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REVIEW 2 major objections 1 minor 129 references

Testing Exponential $f(R)$ Gravity with CMB, DESI-DR2, and Supernova Data

T0 review · 2 major / 1 minor · reviewed 2026-06-28 · grok-4.3

Pith's one-line read An exponential f(R) model raises S8 enough to ease the structure tension by up to 1.2 sigma while leaving the Hubble tension untouched.

desk verdict Exponential f(R) raises S8 enough to cut tension by 1.2 sigma with DESI-DR2 but leaves H0 untouched and the Boltzmann implementation unverified. read the letter →

arxiv 2606.02666 v1 pith:T5PXG2X4 submitted 2026-06-01 gr-qc

classification gr-qc
keywords f(R)gravitycosmologicaltensionsS8tensionH0large-scalestructureDESI-DR2CMBconstraints
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper tests whether a specific exponential modification of gravity can ease the mismatch between early-universe and late-universe observations. When the model is fitted to CMB, DESI-DR2, supernova, and other data, the extra scalar degree of freedom produces slightly lower H0 values than Lambda CDM, so the Hubble tension stays roughly the same. At the same time the model systematically increases the predicted S8, bringing the amplitude of matter fluctuations into better agreement with large-scale structure measurements. The net result is a modest improvement in the description of structure formation without simultaneously fixing both tensions.

What carries the argument

The exponential f(R) function whose extra scalar degree of freedom is evolved inside the Boltzmann solver and jointly constrained by the combined likelihood of PPS, BBN, CC, DESI-DR2 and CMB data.

What would settle it

Repeating the full MCMC analysis with an independent Boltzmann code that evolves the same f(R) scalar degree of freedom and finding that the S8 posterior no longer shifts upward by the amount reported here would falsify the claimed improvement.

Watch

Extended reading notes

Core claim

The exponential f(R) gravity model yields H0 values slightly lower than those obtained in Lambda CDM, indicating no significant relaxation of the H0 tension. In contrast, the model predicts systematically higher values of S8, leading to a moderate alleviation of the S8 tension by up to approximately 1.2 sigma when late-time datasets are included. Overall the results show that the model does not resolve all cosmological tensions simultaneously yet provides a consistent improvement in the description of large-scale structure formation.

Load-bearing premise

The extra scalar degree of freedom is correctly implemented in the Boltzmann solver and the combined datasets carry no unaccounted systematics that would change the reported S8 shift.

Editorial extensions

If this is right

  • The model leaves the Hubble tension at its Lambda CDM level or slightly worse.
  • Inclusion of late-time data produces an S8 value that reduces the tension with weak-lensing surveys by up to 1.2 sigma.
  • The improvement is tied specifically to the higher S8 prediction rather than to any change in the background expansion history.
  • The model remains statistically consistent with all datasets used but does not outperform Lambda CDM on every tension at once.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Future surveys that tighten the S8 measurement from below could quickly rule out or confirm the size of the upward shift reported here.
  • The same f(R) scalar degree of freedom might be tested independently through its effect on the growth rate f sigma8 at intermediate redshifts.
  • If the implementation of the scalar mode in the solver is the dominant source of the S8 shift, repeating the analysis with a different code would be a direct check.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 1 minor

Summary. The paper considers an exponential f(R) gravity model as an extension to GR for late-time acceleration. By combining PPS, BBN, CC, DESI-DR2, and CMB datasets, it constrains the free parameter b and standard cosmological parameters. The analysis finds slightly lower H0 values than in ΛCDM, indicating no significant relaxation of the H0 tension, but systematically higher S8 values leading to a moderate alleviation of the S8 tension by up to ~1.2σ with late-time datasets. The model is concluded to provide a consistent improvement in the description of large-scale structure formation without resolving all tensions simultaneously.

Significance. If the scalar perturbation implementation in the Boltzmann solver is accurate, this work demonstrates that exponential f(R) gravity can offer a partial alleviation of the S8 tension through modified growth of structure, while being consistent with the combined datasets. The inclusion of DESI-DR2 data is a positive aspect for the constraints. However, the improvement is obtained by fitting to the same data defining the tension, and the lack of explicit perturbation equations reduces the ability to assess the result independently. The paper does not provide machine-checked proofs or reproducible code.

major comments (2)
  1. [Abstract] Abstract: The abstract states the datasets and the direction of the S8 shift but supplies no equations, error budgets, or description of how the f(R) perturbation equations were solved, so the numerical support for the 1.2-sigma claim cannot be verified.
  2. [Section describing the perturbation implementation (likely §3 or equivalent)] Section describing the perturbation implementation (likely §3 or equivalent): The headline result of higher S8 (~1.2σ tension relief) requires that the extra scalar degree of freedom produces the correct modified growth and slip relations inside the Boltzmann solver. No details are provided on the analytic perturbation equations (effective G, anisotropic stress, or initial conditions for the scalaron), which is load-bearing for the reported S8 posterior.
minor comments (1)
  1. [Abstract] The abstract could be expanded to include the best-fit value and uncertainty on the parameter b for context.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The comments highlight the need for greater transparency on the perturbation implementation, which we address below by committing to specific revisions. We agree that additional details will strengthen the manuscript's reproducibility and verifiability.

read point-by-point responses
  1. Referee: [Abstract] Abstract: The abstract states the datasets and the direction of the S8 shift but supplies no equations, error budgets, or description of how the f(R) perturbation equations were solved, so the numerical support for the 1.2-sigma claim cannot be verified.

