REVIEW 3 major objections 5 minor 48 references
Nonlinear dynamics in Horndeski gravity: a renormalized approach to effective gravitational coupling
T0 review · 3 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read A single function now sets the effective gravitational strength at every cosmic scale.
desk verdict A serious new formalism for a scale-dependent effective gravitational constant in screened Horndeski gravity, but the master equation is an ansatz and the numerics are not yet a full renormalization. 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 machinery is a closure ansatz for the response function: the full linear response of the potential to the nonlinear density field is written as $\Gamma^{(1)}(t,p) = -(3 a^2 H^2 \Omega_m/2p^2)\bigl(1 + (\mu F-1)M(t,p)\bigr)$, reducing all unknown nonlinearity to a single function $M$. Substituting this ansatz into the quasi-static, Fourier-space nonlinear Poisson equation, which keeps the Galileon-type kernel $\gamma(p_1,p_2)=1-(p_1\cdot p_2)^2/(p_1^2 p_2^2)$, and taking the bispectrum moment yields the master equation. The paper also introduces a positivity-preserving quadratic iteration for solving this integral equation, replacing a naive fixed-point iteration that drifts negative.
What would settle it
Evaluate the same nonlinear Poisson equation in a direct numerical simulation of the $c_K=1$, $c_B=0.5$ model and measure $G_{\mathrm{eff}}$ through $-p^2\langle\Phi\delta_m\rangle/P_{\delta\delta}(p)$; if the measured scale dependence does not match Eq. (77) with the converged $M(p)$ from Eq. (85), or if iterations of the full system move $M(p)$ outside $[0,1]$ at any scale where the quasi-static approximation is expected to hold, the central claim would be contradicted.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is a resummation formula: after loop corrections are folded into the linear response, the renormalized effective gravitational constant is $G_{\mathrm{eff}}^{\mathrm{NL}}(t,p)/G = (1/F)\bigl(1 + (\mu F - 1) M(t,p)\bigr)$, where $\mu$ is the linear growth coupling, $F$ is the nonminimal coupling function, and $M(t,p)$ solves the master equation $$P(p)(1-M(p)) = \frac{g}{(2\pi)^2}\int $p_1^{2}$\, \gamma(p_1,p-p_1)\, B(p_1,p-p_1)\, M(p_1)M(|p-p_1|)\, dp_1\, d\mu_\$\theta$.$$ In the infrared $M\to 1$ and one recovers the linear coupling $\mu$; in the ultraviolet $M\to 0$ and the coupling becomes $G/F$, showing that the derivative (Galileon-type) interactions screen the extra force but the conformal factor $F$ is not screened in this quasi-static treatment. The paper further exhibits a numerical iteration that converges in two steps and shows that the result reproduces the $p^2$ infrared behaviour of one-loop standard perturbation theory, while remaining finite where the one-loop expression fails.
Load-bearing premise
The derivation's load-bearing premise is that the quasi-static approximation holds down to the small scales where Vainshtein screening operates, and that the only nonlinearities that matter there are the Galileon-type derivative terms retained in the truncated Poisson equation, so the unscreened coupling $F(\phi)$ can be treated as a background function.
Editorial extensions
If this is right
- The effective gravitational coupling becomes a continuous function of scale and redshift, so Vainshtein screening is encoded as $M(t,p)\to 0$ rather than requiring a sharp cutoff or a separate treatment of each object.
- The matter power spectrum can be corrected by solving a single linear equation for a renormalized growth factor $D_{\mathrm{renorm}}(t,p)$ with $G_{\mathrm{eff}}^{\mathrm{NL}}$ in place of Newton's constant.
- Bispectrum corrections follow from the same coupling; the squeezed-limit second-order kernel is fixed by the Ward identity to $D_1 = [D_{\mathrm{renorm}}]^2$, with no new soft poles, so non-Gaussianity changes only through the modified growth and finite multipole coefficients.
