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This study reveals that black holes moving through ultralight dark matter (ULDM) solitons can exhibit "stone skipping" behavior, where their orbital radius oscillates quasi-periodically rather than simply decaying as expected from dynamical friction. The researchers demonstrate that this phenomenon is driven by a dipole excitation of the soliton, which they model as a resonance in a forced, damped harmonic oscillator. This coherent response of the soliton can significantly alter the orbital dynamics of black holes that are substantially less massive than the soliton itself.
Why it matters
This finding has important implications for understanding supermassive black hole dynamics, potentially addressing the "final parsec problem" in galaxy mergers, and could affect gravitational wave observations if ultralight dark matter exists in the universe. The results suggest that dark matter halos may play a more active role in black hole orbital evolution than previously thought.
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arXiv:2602.11512v4 Announce Type: replace
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, in isolated near-circular soliton–black-hole systems, single black holes can undergo “stone skipping”, with their orbital radius varying quasi-periodically. We show that, within this controlled setting, stone skipping is driven by a dipole excitation of the soliton. We model the effect as a 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. In this regime, a dipole perturbation of a soliton can modify inspiral timescales when the black hole masses are significantly smaller than the soliton mass, with implications for supermassive black hole dynamics, the final parsec problem and gravitational wave observations in a ULDM cosmology.
Source: Stone Skipping Black Holes in Ultralight Dark Matter Solitons