Cosmological constant problem Observation P≈(2×10eV 4 Zero point energy density kvk+m M dk 4兀 16丌 M=Mn-10°eV,,~102p3 4 (2 10 eV) −     Cosmological constant problem Observation Zero point energy density   + = M v M dk k k m 0 2 4 2 2 2 2 4 16  =   28 121 M Mpl ~10 eV, v ~10