.