REVIEW 3 major objections 4 minor 2 cited by
The paper claims that Regge-pole residues of flat-space scattering amplitudes fix the OPE data of the celestial CFT through an explicit dictionary.
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 →
T0 review · deepseek-v4-flash
2026-08-03 04:54 UTC pith:CPAJ4KIJ
load-bearing objection Well-packaged and honest proposal for a celestial Regge dictionary, but the contour integral at its center is divergent under the stated assumptions and the phase in (5.6) doesn't match the derivation — plausible physics, unfinished mathematics. the 3 major comments →
Celestial Regge theory
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper's central claim is that the Regge limit of flat-space scattering amplitudes has a precise counterpart on the celestial sphere, and that the two are linked by an explicit formula. Using a contour representation of the Mellin transform, the authors derive a dispersion relation for the celestial correlator and then show that, in the limit of small cross-ratio, the celestial Regge contribution takes the same form as the standard CFT Regge limit. Matching the two expansions gives the dictionary sigma±(nu) = i pi e^(2 pi i j(nu)) 2^(2 j(nu) - beta - 5) Delta(nu)^(beta/2 - 1) / sin(pi beta/2) e^(pi i (Delta12 - Delta34)/2) K_{Delta(nu),j(nu)} rho±(nu), where rho±(nu) are the residues of t
What carries the argument
The engine of the argument is the contour-trick Mellin transform. For a function f with no singularities on the integration domain, the paper rewrites the standard Mellin transform as M[f](s) = pi/sin(pi s) times the contour integral of (-r)^(s-1) f(r) around the positive real axis. This turns the energy integral in the celestial correlator into a contour integral in a complex Mandelstam variable, so that poles and discontinuities of the bulk amplitude can be read off directly. The same device converts the Froissart-Gribov partial-wave expansion into a celestial expansion, and after a further deformation of the contour to the principal series mu = 1 + i nu, it produces the Regge-limit integr
Load-bearing premise
The load-bearing premise is the working hypothesis that the Regge residues rho±(mu) are holomorphic and exponentially bounded and that the trajectory is exactly linear with j1 > 0 and 1 - j0 - j1 < 0, so the contour can be deformed from the negative real axis to the principal series; if any part fails, the leading celestial Regge term is not the CFT-like integral and the dictionary collapses.
What would settle it
Evaluate the dictionary in a concrete theory with a known Regge pole: compute the bulk partial-wave residue rho±(nu) from the Froissart-Gribov projection and the celestial OPE residue sigma±(nu) from the conformal partial-wave projection, and check the equality at the pole. A theory whose Regge trajectory is not exactly linear, or whose residues are not exponentially bounded, would make the contour deformation to mu = 1 + i nu invalid, and the leading Regge term would fail, falsifying the dictionary.
If this is right
- Celestial OPE data in the Regge limit are fixed by bulk Regge residues through the explicit dictionary, so the boundary theory is not free in this sector.
- The contour-trick Mellin transform yields a celestial dispersion relation, so celestial correlators can be reconstructed from poles and discontinuities of the bulk amplitude without performing the energy integral explicitly.
- At the level of the Regge pole, the dictionary extends to the partial amplitudes, meaning the complex-spin data of the S-matrix and the conformal data of the celestial CFT become the same object in this limit.
- Because the Regge residues carrying the discontinuity contributions are loop-generated in crossing-symmetric theories, the dictionary implies that celestial OPE data encode genuinely quantum, beyond-tree-level dynamics.
Where Pith is reading between the lines
- An extension the paper leaves implicit is that the same contour trick could be used to define and compute celestial correlators in other kinematic regimes, effectively using contour deformations to implement energy cutoffs.
- If the dictionary survives contact with explicit examples, string-like amplitudes with exactly linear Regge trajectories are a natural laboratory: they avoid the working hypothesis and could yield closed-form celestial OPE coefficients.
- The paper notes that gravity's dominant eikonal limit lies outside its scope; a parallel celestial eikonal dictionary would be the natural next step, and the contour-trick Mellin transform is a natural tool for it.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper proposes a dictionary between the Regge limit of flat-space scattering amplitudes and the Regge limit of the putative celestial CFT. The authors introduce an alternative 'contour trick' representation of the Mellin transform that defines the celestial correlator through a contour integral in a Mandelstam variable, leading to a celestial dispersion relation. Applying this to the Regge amplitude, they obtain a principal-series integral (4.10) whose integrand is governed by the bulk Regge residue ρ±(μ). After reviewing the conformal Regge limit, they compare (4.10) with the CFT expression (5.3) and read off the central relation (5.6), expressing the celestial OPE coefficient σ±(ν) in terms of the bulk Regge residue ρ±(ν). The derivation is explicit and carefully signposted, with the main analyticity assumptions labelled as a working hypothesis.
Significance. If valid, Eq. (5.6) is a concrete and useful entry in the flat-space holography dictionary: it determines boundary conformal data from bulk Regge residues and gives a celestial analogue of conformal Regge theory. The contour-trick representation is elegant and may be of independent use. The paper is honest about its working assumptions and does not overclaim beyond the Regge-pole approximation. However, the central derivation is conditional on a convergence property that is not established (see major comments), so the result is currently a plausible dictionary rather than a proven theorem.
major comments (3)
- [§4.3, Eq. (4.10)] The convergence of the vertical-line integral is not established. With j(μ)=j0+j1μ and μ=1+iν, the factor e^{iπj(ν)}α±(ν) grows like e^{π j1|ν|} for ν→−∞ after the sin/cos denominators are taken into account; the factor ζ^{1−j(ν)} has unit modulus up to a fixed power. Consequently (4.10) converges only if |ρ±(1+iν)| decays at least as e^{−π j1|ν|}. 'Exponentially bounded' does not imply this, and no such decay is derived from S-matrix properties. The same asymptotic controls the horizontal arcs in the deformation from C(R−+iε) to Re μ=1, so the issue also affects (4.7). If (4.10) is not a convergent integral, the comparison with (5.3) and the dictionary (5.6) is not valid. Please prove the additional decay from known analyticity/boundedness assumptions or weaken the claim accordingly.
