  A precise zeta-function calculation shows that the contribution of the vacuum
energy to the observed value of the cosmological constant can possibly have the
desired order of magnitude albeit the sign strongly depends on the topology of
the universe. The non-renormalizable, infinite contributions which have been
recently shown to occur when one physically imposes boundary conditions on
quantum fields (Casimir calculations) are considered. It is shown that using a
Hadamard regularization in addition to the zeta method, the ordinary, finite
results in the literature are exactly recovered.
