Exact Results in Quantum Theory

The Ricci dark energy is a brand of holographic dark energy which follows from black hole arguments ("phenomenological" approach). Instead we propose some arguments referring to quantum field theory, in the spirit of the traditional interpretation of dark energy as vacuum energy. In spite of the fact that our approach is not much more rigorous than the original one but it permits to estimate the free parameter of the Ricci dark energy which appears to be astonishingly close to observational expectations.