Speaker
Description
Standard sirens - gravitational wave (GW) sources with an electromagnetic (EM) counterpart - can be used to measure the Hubble constant directly and thus will help to ease the Hubble tension. However, systems that emit detectable GWs and EM radiation at the same time are rare. Therefore, binary black hole (BBH) mergers inside active galactic nuclei (AGNs) are gaining more and more attention as potential standard sirens. At the same time, these systems require more delicate modelling as they differ significantly from standard field binaries - in particular, as the supermassive black hole in the center of the AGN can lead to significant Doppler and gravitational shifts of the GWs frequency. In this talk, I discuss the modeling of these sources and two recent papers. In the first paper, 18 GW events from LIGO-Virgo-KAGRA observing runs O3 to O4b paired with 28 candidate AGN counterparts are analyzed. Of the 28 candidate pairs, 21 are positive-to-strong-favoured and from the 21 positive-to-strong-favoured BBH-AGN pairs, 13 unique associations can be identified as the preferred ones. In the second paper, the Hubble constant $H_0$ is measured using the 13 BBH mergers associated with AGN flares. We find $H_0=70.50^{+3.37}_{-2.89} ({\rm stat})\pm1.56 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($4.4\%$ precision), consistent with both Planck 2018 ($0.98\sigma$) and SH0ES 2024 ($0.76\sigma$), with no significant preference between the two. Combining with the binary neutron star merger GW170817 sharpens the constraint to $H_0=70.31^{+3.00}_{-2.85} ({\rm stat})\pm1.55 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($4.2\%$ precision), and further combining with an independent dark-and-bright-siren sample tightens it to $H_0=69.71^{+2.55}_{-2.40} ({\rm stat})\pm1.54 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($3.5\%$ precision).