Holographic Dark Energy Characterized by the Total Comoving Horizon and
  Insights to Cosmological Constant and Coincidence Problem
  The observed acceleration of the present universe is shown to be well
explained by the holographic dark energy characterized by the total comoving
horizon of the universe ($\eta$HDE). It is of interest to notice that the very
large primordial part of the comoving horizon generated by the inflation of
early universe makes the $\eta$HDE behave like a cosmological constant. As a
consequence, both the fine-tuning problem and the coincidence problem can
reasonably be understood with the inflationary universe and holographical
principle. We present a systematic analysis and obtain a consistent
cosmological constraint on the $\eta$HDE model based on the recent cosmological
observations. It is found that the $\eta$HDE model gives the best-fit result
$\Omega_{m0}=0.270$ ($\Omega_{de0}=0.730$) and the minimal
$\chi^2_{min}=542.915$ which is compatible with $\chi^2_{\Lambda {\rm
CDM}}=542.919$ for the $\Lambda$CDM model.
