  An expanding universe is not expected to have a static vacuum energy density.
The so-called cosmological constant $\Lambda$ should be an approximation,
certainly a good one for a fraction of a Hubble time, but it is most likely a
temporary description of a true dynamical vacuum energy variable that is
evolving from the inflationary epoch to the present day. We can compare the
evolving vacuum energy with a Casimir device where the parallel plates slowly
move apart ("expand"). The total vacuum energy density cannot be measured, only
the effect associated to the presence of the plates, and then also their
increasing separation with time. In the universe there is a nonvanishing
spacetime curvature $R$ as compared to Minkowskian spacetime that is changing
with the expansion. The vacuum energy density must change accordingly, and we
naturally expect $\delta\Lambda\sim R\sim H^2$. A class of dynamical vacuum
models that trace such rate of change can be constructed. They are compatible
with the current cosmological data, and conveniently extended can account for
the complete cosmic evolution from the inflationary epoch till the present
days. These models are very close to the $\Lambda$CDM model for the late
universe, but very different from it at the early times. Traces of the inherent
vacuum dynamics could be detectable in our recent past.

General Relativity and Quantum Cosmology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology;High Energy Physics - Theory