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A holographic dark energy model characterized by the conformal-age-likelength scale $L= \frac{1}{a^4(t)}\int_0^tdt' a^3(t') $ is motivated from thefour dimensional spacetime volume at cosmic time $t$ in the flatFriedmann-Robertson-Walker universe. It is shown that when the backgroundconstituent with constant equation of state $w_m$ dominates the universe in theearly time, the fractional energy density of the dark energy scales as$\Omega_{de}\simeq \frac94(3+w_m)^2d^2a^2$ 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 $w_m=0$ inmatter-dominated epoch respectively. When the model parameter $d$ takes thenormal value at order one, the fractional density of dark energy is naturallynegligible in the early universe, $\Omega_{de} \ll 1$ at $a \ll 1$. With suchan analytic feature, the model can be regarded as a single-parameter model likethe $\Lambda$CDM model, so that the present fractional energy density$\Omega_{de}(a=1)$ can solely be determined by solving the differentialequation of $\Omega_{de}$ once $d$ is given. We further extend the model to thegeneral case in which both matter and radiation are present. The scenarioinvolving possible interaction between the dark energy and the backgroundconstituent is also discussed.

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