Reconstructing the Goldberg-Palatini Action from Noether Identities

Abstract

We investigate the reconstruction of the first-order gravitational action from the Noether identities associated with diffeomorphism invariance. The independent fields are a symmetric perturbation $ϕ^{μν}$ of the contravariant metric density and a trace-adjusted connection $G^λ{}_{μν}$, whose transformation laws are prescribed. For a local action polynomial in these fields, with the gravitational coupling as its only dimensionful constant and each additional field carrying one power of it, a dilatation, which is a particular diffeomorphism, shows that in any dimension $d>2$ the action terminates at most at cubic order, its only possible cubic term being a derivative-free $ϕGG$ vertex. Expanding the kernels of the resulting Lorentz-covariant ansatz in tensor bases gives 31 coefficients for $d\geq 4$. The Noether identities with general diffeomorphisms, expanded order by order in the fields, yield a linear system that eliminates the $ϕϕ$ kernel and determines the remaining coefficients up to an overall normalization, recovering the Goldberg-Palatini action. Since the connection can be eliminated exactly, this also fixes the complete second-order action, with its infinite set of graviton vertices. The construction requires no choice of energy-momentum tensor, the source of ambiguity in the self-coupling approach. The kernel bases, the identities and the solution are given in appendices, and the symbolic computations are publicly available.