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arxiv: 2511.19353 · v2 · submitted 2025-11-24 · 🌀 gr-qc · astro-ph.CO

Inflation in theories with broken diffeomorphisms

Pith reviewed 2026-05-17 06:04 UTC · model grok-4.3

classification 🌀 gr-qc astro-ph.CO
keywords inflationdiffeomorphism breakingtransverse diffeomorphismsslow-roll parametersprimordial power spectrumscalar spectral indexquadratic potentialpost-inflationary dynamics
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The pith

Inflaton models invariant only under transverse diffeomorphisms permit slow-roll but produce novel post-inflationary regimes.

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

The paper investigates the consequences of restricting the inflaton sector to invariance under transverse diffeomorphisms while keeping the rest of the theory fully diffeomorphism invariant. It derives the adjusted slow-roll parameters, number of e-folds, and curvature perturbation power spectrum, then compares the resulting scalar spectral index against Planck and ACT data. For the quadratic potential, asymptotic and numerical work shows that the post-inflationary evolution departs sharply from the standard diffeomorphism-invariant case and enters new dynamical regimes. A reader would care because these changes arise from a minimal relaxation of symmetry yet still support inflation, offering a concrete way that small symmetry violations could alter early-universe history without destabilizing the slow-roll phase.

Core claim

By taking the inflaton sector to be invariant only under the transverse subgroup of diffeomorphisms, the authors demonstrate that a consistent slow-roll phase remains possible. They obtain explicit expressions for the slow-roll parameters, the number of e-folds, and the primordial power spectrum of curvature perturbations. The scalar spectral index acquires modifications that are tested against CMB observations. Detailed analysis of the quadratic potential reveals that the post-inflationary behavior differs drastically from the fully diffeomorphism-invariant case and exhibits novel dynamical regimes.

What carries the argument

The inflaton sector restricted to invariance under transverse diffeomorphisms, with the remainder of the theory kept fully diffeomorphism invariant.

If this is right

  • Slow-roll parameters and the number of e-folds receive explicit corrections due to the reduced symmetry.
  • The primordial power spectrum of curvature perturbations is modified, producing a distinct scalar spectral index testable against Planck and ACT data.
  • For the quadratic potential, both asymptotic and numerical analyses uncover new post-inflationary dynamical regimes absent in the standard case.
  • These modifications remain compatible with a viable slow-roll phase while altering the subsequent cosmological evolution.

Where Pith is reading between the lines

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

  • Such symmetry restrictions could be combined with other potentials to generate testable shifts in the tensor-to-scalar ratio.
  • The novel post-inflationary regimes might change the duration or efficiency of reheating in ways that affect the radiation-dominated era.
  • This construction supplies a controlled setting for exploring partial symmetry breaking in other cosmological epochs, such as dark energy.
  • If realized, the framework would imply that cosmological observables can probe the precise subgroup of diffeomorphisms preserved by the inflaton.

Load-bearing premise

The inflaton sector is invariant only under transverse diffeomorphisms while the rest of the theory remains fully diffeomorphism invariant, allowing a consistent slow-roll phase without introducing new instabilities.

What would settle it

A future high-precision measurement of the scalar spectral index or post-inflationary observables that exactly matches the standard diffeomorphism-invariant predictions while deviating from the transverse-invariant expressions derived here would falsify the claim of novel regimes.

Figures

Figures reproduced from arXiv: 2511.19353 by Antonio L. Maroto, Miguel Orbaneja-P\'erez, Prado Mart\'in-Moruno.

Figure 1
Figure 1. Figure 1: FIGURE 1 [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIGURE 2 [PITH_FULL_IMAGE:figures/full_fig_p011_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIGURE 3 [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIGURE 4 [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIGURE 5 [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIGURE 6 [PITH_FULL_IMAGE:figures/full_fig_p018_6.png] view at source ↗
read the original abstract

We analyze the impact of breaking diffeomorphism invariance in the inflaton sector. In particular, we consider inflaton models which are invariant under the subgroup of transverse diffeomorphisms and address the possibility of implementing a slow-roll phase. We obtain the corresponding expressions for relevant quantities such as the slow-roll parameters and the number of $e$-folds, and derive the primordial power-spectrum of curvature perturbations. The scalar spectral index features modifications which are confronted with CMB data from Planck and ACT. We study in detail the quadratic potential model, combining asymptotic and numerical analysis. We show that the post-inflationary behavior can be drastically different from the diffeomorphism-invariant case, exhibiting novel dynamical regimes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper analyzes inflation models in which diffeomorphism invariance is broken in the inflaton sector while preserving invariance under transverse diffeomorphisms. It derives modified expressions for the slow-roll parameters, the number of e-folds, and the primordial power spectrum of curvature perturbations. The scalar spectral index is modified and compared to Planck and ACT CMB data. A detailed study of the quadratic potential combines asymptotic and numerical methods, revealing post-inflationary dynamics that differ markedly from the standard diffeomorphism-invariant case and exhibit novel regimes.

