Constraints on Redshift-Binned Dark Energy using DESI BAO Data

We parameterize the equation of state of late-time dark energy as wbin(z), with three redshift bins, characterized by a constant equation of state in each bin. Then, we constrain the parameters of the wbinCDM model using datasets from DESI BAO data, Planck CMB power spectrum, ACT DR6 lensing power spectrum, and type Ia supernova distance-redshift data of Pantheon Plus/DES Y5/Union3. The significances for w₁>-1 is 1.9σ, 2.6σ and 3.3σ, and w₂ is consistent with $-1$ within 1σ level, while 1.6σ, 1.5σ and 1.5σ for w₃<-1 in these three data combinations with different choices of type Ia supernova datasets, respectively. Additionally, to alleviate H₀ tension, we incorporate the early dark energy (EDE) model in the early-time universe and add the SH0ES absolute magnitude Mb prior (or H₀ prior) to further constrain the wbinEDE model. In the wbinEDE model, we find a wbin(z) pattern similar to that in the wbinCDM model. The results of the three data combinations exhibit w₁>-1 at 1.9σ, 1.7σ and 2.9σ level, meanwhile w₃<-1 at 1.3σ, 1.3σ and 1.3σ level, respectively. In all, our results indicate that the transition of dark energy from phantom at high redshifts to quintessence at low redshifts is not conclusive.