We present a privileged Fock quantization of a massive Dirac field in a
closed Friedmann-Robertson-Walker cosmology, partially selected by the criteria
of invariance of the vacuum under the symmetries of the field equations, and
unitary implementation of the dynamics. When quantizing free scalar fields in
homogeneous and isotropic spacetimes with compact spatial sections, these
criteria have been shown to pick out a unique Fock representation (up to
unitary equivalence). Here, we employ the same criteria for fermion fields and
explore whether that uniqueness result can be extended to the case of the Fock
quantization of fermions. For the massive Dirac field, we start by introducing
a specific choice of the complex structure that determines the Fock
representation. Such structure is invariant under the symmetries of the
equations of motion. We then prove that the corresponding representation of the
canonical anticommutation relations admits a unitary implementation of the
dynamics. Moreover, we construct a rather general class of representations that
satisfy the above criteria, and we demonstrate that they are all unitarily
equivalent to our previous choice. The complex structures in this class are
restricted only by certain conditions on their asymptotic behavior for modes in
the ultraviolet sector of the Dirac operator. We finally show that, if one
assumes that these asymptotic conditions are in fact trivial once our criteria
are fulfilled, then the time-dependent scaling in the definition of the
fermionic annihilation and creation-like variables is essentially unique.Comment: 24 page
