AI 中文总结
该研究在DESI DR2数据背景下，引入CPL参数化的密度级枢轴构造，对比$f_af_b$CDM与$w_0w_a$CDM模型，发现枢轴化的$w_0w_a$CDM模型约束更优，是描述暗能量的最佳框架。
AI 中文摘要
我们为Chevallier-Polarski-Linder（CPL）参数化引入了密度级枢轴构造，通过在优化的枢轴标度因子$a_p$处定义归一化暗能量密度$f_p\equiv f_{\rm DE}(a_p)$和状态方程$\omega_p\equiv \omega(a_p)$。这种重新参数化保留了基础CPL宇宙学不变，使我们能够利用数据最敏感时期具有直接物理解释的参数来比较两个模型。因此，在DESI DR2结果发布后，我们将基于归一化暗能量密度二阶泰勒展开的新提出的$f_a f_b$CDM参数化与标准$w_0w_a$CDM模型进行比较。我们使用压缩宇宙微波背景（CMB）、DESI DR2重子声学振荡（BAO）、宇宙计时仪（CC）和Pantheon+ Ia型超新星数据，并施加SH0ES对$H_0$的先验，对原始和枢轴参数化进行约束。我们发现，对$w_0w_a$CDM模型应用相同的密度级枢轴方案，可大幅降低其暗能量参数之间的相关性，并提供更紧密、更稳定的约束。统计比较显示，即使在优化的参数基下分析两个模型，该模型仍比暗能量密度的泰勒展开更受青睐。此外，枢轴化的CPL参数化能准确再现精质模型的背景演化，为基础暗能量动力学提供可靠的唯象近似，优于$f_af_b$CDM模型。我们得出结论，改变参数基可提升$w_0w_a$CDM模型的性能，在本研究考虑的模型类别中，该模型成为描述暗能量部分最合适的框架。
英文摘要
We introduce a density-level pivot construction for the Chevallier-Polarski-Linder (CPL) parameterization by defining the normalized dark energy density $f_p\equiv f_{\rm DE}(a_p)$ and the equation of state $ω_p\equiv ω(a_p)$ at an optimized pivot scale factor $a_p$. This reparameterization leaves the underlying CPL cosmology unchanged and allows the two models to be compared using parameters with a direct physical interpretation at the epoch where the data are most sensitive. Accordingly, following the DESI DR2 results, we compare a newly proposed $f_a f_b$CDM parameterization, based on a second-order Taylor expansion of the normalized dark energy density, with the standard $w_0w_a$CDM model. In particular, we constrain the original and pivoted parameterizations using compressed cosmic microwave background (CMB), DESI DR2 baryon acoustic oscillations (BAO), cosmic chronometers (CC), and Pantheon+ Type Ia supernovae data, with the SH0ES prior imposed on $H_0$. We find that applying the same density-level pivot prescription to the $w_0w_a$CDM model substantially reduces the correlation between its dark energy parameters and provides tighter and more stable constraints. The statistical comparison shows that this model remains favored over the Taylor expansion of the dark energy density, even when both models are analyzed in the optimized parameter basis. Moreover, the pivoted CPL parameterization accurately reproduces the background evolution of quintessence models, providing a reliable phenomenological approximation to the underlying dark energy dynamics, better than the $f_af_b$CDM model. We conclude that changing the parameter basis improves the performance of the $w_0w_a$CDM model, which emerges as the most suitable framework to describe the dark energy sector within the class of models considered here.
Comments15 pages, 7 figures, 4 tables
