A Simple Phenomenological Emergent Dark Energy Model can Resolve the
  Hubble Tension
  Motivated by the current status of the cosmological observations and
significant tensions in the estimated values of some key parameters assuming
the standard $\Lambda$CDM model, we propose a simple but radical
phenomenological emergent dark energy model where dark energy has no effective
presence in the past and emerges at the later times. Theoretically, in this
phenomenological dark energy model with zero degree of freedom (similar to a
$\Lambda$CDM model), one can derive that the equation of state of dark energy
increases from $-\frac{2}{3 {\rm{ln}}\, 10} -1$ in the past to $-1$ in the
future. We show that by setting a hard-cut 2$\sigma$ lower bound prior for the
$H_0$ that associates with $97.72\%$ probability from the recent local
observations~\citep{riess2019large}, this model can satisfy different
combinations of cosmological observations at low and high redshifts (SNe Ia,
BAO, Ly{$\alpha$} BAO and CMB) substantially better than the concordance
$\Lambda$CDM model with $\Delta \chi^2_{bf} \sim -41.08$ and
$\Delta\,{\rm{DIC}} \sim-35.38$. If there are no substantial systematics in SN
Ia, BAO or Planck CMB data and assuming reliability of the current local $H_0$
measurements, there is a very high probability that with slightly more precise
measurement of the Hubble constant our proposed phenomenological model rules
out the cosmological constant with decisive statistical significance and is a
strong alternative to explain combination of different cosmological
observations. This simple phenomenologically emergent dark energy model can
guide theoretically motivated dark energy model building activities.
