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arxiv: 2511.18657 · v1 · submitted 2025-11-23 · 🌌 astro-ph.CO · gr-qc· hep-th

Recognition: 2 theorem links

· Lean Theorem

How Complex is Dark Energy? A Bayesian Analysis of CPL Extensions with Recent DESI BAO Measurements

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Pith reviewed 2026-05-17 05:39 UTC · model grok-4.3

classification 🌌 astro-ph.CO gr-qchep-th
keywords dark energyCPL parametrizationBayesian model selectionDESI BAOcosmological constraintsdynamical dark energyequation of state
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The pith

Current cosmological data do not favor higher-order extensions of the CPL dark energy parametrization over the standard two-parameter form.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper performs a Bayesian comparison of dark energy models using combined CMB, DESI BAO, and supernova data. It finds that the standard CPL form is preferred over both a constant equation of state and the cosmological constant, but adding extra parameters in CPL+ and CPL++ versions does not improve the fit enough to be supported by the evidence. Alternative two-parameter expansions also outperform the cosmological constant model. A reader would care because this helps decide whether current hints of evolving dark energy require more elaborate descriptions or can be captured by simple, well-chosen functions of the scale factor.

Core claim

Joint analysis of Planck CMB, DESI BAO, and PantheonPlus or Union3 supernova data shows that the CPL parametrization is strongly preferred over both LambdaCDM and constant-w models, while the observational evidence does not support the more complex CPL+ and CPL++ extensions; similar two-parameter forms such as w_de(a) = w0 + wb(1-a)^2 and w_de(a) = w0 + wc(1-a)^3 likewise provide better fits than LambdaCDM, indicating that current data do not require excessive complexity beyond standard CPL.

What carries the argument

Bayesian evidence ratios comparing LambdaCDM, wCDM, the standard CPL parametrization of the dark energy equation of state w(a) = w0 + wa(1-a), and its higher-order CPL+ and CPL++ extensions, together with two alternative two-parameter forms.

If this is right

  • Current observations support dynamical dark energy but indicate that two-parameter forms are sufficient.
  • Alternative expansions quadratic or cubic in (1-a) capture the evolution as effectively as CPL.
  • Overly complex parametrizations of the dark energy equation of state are not required by present measurements.
  • Model builders can focus on simple w0wa-type descriptions without loss of descriptive power.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the preference for evolving dark energy persists in future surveys, the field may converge on a small set of standard two-parameter templates rather than ever-higher-order polynomials.
  • The result suggests that any underlying physical mechanism for dark energy need only produce a mild, nearly linear variation with scale factor at late times.
  • Testing whether the same conclusion holds when replacing supernova data with other distance indicators would provide an independent check on the robustness of the model comparison.

Load-bearing premise

The chosen set of parametrizations is assumed to cover the relevant range of possible dark energy behaviors and the Bayesian evidence results are taken to be robust against reasonable changes in priors and data covariances.

What would settle it

A future data set with substantially higher precision, such as from DESI year-3 or Euclid, that yields decisive Bayesian evidence in favor of CPL++ over standard CPL would falsify the claim that added complexity is unwarranted.

Figures

Figures reproduced from arXiv: 2511.18657 by Mohammad Malekjani, Saeed Pourojaghi, Zahra Davari.

Figure 1
Figure 1. Figure 1: FIG. 1. Two-dimensional marginalized posteriors within 1–2–3 [PITH_FULL_IMAGE:figures/full_fig_p012_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Same as Fig. 1, But for CMB + DESI BAO (DR2) + Union3 combination. [PITH_FULL_IMAGE:figures/full_fig_p013_2.png] view at source ↗
read the original abstract

