Melia
[7] has proposed an alternate model of dark energy, namely, $R_h=ct$ model. Melia's model suggests that the Hubble horizon of the universe is always equal to the light traveling distance throughout the whole history of universe. It was shown that various astrophysical data are well consistent with the $R_h=ct$ model.
[8-12] In fact, the $R_h=ct$ model has connections with holographic dark energy (HDE) model.
[13] The HDE model is inspired by the holographic principle,
[14] and it assumes that the infrared cut-off of vacuum energy density, which is relevant to dark energy is the size of the future event horizon of our universe. In an early work, Zhang and Wu
[15] constrained the HDE model using the gold sample of SN Ia combined with CMB and the large-scale structure (LSS), and the results can be fitted well with the observational data. In Refs.
[16-17], various dark energy models have been discussed by using the observational data, and the best fitting results of the HDE model can also be fitted well with the observational data. Moreover, in Ref.
[18] for recent review, the HDE model is in good agreement with several cosmological observations.