- [§5.2 / Appendix E] The matching of the CFT and CCFT Regge limits requires an analytic continuation of the distribution δ(z−z̄). The paper defines this via the restriction homomorphism D(C)→D(R+) in Appendix E, so that monodromies act only on the function h(r) on the support. This is a prescription, not a consequence of the bulk amplitude; the phase factors in (5.6) (e.g. e^{2πij(ν)} and e^{πi(Δ12−Δ34)/2}) depend on it. The text acknowledges that different prescriptions may change overall factors, but (5.6) is presented as an equality. Please either show that the relation is independent of the distribution prescription, or state clearly which prescription defines the dictionary and what ambiguity remains.
- [§4.3, working hypothesis] The 'working hypothesis' on ρ±(μ) and the linear trajectory j(μ)=j0+j1μ is imported without physical justification. The derivation of the celestial Regge term (4.10) depends on these conditions, and no example or argument is given that they hold for amplitudes with Regge poles beyond the general motivation in Appendix B.4. If these hypotheses fail, the central claim is conditional. I would ask the authors to either justify them from S-matrix analyticity or provide a class of amplitudes where they hold.
minor comments (4)
- [§2] Typo: 'weather' should be 'whether' in the sentence 'on weather the momenta are incoming or outgoing.'
- [Introduction, Eq. for α−] The displayed expression for α−(ν) in the Introduction uses ρ−(ν), while later in §4.3 the notation is ρ−(µ). Make the argument of ρ± consistent.
- [§5.1, Eqs. (5.4)–(5.5)] The notation 'iπ2' is ambiguous (iπ² vs iπ·2). In the subsequent derivation it appears to mean iπ², but this should be written as iπ^2 for clarity.
- [§5.2] When setting μ=1+iν in the principal series, the branch of (−iε+μ)^{β/2−1} must be specified. The phase in (5.6) may depend on this choice, so please state the chosen branch and check consistency with the contour orientation.
Circularity Check
No significant circularity found: (5.6) is a genuine dictionary between bulk Regge residues and celestial OPE data, not a redefinition.
full rationale
The paper's central relation (5.6) connects two objects defined by different expansions: the bulk Regge residues ρ±(ν), arising from the Froissart–Gribov/Sommerfeld–Watson decomposition of T(s,t), and the conformal OPE data σ±(ν), defined as residues of c(Δ,J) in the conformal partial wave expansion of the celestial correlator. These are not the same quantity by definition; the relation follows by explicitly matching the bulk Mellin transform (4.10) with the CFT Regge expression (5.3), producing nontrivial prefactors (powers of 2, K_{Δ,j}, phases) rather than a tautology. There are no author self-citations used as load-bearing support; the cited Regge/CFT results [7,8,9,15,18,19] are external literature. The 'working hypothesis' on holomorphy and exponential boundedness of ρ± in §4.3 is a convergence/validity assumption for the contour deformation, not a circular redefinition. The paper's own caveat in §5.2 that the matching prescription is 'motivated purely by an educated guess' concerns non-uniqueness of the dictionary, not circularity. The skeptical concern about e^{iπj(ν)} growth making the integral in (4.10) divergent is a technical correctness question; even if fatal, it would invalidate the derivation rather than make it circular. No step in the derivation reduces to its own inputs.
Axiom & Free-Parameter Ledger
free parameters (2)
- j0 (Regge intercept)
- j1 (Regge slope)
axioms (5)
- domain assumption Scattering amplitude is analytic except for poles and branch cuts; arcs at infinity can be neglected in contour manipulations
- ad hoc to paper Regge residues ρ±(μ) are holomorphic and exponentially bounded
- ad hoc to paper Regge trajectory is linear: j(μ)=j0+j1μ with j1>0 and 1-j0-j1<0
- domain assumption The celestial correlator admits an SL(2,C) conformal partial wave expansion over the principal series with coefficients σ±(ν)
- ad hoc to paper A δ(z-z̄)-supported distribution can be analytically continued by restricting to its support
read the original abstract
Exploiting the analytic properties of scattering amplitudes, we provide an alternative but equivalent definition of the standard Mellin transform used to obtain celestial correlation functions. From this representation, we identify a celestial dispersion relation that relates the reduced correlation function to the poles and discontinuities of the bulk amplitude, and we present a novel expansion for the celestial correlator from an integral transform of the Froissart-Gribov expansion on the bulk. By drawing an analogy with the standard CFT case, we define the celestial Regge limit and identify the relevant celestial CFT data in terms of the partial amplitudes governing the bulk Regge limit.
Figures
Forward citations
Cited by 2 Pith papers
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Topics in Celestial holography: A bottom-up perspective
Review of symmetries, celestial CFT, twistor interplay, and AdS/CFT connections in the search for a celestial dual to flat-spacetime quantum gravity.
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Topics in Celestial holography: A bottom-up perspective
A review of symmetries, celestial CFT, twistor theory interplay, and AdS/CFT connections in the bottom-up search for a celestial dual to flat-space quantum gravity.
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discussion (0)
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