Intermediate behavior of Kerr tails

TL;DR

This paper investigates the intermediate decay behavior of scalar fields in Kerr spacetime, explaining the observed splitting phenomena and proposing a formula for decay rates that accounts for mode interactions.

Contribution

It provides a detailed explanation of the splitting phenomena in Kerr tails and introduces a decay rate formula considering mode excitation effects.

Findings

Splitting is due to mode competition and excitation.

Asymptotic decay rates are consistent across finite distances.

A new decay rate formula accounts for inter-mode excitation effects.

Abstract

The numerical investigation of wave propagation in the asymptotic domain of Kerr spacetime has only recently been possible thanks to the construction of suitable hyperboloidal coordinates. The asymptotics revealed an apparent puzzle in the decay rates of scalar fields: the late-time rates seemed to depend on whether finite distance observers are in the strong field domain or far away from the rotating black hole, an apparent phenomenon dubbed "splitting". We discuss far-field "splitting" in the full field and near-horizon "splitting" in certain projected modes using horizon-penetrating, hyperboloidal coordinates. For either case we propose an explanation to the cause of the "splitting" behavior, and we determine uniquely decay rates that previous studies found to be ambiguous or immeasurable. The far-field "splitting" is explained by competition between projected modes. The near-horizon "splitting" is due to excitation of lower multipole modes that back excite the multipole mode for which "splitting" is observed. In both cases "splitting" is an intermediate effect, such that asymptotically in time strong field rates are valid at all finite distances. At any finite time, however, there are three domains with different decay rates whose boundaries move outwards during evolution. We then propose a formula for the decay rate of tails that takes into account the inter--mode excitation effect that we study.