Hubble parameter estimation via dark sirens with the LISA-Taiji network
  The Hubble parameter is one of the central parameters in modern cosmology,
which describes the present expansion rate of the universe. Their values
inferred from the late-time observations are systematically higher than those
from the early-time measurements by about $10\%$. To come to 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 5 years
operation time, the LISA-Taiji network is able to constrain the Hubble
parameter within $1\%$ accuracy, and possibly beats the scatters down to
$0.5\%$ or even better.
