We assume the DE state equations w ( a ) = w _ { 0 } + w _ { a } ( a _ { p } - a ) , and study the dependence of the constraints on w _ { 0 } and w _ { a } coefficients on the pivoting redshift 1 + z _ { p } = 1 / a _ { p } .
Coefficients are fitted to data including WMAP7 , SNIa ( Union 2.1 ) , BAO ’ s ( including WiggleZ and SDSS results ) and H _ { 0 } constraints .
The fitting algorithm is CosmoMC .
We find specific differences between the cases when \nu –mass is allowed or disregarded .
More in detail : ( i ) The z _ { p } value yielding uncorrelated constraints on w _ { 0 } and w _ { a } is different in the two cases , holding \sim 0.25 and \sim 0.35 , respectively .
( ii ) If we consider the intervals allowed to w _ { 0 } , we find that they shift when z _ { p } increases , in opposite directions for vanishing or allowed \nu –mass .
This leads to no overlap between 1 \sigma intervals already at z _ { p } > \sim 0.4 .
( iii ) The known effect that a more negative state parameter is required to allow for \nu mass displays its effects on w _ { a } , rather than on w _ { 0 } .
( iv ) The w _ { 0 } – w _ { a } constraints found by using any pivot z _ { p } can be translated into constraints holding at a specific z _ { p } value ( 0 or the z _ { p } where errors are uncorrelated ) .
When we do so , error ellipses exhibit a satisfactory overlap .