Impacts of dark energy on weighing neutrinos: Mass hierarchies considered

Taking into account the mass splittings between three active neutrinos, we investigate the impacts of dark energy on constraining the total neutrino mass m_ by using recent cosmological observations. We consider two typical dark energy models, namely, the wCDM model and the holographic dark energy (HDE) model, which both have an additional free parameter compared with the model. We employ the Planck 2015 data of CMB temperature and polarization anisotropies, combined with low-redshift measurements on BAO distance scales, type Ia supernovae, the Hubble constant, and Planck lensing. Compared to the model, our study shows that the upper limit on m_ becomes much looser in the wCDM model but much tighter in the HDE model. In the HDE model, we obtain the 95% confidence level upper limit m_<0. 105 eV for three degenerate neutrinos. This might be the most stringent constraint on m_ by far, and it is on the verge of diagnosing the neutrino mass hierarchies in the HDE model. However, the difference of ^2 is still not significant enough to distinguish the neutrino mass hierarchies, even though the minimal ^2 of the normal hierarchy is slightly smaller than that of the inverted hierarchy.