REVIEW 4 major objections 5 minor 40 references
The Regular Ricci-Inverse Cosmology with Multiple Anticurvature Scalars
T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read A new Ricci-inverse gravity with two anticurvature scalars claims to be singularity-free and, in the linear model, to reduce to general relativity with a shifted Hubble parameter.
desk verdict A genuinely new algebraic construction and a clean GR-like background result, but the singularity-free claim is not supported at the perturbation level, especially at de Sitter. 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 load-bearing object is the ratio-symmetric scalar $L_n$ defined in equation (9), built from the Ricci powers $R_n$, $R_{-n}$, and $R_{-2n}$ so that its FLRW value reduces to $a^{-2n}(X^n+Y^n)$ instead of a rational function with poles. In FLRW, $X = a^2 R^0{}_0$ and $Y = a^2 R^i{}_i/(D-1)$; the three apparent poles $X=0$, $Y=0$, and $X=Y$ are all removable. The unified variational formula (4) with intermediate tensors $P^\mu{}_\nu$ and $Q^\mu{}_\nu$ keeps the equations compact, and the background system uses the modified Friedmann equation together with energy conservation, mirroring the standard $f(R_1)$ treatment.
What would settle it
Solve the full fourth-order tensor perturbation equations numerically across a transition into the exponential-expansion phase without assuming $\phi'^{-2}\to 0$. If no finite, regular solution exists unless the two conflicting equations (30) and (31) are both satisfied with $v_g=1$, the claim that the theory is safe for fluctuations in that phase is refuted.
Extended reading notes
Core claim
The paper's central claim is that the combination $L_n := (R_n - D R_{-n}/R_{-2n})/(D - R_{-n}^2/R_{-2n})$ forms a scalar with the same dimension as $R_n$ whose FLRW value is simply $a^{-2n}(X^n + Y^n)$, with no rational-function poles. For $n=1$ and with $L_1$ added linearly to the Ricci-scalar action, the background equations of motion reduce to $\rho = 3(1+\alpha) H^2$ and $p = -(1+\alpha)(2\xi+1) H^2$, i.e. general relativity with the replacement $H_0 \to \sqrt{1+\alpha}\,H_0$. The three singular points $X=0$, $Y=0$, and $X=Y$ are all removable, so the cosmic deceleration/acceleration boundary and the exponential-expansion (de Sitter) limit are not obstructions. On the perturbation side, the claim is that although the action is fourth-order and carries an extra scalar degree of freedom, in the limits $J\to 0$ and the exponential-expansion phase the extra mode decouples and scalar and tensor spectra reduce to the GR form with a modified group velocity $v_g$; the paper notes an unresolved conflict in the tensor-mode equations in that phase.
Load-bearing premise
The main bet is that the extra wobbling mode that appears in the perturbed equations quietly disappears in the $J\to 0$ limit and in the exponential-expansion phase, so the simplified 'pseudo GR' behavior is valid; the paper itself finds conflicting equations for gravitational waves in the exponential-expansion limit, so this bet is not settled.
Editorial extensions
If this is right
- The background evolution of the linear $L_1$ model is identical to general relativity with $H_0 \to \sqrt{1+\alpha}\,H_0$, so standard cosmological solutions carry over with rescaled expansion rates.
- The apparent singularities at $X=0$, $Y=0$, and $X=Y$ are removable, so the deceleration-acceleration transition and the exponential-expansion phase are not barriers for the background as long as the equation-of-state parameter lies between $-1$ and $1$.
- At the perturbation level the theory is fourth-order with an extra scalar degree of freedom; in the $J\to 0$ limit, corresponding to $w=-1/3$ or $w=-1/11$ (redshifts $z\approx 0.67$ or $1.8$), the extra mode decouples and scalar perturbations follow the GR form with a modified group velocity.
- Tensor modes reduce to the same pseudo-GR form in the $J\to 0$ limit, but in the exponential-expansion limit the two natural tensor equations conflict, leaving the fate of tensor fluctuations open.
Reading between the lines
- Beyond the paper, the $J=0$ redshifts $z\approx 0.67$ and $1.8$ are structure-formation epochs, so the model predicts a change in the propagation speed of curvature perturbations during those epochs; large-scale-structure surveys could in principle look for this signature.
