Analysis on a General Class of Holographic Type Dark Energy Models

We present a detail analysis on a general class of holographic type dark energy models characterized by the length scale $L=\frac1{a^n(t)}\int_0^t dt' a^m(t')$. We show that $n \geq 0$ is required by the recent cosmic accelerated expansion of universe. In the early universe dominated by the constituent with constant equation of state $w_m$, we have $w_{de}\simeq -1-\frac{2n}{3}$ for $n \geq 0$ and $m<0$, and $w_{de}\simeq-\frac23(n-m)+w_m$ for $n > m \geq 0$. The models with $n > m \geq 0$ become single-parameter models like the $Λ$CDM model due to the analytic feature $Ω_{de}\simeq \frac{d^2}4(2m+3w_m+3)^2a^{2(n-m)}$ at radiation- and matter-dominated epoch. Whereas the cases $n=m\geq 0$ should be abandoned as the dark energy cannot dominate the universe forever and there might be too large fraction of dark energy in early universe, and the cases $m> n \geq 0$ are forbidden by the self-consistent requirement $Ω_{de}\ll1$ in the early universe. Thus a detailed study on the single-parameter models corresponding to cases $n >m \geq 0$ is carried out by using recent observations. The best-fit analysis indicates that the conformal-age-like models with $n=m+1$, i.e. $L\propto\frac1{Ha}$ in early universe, are more favored and also the models with smaller $n$ for the given $n-m$ are found to fit the observations better. The equation of state of the dark energy in models with $n=m+1 >0$ transits from $w_{de}<-1$ during inflation to $w_{de}>-1$ in radiation- and matter-dominated epoch, and then back to $w_{de}<-1$ eventually. The best-fit result of the case $(n=0, m=-1)$ which is so-called $η$HDE model proposed in \cite{Huang:2012xm} is the most favorable model and compatible with the $Λ$CDM model.

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