On symmetries of gravitational on-shell boundary action at null infinity
Pith reviewed 2026-05-23 05:58 UTC · model grok-4.3
The pith
The on-shell gravitational boundary action at null infinity reproduces the subleading soft graviton theorem once the BMS group includes superrotations.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By fixing the corner terms in the boundary action through the 5-point eikonal constraint, the subleading soft graviton theorem emerges naturally from the on-shell action when superrotations are admitted; generalizing the Geroch tensor to a tower of divergence-free symmetric traceless tensors on the sphere then generates an infinite sequence of sub^n-leading soft insertions in the tree-level S-matrix.
What carries the argument
The generalized Geroch tensor, a set of divergence-free symmetric traceless tensors on the sphere that label an infinite tower of Goldstone modes and produce the higher-order soft insertions.
If this is right
- The subleading soft graviton theorem is recovered directly from the on-shell boundary action.
- An infinite tower of sub^n-leading soft insertions appears in the tree-level S-matrix.
- The extended BMS group with superrotations is realized through the boundary action.
- The construction supplies a uniform origin for the entire hierarchy of tree-level soft symmetries.
Where Pith is reading between the lines
- Higher soft theorems beyond the subleading order may be derivable from the same boundary action by including still more tensors in the generalized Geroch tower.
- The framework could be tested by checking whether the action reproduces known higher-point amplitudes that involve multiple soft insertions.
- Similar boundary-action treatments might apply to other asymptotic symmetry groups, such as those in higher dimensions or with additional matter fields.
Load-bearing premise
Corner ambiguities in the boundary action can be fixed by demanding that the exponential of the on-shell action reproduces the tree-level 5-point amplitude in eikonal approximation in a generic supertranslated vacuum.
What would settle it
An explicit computation showing that the on-shell action, after the proposed corner fixing, fails to reproduce the known subleading soft factor when superrotations are included, or fails to match the 5-point eikonal amplitude.
read the original abstract
We revisit the gravitational boundary action at null infinity of asymptotically flat spacetimes. We fix the corner ambiguities in the boundary action by using the constraint that (exponential of) the on-shell action leads to tree-level 5-point amplitude in eikonal approximation in a generic supertranslated vacuum. The subleading soft graviton theorem follows naturally from the on-shell action when the BMS group is extended to include superrotations. An infinite tower of Goldstone modes is proposed by `generalizing' the Geroch tensor to incorporate a set of divergence-free symmetric traceless tensors on the sphere. This generalization leads to $\text{sub}^{n}$-leading soft insertions in the tree-level $\mathcal{S}$ matrix, thus paving the way to understanding the infinite tower of tree-level symmetries within this framework.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript revisits the gravitational on-shell boundary action at null infinity. Corner ambiguities are fixed by the requirement that exp(S_on-shell) reproduces the tree-level 5-point amplitude in eikonal approximation inside a generic supertranslated vacuum. Extending the BMS group to superrotations, the subleading soft graviton theorem is asserted to follow from the action. An infinite tower of Goldstone modes is introduced by generalizing the Geroch tensor to a set of divergence-free symmetric traceless tensors on the sphere; this is claimed to generate sub^n-leading soft insertions in the tree-level S-matrix.
Significance. If the fixing procedure can be shown to be independent of the soft factors being derived and the generalization of the Geroch tensor is placed on a firmer footing, the work would supply a concrete link between boundary actions, extended BMS symmetries, and the infinite tower of soft theorems. Such a framework could clarify the origin of higher-order soft insertions without ad-hoc additions to the symmetry group.
major comments (2)
- [Abstract] Abstract (first sentence after 'revisit'): the corner ambiguities are fixed by demanding that the on-shell action reproduce the known 5-point eikonal amplitude in a supertranslated vacuum. Because this amplitude already encodes the leading soft graviton factor (or its Ward identity under supertranslations), the subsequent claim that the subleading soft theorem 'follows naturally' from the same action risks circularity. The manuscript must demonstrate explicitly that the subleading insertion is not already presupposed by the 5-point constraint.
- [Abstract] Abstract (paragraph on infinite tower): the generalization of the Geroch tensor to an infinite set of divergence-free symmetric traceless tensors is introduced without a derivation from the boundary action or from the extended BMS algebra. This step appears to be an additional assumption rather than a consequence; the manuscript should supply the explicit map from the action (after the 5-point fixing) to these modes and to the sub^n soft factors.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive comments. We address the two major points raised in the report below, with clarifications on the logical structure of our arguments and indications of where the manuscript will be revised for greater precision.
read point-by-point responses
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Referee: [Abstract] Abstract (first sentence after 'revisit'): the corner ambiguities are fixed by demanding that the on-shell action reproduce the known 5-point eikonal amplitude in a supertranslated vacuum. Because this amplitude already encodes the leading soft graviton factor (or its Ward identity under supertranslations), the subsequent claim that the subleading soft theorem 'follows naturally' from the same action risks circularity. The manuscript must demonstrate explicitly that the subleading insertion is not already presupposed by the 5-point constraint.