Significance. If the perturbation sector remains stable, the results would be significant for exploring symmetry-breaking effects in inflationary cosmology, offering potential new dynamical regimes and observable modifications to the spectral index that can be tested against data. The explicit derivations for slow-roll quantities and the numerical treatment of the quadratic potential provide concrete, falsifiable predictions. However, the lack of explicit checks for the quadratic action undermines the ability to assess whether these claims hold.

major comments (2)
  1. [section deriving the primordial power-spectrum of curvature perturbations] The derivation of the primordial power spectrum (mentioned in the abstract and the section on curvature perturbations) assumes a healthy quadratic action for scalar perturbations, yet no explicit second-order action or dispersion relation is provided. Given that only transverse diffeomorphisms are preserved, the inflaton stress tensor is not covariantly conserved; this must be balanced by the metric sector and can alter the kinetic matrix or sound speed squared. Without this calculation it is impossible to confirm the absence of ghosts or gradient instabilities during slow-roll or in the claimed novel post-inflationary regimes.
  2. [quadratic potential model analysis] The claim of novel post-inflationary dynamical regimes (abstract and quadratic-potential analysis) is load-bearing for the paper's strongest result, but the manuscript does not demonstrate that the modified background evolution preserves a positive-definite kinetic term or positive sound speed squared in these regimes. The Einstein equations imply a non-zero divergence that must be compensated, which typically modifies the perturbation kinetic matrix; an explicit check is required before the regimes can be considered physically viable.
minor comments (2)
  1. [introduction and model setup] Clarify the precise definition of the transverse-diffeomorphism subgroup and how the action is constructed to preserve it while breaking full diffeomorphism invariance; this would aid reproducibility.
  2. [comparison with CMB data] Include error budgets or explicit comparison tables when confronting the modified spectral index with Planck and ACT data.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the constructive comments, which help clarify important aspects of the perturbation sector. We address each major comment below and will incorporate the requested explicit calculations into the revised version.

read point-by-point responses
  1. Referee: [section deriving the primordial power-spectrum of curvature perturbations] The derivation of the primordial power spectrum (mentioned in the abstract and the section on curvature perturbations) assumes a healthy quadratic action for scalar perturbations, yet no explicit second-order action or dispersion relation is provided. Given that only transverse diffeomorphisms are preserved, the inflaton stress tensor is not covariantly conserved; this must be balanced by the metric sector and can alter the kinetic matrix or sound speed squared. Without this calculation it is impossible to confirm the absence of ghosts or gradient instabilities during slow-roll or in the claimed novel post-inflationary regimes.

    Authors: We agree that an explicit derivation of the second-order action for scalar perturbations is required to rigorously confirm the absence of ghosts and gradient instabilities, particularly given the non-conservation of the inflaton stress tensor under broken diffeomorphisms. While our power-spectrum derivation adapts the standard Mukhanov-Sasaki variable to the modified background equations, we did not present the quadratic action or dispersion relation. In the revised manuscript we will derive the quadratic action for curvature perturbations, obtain the corresponding dispersion relation, and explicitly verify that the kinetic term remains positive definite with positive sound speed squared throughout slow-roll and the post-inflationary regimes discussed. revision: yes

  2. Referee: [quadratic potential model analysis] The claim of novel post-inflationary dynamical regimes (abstract and quadratic-potential analysis) is load-bearing for the paper's strongest result, but the manuscript does not demonstrate that the modified background evolution preserves a positive-definite kinetic term or positive sound speed squared in these regimes. The Einstein equations imply a non-zero divergence that must be compensated, which typically modifies the perturbation kinetic matrix; an explicit check is required before the regimes can be considered physically viable.

