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REVIEW 3 major objections 5 minor 192 references

A fractional gravity model with varying G and a scalar field can match late-time data when the quartic potential term is switched off, producing an overdamped relaxation of about 9 Gyr and cosmographic signatures that may ease the H0 and S8

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

A fractional gravity model with dynamical G and scalar field fits late-time data only in the μ=0 case, mimics ΛCDM expansion, but is disfavored by BIC and yields an unphysically large universe age.

T0 review reviewed 2026-07-14 challenge →

load-bearing objection Solid dynamical-systems and multi-probe MCMC work on a fractional varying-G model, but the preferred age is ~40 Gyr and the H(z) pipeline rests on a convenience ansatz. the 3 major comments →

arxiv 2607.09722 v1 pith:ZVBQIVDP submitted 2026-06-27 physics.gen-ph astro-ph.COgr-qc

Varying Gravity from a Modified Fractional Model: Observational Constraints and Slow-Fast Dynamics

classification physics.gen-ph astro-ph.COgr-qc
keywords fractional action cosmologyvarying gravitational constantslow-fast dynamicsH0 tensionS8 tensioncosmographic parametersBayesian model comparisonoverdamped scalar field
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

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 builds a fractional-action cosmology in which both the Hubble rate and Newton’s constant G evolve because of renormalization-group and nonlocal fractional corrections, driven by a scalar field. After reducing the field equations to a regularized autonomous system, the authors integrate H(z) and confront it with supernovae, cosmic chronometers, BAO, strong lensing and black-hole shadows. Bayesian model comparison shows that only the truncated model with vanishing quartic coupling (μ=0) is statistically viable: it returns a stable h≈0.72, tightly constrained fractional parameters α≈1.20 and ζ≈0.43, and an overdamped scalar decay with relaxation time ≈9 Gyr. The same model reproduces late-time acceleration and tracks ΛCDM cosmography while leaving distinctive imprints in the jerk and snap parameters. The dynamical-systems analysis further reveals a slow–fast structure controlled by the relative variation of G, which the authors argue can modulate structure growth and the local expansion rate, thereby offering a geometric pathway toward relieving the H0 and S8 tensions.

Core claim

When the scalar potential is purely quadratic (μ=0), the fractional model with dynamically evolving G and H becomes the only variant that is both statistically competitive with ΛCDM and dynamically consistent: it yields an overdamped relaxation timescale τ_rel≈9 Gyr, well-constrained fractional exponents, and late-time cosmographic functions that closely track but do not coincide with those of ΛCDM, while the full quartic model is disfavored by BIC and by parameter degeneracies.

What carries the argument

The closed evolution equation for the diagnostic R ≡ (Ġ/G)/H, together with the slow–fast decomposition of the regularized autonomous system in the variables (u,v1,v2,v3,R). R organizes both the modified continuity equation and the growth of density perturbations, converting the nonlocal fractional corrections into observable shifts of H(z) and fσ8.

Load-bearing premise

The entire reconstruction rests on a specific three-term ansatz for the Hubble rate, H = H0 + ξφ + ε/t with ε fixed to (α−1)/3, that eliminates the time-dependent friction and forces the scalar equation into a solvable Levinson–Smith form; if that functional form is not a faithful description of the true expansion history, the fitted posteriors and cosmographic signatures lose their foundation.

What would settle it

A direct measurement of the present-day logarithmic growth rate fσ8 (or of the cosmographic jerk and snap at z≲1) that is incompatible with the μ=0 posterior predictions at more than 3σ would rule out the claim that the fractional model is observationally viable and dynamically preferred.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

If this is right

  • Late-time acceleration can be reproduced without a pure cosmological constant once fractional nonlocal corrections and a slowly varying G are admitted.
  • The overdamped 9 Gyr relaxation timescale sets a concrete, observationally accessible damping scale for the scalar field that can be tested against future growth-rate surveys.
  • Distinctive departures of the jerk and snap from their ΛCDM values become diagnostic signatures that future cosmographic reconstructions can hunt for.
  • The sign and magnitude of R on the slow manifold simultaneously control the local expansion rate and the amplitude of structure growth, linking the H0 and S8 tensions inside a single geometric mechanism.
  • BBN abundance measurements can be used as a high-redshift filter on the allowed range of α and β because rapid early variations of G are tightly constrained by light-element yields.