- The same algebraic identity that builds $L_1$ can be applied at higher $n$ or to other combinations of anticurvature scalars, making the construction a general template for singularity-free Ricci-inverse theories rather than a single tuned model.
- If the tensor-mode conflict in the exponential-expansion phase persists under a full analysis, the theory would suppress primordial tensor perturbations, so an observed primordial $B$-mode signal at CMB scales would rule out this simple version.
- Reading $\rho_{\rm eff} = -3\alpha H^2$ as effective cold dark matter implies that background expansion data alone could constrain $\alpha$, because the model changes the effective matter density without altering the equation of state.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper constructs a class of Ricci-inverse gravity theories whose Lagrangian is a function of traces of powers of the Ricci tensor, including negative powers (anticurvature). A combined quantity L_n is introduced via an algebraic identity involving R_n, R_{-n}, and R_{-2n}. In FLRW spacetime, L_n simplifies to a closed algebraic form, and for the linear model f=R1+αL1 the background Friedmann equations are shown to coincide with GR up to the rescaling H→√(1+α)H. The paper claims that this construction is free from the singularity problem for both the background and linear perturbations for equations of state with w between -1 and 1. Perturbation actions for scalar and tensor modes are presented, but the scalar constraint is not solved and the de Sitter limit is left with an acknowledged conflict.
Significance. The construction of L_n is elegant, and the background dynamics of the linear model being GR-like is a clean and non-trivial result. The algebraic identity underlying L_n is a useful observation for the Ricci-inverse literature. However, the central safety claim is only demonstrated at the background level. The perturbation analysis, which is essential for the claim, is incomplete: the second-order actions are not derived, the scalar constraint is left unsolved, and the de Sitter tensor sector yields contradictory equations. As presented, the manuscript does not support the conclusion that the theory is free from singularities for fluctuations over the stated range of w. The main value of the paper would be in the background construction if the perturbative issues are either resolved or explicitly excluded from the claims.
major comments (4)
- [Perturbations, Eq. (21)] The second-order expansion of L1 in Eq. (21) has denominators (x-y). At the de Sitter endpoint (w=-1) one has x=y and φ'=0, so the perturbation terms and the 1/φ'^2 prefactors in Eqs. (24) and (28) diverge unless the associated perturbation combinations vanish identically. The paper's ansatz that these combinations vanish is not proven, and for tensor modes it leads to the contradictory conditions in Eqs. (30)-(31). Therefore the claim that fluctuations are safe for -1≤w≤1 is not supported; the endpoint must either be analyzed or explicitly excluded.
- [Perturbations, Eqs. (30)-(31)] In the de Sitter limit, requiring the singular 1/φ'^2 term in the tensor action (28) to vanish gives Eq. (30), while the remaining GR-like part of the action demands Eq. (31). For α≠0, v_g^2=(1+α/3)/(1+α)≠1, so no nonzero tensor perturbation can satisfy both equations. The paper acknowledges this conflict but does not resolve it; nevertheless the conclusion includes w=-1 in the claimed safe range. This inconsistency is load-bearing for the singularity-free claim and must be resolved before the claim can stand.
- [Perturbations, Eq. (25)] The scalar constraint equation (25) is not solved; the authors explicitly defer this, noting that solving it would introduce k in the denominator. Since the regularity of scalar perturbations at x=y, φ'=0, and J=0 depends on the behavior of the lapse perturbation A after solving (25), the unsolved constraint leaves the safety claim for scalar modes unverified. The pseudo-GR argument for J→0 does not cover the de Sitter point or generic intermediate values of w.
- [Perturbations, Eqs. (24) and (28)] The second-order actions (24) and (28) are presented as the outcome of 'tedious integration by parts' but no derivation is provided. These actions are the sole basis for the perturbation claims, and they contain the divergent 1/φ'^2 terms. The authors should supply the perturbed Ricci tensor components, the gauge-fixing conditions, and the reduction steps, or at least a detailed outline, so that the presence and form of these singular terms can be independently checked.
minor comments (5)
- [Specific Model, Eq. (8)] The algebraic identity (8) and the definition of L_n in (9) are typeset in a garbled way that makes them difficult to read; they should be rewritten with explicit fractions and parentheses.
- [Specific Model, Eq. (11)] The variables ξ_N and ξ_NN in Eqs. (11)-(12) are not explicitly defined; please define them as derivatives with respect to ln a.