    Authors: We acknowledge that the viability of the novel post-inflationary regimes rests on the stability of perturbations and that the manuscript currently focuses on background dynamics without an explicit check of the perturbation kinetic matrix in those regimes. The non-zero divergence of the inflaton stress tensor is indeed compensated by the metric sector, which can affect the sound speed. In the revision we will extend the analysis of the quadratic potential to include the second-order perturbation action evaluated along the numerically obtained background trajectories, demonstrating that the kinetic term stays positive definite and the sound speed squared remains positive in the claimed regimes (or identifying the parameter ranges where this holds). revision: yes

Circularity Check

0 steps flagged

Derivations begin from modified action and yield independent expressions for slow-roll parameters and spectra

full rationale

The paper starts from an action invariant only under transverse diffeomorphisms in the inflaton sector and derives the background equations, slow-roll parameters, e-fold number, and curvature power spectrum directly from that action. These quantities are then compared to Planck/ACT data rather than fitted to them and re-labeled as predictions. No self-citation is invoked to justify uniqueness or to close the derivation loop, and the central results (modified spectral index, novel post-inflationary regimes) remain independent of the observational confrontation step. The provided abstract and skeptic analysis give no evidence that any load-bearing equation reduces by construction to a prior fit or self-referential definition.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the assumption that a consistent slow-roll phase exists once full diffeomorphism invariance is reduced to transverse diffeomorphisms; no explicit free parameters or new entities are mentioned in the abstract, but the symmetry reduction itself functions as an ad-hoc domain assumption.

axioms (1)
  • domain assumption The inflaton sector is invariant under transverse diffeomorphisms while the gravitational sector remains fully diffeomorphism invariant.
    Stated in the abstract as the starting point for the models considered.

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Reference graph

Works this paper leans on

45 extracted references · 45 canonical work pages · 9 internal anchors

  1. [1]

    π 2 p −cskτ H(1) ν (−cskτ).(108) with ¯Vthe finite spatial volume. In the super-Hubble regime|c skτ| ≪1, the solution reads vk(τ) = 2ν− 3 2 p 2csk ¯V Γ(ν) Γ 3 2 (−cskτ) 1 2 −ν .(109) The primordial power-spectrum for the curvature per- turbationζcan be now obtained. For this purpose, we can combine the variables (85) and (88), so that v z =H δϕ ϕ′ 0 + Φ =...

  2. [2]

    In addition, the quadratic potential predictions are slightly improved for certain values ofα >1/2

    However, we still find that potentials withp <2 are fa- vored with respect to potentials with larger exponents. In addition, the quadratic potential predictions are slightly improved for certain values ofα >1/2. We stress that the regionα→0, corresponding to equations (148) and (149) for allp, overlaps with the quadratic potential line forα≤1/2. As a brie...

  3. [3]

    brick-wall

    Potential domination:c 2 =−1 Firstly, we address the relevant case for inflation. Star- ting now withα >1/2 in figure 5, the top left panel, we can compare it to the Diff oscillator case, the bot- tom right panel. In contrast to a perfect circle, the TDiff damping makes the field tend to the origin. Thus, if the field starts with{ϕ, ˙ϕ}>0, it will follow ...

  4. [4]

    Continuing now with the kinetic domi- nation, we shall restrict to values 1> α >1/2, so that there is a maximumY max (see table I)

    Kinetic domination:c 2 = +1 For the sake of completeness, we shall briefly review the rest of scenarios. Continuing now with the kinetic domi- nation, we shall restrict to values 1> α >1/2, so that there is a maximumY max (see table I). The behavior of the caseα >1 shall be studied in further research works. The analyzed case is pictured in figure 5, the ...

  5. [5]

    As before, if the field starts away from the origin, the initial conditions determine whetherϕslowly tends to the origin or moves further away

    Intermediate case:c 2 = 0 Lastly, the phase portrait in figure 5, the top right panel, is valid forαabove and below 1/2, sinceϕ= ˙ϕ. As before, if the field starts away from the origin, the initial conditions determine whetherϕslowly tends to the origin or moves further away. In this case, we also have the time expression from equation (168), so the field...

  6. [6]

    Starting with the top left panel, the Hubble parameterHand|H Y |feature a rather distinct behavior over time

    Numerical analysis The ODEs system (155a)–(155c) has been numerically solved for different values ofα >1/2 and for the Diff case (α= 1/2), with initial conditions a(0) = 1, Y(0) = 1, ϕ(0) = 1, ˙ϕ(0) = 0.(173) The results are shown in figure 6. Starting with the top left panel, the Hubble parameterHand|H Y |feature a rather distinct behavior over time. On ...

  7. [7]

    In both cases, the most relevant feature was that, in two spatial dimensions, density shows spatially periodic patterns of hexagonal symmetry ifpis large enough

    Asymptotic behavior: post-inflationary phase The assumption of reaching a strong TDiff regime, des- cribed in section VI A, is now confirmed. Indeed, du- ring inflation the standard cosmological expansionH dominates but, in light of the figure 6,Hbecomes ne- gligible in the long term compared toH Y . Thus, the post-inflationary phase is then dominated byH...

