Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 354 / 1 / c / Solution

Translation invariance supplies the momentum-conserving delta function. Lorentz invariance, symmetry of $T_{ij}$, stress-tensor conservation, and tracelessness then fix the remaining tensor structure to the transverse traceless spin-two projector shown in the question, up to an overall theory-dependent coefficient. Since the stress-energy tensor has scaling dimension $d$, its momentum transform has dimension zero. The delta function has momentum dimension $-d$, so scale invariance requires

For the Wightman function, the spectral condition restricts support to the appropriate future-directed timelike momenta, with convention-dependent distributions on the null boundary. It vanishes for spacelike momentum. A time-ordered or Euclidean correlator is obtained by analytic continuation and exists more broadly, but polynomial contact terms are renormalization-scheme dependent; in even dimensions the nonlocal power is accompanied by a logarithm.

In a large-$N$ holographic CFT, the single-trace state $T_{ij}(p)|0⟩$ is dual at leading order to a one-graviton bulk state with the matching boundary momentum and polarization. Multiparticle intermediate states and graviton interactions enter at subleading orders in $1/N$.