- [Perturbations, Eq. (24)] The definition of J from Eq. (21) should be restated near Eq. (24) for readability.
- [Conclusion] The phrase 'as long as the EoS parameter w ranges between -1 and 1' should be clarified to state whether the endpoints are included, given that the de Sitter endpoint is exactly where the perturbation analysis encounters the unresolved conflict described in the text.
- [Introduction] There is a typo: 'Talor/Laurent expasion' should be 'Taylor/Laurent expansion'. Additionally, the no-go theorem for L=R1+αR_{-1}^l mentioned in the introduction is stated without a citation; a reference should be provided.
Circularity Check
No significant circularity: the singularity-free background follows by explicit algebraic construction and direct variation, while the open de Sitter perturbation problem is an unsupported conclusion, not a circular one.
full rationale
The paper's derivation chain is self-contained. Starting from the action (2) and the unified EoM (4), the FLRW reduction gives Rn = a^{-2n}[X^n + (D-1)Y^n] in (7), and the algebraic identity (8) then defines Ln in (9) as a combination whose denominators cancel, leaving Ln = a^{-2n}[X^n + Y^n]. The statement that X=0, Y=0 and X=Y are removable singularities is therefore a direct consequence of the definition; the paper openly says 'we design' this class and calls the singularities removable. This is model-building by ansatz rather than a fitted parameter renamed as a prediction, so it does not meet the threshold for circularity. The background EoM (11)-(13) and their linear-L1 reduction (15) follow by substituting f = R1 + alpha L1 into the variational principle; the GR-like form with shifted Hubble parameter (16) is a computed consequence of FLL=0, not an input. No data are fitted, no external result is invoked through self-citation, and no uniqueness theorem is imported. The genuine weakness is in the perturbation section: expressions (21), (24) and (28) contain denominators x-y and phi'^2 that diverge at the de Sitter limit (w=-1, x=y, phi'=0), and the paper's own ansatz produces the contradictory tensor equations (30) and (31), with the text explicitly stating 'there cannot be tensor mode fluctuations for de Sitter limit in our theory' and deferring detailed analysis. The conclusion's claim that the theory is safe 'both for background and fluctuations' for all w in [-1,1] is therefore broader than the demonstrated results. That is an evidentiary gap and an overbroad claim, but it is not circular: the perturbation results were derived, not assumed, and the contradiction is disclosed rather than hidden. Accordingly, the circularity score is 0.
Assumptions & free parameters
free parameters (2)
- α
- F(L1)
assumptions (5)
- standard math The algebraic identity in Eq. (8) holds for the FLRW Ricci eigenvalues X and Y.
- domain assumption The unified field equations (4) follow from varying the action (2) with respect to the metric.
- domain assumption The FLRW metric with maximally symmetric Ricci tensor is the correct background for cosmological analysis.
- domain assumption The scalar perturbation action (24) is obtained after 'tedious integration by parts' and is correct.
- ad hoc to paper The ansatz that in the de Sitter limit the perturbation should not blow up, requiring the square-bracketed parts in (24) to vanish.
invented entities (1)
-
L_n
Cite this review
Pith. "Pith review of The Regular Ricci-Inverse Cosmology with Multiple Anticurvature Scalars." pith.science (2026). https://pith.science/paper/DGWN47QU
@misc{pith2026250116628,
author = {Pith},
title = {Pith review of: The Regular Ricci-Inverse Cosmology with Multiple Anticurvature Scalars},
year = {2026},
howpublished = {\url{https://pith.science/paper/DGWN47QU}},
note = {Machine review of arXiv:2501.16628}
}
read the original abstract
We investigate the modified gravity in which the Lagrangian of gravity is a function of the trace of the n-th matrix power of Ricci tensor in a Friedmann-Lemaitre-Robertson-Walker(FLRW) spacetime. When n is negative, the inverse of Ricci tensor, also called the anticurvature tensor, will be introduced. We design a new class of Ricci-inverse theory containing two anticurvature scalars and resulting to be free from the singularity problem.