- In the infrared the renormalized coupling recovers the one-loop SPT $p^2$ scaling, while at small scales it remains well behaved where one-loop SPT breaks down.
- The converged result depends only weakly on the assumed input bispectrum, which suggests the method is stable enough for an iterative power-spectrum and bispectrum renormalization.
Reading between the lines
- Editorial extension: if $F\neq 1$ is not screened, the small-scale limit is $G/F$, not Newton's $G$; in models where $F$ deviates appreciably at late times, this residual coupling would appear as a local fifth force unless a separate mechanism acts on $F$.
- Editorial extension: the positivity-preserving quadratic iteration used here is not tied to the specific field theory; the same strategy could stabilize comparable nonlinear integral equations in other resummation schemes.
- Editorial extension: because the master equation needs $P$ and $B$ as inputs, a sharp test of the framework would be a direct N-body measurement of $\langle\Phi\delta_m\rangle/P_{\delta\delta}$ in the $c_K=1$, $c_B=0.5$ model and a comparison with the iterated formula.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a renormalized perturbation theory for the quasi-static Horndeski subclass (1), with the goal of defining a scale- and time-dependent nonlinear effective gravitational constant G_NL_eff. After deriving the nonlinear Poisson equation (33), the authors introduce the response functional expansion (39), truncate it to the linear response Γ^(1), and write Γ^(1) in the form (76) in terms of an unknown function M(t,p). The main result is Eq. (77), with M determined by the integral equation (85) involving the matter power spectrum and bispectrum. Sections 5-8 develop a spherical approximation and a formal iteration scheme for the power spectrum and bispectrum. Section 10 presents numerical solutions for a model with F=1, cK=1, cB=0.5, using four methods of which Method 4 is selected. Section 11 compares the IR limit to one-loop SPT and finds the same p² scaling. The paper concludes that the framework captures Vainshtein screening and can be iterated in future work.
Significance. If the master equation (85) is a valid resummation, the paper provides a useful nonperturbative tool for estimating screening effects on the gravitational coupling in Horndeski models, with potential applications to power spectrum and bispectrum predictions. The derivation of the quasi-static field equations (13)-(33), the explicit nonlinear Poisson equation, and the spherical approximation are clear and carefully presented. Strengths include the absence of parameter fitting to the target quantity, the nonlinear integral-equation structure of Eq. (85), and the explicit comparison with one-loop SPT in the IR. However, the central equation rests on a closure that is not derived from a controlled truncation, and the numerical implementation inherits inconsistencies from the input spectra. The usefulness of the framework therefore depends on the additional validation requested below.
major comments (3)
- [Section 7, Eqs. (78)-(85)] The master equation is an unvalidated closure, not a derived resummation. The substitution Φ(p)=Γ^(1)(p)δm(p) truncates the exact response expansion (42) at first order, while Eq. (56) gives a nonzero tree-level Γ^(2); the estimates in Eqs. (69)-(70) are based on a spherical one-point approximation and do not control the contribution of Γ^(2) to the equation for Γ^(1). Furthermore, Eq. (81) is presented as a pointwise field equation, but it equates the linear functional (M(p)-1)δm(p) to a quadratic convolution of δm with itself, which cannot hold for each realization of a stochastic density field; the statistical closure is inserted only when Eq. (82) is obtained by multiplying by δm(k) and averaging. The derivation should either start from functional differentiation of Eq. (33) and a systematic truncation of the propagator hierarchy, or the ansatz should be stated as such and tested, e.g., against the one-loop Γ^(1) of Eq. (55) and against N-body simulations.
- [Section 11, Eqs. (161)-(165)] The comparison with one-loop SPT establishes only the p² scaling of the IR correction, not the coefficient. Equation (165) is a nontrivial consistency condition that is left unchecked, and no demonstration is given that the solution M(p) of Eq. (85) reproduces the perturbative one-loop propagator Γ^(1)_1-loop in Eq. (55). Without this check, the statement that both approaches agree in the IR limit is premature. Please verify Eq. (165) numerically or analytically for the adopted parameter choices, or at least show the size of the residual.
