  We consider an open universe created by bubble nucleation, and study possible
effects of an "ancestor vacuum" (de Sitter space in which bubble nucleation
occurred) on the present universe. We compute vacuum expectation values of
energy-momentum tensor for a minimally coupled scalar field, carefully taking
into account the effect of the ancestor vacuum by the Euclidean prescription.
In the study of the time evolution, an important role is played by the
so-called supercurvature mode, which is non-normalizable on a spatial slice of
open universe and decays in time most slowly. We point out that vacuum energy
of a quantum field can be regarded as dark energy if mass of the field is of
order the present Hubble parameter or smaller. We obtain preliminary results
for the dark energy equation of state w(z) as a function of the redshift.