Reference graph
Works this paper leans on
-
[1]
The Mathematical Theory of Relativ- ity,
A.S. Eddington, “The Mathematical Theory of Relativ- ity,” Cambridge Univ. Press, 1923
work page 1923
-
[2]
H. Weyl, Annalen Phys. 59, 101-133 (1919) doi:10.1002/andp.19193641002
-
[3]
C. Brans and R. H. Dicke, Phys. Rev. 124, 925-935 (1961) doi:10.1103/PhysRev.124.925
-
[4]
R. H. Dicke, Phys. Rev. 125, 2163-2167 (1962) doi:10.1103/PhysRev.125.2163
-
[5]
F. W. Hehl, P. Von Der Heyde, G. D. Kerlick and J. M. Nester, Rev. Mod. Phys. 48, 393-416 (1976) doi:10.1103/RevModPhys.48.393
-
[6]
A. G. Riess et al. [Supernova Search Team], Astron. J. 116, 1009-1038 (1998) doi:10.1086/300499 [arXiv:astro- ph/9805201 [astro-ph]]
arXiv 1998
-
[7]
J. L. Bernal, L. Verde and A. G. Riess, JCAP 10, 019 (2016) doi:10.1088/1475-7516/2016/10/019 [arXiv:1607.05617 [astro-ph.CO]]
arXiv 2016
-
[8]
R. Abuter et al. [GRA VITY], Astron. Astrophys. 657, L12 (2022) doi:10.1051/0004-6361/202142465 [arXiv:2112.07478 [astro-ph.GA]]
arXiv 2022
Show all 40 references
- [9]
-
[10]
Sakstein and M
J. Sakstein and M. Trodden, Phys. Rev. Lett. 124, no.16, 161301 (2020) doi:10.1103/PhysRevLett.124.161301 [arXiv:1911.11760 [astro-ph.CO]]
2020 arXiv
-
[11]
S. M. Carroll, A. De Felice, V. Duvvuri, D. A. Eas- son, M. Trodden and M. S. Turner, Phys. Rev. D 71, 063513 (2005) doi:10.1103/PhysRevD.71.063513 [arXiv:astro-ph/0410031 [astro-ph]]
2005 arXiv
-
[12]
Capozziello and M
S. Capozziello and M. De Laurentis, Phys. Rept. 509, 167-321 (2011) doi:10.1016/j.physrep.2011.09.003 [arXiv:1108.6266 [gr-qc]]
2011 arXiv
-
[13]
Glavan and C
D. Glavan and C. Lin, Phys. Rev. Lett. 124, no.8, 081301 (2020) doi:10.1103/PhysRevLett.124.081301 [arXiv:1905.03601 [gr-qc]]
2020 arXiv
-
[14]
Mustafa, X
G. Mustafa, X. Tie-Cheng, M. F. Shamir and M. Javed, Eur. Phys. J. Plus 136, no.2, 166 (2021) doi:10.1140/epjp/s13360-021-01083-x
2021 doi
-
[15]
Lee and G
S. Lee and G. Tumurtushaa, JCAP 06, 029 (2020) doi:10.1088/1475-7516/2020/06/029 [arXiv:2001.07021 [astro-ph.CO]]
2020 arXiv
-
[16]
T. P. Sotiriou and V. Faraoni, Rev. Mod. Phys. 82, 451-497 (2010) doi:10.1103/RevModPhys.82.451 [arXiv:0805.1726 [gr-qc]]
2010 arXiv
-
[17]
Clifton, P
T. Clifton, P. G. Ferreira, A. Padilla and C. Skordis, Phys. Rept. 513, 1-189 (2012) 5 doi:10.1016/j.physrep.2012.01.001 [arXiv:1106.2476 [astro-ph.CO]]
2012 arXiv
-
[18]
De Felice and S
A. De Felice and S. Tsujikawa, Living Rev. Rel. 13, 3 (2010) doi:10.12942/lrr-2010-3 [arXiv:1002.4928 [gr-qc ]]
2010 arXiv
-
[19]
Noh and J
H. Noh and J. c. Hwang, Phys. Rev. D 59, 047501 (1999) doi:10.1103/PhysRevD.59.047501 [arXiv:gr-qc/9811013 [gr-qc]]
1999 arXiv
-
[20]
N. Ohta, R. Percacci and A. D. Pereira, Phys. Rev. D 97, no.10, 104039 (2018) doi:10.1103/PhysRevD.97.104039 [arXiv:1804.01608 [hep-th]]
2018 arXiv
-
