Memory of Robinson-Trautman waves

TL;DR

This paper explicitly computes the memory effect for Robinson-Trautman waves, analyzing their asymptotic flatness, mass aspects, and invariance properties, revealing connections to Euclidean Liouville theory and black hole solutions.

Contribution

It provides a detailed construction of the memory effect for Robinson-Trautman waves, including a new generalized mass aspect and invariance results under BMS transformations.

Findings

Memory effects are invariant under supertranslations and Lorentz transformations.

News-free solutions correspond to boosted and rescaled Schwarzschild black holes.

A new interpretation of flows controlling low harmonics is introduced.

Abstract

The memory effect for Robinson-Trautman waves is explicitly worked out. In a first step, we construct the combined frame rotation and coordinate transformation in which Robinson-Trautman waves are manifestly locally asymptotically flat at future null infinity. This allows us to apply well-established results on how to derive the memory effect in this context. In a second step, we construct a suitably improved generalized mass aspect that provides a local Lyapunov function for the flow in the sense that it is manifestly positive. News-free solutions are studied in detail and shown to coincide with the vacuum sector of Euclidean Liouville theory. They correspond to a boosted and rescaled Schwarzschild black hole. As a by-product, we show that the displacement and non-linear memory effects in locally asymptotically flat spacetimes at future null infinity are invariant under supertranslations and covariant under $\mathrm{BMS}_4$ Lorentz transformations and constant rescalings. A novel interpretation of modified flows that control the low harmonics in terms of keeping the system in its instantaneous rest frame is provided.