Astro/GEI Seminar
Depletion of Dark-Matter Spikes Around Massive Black Holes by Stars and Extreme Mass-Ratio In-Spirals
Speaker: Charles Sharpe (Oxford)
Date: Wednesday 18 February 2026
Time: 14:00
Venue: N/4.07 Prince Phillip Lecture Theatre
Dark matter (DM) over densities surrounding black holes provide a compelling opportunity to probe the fundamental nature of dark matter. In particular, the classic Gondolo–Silk scenario predicts the formation of a steep, dense DM spike through the adiabatic growth of a black hole at the centre of a pre-existing DM cusp. This framework has received significant attention due to its potential observational signatures, initially motivated by observing gamma-ray emissions from DM self-annihilation. More recently, interest has turned to gravitational-wave signatures. A black hole binary evolving within such a spike experiences dynamical friction and gravitational backreaction from the surrounding DM, leading to measurable dephasing of the waveform. This effect is expected to be especially pronounced for extreme mass-ratio inspirals (EMRIs), which are among the primary targets of future space-based detectors such as LISA. In this work, we revisit this paradigm by embedding the DM spike within a realistic galactic environment. We model the inward relaxation of stars from the outer regions of the sphere of influence into the central few mpcs and investigate the cumulative impact of repeated EMRI events on the spike structure. We find that these processes substantially deplete the spike, reducing the DM density by at least two orders of magnitude, and in some cases considerably more. This depletion has important implications for the prospects of precision gravitational-wave measurements in these systems, as well as for the detectability of dark matter through such observations.