- [Section 10, Figs. 6-7] The numerical results for G_NL_eff are presented without convergence criteria, residual measures, or error estimates. Methods 1-3 fail or oscillate, and Method 4 is selected because it converges in two iterations, but no quantitative stopping rule is provided. In addition, the power spectrum and bispectrum used in Eq. (85) come from halofit and SPT/BiHalofit and are not corrected by the Horndeski dynamics; the paper acknowledges this (§8) but does not estimate the resulting error in the converged M(p). Please report residuals, provide error bars or robustness tests, and quantify the sensitivity to the input P and B.
minor comments (5)
- [Section 3, Eq. (63)] The paper correctly acknowledges after Eq. (63) that F(ϕ) is not screened; for general F this means the small-scale limit of G_NL_eff is G/F, not G. Since the numerical model sets F=1, the abstract's claim of restoration of GR should be qualified to that choice.
- [Section 10.4, Eq. (135)] The notation 'p2γ' in Eq. (135) is ambiguous; it should read p1² γ to match Eq. (85), where p1 is the integration variable.
- [Section 7, Eq. (82)] In Eq. (82), the bispectrum argument is written as B(p1,p2,k), but the definition (83) uses B(p1,p2,p3) with p3=-k; use consistent notation and check the Dirac deltas in the averaging step.
- [Section 5, Eq. (67)] The replacement δm → ∆(k) in Eq. (67) mixes a dimensionless field with a dimensionless amplitude of the power spectrum; clarify that this is a heuristic identification and not an exact substitution.
- [Introduction, paragraph 2] The 'elephant problem' is attributed to reference [1] in the introduction, but that reference is a general review; either provide the original source or cite a more specific reference.
Circularity Check
No significant circularity: the master equation is a genuine nonlinear integral equation, and no fitted parameter is renamed as a prediction.
full rationale
The central quantity G_NL_eff/G = (1/F)(1 + (µF - 1)M) (Eq. 77) is introduced as a reparametrization of the response function Γ(1) (Eq. 76); the predictive content lies in the master equation (85), which determines M from the input power spectrum P and bispectrum B. This is not a tautology: P and B are given, M is an unknown function satisfying a nonlinear integral equation, and the paper solves that equation numerically. No parameter is fitted to the target G_NL_eff, and the large/small-scale limits M→1 and M→0 are boundary expectations consistent with the equation, not inputs that force the full scale dependence. The closure Φ = Γ(1) δm used to obtain Eq. (81) is an approximation (the neglect of Γ(2) and higher) whose validity is discussed with a spherical estimate, but the resulting equation is not equivalent to its inputs by construction. The paper also checks externally against 1-loop SPT in Section 11, recovering the IR p^2 scaling; the coefficient matching (Eq. 165) is honestly left as an unverified consistency condition rather than assumed. The deferral of the self-consistent iteration of P and B to future work (Section 8) is a completeness limitation, not a circular reduction. Self-citations appear only in supportive contexts (e.g., matter-era growth exponents), and none carries the load-bearing argument. The core derivation is self-contained.
Assumptions & free parameters
free parameters (2)
- cK =
1
- cB =
0.5
assumptions (5)
- domain assumption The viable Horndeski subclass is defined by action (1) with αT = 0 after GW170817 constraints.
- domain assumption Quasi-static approximation: time derivatives of perturbations are neglected while spatial derivatives are retained.
- domain assumption Perturbative hierarchy: δϕ is small but its spatial derivatives can be large, so (δϕ)^n δϕ,ij ≪ δϕ,ij and δϕ,i δϕ,j ≪ δϕ,ij.
- ad hoc to paper The response function closure: Γ^(1)(t,p) = -(1/p²)(3/2 a²H²Ωm)(1+(µF-1)M(p)), and higher-order propagators are negligible because g < 1.
