PHYS/ASTR Colloquium: "Probing the nature of dark matter using super massive black holes" - Dr. Chris Gordon (Senior Lecturer, U. Canterbury)

San Francisco State University

Physics & Astronomy Colloquium Series

Monday, March 9, 2026

SEIC 210, 3:30 PM

Dr. Chris Gordon (Senior Lecturer, U. Canterbury)

Title: Probing the nature of dark matter using super massive black holes

Abstract: Axions are hypothetical, extremely light particles originally proposed to solve the strong-CP problem in QCD, and they are also a well-motivated candidate for the Universe’s dark matter. In many axion dark-matter scenarios, the axion density is expected to clump early into dense “miniclusters” and then merge into somewhat larger bound clumps (“minihalos”), with typical masses in the asteroid-to-planetary range. In the Milky Way, repeated stellar flybys and disk passages can disrupt these structures, altering the local distribution of axions and the expectations for laboratory searches. I will present a new treatment of this disruption that accounts for multiple encounters and for the minihalos’ internal relaxation between encounters, calibrated with orbit ensembles evolved in a Galactic potential. We find that stellar interactions are substantially more destructive than previously estimated: minihalos retain only ∼ 30% of their mass near the Solar System, compared to earlier values of ∼ 60%. I will explain the physical origin of this enhanced stripping, and discuss what it implies for the local smooth axion density and for microwave-based axion detection experiments.