  It is shown that the first law of thermodynamics and the holographic
principle applied to an arbitrary large cosmic causal horizon naturally demand
the zero cosmological constant and non-zero dynamical dark energy in the form
of the holographic dark energy. Semiclassical analysis shows that the
holographic dark energy has a parameter $d=1$ and an equation of state
comparable to current observational data, if the entropy of the horizon
saturates the Bekenstein-Hawking bound. This result indicates that quantum
field theory should be modified at large scale to explain dark energy. The
relations among dark energy, quantum vacuum energy and entropic gravity are
also discussed.
