Applications of the close-limit approximation: horizonless compact objects and scalar fields

TL;DR

This paper applies the close-limit approximation to study horizonless compact objects and scalar fields during mergers, revealing echoes and energy distribution, thus extending perturbation theory's applicability in gravitational wave modeling.

Contribution

It demonstrates the effectiveness of the close-limit approximation in modeling matter-involved mergers and scalar field interactions, expanding beyond vacuum black hole scenarios.

Findings

Echoes appear in horizonless object mergers.

A significant fraction of energy is emitted as late-time echoes.

Perturbation theory can describe parts of the merger phase.

Abstract

The ability to model the evolution of compact binaries from the inspiral to coalescence is central to gravitational wave astronomy. Current waveform catalogues are built from vacuum binary black hole models, by evolving Einstein equations numerically and complementing them with knowledge from slow-motion expansions. Much less is known about the coalescence process in the presence of matter, or in theories other than General Relativity. Here, we explore the Close Limit Approximation as a powerful tool to understand the coalescence process in general setups. In particular, we study the head-on collision of two equal-mass, compact but horizonless objects. Our results show the appearance of ``echoes'' and indicate that a significant fraction of the merger energy goes into these late-time repetitions. We also apply the Close Limit Approximation to investigate the effect of colliding black holes on surrounding scalar fields. Notably, our results indicate that observables obtained through perturbation theory may be extended to a significant segment of the merger phase, where in principle only a numerical approach is appropriate.