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 $\mathcal{O} (10\%)$. 
Maria Archidiacono, Laura Lopez-Honorez and Olga Mena
astro-ph.CO