  An important hurdle to be faced by any model proposing a resolution to the
cosmological constant problem is Weinberg's venerable no go theorem. This
theorem states that no local field equations including classical gravity can
have a flat Minkowski solution for generic values of the parameters, in other
words, the no go theorem forbids the existence of any solution to the
cosmological constant problem within local field theories without fine tuning.
Though the original Weinberg theorem is valid only in classical gravity, in
this article we prove that this theorem holds even in quantum gravity. Our
proof is very general since it makes use of the BRST invariance emerging after
gauge-fixing of general coordinate invariance and does not depend on the detail
of quantum gravity.
