Cosmological constant and vacuum energy
  The general thermodynamic analysis of the quantum vacuum, which is based on our knowledge of the vacua in condensed-matter systems, is consistent with the Einstein earlier view on the cosmological constant. In the equilibrium Universes the value of the cosmological constant is regulated by matter. In the empty Universe, the vacuum energy is exactly zero, lambda=0. The huge contribution of the zero point motion of the quantum fields to the vacuum energy is exactly cancelled by the higher-energy degrees of freedom of the quantum vacuum. In the equilibrium Universes homogeneously filled by matter, the vacuum is disturbed, and density of the vacuum energy becomes proportional to the energy density of matter. This consideration applies to any equilibrium vacuum irrespective of whether the vacuum is false or true, and is valid both in the Einstein general theory of relativity and within the special theory of relativity, i.e. in the world without gravity. 
G.E. Volovik