Gravitational radiation at infinity with non-negative cosmological constant

Abstract

The existence of gravitational radiation arriving at null infinity -- i.e. escaping from the physical system -- is addressed in the presence of a non-negative cosmological constant $\Lambda\geq 0$. The case with vanishing $\Lambda$ is well understood and relies on the properties of the News tensor field (or the News function) defined at infinity. The situation is drastically different when $\Lambda>0$ where there is no known notion of `News' with similar good properties. In this paper both situations are considered jointly from a {\em tidal} point of view, that is, taking into account the strength (or energy) of the curvature tensors. The fundamental object used for that purposes in the {\em asymptotic (radiant) super-momentum}, a causal vector defined at infinity with remarkable properties. This leads to a novel characterization of gravitational radiation valid for the general case with $\Lambda\geq 0$ that has been proven to be equivalent, when $\Lambda =0$, to the standard one based on News. The implications of this result are analyzed in some detail when $\Lambda >0$. A general procedure to construct `news tensors' when $\Lambda >0$ is depicted, a proposal for asymptotic symmetries provided, and an example of a conserved charge that may detect gravitational radiation at infinity exhibited. A series of illustrative examples is listed.