Prospects of the local Hubble parameter measurement using gravitational
  waves from double neutron stars
  Following the detection of the GW170817 signal and its associated
electromagnetic emissions, we discuss the prospects of the local Hubble
parameter measurement using double neutron stars (DNSs). The kilonova emissions
of GW170817 are genuinely unique in terms of the rapid evolution of color and
magnitude and we expect that, for a good fraction $\gtrsim 50\%$ of the DNS
events within $\sim 200$Mpc, we could identify their host galaxies, using their
kilonovae. At present, the estimated DNS merger rate $(1.5^{+3.2}_{-1.2})\times
10^{-6} {\rm Mpc^{-3} yr^{-1}}$ has a large uncertainty. But, if it is at the
high end, we could measure the local Hubble parameter $H_L$ with the level of
$\Delta H_L/H_L\sim 0.042$ ($1\sigma$ level), after the third observational run
(O3). This accuracy is four times better than that obtained from GW170817
alone, and we will be able to examine the Hubble tension at $2.1\sigma$ level.
