To CPL, or not to CPL? What we have not learned about the dark energy equation of state (2503.22529v1)

Abstract: We show that using a Taylor expansion for the dark energy equation-of-state parameter and limiting it to the zeroth and first-order terms, i.e., the so-called Chevallier-Polarski-Linder (CPL) parametrization, instead of allowing for higher-order terms and then marginalizing over them, adds extra information not present in the data and leads to markedly different and potentially misleading conclusions. Fixing the higher-order terms to zero, one concludes that vacuum energy that is currently non-dynamical (e.g., the cosmological constant) is excluded at several $\sigma$ significance as the explanation of cosmic acceleration, even in Dark Energy Spectroscopic Survey (DESI) DR1 data. Meanwhile, instead marginalizing over the higher-order terms shows that we know neither the current dark energy equation of state nor its current rate of change well enough to make such a claim. This issue has become more prominent now with the recent release of high-quality Stage IV galaxy survey data. The results of analyses using simple Taylor expansions should be interpreted with great care.

Examination of the CPL Parametrization in Describing Dark Energy's Role in Cosmic Acceleration

The paper titled "To CPL, or not to CPL? What we have not learned about the dark energy equation of state" critically evaluates the utilization of the Chevallier-Polarski-Linder (CPL) parametrization in probing the nature of dark energy, which is instrumental in explaining cosmic acceleration. This paper, authored by Savvas Nesseris, Yashar Akrami, and Glenn D. Starkman, delineates how employing the CPL parametrization with a truncated Taylor expansion can lead to inaccurate conclusions about the dark energy equation of state (EoS).

The pivotal examination centers on the tendency of the CPL parametrization to disproportionately exclude the suitability of a constant vacuum energy as a possible driver of cosmic acceleration. By restricting the EoS parameter, $w_{\text{DE}}(a)$, to a linear form, i.e., considering only the zeroth and first-order terms, the inferred constraints ostensibly suggest deviations from $w = -1$ at the level of several standard deviations. This conclusion emerges from comparing results where higher-order terms in the Taylor expansion are either fixed to zero or marginalized over.

The authors utilized a robust dataset comprising the Dark Energy Spectroscopic Instrument (DESI) DR1 baryon acoustic oscillations data, the Pantheon+ compilation of Type Ia supernovae, and the Planck cosmic microwave background (CMB) data. Through an extensive Markov chain Monte Carlo (MCMC) analysis, several parameterizations were compared: CPL where only $w_0$ and $w_a$ are varied, and extended versions (CPL$^+$ and CPL$^{++}$) which incorporate additional terms $w_b$ and $w_c$, marginalized over them.

The analysis reveals that once the higher-order terms are considered, the exclusion strength of a cosmological constant as the explanation for cosmic acceleration diminishes significantly. Notably, the evidence against a constant vacuum energy $w=-1$ falls below a $1\sigma$ significance with the CPL$^{++}$ model compared to several σ with the regular CPL model. This broadens the scope for concordance models, such as $\Lambda$CDM, aligning the data with the possibility of constant dark energy being dominant in the current Universe.

In terms of model comparison, Akaike information criterion (AIC) assessments suggest that while there is indeed some preference for evolving dark energy, distinct against a pure cosmological constant, the results are not exceedingly compelling. AIC reductions were observed; however, the disparity was not extensive to exacerbate certainty over dynamical models.

The paper concludes that while galaxy surveys continue to enhance precision constraints on cosmic acceleration, caution is advised in overinterpreting parameterized fits like CPL. The truncation at first order in CPL unnaturally injects extraneous information absent in the dataset, which could lead to dismissing viable cosmological scenarios like constant dark energy. Thus, the authors advocate for careful consideration of extended parametrizations or data-driven approaches, like principal components, to better encapsulate the full scale of observational uncertainties and make informed decisions about potential dark energy models beyond $\Lambda$CDM.

Future work could involve enhanced data acquisitions and theoretical model developments, where a refined focus on multiple parameters and higher-order behaviors substantiates or challenges the axiomatic viability of the cosmological constant with greater empirical grounds. The upshot envisages nuanced insights contributing to the perennial endeavor of unraveling the dynamics of dark energy—an archetypal inquiry in the current cosmological paradigm.