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

Abstract

A holographic dark energy model characterized by the conformal-age-like length scale [Formula: see text] 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 wm dominates the universe in the early time, the fractional energy density of the dark energy scales as [Formula: see text] with the equation of state given by [Formula: see text]. The value of wm is taken to be wm≃-1 during inflation, wm = ⅓ in radiation-dominated epoch and wm = 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 Ωde 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.