Where Pith is reading between the lines

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

  • If the slow-manifold value of R remains negative at late times, the same mechanism that lowers σ8 would also pull the local H0 toward the lower CMB-inferred value, potentially reconciling both tensions with a single sign choice.
  • The requirement that μ vanish for statistical viability suggests that the quartic self-interaction of the scalar is radiatively suppressed or irrelevant at late times, a prediction that could be checked in a UV completion of the fractional RG flow.
  • Because the model already produces cyclic and oscillatory early phases, a dedicated primordial-nucleosynthesis likelihood analysis could turn the present BBN consistency arguments into a quantitative prior on the fractional order α.
  • The geometric slow–fast structure implies that any future detection of a non-constant G on cosmological scales would automatically select a preferred region of the (α,ζ) plane already constrained by the MCMC chains.
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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

3 major / 5 minor

Summary. The manuscript constructs a fractional-action cosmological model (FRGIC/FEG) with a scalar field and a time-varying gravitational constant G, motivated by renormalization-group ideas. After imposing a phenomenological Hubble ansatz H = H0 + ξφ + ε/t with the special choice ε = (α−1)/3, the authors reduce the dynamics to a regularized autonomous system, analyze its slow–fast structure and critical points, and reconstruct H(z). Bayesian MCMC fits to SNe Ia (Pantheon+), cosmic chronometers, DESI BAO, H0LiCOW lensing and black-hole shadows are presented for two variants (μ ≠ 0 and μ = 0) and compared with flat ΛCDM. The authors conclude that only the μ = 0 model is statistically competitive (slightly lower χ²_min, well-constrained α and ζ, overdamped relaxation τ_rel ≃ 9 Gyr), that it reproduces late-time acceleration while producing distinctive cosmographic signatures, and that fractional nonlocal corrections may help address the H0 and S8 tensions.

Significance. If the reduction and the observational pipeline were robust, the work would supply a concrete, observationally constrained fractional-gravity scenario with an explicit slow–fast geometric structure and a diagnostic R ≡ (Ġ/G)/H that links early-universe (BBN) and late-time (growth, H0) physics. The dynamical-systems analysis (regularization via u = H0/H, Puiseux expansions, critical-point classification, desingularization of u = 0) is carefully executed and of independent interest for fractional cosmologies. The MCMC implementation uses standard data sets and reports BIC comparisons honestly. These technical strengths are real; the central viability claim, however, rests on an ansatz whose status is not derived from the fractional field equations, which limits the present significance for the H0/S8 problem.