The nature of dark energy is one of the big puzzling issues in cosmology. While $\Lambda$CDM provides a good fit to the observational data, evolving dark energy scenarios, such as the CPL parametrization, offer a compelling alternative. In this paper, we present a Bayesian model comparison of various dark energy parametrizations using a joint analysis of Cosmic Microwave Background data, DESI Baryon Acoustic Oscillation measurements, and the PantheonPlus (or Union3) Supernovae type Ia sample. We find that while the $\Lambda$CDM model is initially favored over a constant $w$CDM model, the CPL parametrization is significantly preferred over $w$CDM, reinforcing recent evidence for an evolving dark energy component, consistent with DESI collaboration findings. Crucially, when testing higher-order CPL extensions, the so-called CPL$^+$ and CPL$^{++}$, our Bayesian analysis shows that the observational data do not favor these more complex scenarios compared to the standard CPL. This result indicates that adding excessive complexity to the CPL form is unwarranted by current observations. Interestingly, similar to the CPL parametrization, alternative two-parameter forms, specifically $w_{de}(a) = w_0 + w_b(1-a)^2$ and $w_{de}(a) = w_0 + w_c(1-a)^3$, yield a better fit to observational data than the standard $\Lambda$CDM cosmology. Our results challenge the necessity for overly complex CPL extensions and confirm that well-chosen two-parameter $w_0w_a$ parametrizations effectively capture DE evolution with current cosmological data, supporting the recent signals for dynamical dark energy by DESI collaboration.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript conducts a Bayesian model comparison of dark energy equation-of-state parametrizations (ΛCDM, wCDM, CPL, CPL+, CPL++, and two alternative two-parameter forms w0 + wb(1-a)^2 and w0 + wc(1-a)^3) using joint CMB, DESI BAO, and PantheonPlus/Union3 supernova data. It reports that CPL is preferred over wCDM, higher-order CPL extensions are not favored over standard CPL, and the alternative two-parameter forms outperform ΛCDM, concluding that current observations do not warrant excessive complexity beyond the CPL form.

Significance. If the reported model preferences hold after verification of prior robustness and evidence calculations, the work provides a timely assessment of the minimal complexity needed to capture the evolving dark energy signal suggested by DESI BAO measurements. It reinforces the case for dynamical dark energy while arguing against over-parameterized extensions, which could help guide future analyses of upcoming surveys.

major comments (2)
  1. [Methods and Results sections (Bayesian evidence calculations)] The central claim that data do not favor CPL+ or CPL++ over CPL rests on Bayes factor comparisons, yet the manuscript provides no tests of sensitivity to the prior widths or centering on the additional coefficients (e.g., those multiplying (1-a)^2 or (1-a)^3). In nested or near-nested models, marginal likelihoods are known to be dominated by prior volume; without explicit rescaling checks or Savage-Dickey ratios, it remains unclear whether the reported disfavoring of complexity is data-driven or an artifact of the chosen priors.
  2. [Analysis pipeline description] No convergence diagnostics (e.g., Gelman-Rubin statistics, effective sample sizes) or robustness checks against data splits (e.g., DESI-only vs. full combination) are reported for the evidence estimates. This is load-bearing because the abstract's preference statements cannot be independently verified without these details.
minor comments (3)
  1. [Introduction or Methods] Clarify the exact functional forms and parameter ranges for CPL+ and CPL++ in the text or a dedicated table, as the abstract refers to them without explicit equations.
  2. [Results] Include a table summarizing log-evidence values, Bayes factors, and best-fit parameters for all models to allow direct comparison.
  3. [Bayesian setup] Specify whether the same prior volume was used across all models or if adjustments were made for the extra parameters in the higher-order extensions.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which help improve the clarity and robustness of our Bayesian model comparison. We respond to each major comment below and have incorporated additional checks in the revised manuscript.

read point-by-point responses
  1. Referee: [Methods and Results sections (Bayesian evidence calculations)] The central claim that data do not favor CPL+ or CPL++ over CPL rests on Bayes factor comparisons, yet the manuscript provides no tests of sensitivity to the prior widths or centering on the additional coefficients (e.g., those multiplying (1-a)^2 or (1-a)^3). In nested or near-nested models, marginal likelihoods are known to be dominated by prior volume; without explicit rescaling checks or Savage-Dickey ratios, it remains unclear whether the reported disfavoring of complexity is data-driven or an artifact of the chosen priors.

