Holographic Principle, Cosmological Constant and Cyclic Cosmology

The holographic principle provides a deep insight into quantum gravity and resolves the fine-tuning crisis concerning the cosmological constant. Holographic dark energy introduces new ultra-violet (UV) and infra-red (IR) cutoffs into quantum gravity which are necessarily strongly related. The equation of state for dark energy $\omega = p/\rho$ is discussed from the holographic point of view. The phantom option of $\omega < -1$ is resurrected, as in an earlier cyclic cosmology. Such a cyclic model can, however, equally use the cosmological constant with $\omega = -1$.