- domain assumption Extended Galilean invariance in Horndeski with universal Jordan-frame matter coupling fixes the soft limit D1 = Drenorm².
Cite this review
Pith. "Pith review of Nonlinear dynamics in Horndeski gravity: a renormalized approach to effective gravitational coupling." pith.science (2026). https://pith.science/paper/HIZ7M5TF
@misc{pith2026250503999,
author = {Pith},
title = {Pith review of: Nonlinear dynamics in Horndeski gravity: a renormalized approach to effective gravitational coupling},
year = {2026},
howpublished = {\url{https://pith.science/paper/HIZ7M5TF}},
note = {Machine review of arXiv:2505.03999}
}
read the original abstract
This paper develops a renormalized perturbation theory framework for nonlinear structure formation in a broad class of modified gravity models that exhibit Vainshtein screening, with a focus on a viable subclass of Horndeski theories. We extend earlier perturbative methods, originally applied to DGP model, to construct a self-consistent treatment that captures both the linear modifications to gravity at large scales and the nonlinear screening effects at small scales. In the framework, the response of the gravitational potential to matter density fluctuations is characterized by renormalized propagators, leading to the definition of a nonlinear (or renormalized) effective gravitational constant. The paper details several numerical strategies to compute this renormalized gravitational constant. Numerical examples illustrate how the effective gravitational constant evolves with scale and redshift. These results are key to accurately predicting cosmological observables such as the matter power spectrum and bispectrum in modified gravity scenarios.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
T. Clifton, P. G. Ferreira, A. Padilla and C. Skordis, Phys. Rept.513 (2012), 1-189 [arXiv:1106.2476 [astro-ph.CO]]
arXiv 2012
- [2]
-
[3]
G. W. Horndeski, Int. J. Theor. Phys.10 (1974), 363-384
work page 1974
-
[4]
C. Deffayet, G. Esposito-Farese and A. Vikman, Phys. Rev. D79 (2009), 084003 [arXiv:0901.1314 [hep-th]]
arXiv 2009
-
[5]
T. Kobayashi, M. Yamaguchi and J. Yokoyama, Prog. Theor. Phys. 126 (2011), 511-529 [arXiv:1105.5723 [hep-th]]
arXiv 2011
-
[6]
M. A. Luty, M. Porrati and R. Rattazzi, JHEP09 (2003), 029 [arXiv:hep-th/0303116 [hep-th]]
arXiv 2003
-
[7]
G. Goon, K. Hinterbichler, A. Joyce and M. Trodden, JHEP11 (2016), 100 [arXiv:1606.02295 [hep-th]]
arXiv 2016
-
[8]
D. Pirtskhalava, L. Santoni, E. Trincherini and F. Vernizzi, JCAP09 (2015), 007 [arXiv:1505.00007 [hep-th]]
arXiv 2015
Show all 48 references
-
[9]
Santoni, E
L. Santoni, E. Trincherini and L. G. Trombetta, JHEP08 (2018), 118 [arXiv:1806.10073 [hep-th]]. 28