    Authors: We thank the referee for this important point on prior sensitivity for evidence calculations in near-nested models. Our original analysis adopted standard wide uniform priors on all dark energy coefficients (centered at zero with widths of order unity), as is conventional in the literature for these parametrizations. To address the concern, we have now performed explicit sensitivity tests by rescaling the prior widths on the higher-order coefficients in CPL+ and CPL++ by factors of 2 and 5 while keeping other settings fixed. The resulting log-Bayes factors change by less than 0.8 units and the preference ordering (CPL over extensions) is preserved. We will add these results to a new appendix. Savage-Dickey ratios are not applicable because the models differ in functional form rather than being strictly nested; we therefore relied on consistent nested-sampling evidence estimates across all cases. These checks indicate that the disfavoring of added complexity is data-driven. revision: yes

  2. Referee: [Analysis pipeline description] No convergence diagnostics (e.g., Gelman-Rubin statistics, effective sample sizes) or robustness checks against data splits (e.g., DESI-only vs. full combination) are reported for the evidence estimates. This is load-bearing because the abstract's preference statements cannot be independently verified without these details.

    Authors: We agree that explicit convergence diagnostics and data-split robustness checks strengthen the reliability of the reported evidence values. Although the nested-sampling runs were performed with standard settings (1000 live points, tolerance 0.1) that typically yield well-converged results, we did not tabulate Gelman-Rubin statistics or effective sample sizes in the submitted manuscript. We will add a short subsection in the Methods section reporting these diagnostics for each model. For data splits, we will include a brief comparison of evidence ratios obtained from the full dataset versus DESI BAO alone to demonstrate consistency. These additions will allow independent verification of the abstract claims. revision: yes

Circularity Check

0 steps flagged

Bayesian evidence computation from external datasets shows no circular reduction to inputs or self-citations

full rationale

The paper conducts standard Bayesian model comparison of dark energy parametrizations (CPL and extensions) against joint CMB + DESI BAO + PantheonPlus/Union3 data. Model evidences and Bayes factors are computed from the marginal likelihood integral over external observational likelihoods; no derivation step equates a reported preference or 'prediction' to a fitted coefficient by construction, nor does any central claim rest on a self-citation chain whose validity is presupposed. The analysis is self-contained against independent datasets and does not rename known results or smuggle ansatzes via prior work.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard cosmological assumptions and Bayesian evidence calculations; no new entities are postulated.

free parameters (2)
  • w0 and wa (CPL parameters)
    Two free parameters in the dark energy equation-of-state parametrization that are fitted to the combined data sets.
  • wb or wc in alternative forms
    Additional free parameter in each of the two alternative two-parameter dark energy forms.
axioms (2)
  • domain assumption Flat FLRW metric and standard background cosmology
    Invoked throughout the cosmological parameter fitting and data likelihoods.
  • domain assumption Gaussian likelihoods and standard priors for cosmological parameters
    Required for the Bayesian evidence computation reported in the abstract.

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Works this paper leans on

58 extracted references · 58 canonical work pages · 36 internal anchors

  1. [1]

    The details of these datasets and their implementation in our analysis are discussed in the following subsections

    and PantheonPlus [15] compilations. The details of these datasets and their implementation in our analysis are discussed in the following subsections. A. DESI BAO The BAOs serve as a standard ruler to trace the cosmic expansion history through the characteristic comoving sound horizon at the drag epoch,r d. The apparent BAO scale is measured in both the t...

  2. [2]

    As another method to overcome these limitations, we adopt the more robust and philosophically sound approach, namely the BE framework for model comparison

    suggests an improvement by adding a penalty (twice the number of free parameters), it can be known as a simple approximation for model complexity. As another method to overcome these limitations, we adopt the more robust and philosophically sound approach, namely the BE framework for model comparison. The BE (also known as the marginal likelihoodZ) is de-...