  8. [8]

    E. W. Kolb and M. S. Turner, The Early Universe, Vol. 69 (Taylor and Francis, 2019)

  9. [9]

    A. H. Guth, The Inflationary Universe: A Possible Solu- tion to the Horizon and Flatness Problems, Phys. Rev. D23, 347 (1981)

  10. [10]

    A. D. Linde, Chaotic Inflation, Phys. Lett. B129, 177 (1983)

  11. [11]

    Inflation and the Theory of Cosmological Perturbations

    A. Riotto, Inflation and the theory of cosmological perturbations, ICTP Lect. Notes Ser.14, 317 (2003), arXiv:hep-ph/0210162

  12. [12]

    F. L. Bezrukov and M. Shaposhnikov, The Standard Mo- del Higgs boson as the inflaton, Phys. Lett. B659, 703 (2008), arXiv:0710.3755 [hep-th]

  13. [13]

    A. A. Starobinsky, A New Type of Isotropic Cosmological Models Without Singularity, Phys. Lett. B91, 99 (1980)

  14. [14]

    Planck 2018 results. X. Constraints on inflation

    Y. Akrami et al. (Planck), Planck 2018 results. X. Cons- traints on inflation, Astron. Astrophys.641, A10 (2020), arXiv:1807.06211 [astro-ph.CO]

  15. [15]

    The Atacama Cosmology Telescope: DR6 Constraints on Extended Cosmological Models

    E. Calabrese et al. (ACT), The Atacama Cosmology Te- lescope: DR6 Constraints on Extended Cosmological Mo- dels, (2025), arXiv:2503.14454 [astro-ph.CO]

  16. [16]

    SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field

    E. Camphuis et al. (SPT-3G), SPT-3G D1: CMB tem- perature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field, (2025), arXiv:2506.20707 [astro-ph.CO]

  17. [17]

    Kallosh, A

    R. Kallosh, A. Linde, and D. Roest, Atacama Cosmology Telescope, South Pole Telescope, and Chaotic Inflation, Phys. Rev. Lett.135, 161001 (2025), arXiv:2503.21030 [hep-th]

  18. [18]

    Q. Gao, Y. Gong, Z. Yi, and F. Zhang, Nonminimal coupling in light of ACT data, Phys. Dark Univ.50, 102106 (2025), arXiv:2504.15218 [astro-ph.CO]

  19. [19]

    Einstein, Spielen Gravitationsfelder im Aufbau der materiellen Elementarteilchen eine wesentliche Rolle?, Sitzungsber

    A. Einstein, Spielen Gravitationsfelder im Aufbau der materiellen Elementarteilchen eine wesentliche Rolle?, Sitzungsber. Preuss. Akad. Wiss. Berlin (Math. Phys. ) 1919, 349 (1919)

  20. [20]

    W. G. Unruh, Unimodular theory of canonical quantum gravity, Phys. Rev. D40, 1048 (1989)

  21. [21]

    Carballo-Rubio, L

    R. Carballo-Rubio, L. J. Garay, and G. Garc´ ıa-Moreno, Unimodular gravity vs general relativity: a status report, Class. Quant. Grav.39, 243001 (2022), arXiv:2207.08499 [gr-qc]

  22. [22]

    Ellis, H

    G. Ellis, H. van Elst, J. Murugan, and J.-P. Uzan, On the trace-free einstein equations as a viable alternative to general relativity, Classical and Quantum Gravity28, 225007 (2011)

  23. [23]

    Transverse Fierz-Pauli symmetry

    E. Alvarez, D. Blas, J. Garriga, and E. Verdaguer, Trans- verse Fierz-Pauli symmetry, Nucl. Phys. B756, 148 (2006), arXiv:hep-th/0606019

  24. [24]

    Y. F. Pirogov, Unimodular bimode gravity and the cohe- rent scalar-graviton field as galaxy dark matter, Eur. 21 Phys. J. C72, 2017 (2012), arXiv:1111.1437 [gr-qc]

  25. [25]

    A. G. Bello-Morales and A. L. Maroto, Cosmology in gravity models with broken diffeomorphisms, Phys. Rev. D109, 043506 (2024), arXiv:2308.00635 [gr-qc]

  26. [26]

    A. G. Bello-Morales, J. Beltr´ an Jim´ enez, A. Jim´ enez Cano, A. L. Maroto, and T. S. Koivisto, A class of ghost-free theories in symmetric teleparallel geometry, (2024), arXiv:2406.19355 [gr-qc]

  27. [27]