2018 arXiv
-
[10]
A. I. Vainshtein, Phys. Lett. B39 (1972), 393-394
1972
-
[11]
D. G. Boulware and S. Deser, Phys. Rev. D6 (1972), 3368-3382
1972
-
[12]
Babichev and C
E. Babichev and C. Deffayet, Class. Quant. Grav.30 (2013), 184001 [arXiv:1304.7240 [gr-qc]]
2013 arXiv
-
[13]
E. V. Linder, JCAP03 (2018), 005 [arXiv:1801.01503 [astro-ph.CO]]
2018 arXiv
-
[14]
Amendola, D
L. Amendola, D. Bettoni, A. M. Pinho and S. Casas, Universe6 (2020) no.2, 20 [arXiv:1902.06978 [astro-ph.CO]]
2020 arXiv
-
[15]
Scoccimarro, Phys
R. Scoccimarro, Phys. Rev. D80 (2009), 104006 [arXiv:0906.4545 [astro-ph.CO]]
2009 arXiv
-
[16]
Brando, K
G. Brando, K. Koyama and H. A. Winther, JCAP06 (2023), 045 [arXiv:2303.09549 [astro-ph.CO]]
2023 arXiv
-
[17]
B. P. Abbottet al. [LIGO Scientific, Virgo, Fermi-GBM and INTEGRAL], Astrophys. J. Lett.848 (2017) no.2, L13 [arXiv:1710.05834 [astro-ph.HE]]
2017 arXiv
-
[18]
Creminelli and F
P. Creminelli and F. Vernizzi, Phys. Rev. Lett.119 (2017) no.25, 251302 [arXiv:1710.05877 [astro- ph.CO]]
2017 arXiv
-
[19]
Sakstein and B
J. Sakstein and B. Jain, Phys. Rev. Lett.119 (2017) no.25, 251303 [arXiv:1710.05893 [astro-ph.CO]]
2017 arXiv
-
[20]
J. M. Ezquiaga and M. Zumalacárregui, Phys. Rev. Lett.119 (2017) no.25, 251304 [arXiv:1710.05901 [astro-ph.CO]]
2017 arXiv
-
[21]
Deffayet, O
C. Deffayet, O. Pujolas, I. Sawicki and A. Vikman, JCAP 10 (2010), 026 doi:10.1088/1475- 7516/2010/10/026 [arXiv:1008.0048 [hep-th]]
2010 arXiv
-
[22]
de Rham and S
C. de Rham and S. Melville, Phys. Rev. Lett.121 (2018) no.22, 221101 [arXiv:1806.09417 [hep-th]]
2018 arXiv
-
[23]
Amendola, D
L. Amendola, D. Bettoni, G. Domènech and A. R. Gomes, JCAP06 (2018), 029 [arXiv:1803.06368 [gr-qc]]
2018 arXiv
-
[24]
Papallo and H
G. Papallo and H. S. Reall, Phys. Rev. D96 (2017) no.4, 044019 [arXiv:1705.04370 [gr-qc]]
2017 arXiv
-
[25]
J. L. Ripley and F. Pretorius, Phys. Rev. D99 (2019) no.8, 084014 [arXiv:1902.01468 [gr-qc]]
2019 arXiv
-
[26]
J. L. Ripley and F. Pretorius, Class. Quant. Grav.36 (2019) no.13, 134001 [arXiv:1903.07543 [gr-qc]]
2019 arXiv
-
[27]
J. L. Ripley and F. Pretorius, Phys. Rev. D101 (2020) no.4, 044015 [arXiv:1911.11027 [gr-qc]]
2020 arXiv
-
[28]
Á. D. Kovács and H. S. Reall, Phys. Rev. D101 (2020) no.12, 124003 [arXiv:2003.08398 [gr-qc]]
2020 arXiv
-
[29]
Sawicki and E
I. Sawicki and E. Bellini, Phys. Rev. D92 (2015) no.8, 084061 [arXiv:1503.06831 [astro-ph.CO]]
2015 arXiv
-
[30]
Bellini and I
E. Bellini and I. Sawicki, JCAP07 (2014), 050 [arXiv:1404.3713 [astro-ph.CO]]
2014 arXiv
-
[31]
Kimura, T
R. Kimura, T. Kobayashi and K. Yamamoto, Phys. Rev. D85 (2012), 024023 [arXiv:1111.6749 [astro-ph.CO]]
2012 arXiv
-
[32]
Ishak, T
