Gravitational Radiation at Infinity with Non-Negative Cosmological Constant

The existence of gravitational radiation arriving at null infinity J+, i.e., escaping from the physical system, is addressed in the presence of a non-negative cosmological constant Lambda >= 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 J+. 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 tidal point of view, that is, taking into account the strength (or energy) of the curvature tensors. The fundamental object used for this purposes is the 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 >= 0, which has been proven to be equivalent when Lambda=0 to the standard one based on News. Here, the implications of this result when Lambda>0 are analyzed in detail. A general procedure to construct 'News tensors' when Lambda>0 is depicted, a proposal for asymptotic symmetries is provided, and an example of a conserved charge that may detect gravitational radiation at J+ is exhibited. A series of illustrative examples is listed as well.