Stone Skipping Black Holes in Ultralight Dark Matter Solitons

TL;DR

This paper reveals that black holes within ultralight dark matter solitons can exhibit 'stone skipping' behavior due to dipole excitations, significantly affecting their orbital dynamics and inspiral timescales.

Contribution

It introduces a resonance model showing how dipole excitations induce stone skipping, altering black hole orbital decay in ULDM solitons.

Findings

Stone skipping causes quasi-periodic orbital variations.

Dipole excitations can modify inspiral timescales.

Implications for supermassive black hole dynamics and gravitational waves.

Abstract

The orbit of a black hole moving within an ultralight dark matter (ULDM) soliton is naively expected to decay due to dynamical friction. However, single black holes can undergo ``stone skipping'', with their orbital radius varying quasi-periodically. We show that stone skipping is induced by the dipole excitation of the soliton. We model it as resonance in a forced, damped harmonic oscillator, demonstrating that the coherent response of the soliton can significantly modify the dynamics of objects orbiting within it. This suggests that a dipole perturbation of a soliton can modify inspiral timescales if the black holes masses are significantly less than the soliton mass, with implications for supermassive black hole dynamics, the final parsec problem and gravitational wave observations in a ULDM cosmology.