Cosmological prospects for multiband detection of intermediate-mass binary black holes with Taiji and ground-based detectors

TL;DR

This paper explores how combining space-based and ground-based gravitational-wave detectors enhances the detection and cosmological measurement of intermediate-mass black hole binaries, especially improving constraints on the Hubble constant.

Contribution

It demonstrates that multiband gravitational-wave observations significantly improve detection prospects and parameter estimation for IMBH binaries compared to single-detector approaches.

Findings

Multiband detection expands the observable parameter space.

Combining detectors improves the constraint on H_0 by over 30%.

Parameter estimation improves most for small sample sizes.

Abstract

Intermediate-mass black holes (IMBHs) bridge the gap between stellar-mass and supermassive black holes, but remain challenging to detect electromagnetically. Gravitational-wave observations provide a direct means of detecting IMBHs and their mergers. We simulate the gravitational-wave signals of IMBH binaries under different population models and assess their detectability with the space-based detector Taiji alone and in a multiband network combining Taiji with third-generation ground-based detectors. Taiji performs well in detecting high-mass IMBH binaries, while ground-based detectors compensate for its reduced sensitivity to lower-mass systems. Their combination expands the accessible parameter space and improves the constraints on cosmological parameters. In particular, multiband observations improve the constraint accuracy on $H_0$ by $36.5\%$ and $31.0\%$ compared with Taiji and ET2CE alone, respectively. We further examine the dependence of parameter accuracy on the number of simulated events, finding that improvements are most pronounced for small samples and gradually saturate as the number of events increases. We conclude that multiband observations enhance the detectability of IMBH binaries and reinforce their role as probes of precision cosmology.