  The cosmological constant is the most economical candidate for dark energy.
No other approach really alleviates the difficulties faced by the cosmological
constant because, in all other attempts to model the dark energy, one still has
to explain why the bulk cosmological constant (treated as a low-energy
parameter in the action principle) is zero. I argue that until the theory is
made invariant under the shifting of the Lagrangian by a constant, one cannot
obtain a satisfactory solution to the cosmological constant problem. This is
impossible in any generally covariant theory with the conventional low-energy
matter action, if the metric is varied in the action to obtain the field
equations. I review an alternative perspective in which gravity arises as an
emergent, long wavelength phenomenon and can be described in terms of an
effective theory using an action associated with null vectors in the spacetime.
This action is explicitly invariant under the shift of the energy momentum
tensor $T_{ab}\to T_{ab}+\Lambda g_{ab}$ and any bulk cosmological constant can
be gauged away. Such an approach seems to be necessary for addressing the
cosmological constant problem and can easily explain why its bulk value is
zero. I describe some possibilities for obtaining its observed value from
quantum gravitational fluctuations.