[21]
A. Z. Kaczmarek and D. Szcze´ sniak, Sci. Rep. 11, no.1, 18363 (2021) doi:10.1038/s41598-021-97907-y [arXiv:2105.05050 [gr-qc]]
2021 arXiv
-
[22]
Deser and R
S. Deser and R. P. Woodard, Phys. Rev. Lett. 99, 111301 (2007) doi:10.1103/PhysRevLett.99.111301 [arXiv:0706.2151 [astro-ph]]
2007 arXiv
-
[23]
Deser and R
S. Deser and R. P. Woodard, JCAP 06, 034 (2019) doi:10.1088/1475-7516/2019/06/034 [arXiv:1902.08075 [gr-qc]]
2019 arXiv
-
[24]
Amendola, L
L. Amendola, L. Giani and G. Laverda, Phys. Lett. B 811, 135923 (2020) doi:10.1016/j.physletb.2020.135923 [arXiv:2006.04209 [astro-ph.CO]]
2020
-
[25]
T. Q. Do, Eur. Phys. J. C 82, no.1, 15 (2022) doi:10.1140/epjc/s10052-021-09974-0 [arXiv:2101.0853 8 [gr-qc]]
2022
-
[26]
Jawad and A
A. Jawad and A. M. Sultan, EPL 138, no.2, 29001 (2022) doi:10.1209/0295-5075/ac6977
2022 doi
-
[27]
J. C. R. de Souza and A. F. Santos, Eur. Phys. J. C 83, no.9, 834 (2023) doi:10.1140/epjc/s10052-023-12020- w [arXiv:2309.05439 [gr-qc]]
2023 arXiv
-
[28]
Malik, A
A. Malik, A. Arif and M. F. Shamir, Int. J. Theor. Phys. 62, no.11, 243 (2023) doi:10.1007/s10773-023-05499-2
2023 doi
-
[29]
Ahmed, J
F. Ahmed, J. C. R. de Souza and A. F. Santos, Annals Phys. 461, 169578 (2024) doi:10.1016/j.aop.2023.169578 [arXiv:2312.16123 [gr-qc]]
2024
-
[30]
Ahmed and A
F. Ahmed and A. Guvendi, Chin. J. Phys. 89, 69-85 (2024) doi:10.1016/j.cjph.2024.03.009
2024 doi
-
[31]
Ahmed, J
F. Ahmed, J. C. R. de Souza and A. F. San- tos, Nucl. Phys. B 1004, 116573 (2024) doi:10.1016/j.nuclphysb.2024.116573
2024
-
[32]
Ahmed, J
F. Ahmed, J. C. R. de Souza and A. F. Santos, JCAP 10, 015 (2024) doi:10.1088/1475-7516/2024/10/015 [arXiv:2407.11513 [gr-qc]]
2024 arXiv
-
[33]
Ahmed and A
F. Ahmed and A. Bouzenada, Eur. Phys. J. C 84, no.12, 1271 (2024) doi:10.1140/epjc/s10052-024-13637-1 [arXiv:2410.11922 [gr-qc]]
2024 arXiv
-
[35]
Malik, A
A. Malik, A. Hussain, M. Ahmad and M. F. Shamir, Chin. J. Phys. 91, 560-574 (2024) doi:10.1016/j.cjph.2024.08.005
2024 doi
- [36]
-
[37]
I. Das, J. P. Johnson and S. Shankaranarayanan, Eur. Phys. J. Plus 137, no.11, 1265 (2022) doi:10.1140/epjp/s13360-022-03472-2 [arXiv:2108.0099 2 [gr-qc]]
2022
-
[38]
J. C. R. de Souza, A. F. Santos and F. Ahmed, Eur. Phys. J. C 84, no.6, 559 (2024) doi:10.1140/epjc/s10052-024- 12934-z
2024 doi
-
[39]
Ahmed, J
F. Ahmed, J. C. R. de Souza and A. F. Santos, Int. J. Mod. Phys. A 39, no.30, 2450120 (2024) doi:10.1142/S0217751X24501203
2024 doi
-
[40]
Nojiri and S
S. Nojiri and S. D. Odintsov, Phys. Rev. D 78, 046006 (2008) doi:10.1103/PhysRevD.78.046006 [arXiv:0804.3519 [hep-th]]
2008 arXiv
-
[41]
S. A. Appleby, R. A. Battye and A. A. Starobin- sky, JCAP 06, 005 (2010) doi:10.1088/1475- 7516/2010/06/005 [arXiv:0909.1737 [astro-ph.CO]]
2010 arXiv
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