  In this paper the Feynman Green function for Maxwell's theory in curved
space-time is studied by using the Fock-Schwinger-DeWitt asymptotic expansion;
the point-splitting method is then applied, since it is a valuable tool for
regularizing divergent observables. Among these, the stress-energy tensor is
expressed in terms of second covariant derivatives of the Hadamard Green
function, which is also closely linked to the effective action; therefore one
obtains a series expansion for the stress-energy tensor. Its divergent part can
be isolated, and a concise formula is here obtained: by dimensional analysis
and combinatorics, there are two kinds of terms: quadratic in curvature tensors
(Riemann, Ricci tensors and scalar curvature) and linear in their second
covariant derivatives. This formula holds for every space-time metric; it is
made even more explicit in the physically relevant particular cases of
Ricci-flat and maximally symmetric spaces, and fully evaluated for some
examples of physical interest: Kerr and Schwarzschild metrics and de Sitter
space-time.
