Probing Ultralight Bosons in the Environment of Supermassive Black Holes

Abstract

The unresolved mysteries of the universe, such as the nature of dark matter and the dynamics of supermassive black holes (SMBHs), reveal the limitations of the Standard Model of particle physics, driving the search for new physics. This thesis investigates ultralight bosons—axions and dark photons—as leading candidates for wave-like dark matter, using SMBHs as astrophysical laboratories. We explore two distinct detection strategies, each leveraging unique phenomena. First, we analyze observations of the UGC4211 system, which suggest the presence of a soliton, a self-gravitating wave-like dark matter structure. We extend this concept to all supermassive black hole binaries (SMBHBs), evaluating the potential of the stochastic gravitational wave background (SGWB) at low frequencies to probe soliton existence. By combining latest pulsar timing data from NANOGrav and EPTA with constraints from dwarf galaxies, we derive the best-fit axion mass and decay constant ranges of {m_a, f_a} ∼{10^−21.7eV, 10^15.5GeV} and {m_a, f_a} ∼{10^−20.5eV, 10^16.8GeV}. Second, we investigate polarimetric detection of dark photons, utilizing superradiance around SMBHs like M87* and Sgr A*. This process may produce a large number of dark photons, which induce oscillating electromagnetic fields via kinetic mixing with photons. These extra fields generate optical effects, such as Faraday rotation, that leave observable imprints on black hole images through alterations in polarization patterns. Using Event Horizon Telescope (EHT) measurements of Stokes parameters, we establish constraints on the photon-dark photon mixing parameter: for M87*, limits span dark photon masses from 10⁻²²eV to 10⁻²⁰eV, with a peak sensitivity of ∼10⁻⁹; for Sgr A*, constraints cover 10⁻¹⁹eV to 10⁻¹⁷eV, achieving ∼10⁻¹¹. These approaches highlight the power of SMBH-related phenomena to probe ultralight bosons and their role in dark matter, advancing our understanding of the universe’s fundamental constituents.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Tao LIU (Supervisor)