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Holographic Dark Energy Model Characterized by the Conformal-age-like Length

A holographic dark energy model characterized by the conformal-age-like length scale $L= \frac{1}{a^4(t)}\int_0^tdt' a^3(t') $ is motivated from the four dimensional spacetime volume at cosmic time $t$ in the flat Friedmann-Robertson-Walker universe. It is shown that when the background constituent with constant equation of state $w_m$ dominates the universe in the early time, the fractional energy density of the dark energy scales as $Ω_{de}\simeq \frac94(3+w_m)^2d^2a^2$ with the equation of state given by $w_{de}\simeq-\frac23 +w_m$. The value of $w_m$ is taken to be $w_m\simeq-1$ during inflation, $w_m=\frac13$ in radiation-dominated epoch and $w_m=0$ in matter-dominated epoch respectively. When the model parameter $d$ takes the normal value at order one, the fractional density of dark energy is naturally negligible in the early universe, $Ω_{de} \ll 1$ at $a \ll 1$. With such an analytic feature, the model can be regarded as a single-parameter model like the $Λ$CDM model, so that the present fractional energy density $Ω_{de}(a=1)$ can solely be determined by solving the differential equation of $Ω_{de}$ once $d$ is given. We further extend the model to the general case in which both matter and radiation are present. The scenario involving possible interaction between the dark energy and the background constituent is also discussed.