Dark energy from quantum fluctuations
  We have derived the quantum vacuum pressure P_vac as a primary entity,
removing a trivial and a gauge terms from the cosmological constant-like part
(the zeroth term) of the effective action for a free matter field. The quantum
vacuum energy density G_vac appears a secondary entity, but both are of
expected order. Moreover P_vac and G_vac are dynamical, and therefore they can
be used in the Einstein equations. In particular, they could dynamically
support holographic dark energy model as well as the `thermodynamic' one.
