Prospects for measuring dark energy with 21 cm intensity mapping
  experiments
  Using the 21 cm intensity mapping (IM) technique can efficiently perform
large-scale neutral hydrogen surveys, and this method has great potential for
measuring dark-energy parameters. Some 21 cm IM experiments aiming at measuring
dark energy in the redshift range of $0<z<3$ have been proposed and performed,
in which the typical ones using single-dish mode include e.g., BINGO, FAST, and
SKA1-MID, and those using interferometric mode include e.g., HIRAX, CHIME, and
Tianlai. In this work, we make a forecast for these typical 21 cm IM
experiments on their capability of measuring parameters of dark energy. We find
that the interferometers have great advantages in constraining cosmological
parameters. In particular, the Tianlai cylinder array alone can achieve the
standard of precision cosmology for the $\Lambda$CDM model (i.e., the precision
of parameters is better than 1%). However, for constraining dynamical dark
energy, we find that SKA1-MID performs very well. We show that the simulated 21
cm IM data can break the parameter degeneracies inherent in the CMB data, and
CMB+SKA1-MID offers $\sigma(w)=0.013$ in the $w$CDM model, and
$\sigma(w_0)=0.080$ and $\sigma(w_a)=0.25$ in the CPL model. Compared with
CMB+BAO+SN, Tianlai can provide tighter constraints in $\Lambda$CDM and $w$CDM,
but looser constraints (tighter than CMB+BAO) in CPL, and the combination
CMB+BAO+SN+Tianlai gives $\sigma(w)=0.013$, $\sigma(w_0)=0.055$, and
$\sigma(w_a)=0.13$. In addition, it is found that the synergy of FAST
($0<z<0.35$)+SKA1-MID ($0.35<z<0.77$)+Tianlai ($0.77<z<2.55$) offers a very
promising survey strategy. Finally, we find that the residual foreground
contamination amplitude has a considerable impact on constraint results. We
show that in the future 21 cm IM experiments will provide a powerful probe for
exploring the nature of dark energy.