    A. L. Maroto, TDiff invariant field theories for cosmology, JCAP04, 037, arXiv:2301.05713 [gr-qc]

  28. [28]

    Jaramillo-Garrido, A

    D. Jaramillo-Garrido, A. L. Maroto, and P. Mart´ ın- Moruno, TDiff in the dark: gravity with a scalar field invariant under transverse diffeomorphisms, JHEP03, 084, arXiv:2307.14861 [gr-qc]

  29. [29]

    de Cruz P´ erez and A

    J. de Cruz P´ erez and A. L. Maroto, ΛCDM from bro- ken diffeomorphisms, Phys. Rev. D111, 123555 (2025), arXiv:2504.02541 [gr-qc]

  30. [30]

    Alonso-L´ opez, J

    D. Alonso-L´ opez, J. de Cruz P´ erez, and A. L. Maroto, Unified transverse diffeomorphism invariant field theory for the dark sector, Phys. Rev. D109, 023537 (2024), arXiv:2311.16836 [astro-ph.CO]

  31. [31]

    Jaramillo-Garrido, A

    D. Jaramillo-Garrido, A. L. Maroto, and P. Mart´ ın- Moruno, Symmetry restoration in transverse diffeo- morphism invariant scalar field theories, Phys. Rev. D 110, 044009 (2024), arXiv:2402.17422 [gr-qc]

  32. [32]

    Tessainer, A

    D. Tessainer, A. L. Maroto, and P. Mart´ ın-Moruno, Multi-field TDiff theories for cosmology, Phys. Dark Univ.47, 101769 (2025), arXiv:2409.11991 [gr-qc]

  33. [33]

    A. L. Maroto, P. Mart´ ın-Moruno, and D. Tessainer, Multi-field TDiff theories: the mixed regime case, (2025), arXiv:2507.16616 [gr-qc]

  34. [34]

    A. L. Maroto and A. D. Miravet, Transverse- diffeomorphism invariant gauge fields in cosmology, Phys. Rev. D109, 103504 (2024), arXiv:2402.18368 [gr-qc]

  35. [35]

    A. L. Maroto and A. D. Miravet, Cosmic magnetic fields invariant under transverse diffeomorphisms, Phys. Rev. D110, 063530 (2024), arXiv:2407.04647 [astro-ph.CO]

  36. [36]

    Beltr´ an Jim´ enez, T

    J. Beltr´ an Jim´ enez, T. Borislavov Vasilev, D. Jaramillo- Garrido, A. L. Maroto, and P. Mart´ ın-Moruno, K- nonizing, (2025), arXiv:2509.07715 [gr-qc]

  37. [37]

    Baumann, Cosmology (Cambridge University Press, 2022)

    D. Baumann, Cosmology (Cambridge University Press, 2022)

  38. [38]

    A. D. Di Marco, E. Orazi, and G. Pradisi, Introduction to the Number of e-Folds in Slow-Roll Inflation, Universe 10, 284 (2024), arXiv:2408.01854 [astro-ph.CO]

  39. [39]

    A. R. Liddle, P. Parsons, and J. D. Barrow, Formalizing the slow roll approximation in inflation, Phys. Rev. D50, 7222 (1994), arXiv:astro-ph/9408015

  40. [40]

    Mukhanov, Physical Foundations of Cosmology (Cambridge University Press, Oxford, 2005)

    V. Mukhanov, Physical Foundations of Cosmology (Cambridge University Press, Oxford, 2005)

  41. [41]

    Martin, C

    J. Martin, C. Ringeval, and V. Vennin, Encyclopædia Inflationaris: Opiparous Edition, Phys. Dark Univ.5-6, 75 (2014), arXiv:1303.3787 [astro-ph.CO]

  42. [42]

    DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

    M. Abdul Karim et al. (DESI), DESI DR2 results. II. Measurements of baryon acoustic oscillations and cos- mological constraints, Phys. Rev. D112, 083515 (2025), arXiv:2503.14738 [astro-ph.CO]

  43. [43]

    Demirel, Planck-constraints-r-vs-ns- plot,https://github.com/kdemirel/ Planck-constraints-r-vs-ns-plot(2025)

    K. Demirel, Planck-constraints-r-vs-ns- plot,https://github.com/kdemirel/ Planck-constraints-r-vs-ns-plot(2025)

  44. [44]

    J. M. Cline, Comment on ”Dark Energy from Time Crys- tals”, (2025), arXiv:2502.19448 [hep-ph]

  45. [45]

    M. S. Turner, Coherent Scalar Field Oscillations in an Expanding Universe, Phys. Rev. D28, 1243 (1983)