A Note on Agegraphic Dark Energy

Recently a new model of dynamical dark energy, or time-varying $\Lambda$, was proposed by Cai [arXiv:0707.4049] by relating the energy density of quantum fluctuations in a Minkowski space-time, namely $\rho_q \equiv 3 n^2 m_P^2/t^2$, where $n\sim {\cal O}(1)$ and t is the cosmic time, to the present day dark energy density. In this note, we show that the model can be adjusted to the present values of dark energy density parameter $\Omega_q$ ($\simeq 0.73$) and the equation of state ${\rm w}\Z{q}$ ($\simeq -1$) only if the numerical coefficient $n$ takes a reasonably large value ($n\gtrsim 3$) or the present value of the gravitational coupling of q-field to (dark) matter is also nonzero, namely, $\tilde{Q}\simeq \frac{2}{n}(\Omega\Z{q0})^{3/2}>0$ where $\Omega\Z{q0}$ is the present value of dark energy density fraction. We also discuss some of the difficulties of this proposal as a viable dark energy model with a constant $n$; especially, the bound imposed on the dark energy density parameter $\Omega_q <0.1$ during big bang nucleosynthesis (BBN) requires $n< 1/6$. To overcome this drawback, we outline a few modifications where such constraints can be weakened or relaxed. Finally, by establishing a correspondence between the agegraphic dark energy scenario and the standard scalar-field model, we also point out some interesting features of an agegraphic quintessence model.