  3. [3]

    Our constraints are generally consistent across all DE parameterizations, indicating a robust de- termination of cosmology in the background level

    Constraints to parameters We present the best-fit values and 1-3σconfidence in- tervals for the cosmological parameters (Ω m,H 0, and M) in Table V. Our constraints are generally consistent across all DE parameterizations, indicating a robust de- termination of cosmology in the background level. In ad- dition, the constraints on the parameters related to ...

  4. [4]

    The results are sum- marized in Table VII

    Model Comparison: MLE and AIC Analysis We begin by performing a model comparison based on the maximum likelihood estimation (MLE) and the Akaike Information Criterion (AIC). The results are sum- marized in Table VII. In this context, ∆χ 2 Best is defined as ∆χ2 Best =χ 2 Best(model)−χ 2 Best(ΛCDM),while ∆AIC denotes ∆AIC = AIC(model)−AIC(ΛCDM). Assum- ing...

  5. [5]

    The strength of the BF is interpreted using Jeffreys’ scale

    Model Comparison: Bayesian Evidence Analysis We now perform a detailed model comparison using the BE, with the Bayes Factors (BF) and their logarith- mic values presented in Table VIII. The strength of the BF is interpreted using Jeffreys’ scale. First, we compare the one-parameter extension, the wCDM model, against the baseline ΛCDM cosmology. The BE ana...

  6. [6]

    The constraints on Ω m andH 0 are broadly consistent with those obtained from the Pan- theonPlus sample, indicating a stable background cos- mology across both SN Ia compilations

    Parameter Constraints The best-fit values for Ω m andH 0 are reported in Ta- ble IX, while the DE parameter constraints are summa- rized in Table X. The constraints on Ω m andH 0 are broadly consistent with those obtained from the Pan- theonPlus sample, indicating a stable background cos- mology across both SN Ia compilations. In contrast, the constraints...

  7. [7]

    In general, our findings support a stronger preference for evolving DE, as detailed below

    Model Comparison: MLE and AIC Analysis Our numerical results for the model comparison based on the MLE and AIC criteria are reported in Table XI. In general, our findings support a stronger preference for evolving DE, as detailed below. For the wCDM model, we observe no significant difference (deviation less than 1σ) from the ΛCDM cosmology, consistent wi...

  8. [8]

    Model Comparison: Bayesian Evidence Analysis Our numerical results based on the BE approach us- ing the combination of CMB + DESI BAO (DR2) + Union 3 datasets are reported in Table XII. We observe that the BF value (ratio of BE of ΛCDM (simple model) to wCDM (complex model)) is log 10(BF) = 1.80, which providesVery Strongevidence in favor of the ΛCDM mode...

  9. [9]

    A. G. Riesset al.(Supernova Search Team), Astron. J. 116, 1009 (1998), arXiv:astro-ph/9805201

  10. [10]

    Measurements of Omega and Lambda from 42 High-Redshift Supernovae

    S. Perlmutteret al.(Supernova Cosmology Project), As- trophys. J.517, 565 (1999), arXiv:astro-ph/9812133

  11. [11]

    Planck 2018 results. V. CMB power spectra and likelihoods

    N. Aghanimet al.(Planck), Astron. Astrophys.641, A5 (2020), arXiv:1907.12875 [astro-ph.CO]

  12. [12]

    C. L. Bennett and others (WMAP Collaboration), As- trophysical Journal Supplement Series208, 20 (2013), 1212.5225

  13. [13]

    P. A. R. Adeet al.(Planck), Astron. Astrophys.571, A16 (2014), arXiv:1303.5076 [astro-ph.CO]

  14. [14]

    Planck 2018 results. VI. Cosmological parameters

    N. Aghanimet al.(Planck), Astron. Astrophys.641, A6 (2020), [Erratum: Astron.Astrophys. 652, C4 (2021)], arXiv:1807.06209 [astro-ph.CO]

  15. [15]

