Hubble constant and sound horizon from the late-time Universe
  We measure the expansion rate of the recent Universe and the calibration
scale of the baryon acoustic oscillation (BAO) from low-redshift data. BAO
relies on the calibration scale, i.e., the sound horizon at the end of drag
epoch $r_d$, which often imposes a prior of the cosmic microwave background
(CMB) measurement from the Planck satellite. In order to make really
independent measurements of $H_0$, we leave $r_d$ completely free and use the
BAO data sets combined with the 31 observational $H(z)$ data, GW170817 and
Pantheon sample of Type Ia supernovae. In $\Lambda$CDM model, we get
$H_0=68.63^{+1.75}_{-1.77}$ km s$^{-1}$ Mpc$^{-1}$,
$r_d=146.85^{+3.29}_{-3.77}$ Mpc. For the two model-independent reconstructions
of $H(z)$, we obtain $H_0=68.02\pm1.82$ km s$^{-1}$ Mpc$^{-1}$,
$r_d=148.18^{+3.36}_{-3.78}$ Mpc in the cubic expansion, and $H_0=68.58\pm1.76$
km s$^{-1}$ Mpc$^{-1}$, $r_d=148.02^{+3.63}_{-3.60}$ Mpc in the polynomial
expansion. The values of Hubble constant $H_0$ and sound horizon $r_d$ are
consistent with the estimate derived from the Planck CMB data assuming a flat
$\Lambda$CDM model, but $H_0$ is in $2.4\sim2.6$ $\sigma$ tension with SH0ES
2019, respectively.
