Abstract
The Hubble parameter is one of the central parameters in modern cosmology, and describes the present expansion rate of the universe. The values of the parameter inferred from late-time observations are systematically higher than those inferred from early-time measurements by about n}{}10%. To reach a robust conclusion, independent probes with accuracy at percent levels are crucial. Gravitational waves from compact binary coalescence events can be formulated into the standard siren approach to provide an independent Hubble parameter measurement. The future space-borne gravitational wave observatory network, such as the LISA-Taiji network, will be able to measure the gravitational wave signals in the millihertz bands with unprecedented accuracy. By including several statistical and instrumental noises, we show that, within a five-year operation time, the LISA-Taiji network is able to constrain the Hubble parameter within n}{}1% accuracy, and possibly beats the scatters down to n}{}0.5% or even better.
Highlights
In this article we forecast the ability in estimating the Hubble parameter by using GW sirens data from the future space-based GW observatories
Unlike stellar-mass binary black holes detected with aLIGO/Virgo [23], for which the merger rate is observationally measured, there is no conclusive observational evidence for merging massive binary black holes (MBHs)
The models are built by combining the cosmological galaxy formation history with the massive black hole binary (MBHB) formation dynamics
Summary
We consider 3 different massive black hole formation models with different black hole seedings and time delays. In the “heavy-seed” scenario (assuming the critical Toomre parameter Qc = 3), MBHs arise from the collapse of protogalactic disks and already have the masses around 105M at high redshifts z = 15 ∼ 20. Depending on whether there exist the delays between MBHs and galaxy mergers or not, these “heavy-seed” models are named as “Q3d” and “Q3nod”, respectively. For each of combinations of the model, the mission configuration and the observation time, we generate 40 sets of simulations including both the instrumental noise [27, 28] and lensing noise [29, 30]. Each set of simulations contains a few tens or a few hundreds CBC events according to different MBH formation models. It might be worth noting here that one should pay attention to the accuracy of the Hubble parameter H0 through our simulations, rather than the resulted H0 value itself in this work
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