Where are linearized gauge invariants encoded for plane waves in linearized gravity?

Abstract

The Newman--Penrose (NP) formalism is traditionally used to analyze the polarization content of gravitational waves, while the gauge-invariant Bardeen formalism provides a complementary, and often simpler, description based on the irreducible scalar, vector, and tensor perturbations of the metric. In this work we apply the Bardeen formalism to plane gravitational waves in Minkowski spacetime, computing all scalar, vector, and tensor gauge-invariant variables explicitly and demonstrating that only the two transverse-traceless tensor modes survive, as expected for vacuum waves in general relativity. We then compare these Bardeen variables with curvature-based invariants constructed using the linearized Cartan--Karlhede (CK) algorithm. Although one might anticipate a correspondence, our analysis shows that the CK invariants do \emph{not} capture the polarization modes: the Weyl tensor possesses only a single non-zero Newman--Penrose scalar and the CK algorithm terminates without producing invariants that distinguish the $\oplus$ and $\otimes$ states. However, by computing invariant quantities obtained from the translational Killing vector fields of the Minkowski background that are retained under linear perturbation, we provide an algorithmic approach that reproduces the same physical tensor degrees of freedom captured by the Bardeen variables.