We investigate the bounds on the sum of neutrino masses in a cosmic-acceleration scenario where the equation of state w ( z ) of dark energy ( DE ) is constructed in a model-independent way , using a basis of principal components ( PCs ) that are allowed to cross the phantom barrier w ( z ) = -1 .
We find that the additional freedom provided to w ( z ) means the DE can undo changes in the background expansion induced by massive neutrinos at low redshifts .
This has two significant consequences : ( 1 ) it leads to a substantial increase in the upper bound for the sum of the neutrino masses ( M _ { \nu } < 0.33 - 0.55 eV at the 95 % C.L .
depending on the data sets and number of PCs included ) compared to studies that choose a specific parametrization for w ( z ) ; and ( 2 ) it causes \sim 1 \sigma deviations from \Lambda CDM in the luminosity distance and the Hubble expansion rate at higher redshifts ( z \gtrsim 2 ) , where the contribution of DE is subdominant and there is little constraining data .
The second point consequently means that there are also observable deviations in the shear power spectrum and in the matter power spectrum at low redshift , since the clustering of matter throughout cosmic time depends on the expansion rate .
This provides a compelling case to pursue high- z BAO and SN measurements as a way of disentangling the effects of neutrinos and dark energy .
Finally , we find that the additional freedom given to the dark energy component has the effect of lowering S _ { 8 } with respect to \Lambda CDM .