REVIEW 4 major objections 4 minor 6 cited by
Brans-Dicke gravity with a cosmological constant smoothes out $\Lambda$CDM tensions
T0 review · 4 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read A Brans-Dicke gravity model with a bare cosmological constant and no extra dark energy fluid fits the combined cosmological data better than GR-based ΛCDM and dissolves the H0 and σ8 tensions.
desk verdict A technically solid BD-LCDM fit that softens H0 and sigma8 tensions, but its viability leans on an unproven screening assumption that lets |omega_BD|~300 evade Cassini bounds; deserves a real referee. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The carrying object is the dimensionless Brans-Dicke field $\phi(t)=G_N/G(t)$, with $\epsilon_{\rm BD}=1/\omega_{\rm BD}$ measuring the departure from GR. Rewriting the BD field equations as Friedmann equations produces an effective BD-fluid energy density and pressure, whose combined equation of state with the vacuum term is $w_{\rm eff}(t)$; near $z=0$ this takes the approximate form $w_{\rm eff}\simeq -1 + \frac{\Delta\phi\,\Omega_{m0}}{1-\Omega_{m0}}(1+z)^3$. On the perturbation side, the linear matter-density contrast obeys a modified growth equation in which the effective gravitational constant is $(G_N/\phi)(2+4\epsilon_{\rm BD})/(2+3\epsilon_{\rm BD})$. These two pieces — background equation of state and growth equation — let a single parameter $\epsilon_{\rm BD}$ shift both $H_0$ and $\sigma_8$ in the observed directions.
What would settle it
A cosmological-scale constraint that forced $|\omega_{\rm BD}|\gtrsim10^4$ at all epochs, or a null measurement of $\dot G/G$ at the level of $10^{-14}\,\mathrm{yr}^{-1}$, would exclude the fitted $\epsilon_{\rm BD}\simeq -0.003$ and remove the claimed tension relief.
Extended reading notes
Core claim
The central claim is that BD-ΛCDM — Brans-Dicke gravity with a constant vacuum term and no interaction between the scalar field and matter — provides a better fit to the combined cosmological dataset than the GR-based ΛCDM, with the H0 and σ8 tensions essentially disappearing. In the best fit, $\epsilon_{\rm BD}\simeq -0.003$ (i.e. $\omega_{\rm BD}\simeq -300$), $\phi(0)\simeq 0.90$, so G is slightly larger and growing at present, $\dot G/G\simeq +3\times10^{-13}\,\mathrm{yr}^{-1}$. The model yields $H_0\simeq 71$–$72\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, in line with the local distance ladder at the 1–2σ level, and $\sigma_8\simeq 0.80$, in between Planck and weak-lensing values. In the GR frame the BD field contributes an effective dark-energy fluid with a time-varying equation of state that near $z=0$ takes $w_{\rm eff}\simeq -0.96$, mimicking quintessence at more than 3σ. The DIC and AIC differences between the models lie between 5 and 10, which the authors read as strong evidence for BD-ΛCDM.
Load-bearing premise
The whole fit rests on assuming the Solar System bound on the Brans-Dicke parameter, $|\omega_{\rm BD}|>40000$, does not apply at cosmological scales, allowing $|\omega_{\rm BD}|\simeq 300$ through screening.
Editorial extensions
If this is right
- The tension with the local distance-ladder value of $H_0$ drops from about 4.4σ in ΛCDM to roughly $1.8\sigma$ (DS1) and $1.1\sigma$ (DS2).
- The mass-fluctuation amplitude stays at $\sigma_8\simeq 0.80$, sitting between the higher Planck value and the lower cosmic-shear value and thereby easing the $\sigma_8$ tension.
- Near the present epoch the effective dark energy equation of state is $w_{\rm eff}\simeq -0.96$, a quintessence-like signal at $\gtrsim 3\sigma$, which the authors describe as a smoking gun of the underlying BD dynamics.
- The model is favored over ΛCDM by both DIC and AIC with differences between 5 and 10, which standard model-selection practice treats as strong evidence.
- The fitted value of the Brans-Dicke parameter, $|\omega_{\rm BD}|\simeq 300$, keeps the model close enough to GR to be phenomenological while predicting a mild, positive present-day time variation of $G$.
Reading between the lines
- The paper leaves implicit that the same scalar field can in principle absorb both tensions at once, which suggests a common physical origin — a mildly running gravitational coupling — rather than two independent systematics.
- If the model is right, future BAO and weak-lensing surveys should see the effective dark energy equation of state deviate from $-1$ at low redshift in the direction of quintessence, with the deviation growing roughly as $(1+z)^3$.
- A null detection of the predicted present-day growth of $G$, $\dot G/G\simeq 3\times10^{-13}\,\mathrm{yr}^{-1}$, with upcoming lunar-laser-ranging or atomic-clock experiments would put the model under direct observational pressure.
