  The energy density associated with Planck length is $\rho_{uv}\propto
L_P^{-4}$ while the energy density associated with the Hubble length is
$\rho_{ir}\propto L_H^{-4}$ where $L_H=1/H$. The observed value of the dark
energy density is quite different from {\it either} of these and is close to
the geometric mean of the two: $\rho_{vac}\simeq \sqrt{\rho_{uv} \rho_{ir}}$.
It is argued that classical gravity is actually a probe of the vacuum {\it
fluctuations} of energy density, rather than the energy density itself. While
the globally defined ground state, being an eigenstate of Hamiltonian, will not
have any fluctuations, the ground state energy in the finite region of space
bounded by the cosmic horizon will exhibit fluctuations $\Delta\rho_{\rm
vac}(L_P, L_H)$. When used as a source of gravity, this $\Delta \rho$ should
lead to a spacetime with a horizon size $L_H$. This bootstrapping condition
leads naturally to an effective dark energy density $\Delta\rho\propto
(L_{uv}L_H)^{-2}\propto H^2/G$ which is precisely the observed value. The model
requires, either (i) a stochastic fluctuations of vacuum energy which is
correlated over about a Hubble time or (ii) a semi- anthropic interpretation.
The implications are discussed.
