A note on agegraphic dark energy

Recently a new model of dynamical dark energy, or time-varying A. was proposed by Cai [R.G. Cai, Phys. Lett. B 657 (2007) 228, arXiv:0707.4049 [hep-th]] by relating the energy density of quantum fluctuations in a Minkowski space-time, namely rho(q) = 3(n)(2)m(P)(2)/t(2), where n similar to O(1) and t is the cosmic time, to the present day (lark energy density. In this note, we show that the model can be adjusted to the present values of dark energy density parameter Omega(q) (similar or equal to 0.73) and the equation of state W-q (similar or equal to -1) only if the numerical coefficient n takes a reasonably large Value (n greater than or similar to 3) or the present value of the gravitational coupling of q-field to (dark) matter is also nonzero, namely, (Q) over tilde similar or equal to 2/n (Omega(q0))(3/2) > 0 where Omega(q0) is the present Value of dark energy density fraction. We also discuss some of the difficulties of [his proposal as a viable dark energy model with a constant it; especially, the bound imposed oil the (lark 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. (C) 2009 Elsevier B.V. All rights reserved.