- The screening assumption could be tested cross-model: if the cosmological Brans-Dicke parameter really is as small as $|\omega_{\rm BD}|\simeq 300$, comparable tension relief should appear in other screened modified-gravity theories fitted to the same data.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript analyzes Brans-Dicke gravity with a bare cosmological constant and cold dark matter (BD-ΛCDM) against a combination of SnIa, H(z), BAO, RSD, weak lensing, and the full Planck 2015 CMB likelihood. The authors use two data sets, DS1 and DS2, with the public codes CLASS and MontePython, fitting eight parameters including the BD parameters ε_BD and φ_ini. They report that BD-ΛCDM is preferred over GR-based ΛCDM by ΔDIC ≈ 8.3–9.9 and ΔAIC ≈ 7.7–9.9, with H0 ≈ 71–72 km/s/Mpc, σ8 ≈ 0.80, and an effective dark-energy equation of state w_eff(0) ≈ −0.95 to −0.96, which they interpret as quintessence-like behavior at ≳3σ. The paper concludes that the H0 and σ8 tensions of ΛCDM essentially disappear in this context. The central load-bearing elements are the quoted matter-perturbation equation, the screening assumption that bypasses solar-system bounds on ω_BD, and the interpretation of the fitted BD field as an effective quintessence component.
Significance. If the central claim holds, BD-ΛCDM would be a minimal and economical modification of the concordance model that resolves two major tensions without introducing a new dark-energy fluid. The paper's strengths are its use of standard public pipelines and the full Planck 2015 likelihood, the explicit reporting of marginalized constraints and information-criterion differences, and a clear statement of the screening assumption. These features make the statistical comparison reproducible in principle. However, the quoted growth equation is not derived, the screening mechanism is not specified, and the quintessence signal is a reparameterization of the fitted BD field rather than an independent observable. As a result, the confidence that can currently be placed in the model-selection preference is limited, and the interpretation of the 3σ w_eff result needs to be framed more carefully.
major comments (4)
- [§3, Eq. (10)] The linear matter-perturbation equation is quoted without derivation, with the details deferred to a future 'more complete presentation.' Equation (10) is the basis for the reported σ8, RSD, WL, and bispectrum fits, so the central model-selection result depends on it. Please provide a derivation or a public implementation that demonstrates how the modified CLASS equations reproduce Eq. (10) in the subhorizon limit, including the treatment of the BD scalar-field perturbations and the (2+4ε)/(2+3ε) coupling factor. As written, the growth sector cannot be independently verified.
- [§3, screening of solar-system bounds] The fitted value ε_BD ≈ −0.003 corresponds to |ω_BD| ≈ 300 and to a PPN parameter γ−1 ≈ 3×10^−3, which is more than two orders of magnitude above the Cassini bound. The manuscript explicitly sets aside local bounds and appeals to screening, but the action in §2 contains no potential for ψ, no nonlinear self-interactions, and no chameleon, Vainshtein, or symmetron mechanism that would make ω_BD environment-dependent. The manuscript should either specify a concrete screening mechanism that reconciles the cosmological and local values, or present the analysis as conditional on this assumption and quantify how severely the Cassini bound would restrict the fitted region if it applies.
- [§2.2, Eq. (8) and §4.2, w_eff(0)] The claim that BD-ΛCDM 'mimics quintessence at more than 3σ' is a restatement of the fitted BD field, not an independent constraint: w_eff is constructed from the same fitted φ that enters the background and growth equations, so the 3σ deviation from −1 is a derived parameter rather than a new observable. In addition, the functions f1 and f2 in Eq. (8) are not given explicitly, and the statement that they are 'numerically negligible' is asserted without a bound. Please provide explicit expressions and a quantitative assessment of the dropped terms near z = 0, and describe the reported significance as a constraint on the derived w_eff rather than as an independent detection of quintessence.
- [§4.2, initial conditions] The numerical solution fixes φ'(z_ini) = 0 at z_ini = 10^14. This is a nontrivial theoretical input: the BD scalar is dynamical, and imposing a zero initial velocity at a finite initial epoch is a prior on the solution space. The paper reports no test of the sensitivity of H0, σ8, or ΔDIC/ΔAIC to the choice of z_ini and to the initial derivative. Please add a convergence/sensitivity test (for example, varying z_ini by an order of magnitude and allowing a non-zero initial derivative) to show that the model-selection result is not driven by this boundary condition.
minor comments (4)
- [§2.1, before Eq. (4)] There is a typo, 'Hearafter', that should read 'Hereafter'.
- [§2.2, Eq. (8)] The sentence 'The two functions f1,2 need not be specified here' is unsatisfactory in a Letter whose central claim is based on w_eff; at minimum, the explicit expressions should be given in an appendix or in a companion public file.
