Homogeneous Spectral Freedom and Hadamard Consistency of Perturbative Green Functions

Abstract

Perturbative mode equations often admit formally different particular solutions whose spectral sums appear to have different short-distance singularities. We formulate a general diagnostic which separates this representational freedom from a genuine change of quantum state: for a fixed normally hyperbolic operator, two Hadamard two-point functions can differ only by a smooth homogeneous bisolution. Thus, a nonsmooth term generated by rearranging a perturbative source must be traced either to a changed operator or to inadmissible homogeneous spectral data. We apply the criterion to two impulsive plane-wave backgrounds. Starting directly from the metrics, we recompute the wave operators, modes, world functions, and Van Vleck determinants. Exact-mode matching and the geometrical construction agree through second order for the first profile. In the second, correcting the metric changes the leading perturbation from quadratic to linear order in the wave parameter and eliminates inverse transverse-momentum poles, while the corrected metric retains an exact aligned null-radiation Einstein source. In both examples the singular Green function is precisely the perturbative expansion of the Hadamard parametrix; the additional logarithms arise from momentum-singular homogeneous data. For the exactly soluble first profile, the standard flat-normalized plane-wave prescription gives a vanishing renormalized stress tensor away from the impulsive surface. Smooth-front regularizations of both profiles reproduce, at the calculated orders and away from the characteristic zero mode, the singular structure of the matched impulsive propagators without generating inverse transverse-momentum poles.