Jordan W. N. Moncrieff

arXiv2026

Weak in the Presence of Beauty: Gravitational waves from the mergers of black holes and neutron stars as a messenger

In 2025, the 4th Gravitational Wave Transient Catalog reported the discovery of over 200 gravitational wave events since 2015. These gravitational waves (GWs) are emitted by compact binary coalescenses (CBCs) between neutron stars and black holes, and detected with a global network of ground-based laser interferometers. GWs represent a new `messenger' -- ripples in spacetime that propagate across cosmological distances, and carry information about the physical properties of compact objects and their location in the Universe. In this review, we consider what information we can learn about black holes and neutron stars using GWs as a messenger. We explore how the GW observation process allows us to transform the outputs of laser interferometers like LIGO, Virgo and KAGRA into knowledge about black holes and neutron stars. We show that the GW source population acts as a messenger that can lead us to breakthroughs in our understanding of stellar evolution, how the Universe has changed across cosmic time, and fundamental physics. Finally, we discuss three key opportunities to learn more about our Universe using GWs as a messenger in the coming decades.

gr-qc↗

arXiv2025

Not all roads lead to merger: AGN disc properties influence the interactions of highly unequal mass black holes

As the number of gravitational-wave detections of black hole binaries grows, so does the diversity of proposed formation channels. The growing sample of systems with highly unequal masses, such as GW190814 with $m_1=23.2\,M_{\odot}$ and $m_2=2.59\,M_{\odot}$ -- corresponding to a mass ratio $q=0.112$ -- cannot be readily explained by isolated binary evolution and may originate through dynamical assembly in an active galactic nucleus (AGN). We investigate AGN discs capable of producing GW190814-like mergers using \texttt{pAGN} to model self-consistent AGN torques, coupled with \texttt{TSUNAMI}, a regularised N-body code including post-Newtonian terms up to 3.5 order. Suites of N-body simulations reveal possible outcomes of binary capture and merger, mean-motion resonance interactions, and other novel dynamical pathways. We develop analytical models linking the branching ratios of captures and mergers to local disc properties, applicable to black hole populations across all mass ratios. Capture probability is primarily governed by $\mathscr{B}$, the ratio of libration time to resonance-width crossing, and is well-described by a log-Gaussian, $P(\rm{capture}|\mathscr{B}) = A \exp[-(\ln \mathscr{B}-μ)^2/2σ^2]$, with $A = 0.41^{+0.04}_{-0.04}$, $μ= 1.09^{+0.08}_{-0.07}$, $σ= 1.05^{+0.08}_{-0.07}$. This fit, while an upper limit, is useful for simplified population synthesis. Finally, we explore the mass ratio AGN luminosity parameter space and find that GW190814 may be formed in a low luminosity AGN of $L_{\rm AGN}\approx 10^{43.5}\ \rm erg\ s^{-1}$. A more systematic parameter space exploration and future population studies will further test our predictions.

astro-ph.HE↗