major comments (3)
  1. The entire H(z) reconstruction and the MCMC posteriors of Table 1 and Figs. 1–7 rest on the phenomenological ansatz H = H0 + ξφ + ε/t together with the special choice ε = (α−1)/3 (Eqs. 8, 12 and the paragraph after Remark 3). That choice is introduced explicitly “to simplify the analytical solution” and to cancel the time-dependent friction term in the scalar equation (10), converting it into a Levinson–Smith oscillator. It is not a consequence of the fractional Euler–Lagrange equations (3)–(6) nor of the RG-improved action. If a generic ε is retained, or if H is obtained by simultaneous integration of the full (G, φ, a) system, the first-order system (55)–(56) used for the emcee chains ceases to be valid. The statistical-viability claim for μ = 0 therefore inherits the status of an untested ansatz. The manuscript should either (i) derive the ansatz from the field equations under control
  2. Table 1 reports τ0 ≃ 2.94 and an inferred cosmic age t0 ≃ 40 Gyr (1σ lower bound still ≃ 24 Gyr) for both Fractional variants. This is grossly inconsistent with stellar ages, globular-cluster ages and the CMB-inferred age of the Universe (≃ 13.8 Gyr). The text attributes the result to a strong degeneracy between α and τ0 (Eqs. 45, 56) and notes that only a 1σ lower bound is obtained, yet still presents t0 ≃ 40 Gyr as a model outcome. A cosmologically viable model cannot leave an age of ∼40 Gyr as an acceptable posterior region. Either a prior that enforces a realistic age must be imposed and the chains re-run, or the degeneracy must be broken by additional data (e.g., high-z BAO or CMB distance priors) so that the age posterior is brought into agreement with independent constraints. Until this is done, the claim that the μ = 0 model is “statistically viable” is incomplete.
  3. The abstract and §12 assert that fractional nonlocal corrections “may offer new pathways toward addressing the H0 and S8 tensions.” The MCMC analysis, however, yields h ≃ 0.72 for all three models (ΛCDM and both Fractional variants) and does not include growth data (fσ8 or weak lensing) in the likelihood; the S8 discussion in §11 remains at the level of qualitative sign arguments for R. Moreover, BIC strongly favors ΛCDM (ΔBIC > 10). The tension-resolution language should be either supported by an explicit joint fit that includes growth observables and a quantitative ΔH0/ΔS8 assessment, or substantially softened to match what the present data actually constrain.
minor comments (5)
  1. Notation for the fractional parameter is occasionally overloaded: α appears both as the FALVA order and (via α = 1 + 3τ_rel) as a derived age-related quantity. A single consistent definition table would help.
  2. Figures 11–14 (G(t) variations) are repeated with the same caption block; panel labels and parameter values should be made unique and legible.
  3. The prior ranges (e.g., α ∈ [1,4], μ ∈ [−3,10]) are stated but not motivated by theoretical bounds; a short justification or sensitivity check would strengthen §4.2.
  4. Several long passages in §§7–10 restate the same slow–fast geometry in three different charts (λ, T, w). Condensing the comparative synthesis would improve readability without loss of content.
  5. Typographical issues: “CNAAR” in Funding, duplicated figure captions, and occasional missing spaces around equation references.

Circularity Check

3 steps flagged

Central H(z) reconstruction, MCMC viability claims, and overdamped 'confirmation' rest on an undervived convenience ansatz H=H0+ξφ+ε/t with ε=(α-1)/3 plus algebraic relations among fitted parameters.

specific steps
  1. other [Section 2, Eq. (8) and paragraph after Remark 3 (point 4)]
    "Motivated by various scalar field and quintessence cosmological models [73, 74, 75], we suggest the generalized ansatz H=H0 +ξφ+ε/t,(8) ... we have selectedε= (α−1)/3 for two main reasons: first, it considerably simplifies the analytical solution, and second, it eliminates the time-dependent friction term in the differential equation, so we will deal only with constant friction."

    The entire numerical pipeline (system (55)–(56), E(τ), H(z) reconstruction, MCMC posteriors in Table 1, cosmographic plots Figs. 4–7, and the claim that μ=0 is the only statistically viable variant) is derived under this special value of ε. The choice is not obtained from the fractional field equations (4)–(6) but imposed for analytic convenience; therefore the reported H(z) evolution and viability statements are those of the simplified ansatz model by construction, not predictions of the general FRGIC/FEG theory.

  2. self definitional [Section 5.1, Eqs. (153)–(157) and surrounding text]
    "Using the best-fit values in Table 1, m≃30.8 +28.0 −20.9 km s−1 Mpc−1, Γ≃108.3±1.1 km s−1 Mpc−1, ... Δ=... Since Δ>0, the system lies in the overdamped regime. ... τd=1/|λ+|≈...≈9.0 Gyr≃τ_rel=9.037+0.091−0.094 Gyr, in excellent agreement with the fitted value reported in Table 1."

    τ_rel is defined from the free parameters via α=1+3τ_rel and λ=1/τ_rel (Eqs. 36, 41); m=ζ H0 and Γ=3/2 H0 are likewise direct functions of the same fitted (ζ,h). The eigenvalues λ± and the derived τ_d are therefore algebraic rearrangements of the best-fit numbers under the overdamped approximation; the 'agreement' and 'confirmation of an overdamped regime' are identities, not independent dynamical results.