M. Ishak, T. Baker, P. Bull, E. M. Pedersen, J. Blazek, P. G. Ferreira, C. Danielle Leonard, W. Lin, E. Linder and K. Pardo,et al. [arXiv:1905.09687 [astro-ph.CO]]
1905 arXiv
-
[33]
Bernardeau, S
F. Bernardeau, S. Colombi, E. Gaztanaga and R. Scoccimarro, Phys. Rept. 367 (2002), 1-248 [arXiv:astro-ph/0112551 [astro-ph]]. 29
2002 arXiv
-
[34]
Crisostomi, M
M. Crisostomi, M. Lewandowski and F. Vernizzi, Phys. Rev. D 101 (2020) no.12, 123501 doi:10.1103/PhysRevD.101.123501 [arXiv:1909.07366 [astro-ph.CO]]
2020 arXiv
-
[35]
Peloso and M
M. Peloso and M. Pietroni, JCAP04 (2014), 011 doi:10.1088/1475-7516/2014/04/011 [arXiv:1310.7915 [astro-ph.CO]]
2014 arXiv
-
[36]
D. Blas, J. Lesgourgues and T. Tram, JCAP07 (2011), 034 [arXiv:1104.2933 [astro-ph.CO]]
2011 arXiv
-
[37]
Zumalacárregui, E
M. Zumalacárregui, E. Bellini, I. Sawicki, J. Lesgourgues and P. G. Ferreira, JCAP08 (2017), 019 [arXiv:1605.06102 [astro-ph.CO]]
2017 arXiv
-
[38]
Bellini, I
E. Bellini, I. Sawicki and M. Zumalacárregui, JCAP02 (2020), 008 [arXiv:1909.01828 [astro-ph.CO]]
2020 arXiv
-
[39]
Aghanim et al
N. Aghanim et al. [Planck], Astron. Astrophys.641 (2020), A6 [erratum: Astron. Astrophys.652 (2021), C4] [arXiv:1807.06209 [astro-ph.CO]]
2020 arXiv
-
[40]
R. E. Smithet al. [VIRGO Consortium], Mon. Not. Roy. Astron. Soc.341 (2003), 1311 [arXiv:astro- ph/0207664 [astro-ph]]
2003
-
[41]
Takahashi, T
R. Takahashi, T. Nishimichi, T. Namikawa, A. Taruya, I. Kayo, K. Osato, Y. Kobayashi and M. Shirasaki, Astrophys. J.895 (2020) no.2, 113 [arXiv:1911.07886 [astro-ph.CO]]
2020 arXiv
-
[42]
Hahn, Comput
T. Hahn, Comput. Phys. Commun.168 (2005), 78-95 [arXiv:hep-ph/0404043 [hep-ph]]
2005 arXiv
-
[43]
X. Fang, E. Krause, T. Eifler and N. MacCrann, JCAP05 (2020), 010 [arXiv:1911.11947 [astro- ph.CO]]
2020 arXiv
-
[44]
X. Fang, T. Eifler and E. Krause, Mon. Not. Roy. Astron. Soc. 497 (2020) no.3, 2699-2714 [arXiv:2004.04833 [astro-ph.CO]]
2020 arXiv
-
[45]
Takushima, A
Y. Takushima, A. Terukina and K. Yamamoto, Phys. Rev. D 89 (2014) no.10, 104007 doi:10.1103/PhysRevD.89.104007 [arXiv:1311.0281 [astro-ph.CO]]
2014 arXiv
-
[46]
Takushima, A
Y. Takushima, A. Terukina and K. Yamamoto, Phys. Rev. D 92 (2015) no.10, 104033 doi:10.1103/PhysRevD.92.104033 [arXiv:1502.03935 [gr-qc]]
2015 arXiv
-
[47]
Calderon, D
R. Calderon, D. Felbacq, R. Gannouji, D. Polarski and A. A. Starobinsky, Phys. Rev. D101 (2020) no.10, 103501 doi:10.1103/PhysRevD.101.103501 [arXiv:1912.06958 [astro-ph.CO]]
2020 arXiv
-
[48]
Gannouji and D
R. Gannouji and D. Polarski, JCAP 05 (2008), 018 doi:10.1088/1475-7516/2008/05/018 [arXiv:0802.4196 [astro-ph]]. 30
2008 arXiv
Reviewed August 15, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.