Current constraints on early and stressed dark energy models and future
  21 cm perspectives
  Despite the great progress of current cosmological measurements, the nature
of the dominant component of the universe, coined dark energy, is still an open
question. Early Dark Energy is a possible candidate which may also alleviate
some fine tuning issues of the standard paradigm. Using the latest available
cosmological data, we find that the 95% CL upper bound on the early dark energy
density parameter is $\Omega_{\textrm{eDE}}$. On the other hand, the dark
energy component may be a stressed and inhomogeneous fluid. If this is the
case, the effective sound speed and the viscosity parameters are unconstrained
by current data. Future omniscope-like $21$cm surveys, combined with present
CMB data, could be able to distinguish between standard quintessence scenarios
from other possible models with $2\sigma$ significance, assuming a
non-negligible early dark energy contribution. The precision achieved on the
$\Omega_{\textrm{eDE}}$ parameter from these $21$ cm probes could be below
$\mathcal{O} (10\%)$.
