Fermionic vacuum polarization by a cosmic string in de Sitter spacetime
  We investigate the fermionic condensate and the vacuum expectation value of
the energy-momentum tensor for a massive spinor field in the geometry of a
straight cosmic string on background of de Sitter spacetime. By using the
Abel-Plana summation formula, we explicitly extract form the expectation values
the contribution associated with purely de Sitter space, remaining the
expectation values induced by the cosmic string. The latter presents
information about de Sitter gravity as well. Because the investigation of the
fermionic quantum fluctuations in de Sitter space have been investigated in
literature, here we are mainly interested in the cosmic string-induced
contributions. For a massless field, the fermionic condensate vanishes and the
presence of the string does not break chiral symmetry of the massless theory.
Unlike to the case of a scalar field, for a massive fermionic field the vacuum
expectation value of the energy-momentum tensor is diagonal and the axial and
radial stresses are equal to the energy density. At large distances from the
string the behavior of the string-induced parts in the vacuum densities is
damping oscillatory with the amplitude decaying as the inverse fourth power of
the distance. This is in contrast to the case of flat spacetime, in which the
string-induced vacuum densities for a massive field decay exponentially with
distance from the string. In the limit of the large curvature radius of de
Sitter space we recover the results for a cosmic string in flat spacetime.
