Constraining coupled quintessence with the 21cm signal
  The 21cm line probes the evolution of matter perturbations over a wide range
of redshifts, from the dark ages down to the completion of reionization.
Observing the 21cm cosmological signal will extend our understanding of the
evolution of the Universe and it is thus important to investigate the
predictions of different cosmological models. In this paper we focus on the
prospect of constraining coupled quintessence models during the Epoch of
Reionization both for global signal experiments and for intensity mapping
surveys. To derive the all-sky 21cm signal and fluctuations in coupled
quintessence, we simulate cosmological volumes of the 21cm signal including the
coupling between dark matter and the quintessence field, where the strength of
the coupling is labeled by the parameter $Q$. We show that the coupling between
dark matter and quintessence modifies structure formation and expedites the
process of reionization. For the upcoming 21cm line surveys like SKA and a
fiducial global 21cm signal experiment, we perform a Fisher matrix analysis to
constrain the coupling $Q$ and the dark matter density parameter
$\Omega_\mathrm{dm}$.
  The results indicate that SKA will be able to place a 68% upper limit $0.04$
to $|Q|$. At the same time, such a global 21cm detector provides a constraint
on the dark matter density parameter $\Omega_\mathrm{dm}$ around
$\Delta\Omega_{\rm dm}\approx 0.005$, whereas SKA sets a quite weaker
constraint around $\Delta\Omega_{\rm dm}\approx0.1$. These constraints are
comparable to those already obtained from the cosmic microwave background, but
explore an entirely different redshift range.
