HOLOGRAPHIC DARK ENERGY MODEL CHARACTERIZED BY THE CONFORMAL-AGE-LIKE LENGTH

A holographic dark energy model characterized by the conformal-age-like length scale is motivated from the four-dimensional space–time volume at cosmic time t in the flat Friedmann–Robertson–Walker (FRW) 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 with the equation of state given by . The value of w_m is taken to be w_m≃-1 during inflation, w_m = ⅓ 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 ≪1 at a ≪1. With such an analytic feature, the model can be regarded as a single-parameter model like the ΛCDM 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.