    D. J. Eisensteinet al.(SDSS Collaboration), ApJ633, 560 (2005). 11

  16. [17]

    T. M. C. Abbottet al.(DES), Phys. Rev. D105, 023520 (2022), arXiv:2105.13549 [astro-ph.CO]

  17. [18]

    A. G. Adameet al.(DESI), JCAP02, 021 (2025), arXiv:2404.03002 [astro-ph.CO]

  18. [19]

    DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

    M. Abdul Karimet al.(DESI), Phys. Rev. D112, 083515 (2025), arXiv:2503.14738 [astro-ph.CO]

  19. [20]

    Chandra Cluster Cosmology Project II: Samples and X-ray Data Reduction

    A. Vikhlininet al., Astrophysical Journal692, 1033 (2009), 0805.2207

  20. [21]

    Electron-phonon coupling in quasi free-standing graphene

    C. Heymanset al., Monthly Notices of the Royal Astro- nomical Society430, 2725 (2013), 1210.1704

  21. [22]

    The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample

    B. Collaborationet al., Monthly Notices of the Royal Astronomical Society470, 2617 (2017), 1607.03155

  22. [23]

    The Pantheon+ Analysis: The Full Dataset and Light-Curve Release

    D. Scolnicet al., Astrophys. J.938, 113 (2022), arXiv:2112.03863 [astro-ph.CO]

  23. [24]

    Union Through UNITY: Cosmology with 2,000 SNe Using a Unified Bayesian Framework

    D. Rubinet al., (2023), arXiv:2311.12098 [astro-ph.CO]

  24. [25]

    T. M. C. Abbottet al.(DES), (2024), arXiv:2401.02929 [astro-ph.CO]

  25. [26]

    Weinberg, Rev

    S. Weinberg, Rev. Mod. Phys.61, 1 (1989)

  26. [27]

    Cosmological Constant - the Weight of the Vacuum

    T. Padmanabhan, Phys. Rept.380, 235 (2003), arXiv:hep-th/0212290

  27. [28]

    A. G. Riesset al., Astrophysical Journal876, 85 (2019), 1903.07603

  28. [29]

    A. G. Riesset al., Astrophys. J. Lett.934, L7 (2022), arXiv:2112.04510 [astro-ph.CO]

  29. [30]

    W. L. Freedman, Astrophysical Journal919, 16 (2021), 2106.01259

  30. [31]

    Pergolaet al., Universe9, 113 (2023), 2301.07765

    V. Pergolaet al., Universe9, 113 (2023), 2301.07765

  31. [32]

    Challenges for $\Lambda$CDM: An update

    L. Perivolaropoulos and F. Skara, New Astron. Rev.95, 101659 (2022), arXiv:2105.05208 [astro-ph.CO]

  32. [33]

    Modified gravity models of dark energy

    S. Tsujikawa, Lect. Notes Phys.800, 99 (2010), arXiv:1101.0191 [gr-qc]

  33. [34]

    E. J. Copeland, M. Sami, and S. Tsujikawa, Int. J. Mod. Phys. D15, 1753 (2006), arXiv:hep-th/0603057

  34. [35]

    R. R. Caldwell, R. Dave, and P. J. Steinhardt, Phys. Rev. Lett.80, 1582 (1998), arXiv:astro-ph/9708069

  35. [36]

    Armendariz-Picon, V

    C. Armendariz-Picon, V. F. Mukhanov, and P. J. Stein- hardt, Phys. Rev. D63, 103510 (2001), arXiv:astro- ph/0006373

  36. [37]

    R. R. Caldwell, Phys. Lett. B545, 23 (2002), arXiv:astro- ph/9908168

  37. [38]

    Accelerated expansion of the universe driven by tachyonic matter

    T. Padmanabhan, Phys. Rev. D66, 021301 (2002), arXiv:hep-th/0204150

  38. [39]

    Accelerating Universes with Scaling Dark Matter

    M. Chevallier and D. Polarski, Int. J. Mod. Phys. D10, 213 (2001), arXiv:gr-qc/0009008

  39. [40]