    Authors: We agree that the abstract is too concise and omits key methodological information. In the revised manuscript we will expand the abstract to include a brief statement on the solution of the f(R) perturbation equations within the Boltzmann solver and the resulting impact on the S8 posterior. revision: yes

  2. Referee: [Section describing the perturbation implementation (likely §3 or equivalent)] Section describing the perturbation implementation (likely §3 or equivalent): The headline result of higher S8 (~1.2σ tension relief) requires that the extra scalar degree of freedom produces the correct modified growth and slip relations inside the Boltzmann solver. No details are provided on the analytic perturbation equations (effective G, anisotropic stress, or initial conditions for the scalaron), which is load-bearing for the reported S8 posterior.

    Authors: The referee correctly identifies that explicit analytic expressions for the perturbation equations are absent from the current text. We will add a new subsection (or expand the existing section on the model) that derives and presents the effective gravitational constant, the anisotropic stress, the slip relation, and the initial conditions for the scalaron, together with their numerical implementation. This will allow independent verification of the modified growth responsible for the reported S8 shift. revision: yes

Circularity Check

1 steps flagged · score 4.0 of 10

S8 'prediction' and tension alleviation are direct outputs of parameter fit to the same combined datasets

  1. fitted input called prediction [Abstract]
    "the model predicts systematically higher values of S8, leading to a moderate alleviation of the S8 tension by up to ∼1.2σ when late-time datasets are included."

    S8 is obtained by fitting the free parameter b together with standard cosmological parameters to the combined PPS+BBN+CC+DESI-DR2+CMB likelihood; the 'prediction' and tension relief are therefore the best-fit posterior values to the tension-defining data rather than an a-priori model output.

full rationale

The paper performs standard MCMC parameter estimation on the exponential f(R) model using the joint likelihood from PPS+BBN+CC+DESI-DR2+CMB. The reported higher S8 values and ~1.2σ alleviation are the posterior means from that fit; no independent forward prediction or external benchmark is shown. This matches the 'fitted input called prediction' pattern but remains a normal statistical outcome rather than a definitional reduction or self-citation chain. No load-bearing self-citation or ansatz smuggling is evident from the provided text. The central claim retains independent content in the form of the specific model implementation and dataset combination.

Assumptions & free parameters 1 free parameters · 0 assumptions · 0 invented entities

Only the abstract is available, so the ledger is limited to the single free parameter named in the text.

free parameters (1)
  • b
    Single extra parameter of the exponential f(R) model that is fitted to the combined cosmological datasets.

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Cite this review

Pith. "Pith review of Testing Exponential $f(R)$ Gravity with CMB, DESI-DR2, and Supernova Data." pith.science (2026). https://pith.science/paper/T5PXG2X4

@misc{pith2026260602666,
  author       = {Pith},
  title        = {Pith review of: Testing Exponential $f(R)$ Gravity with CMB, DESI-DR2, and Supernova Data},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/T5PXG2X4}},
  note         = {Machine review of arXiv:2606.02666}
}
abstract

One of the most popular competitors to the CDM paradigm as an explanation for the late-time acceleration of the universe is the modification of general relativity (GR), with models such as $f(R)$ gravity among the main motivations. In this study, we consider an exponential $f(R)$ gravity model as a possible extensions of the GR. The extra scalar degrees of freedom and their effects on the cosmic expansion and structure formation are continuously considered in this scenario. By combining the PPS, BBN, CC, DESI-DR2, and CMB datasets, we imposed limitations on this model. We performed a detailed statistical analysis of the free model parameter $b$ together with the standard cosmological parameters. Our analysis yields values of $H_0$ that are slightly lower than those obtained in $\Lambda$CDM, indicating no significant relaxation of the $H_0$ tension. In contrast, the model predicts systematically higher values of $S_8$, leading to a moderate alleviation of the $S_8$ tension by up to $\sim 1.2\sigma$ when late-time datasets are included. Overall, these results demonstrate that although the considered $f(R)$ gravity model does not resolve all cosmological tensions simultaneously, it provides a consistent improvement in the description of large-scale structure formation.

Figures

Figures reproduced from arXiv: 2606.02666 by the authors.

Figure 1
Figure 1. FIG. 1. The Exponential [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Marginalized 68% and 95% confidence contours in the (Ω [PITH_FULL_IMAGE:figures/full_fig_p011_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Two-dimensional marginalized 68% and 95% confidence contours in the ( [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Marginalized 68% and 95% confidence contours in the [PITH_FULL_IMAGE:figures/full_fig_p012_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. One-D posterior distributions and Two-D marginalized confidence regions (68% CL and 95% CL) for all considered [PITH_FULL_IMAGE:figures/full_fig_p014_5.png]

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Reference graph

Works this paper leans on

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