  3. fitted input called prediction [Abstract and Section 4.3 / Table 1]
    "A Bayesian analysis shows that the Fractional model with μ=0 is the only statistically viable variant. The inferred Hubble parameter is stable across models (h≃0.72), while the fractional parameters are significantly better constrained in the μ=0 case (α=1.20+0.25−0.14, ζ=0.43+0.39−0.29). ... Although the μ=0 model attains a slightly lower χ2_min than ΛCDM, the BIC strongly favors ΛCDM ... Overall, the model reproduces late-time acceleration and mimics ΛCDM while introducing distinctive cosmographic signatures."

    The free parameters of the ansatz-reduced system (including α, ζ, τ0, q0 that fix the initial conditions (56) and the friction) are fitted by MCMC to the same SNe+CC+BAO+GL+BHS data that are then used to declare the model 'reproduces late-time acceleration' and yields 'distinctive cosmographic signatures'. The H(z), q(z), j(z), s(z) curves shown in Figs. 4–7 are therefore the best-fit realizations of the parametrized ansatz, not out-of-sample predictions.

full rationale

The paper's strongest statistical claim (μ=0 Fractional model is the only viable variant, with quoted posteriors on α, ζ, m, Γ, τ_rel and distinctive cosmographic signatures that may address H0/S8) is obtained by integrating the reduced first-order system (55)–(56) whose form is forced by the phenomenological Hubble ansatz (8) together with the special value ε=(α-1)/3. That choice is not a consequence of the fractional Euler–Lagrange equations (3)–(6) or the RG-improved action; it is imposed explicitly 'to simplify the analytical solution' and to cancel the time-dependent friction term, converting the scalar equation into a Levinson–Smith oscillator. All subsequent numerical H(z), cosmographic functions, emcee chains, Table 1 and Figs. 4–7 therefore describe this simplified ansatz model, not the general fractional theory. In addition, the 'excellent agreement' between the damping timescale τ_d computed from best-fit (m,Γ) and the reported τ_rel is algebraic consistency under the definitions α=1+3τ_rel, Γ=3/2 H0 and the overdamped approximation, not an independent dynamical prediction. These are partial circularities of the fitted-input and self-definitional kinds; the Bayesian comparison itself is otherwise standard and the dynamical-systems analysis of critical points is independent of the data fit. Score 5 reflects that the load-bearing observational claims reduce to the ansatz plus parameter fitting, while the pure phase-space geometry does not.

Axiom & Free-Parameter Ledger

7 free parameters · 5 axioms · 2 invented entities

The central observational claim rests on the fractional-action variational principle, a phenomenological Hubble ansatz with a tuned ε, a power-law relation Λ∝G^β, and a large set of free parameters fitted to late-time data. Early-universe viability further assumes that the rapid-G-variation regime can be kept inside BBN bounds by suitable choices of α, β, γ.

