  In the present paper we investigate thermal fluctuation corrections to the
vacuum energy at zero temperature of a conformally coupled massless scalar
field whose modes propagate in the Einstein universe with a spherical boundary,
characterized by both Dirichlet and Neumann boundary conditions. Thus, we
generalize the results found in literature in this scenario, which has
considered only the vacuum energy at zero temperature. To do this, we use the
generalized zeta function method plus Abel-Plana formula and calculate the
renormalized Casimir free energy as well as other thermodynamics quantities,
namely, internal energy and entropy. For each one of them we also investigate
the limits of high and low temperatures. At high temperatures we found that the
renormalized Casimir free energy presents classical contributions, along with a
logarithmic term. Also in this limit, the internal energy presents a classical
contribution and the entropy a logarithmic term in addition to a classical
contribution as well. Conversely, at low temperatures, it is shown that both
the renormalized Casimir free energy and internal energy are dominated by the
vacuum energy at zero temperature. It is also shown that the entropy obeys the
third law of thermodynamics.
