Abstract
We investigate the generalized Chevallier–Polarski–Linder (CPL) parametrization, which contains the pivoting redshift z p as an extra free parameter, in order to examine whether the evolution of the dark energy equation of state can be better described by a different parametrization. We use various data combinations from cosmic microwave background (CMB), baryon acoustic oscillations (BAO), redshift space distortion (RSD), weak lensing (WL), joint light curve analysis (JLA), and cosmic chronometers (CC), and we include a Gaussian prior on the Hubble constant value, in order to extract the observational constraints on various quantities. For the case of free z p we find that for all data combinations it always remains unconstrained, and there is a degeneracy with the value of the dark energy equation of state w 0 p at z p . For the case where z p is fixed to specific values, and for the full data combination, we find that with increasing z p the mean value of w 0 p slowly moves into the phantom regime, however the cosmological constant is always allowed within 1 σ confidence-level. In fact, the significant effect is that with increasing z p , the correlations between w 0 p and w a (the free parameter of the dark energy equation of state quantifying its evolution with redshift), change from negative to positive, with the case z p = 0 . 35 corresponding to no correlation. The fact that the two parameters describing the dark energy equation of state are uncorrelated for z p = 0 . 35 justifies why a non-zero pivoting redshift needs to be taken into account.
Highlights
According to observational evidences from various sources it has been established that our universe entered a dark energy-dominated era and began a period of accelerated expansion roughly billion years ago [1]
The overall results are similar with respect to the previous fixed pivot redshift cases, p one can clearly see that for cosmic microwave background (CMB) + baryon acoustic oscillations (BAO) + redshift space distortion (RSD) data sets w0 has a mean value in the phantom regime p
We find that the CMB + BAO + RSD combination returns slightly different constraints compared to the remaining two datasets, for this combination we find an interesting pattern in p p the w0 parameter, where we observe that with increasing z p, w0 eventually approaches towards the cosmological constant value and for large z p (z p = 1) it crosses the −1 boundary
Summary
According to observational evidences from various sources it has been established that our universe entered a dark energy-dominated era and began a period of accelerated expansion roughly billion years ago [1]. The second way is to modify the gravitational sector, constructing extended theories of gravity that possess general relativity as a particular limit, but which, in general, present extra degrees of freedom, capable of describing the universe behavior [4,5,6,7,8] Both approaches can be quantified by the introduction of an equation-of-state parameter for the dark energy perfect fluid (effective in the case of modified gravity), namely. In the present work we are interested in investigating the observational constraints on the most well-known parametrization, namely the CPL one, incorporating the pivoting redshift as an extra parameter, assuming it to be either fixed or free The motivation of this choice is to examine if the evolution of the dark energy equation of state can be better described by introducing more freedom in this parametrization.
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