- [§4.2 and Table 1] The compressed notation for the 1σ, 2σ, and 3σ intervals in the Table 1 caption is difficult to parse; please present the 68% intervals in the main table and move the extended intervals to a supplementary table.
- [References] The Hildebrandt et al. entry is given only as an arXiv number; please add the journal reference if published.
Circularity Check
No significant circularity: model comparison is data-driven, and the 'quintessence-like' EoS is a reparameterization of the fitted scalar field rather than a circular derivation.
full rationale
The paper's central claim is a cosmological model comparison performed with CLASS and MontePython against external datasets (SnIa, H(z), BAO, RSD, weak lensing, and Planck 2015 CMB). The fitted values of H0, sigma8, epsilon_BD, and phi_ini are genuinely constrained by these data, and the DIC/AIC preference for BD-LambdaCDM is a standard model-selection result, not a quantity identical by construction to an input. The statement that BD-LambdaCDM 'mimics quintessence at more than 3 sigma' is a postdiction: Eq. (8) defines w_eff directly from the fitted BD field phi and its derivatives, so the reported quintessence signal inherits the fitted phi_ini rather than being an independent first-principles prediction. This is ordinary parameter inference, not a circular reduction of the kind in which a fitted parameter is renamed as a predicted observable. There are minor self-citations (e.g., 'Both features are consistent with previous estimates from analytical power-law solutions found in Sola (2018) and de Cruz Perez & Sola (2018)') and a data-description citation to the same group's 2019 paper, but these are not load-bearing: the numerical fit and the external perturbation equation (Eq. 10, cited to Boisseau et al. 2000) carry the argument. The paper's explicit assumption that solar-system bounds on omega_BD need not apply at cosmological scales ('we take the wider perspective that the BD theory, when applied to the cosmological level, is not restricted by the bounds obtained in the astrophysical neighborhood') is a physical/correctness risk about screening, not a circularity. The deferred perturbation derivation is an omission, not a circular step. Overall, no derivation step reduces to its own input by construction; the modest score reflects the minor self-citations and the fact that the quintessence signal is a fitted-output reparameterization rather than a true prediction.
Assumptions & free parameters
free parameters (8)
- H0 =
68.65 to 72.00 km/s/Mpc across fits
- Omega_m0 =
0.2665 to 0.2955
- Omega_b0 =
0.0443 to 0.0476
- tau =
0.063 to 0.084
- n_s =
0.9700 to 0.9945
- sigma8(0) =
0.801 to 0.804
- epsilon_BD =
-0.00277 +0.00170 -0.00154 (DS1); -0.00315 +0.00168 -0.00175 (DS2)
- phi_ini =
0.924 +0.021 -0.023 (DS1); 0.901 +0.026 -0.025 (DS2)
assumptions (6)
- domain assumption BD field equations (1)-(3) with no potential for the scalar and with a fundamental cosmological constant Lambda
- domain assumption Flat FLRW metric
- domain assumption Separate conservation of baryons, cold dark matter, photons and neutrinos
- domain assumption Subhorizon perturbation equation (10) from Boisseau et al. 2000
- ad hoc to paper Solar-system bounds on omega_BD can be screened at cosmological scales
- ad hoc to paper Initial derivative of phi is zero at z_ini = 10^14
Cite this review
Pith. "Pith review of Brans-Dicke gravity with a cosmological constant smoothes out $\Lambda$CDM tensions." pith.science (2026). https://pith.science/paper/2JJZWK4Y
@misc{pith2026190902554,
author = {Pith},
title = {Pith review of: Brans-Dicke gravity with a cosmological constant smoothes out $\Lambda$CDM tensions},
year = {2026},
howpublished = {\url{https://pith.science/paper/2JJZWK4Y}},
note = {Machine review of arXiv:1909.02554}
}
abstract
We analyze Brans-Dicke gravity with a cosmological constant, $\Lambda$, and cold dark matter (BD-$\Lambda$CDM for short) in the light of the latest cosmological observations on distant supernovae, Hubble rate measurements at different redshifts, baryonic acoustic oscillations, large scale structure formation data, gravitational weak-lensing and the cosmic microwave background under full Planck 2015 CMB likelihood. Our analysis includes both the background and perturbations equations. We find that BD-$\Lambda$CDM is observationally favored as compared to the concordance $\Lambda$CDM model, which is traditionally defined within General Relativity (GR). In particular, some well-known persisting tensions of the $\Lambda$CDM with the data, such as the excess in the mass fluctuation amplitude $\sigma_8$ and specially the acute $H_0$-tension with the local measurements, essentially disappear in this context. Furthermore, viewed from the GR standpoint, BD-$\Lambda$CDM cosmology mimics quintessence at $\gtrsim3\sigma$ c.l. near our time.
Figures
Forward citations
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