  Two-point functions, mean-squared fluctuations, and the vacuum expectation
value of the energy-momentum tensor operator are investigated for a massive
scalar field with an arbitrary curvature coupling parameter, subject to a
spherical boundary in the background of de Sitter spacetime. The field is
prepared in the Bunch-Davies vacuum state and is constrained to satisfy Robin
boundary conditions on the sphere. Both the interior and exterior regions are
considered. For the calculation in the interior region, a mode-summation method
is employed, supplemented with a variant of the generalized Abel-Plana formula.
This allows us to explicitly extract the contributions to the expectation
values which come from de Sitter spacetime without boundaries. We show that the
vacuum energy-momentum tensor is non-diagonal with the off-diagonal component
corresponding to the energy flux along the radial direction. With dependence on
the boundary condition and the mass of the field, this flux can be either
positive or negative. Several limiting cases of interest are then studied. In
terms of the curvature coupling parameter and the mass of the field, two very
different regimes are realized, which exhibit monotonic and oscillatory
behavior of the vacuum expectation values, respectively, far from the sphere.
The decay of the boundary induced expectation values at large distances from
the sphere is shown to be power-law (monotonic or oscillating), independent of
the value of the field mass.