free parameters (7)
  • α (fractional order) = 1.20^{+0.25}_{-0.14}
    Controls nonlocal corrections and the 1/t friction; fitted, best-fit 1.20^{+0.25}_{-0.14} (μ=0).
  • ζ = m/H0 = 0.43^{+0.39}_{-0.29}
    Scalar mass in units of H0; fitted, best-fit 0.43^{+0.39}_{-0.29} (μ=0).
  • μ (quartic coupling) = 0 (preferred) or 4.0^{+2.8}_{-2.5}
    Strength of φ^4 term; set to 0 for the viable model or left free (poorly constrained).
  • τ0 = H0 t0 = 2.94^{+0.77}_{-1.18}
    Dimensionless age; strongly degenerate with α, posterior ~2.94 with large errors.
  • q0 (present deceleration) = −0.502^{+0.027}_{-0.023}
    Initial condition for the scalar system; fitted ~−0.50.
  • h, rd, M = h≃0.72, rd≃139 Mpc, M≃−19.29
    Standard cosmological nuisance parameters jointly fitted with the fractional sector.
  • β (in Λ=Λ0 G^β)
    Phenomenological exponent relating cosmological and gravitational constants; not tightly constrained by the late-time fit.
axioms (5)
  • domain assumption Fractional action-like variational approach (FALVA) replaces the ordinary action integral by a fractional integral of order α.
    Foundational premise of the entire FRGIC construction (Introduction and §2).
  • ad hoc to paper Hubble ansatz H=H0+ξφ+ε/t with the specific choice ε=(α−1)/3.
    Introduced for analytic convenience (Eq. 8 and Remark 3); not derived from the fractional action.
  • ad hoc to paper Phenomenological law Λ=Λ0 G^β.
    Assumed after Eq. 12 to close the system for G(t).
  • domain assumption Flat FLRW metric and a single scalar field dominate the late universe.
    Standard cosmological setting adopted in §2.
  • domain assumption Pre-recombination sound horizon rd can be treated as a free parameter independent of early-universe microphysics.
    Used in the BAO likelihood (§4.1.3).
invented entities (2)
  • Fractional Renormalization-Group Improved Cosmology (FRGIC / FEG) no independent evidence
    purpose: Unifies fractional-action nonlocality with RG running of G and Λ.
    Named and motivated in the Introduction; no independent experimental handle beyond the cosmological fits themselves.
  • Diagnostic R ≡ (Ġ/G)/H no independent evidence
    purpose: Single quantity that controls modified continuity, growth, BBN bounds, and the H0/S8 interpretation.
    Defined in §7–8; its dynamics are derived from the model equations, so it is not an extra free field, but its cosmological interpretation is model-specific.

reviewed 2026-07-14 · how reviews work

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Cite this review

Pith. "Pith review of Varying Gravity from a Modified Fractional Model: Observational Constraints and Slow-Fast Dynamics." pith.science (2026). https://pith.science/paper/ZVBQIVDP

@misc{pith2026260709722,
  author       = {Pith},
  title        = {Pith review of: Varying Gravity from a Modified Fractional Model: Observational Constraints and Slow-Fast Dynamics},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZVBQIVDP}},
  note         = {Machine review of arXiv:2607.09722}
}
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abstract

We investigate a fractional gravity model in which both the Hubble parameter and the gravitational constant evolve dynamically due to fractional renormalization-group effects. The model incorporates a scalar field coupled to a time-varying $G$, generating nonlocal corrections characteristic of fractional--action cosmology. Analytical and numerical solutions reveal oscillatory regimes, cyclic phases, and rapid variations with implications for BBN and early-universe evolution. A robust numerical framework is developed to integrate the regularized system and compare the resulting $H(z)$ evolution with observational data from the Hubble parameter, baryon acoustic oscillations, type Ia supernovae, gravitational lensing, and black hole shadows, thereby enabling a consistent reconstruction of cosmographic quantities. A Bayesian analysis shows that the Fractional model with $\mu=0$ is the only statistically viable variant. The inferred Hubble parameter is stable across models ($h\simeq 0.72$), while the fractional parameters are significantly better constrained in the $\mu=0$ case ($\alpha=1.20^{+0.25}_{-0.14}$, $\zeta=0.43^{+0.39}_{-0.29}$). The dynamical sector yields $m=30.8^{+28.0}_{-20.9}$ and $\Gamma=108.3\pm1.1$, leading to a positive discriminant and a well-determined relaxation timescale $\tau_{\rm rel}\simeq 9$ Gyr, confirming an overdamped regime. Although the $\mu=0$ model attains a slightly lower $\chi^2_{\min}$ than $\Lambda$CDM, the BIC strongly favors $\Lambda$CDM due to its smaller parameter space. Overall, the model reproduces late-time acceleration and mimics $\Lambda$CDM while introducing distinctive cosmographic signatures. The dynamical systems analysis clarifies the stability structure and parameter dependence, indicating that fractional nonlocal corrections may offer new pathways toward addressing the $H_0$ and $S_8$ tensions.

Figures

Figures reproduced from arXiv: 2607.09722 by Esteban Gonz\'alez, Genly Leon, Kevin Marroqu\'in, Rami Ahmad El-Nabulsi.

