We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R. G. Cai, arXiv:0708.0884] where the presentday dark energy density was taken to be ${\rho }_q\equiv 3{\alpha }^2{m}_P^2/{\eta }^2$, where $\eta$ is the conformal time and $\alpha$ is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field $q$, the model may be adjusted to the present values of the dark energy density fraction ${\Omega }_q\simeq 0.73$ and the equation of state parameter $w_q<-0.78$, if the numerical constant $\alpha$ takes a reasonably large value, $\alpha \gtrsim 2.6$. However, in the presence of a nontrivial gravitational coupling of $q$-field to matter, say $\tilde{Q}$, the model may be adjusted to the values ${\Omega }_q\simeq 0.73$ and $w_q\simeq -1$, even if $\alpha \sim \mathcal{O}(1)$, given that the present value of $\tilde{Q}$ is large. Unlike for the model in [R. G. Cai, arXiv:0707.4049], the bound ${\Omega }_q<0.1$ during big bang nucleosynthesis (BBN) may be satisfied for almost any value of $\alpha$. Here we discuss some other limitations of this proposal as a viable dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.

• Received 25 September 2007

DOI:https://doi.org/10.1103/PhysRevD.76.123006