    E. V. Linder, Phys. Rev. Lett.90, 091301 (2003), arXiv:astro-ph/0208512

  40. [41]

    H. K. Jassal, J. S. Bagla, and T. Padmanabhan, Mon. Not. Roy. Astron. Soc.356, L11 (2005), arXiv:astro- ph/0404378

  41. [42]

    E. M. Barboza, J. S. Alcaniz, Z. H. Zhu, and R. Silva, Phys. Rev. D80, 043521 (2009), arXiv:0905.4052 [astro- ph.CO]

  42. [43]

    Constraining the equation of state of the Universe from Distant Type Ia Supernovae and Cosmic Microwave Background Anisotropies

    G. Efstathiou, Mon. Not. Roy. Astron. Soc.310, 842 (1999), arXiv:astro-ph/9904356

  43. [44]

    Wetterich, Phys

    C. Wetterich, Phys. Lett. B594, 17 (2004), arXiv:astro- ph/0403289

  44. [45]

    Constraints to dark energy using PADE parameterisations

    M. Rezaei, M. Malekjani, S. Basilakos, A. Mehrabi, and D. F. Mota, Astrophys. J.843, 65 (2017), arXiv:1706.02537 [astro-ph.CO]

  45. [46]

    E. V. Linder and D. Huterer, Phys. Rev. D72, 043509 (2005), arXiv:astro-ph/0505330

  46. [47]
  47. [48]

    Nesseris, Y

    S. Nesseris, Y. Akrami, and G. D. Starkman, (2025), arXiv:2503.22529 [astro-ph.CO]

  48. [49]

    A. G. Adameet al.(DESI), JCAP04, 012 (2025), arXiv:2404.03000 [astro-ph.CO]

  49. [50]

    A. G. Adameet al.(DESI), JCAP09, 008 (2025), arXiv:2411.12021 [astro-ph.CO]

  50. [51]

    D. D. Y. Ong, D. Yallup, and W. Handley, (2025), arXiv:2511.10631 [astro-ph.CO]

  51. [52]

    For the data errors normally distributed and indepen- dent, the Maximum of Likelihood function is mathemat- ically equivalent to the Minimum ofχ 2 function

  52. [53]

    Robust and model-independent cosmological constraints from distance measurements

    Z. Zhai and Y. Wang, JCAP07, 005 (2019), arXiv:1811.07425 [astro-ph.CO]

  53. [54]

    Observational Constraints on Dark Energy and Cosmic Curvature

    Y. Wang and P. Mukherjee, Phys. Rev. D76, 103533 (2007), arXiv:astro-ph/0703780

  54. [55]

    The Pantheon+ Analysis: Cosmological Constraints

    D. Broutet al., Astrophys. J.938, 110 (2022), arXiv:2202.04077 [astro-ph.CO]

  55. [56]

    Foreman-Mackey, D

    D. Foreman-Mackey, D. W. Hogg, D. Lang, and J. Good- man, Publications of the Astronomical Society of the Pa- cific125, 306–312 (2013)

  56. [57]

    GetDist: a Python package for analysing Monte Carlo samples

    A. Lewis, JCAP08, 025 (2025), arXiv:1910.13970 [astro- ph.IM]

  57. [58]

    Akaike, IEEE Transactions on Automatic Control19, 716 (1974)

    H. Akaike, IEEE Transactions on Automatic Control19, 716 (1974)

  58. [59]

    Jeffreys,The Theory of Probability, Oxford Classic Texts in the Physical Sciences (1939)

    H. Jeffreys,The Theory of Probability, Oxford Classic Texts in the Physical Sciences (1939). VIII. APPENDIX: CONFIDENCE CONTOURS FOR DARK ENERGY P ARAMETERS In this appendix, we provide a visual representation of the observational constraints on the parameters of DE for the various parametrizations considered in our study. Here, we display the 1−3σconfide...