Figure 1
Figure 1. Figure 1: The posterior 1D distributions and joint marginalized regions for the free parameter space [PITH_FULL_IMAGE:figures/full_fig_p020_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The posterior 1D distributions and joint marginalized regions for the free parameter space [PITH_FULL_IMAGE:figures/full_fig_p021_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The posterior 1D distributions and joint marginalized regions for the free parameter space [PITH_FULL_IMAGE:figures/full_fig_p022_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The Hubble parameter (H) as a function of redshift (z) for the ΛCDM (red dashed line) and Fractional models with µ ̸= 0 (blue solid line) and µ = 0 (orange solid line). The shaded regions represent the confidence intervals of the Hubble parameter at the 1σ CL. The figure was generated using the chains from the MCMC analysis described in Section 4. 23 [PITH_FULL_IMAGE:figures/full_fig_p023_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The deceleration parameter (q) as a function of redshift (z) for the ΛCDM (red dashed line) and Fractional models with µ ̸= 0 (blue solid line) and µ = 0 (orange solid line). The shaded regions represent the confidence intervals of the deceleration parameter at the 1σ CL. The figure was generated using the chains from the MCMC analysis described in Section 4. 24 [PITH_FULL_IMAGE:figures/full_fig_p024_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The jerk parameter (j) as a function of redshift (z) for the ΛCDM (red dashed line) and Fractional models with µ ̸= 0 (blue solid line) and µ = 0 (orange solid line). The shaded regions represent the confidence intervals of the jerk parameter at the 1σ CL. Note that, for the ΛCDM model, the jerk parameter is strictly constant with a value of j = 1. The figure was generated using the chains from the MCMC an… view at source ↗
Figure 7
Figure 7. Figure 7: The snap parameter (s) as a function of redshift (z) for the ΛCDM (red dashed line) and Fractional models with µ ̸= 0 (blue solid line) and µ = 0 (orange solid line). The shaded regions represent the confidence intervals of the snap parameter at the 1σ CL. The figure was generated using the chains from the MCMC analysis described in Section 4. From [PITH_FULL_IMAGE:figures/full_fig_p026_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: One-dimensional flow of equation (273) for v1∗ ∈ {0, ±ζ/√µ} and different parameter values [PITH_FULL_IMAGE:figures/full_fig_p047_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Phase portraits of equation (374) for µ = 1, λ = 0.5 and µ = 1, λ = 10. Linearization and relaxation: we define f(R) = 3 µ + (λ − 3)R + 1 2R2 , with derivative f ′ (R∗) = (λ − 3) + R∗. The linearized dynamics are R ≈ − ˙ κ H R − R∗  , κ = − f ′ (R∗) = 3 − λ − R∗. (378) Thus, the relaxation rate toward the fixed point is controlled by κ and modulated by the integral of H. Integrating, R(t) ≃ R∗ + [PITH_FU… view at source ↗
Figure 10
Figure 10. Figure 10: Evolution of the jerk parameter and the deceleration parameter as a function of redshift. 63 [PITH_FULL_IMAGE:figures/full_fig_p063_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Variation of the gravitational constant as a function of cosmic time (cont.). [PITH_FULL_IMAGE:figures/full_fig_p064_11.png] view at source ↗
Figure 11
Figure 11. Figure 11: Variation of the gravitational constant as a function of cosmic time (cont.). [PITH_FULL_IMAGE:figures/full_fig_p065_11.png] view at source ↗
Figure 11
Figure 11. Figure 11: Variation of the gravitational constant as a function of cosmic time (cont.). [PITH_FULL_IMAGE:figures/full_fig_p066_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Variations of the gravitational parameter as a function of cosmic time for [PITH_FULL_IMAGE:figures/full_fig_p067_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Variations of the gravitational parameter as a function of cosmic time for [PITH_FULL_IMAGE:figures/full_fig_p068_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Variations of the gravitational parameter as a function of cosmic time for [PITH_FULL_IMAGE:figures/full_fig_p069_14.png] view at source ↗

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This paper was first reviewed by grok-4.5 on July 14, 2026.