Authors: The 5-point eikonal amplitude constraint is applied exclusively to fix the corner ambiguities so that the on-shell action reproduces the leading soft graviton factor in a supertranslated vacuum. The subleading soft theorem is obtained independently by extending the symmetry group to superrotations and extracting the associated Ward identity from the resulting action. The eikonal 5-point amplitude used for the fixing does not contain or presuppose subleading soft insertions. To eliminate any appearance of circularity, we will revise the abstract and insert an explicit paragraph in Section 3 demonstrating that the 5-point constraint involves only the leading soft factor and is independent of subleading contributions. revision: yes
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Referee: [Abstract] Abstract (paragraph on infinite tower): the generalization of the Geroch tensor to an infinite set of divergence-free symmetric traceless tensors is introduced without a derivation from the boundary action or from the extended BMS algebra. This step appears to be an additional assumption rather than a consequence; the manuscript should supply the explicit map from the action (after the 5-point fixing) to these modes and to the sub^n soft factors.
Authors: The infinite tower is introduced as a proposal that generalizes the Geroch tensor in a manner consistent with the structure of the fixed boundary action and the pattern established by the leading and subleading cases. It is not asserted to follow as a direct derivation within the present work. We agree that an explicit map from the 5-point-fixed action to the full set of modes is not supplied. In revision we will expand the discussion in Section 4 to clarify the status of the proposal, provide the explicit form of the generalized tensors, and sketch how they couple to the action to produce the sub^n soft factors, while noting that a complete algebraic derivation from the extended BMS group remains an open direction. revision: partial
Circularity Check
Boundary action fixed by matching to known 5-point eikonal amplitude; soft theorems then extracted as 'natural' consequence
specific steps
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fitted input called prediction
[Abstract]
"We fix the corner ambiguities in the boundary action by using the constraint that (exponential of) the on-shell action leads to tree-level 5-point amplitude in eikonal approximation in a generic supertranslated vacuum. The subleading soft graviton theorem follows naturally from the on-shell action when the BMS group is extended to include superrotations."
The 5-point eikonal amplitude in a supertranslated vacuum already contains the leading soft graviton insertion (via the supertranslation charge). Fixing the action to reproduce this amplitude therefore builds the soft factor into the boundary term by construction; the subsequent claim that the subleading theorem 'follows naturally' from the same action is a direct readout of the fitted input rather than an independent derivation.
full rationale
The paper explicitly tunes corner terms in the on-shell boundary action so that exp(S) reproduces the tree-level 5-point amplitude in the eikonal limit inside a supertranslated vacuum. This amplitude already encodes the leading soft graviton factor through the supertranslation Ward identity. Once the action is fixed by this constraint, the subleading soft theorem is asserted to follow 'naturally' upon extending BMS to superrotations, and an infinite tower is obtained by generalizing the Geroch tensor. The derivation therefore reduces to reading back an input that was inserted via the fixing step.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Asymptotically flat spacetimes admit a well-defined gravitational boundary action at null infinity with corner ambiguities
- domain assumption The exponential of the on-shell boundary action must reproduce the tree-level 5-point amplitude in eikonal approximation
invented entities (1)
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Infinite tower of Goldstone modes obtained by generalizing the Geroch tensor to divergence-free symmetric traceless tensors on the sphere
no independent evidence
Forward citations
Cited by 1 Pith paper
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The gravitational S-matrix from the path integral: asymptotic symmetries and soft theorems
A path integral with asymptotic boundary conditions produces the gravitational S-matrix and derives soft graviton theorems from extended BMS symmetry Ward identities.
Reference graph
Works this paper leans on
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On-shell boundary term in general relativity 4
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Boundary terms at null infinity 8
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Corner term ambiguity 13
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Subleading symmetry 16
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subn-leading symmetry 19
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Conformal Gaussian null coordinates/Newman-Unti gauge 27 References 28
Action of superrotations 26 B. Conformal Gaussian null coordinates/Newman-Unti gauge 27 References 28
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INTRODUCTION Over the past decade, there have been several seminal developments in our understanding of the so-called infra-red triangle[1, 2]. Infra-red triangle is a conceptual edifice that con- nects three disparate ideas, namely low-frequency gravitational observables[3–5], asymptotic symmetries[6–10], and quantum soft theorems[11, 12]. Through these ...
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to construct an on-shell boundary action at null infinity in a generic supertranslated vacuum. We show that this action has the so-called corner term ambiguities3. The constraint that the (exponential of this action) leads to 5-point eikonal scattering in a supertranslated background fixes the ambiguities in the corner term. We compute the boundary terms ...
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ON-SHELL BOUNDAR Y TERM IN GENERAL RELA TIVITY The path integral formulation for theS matrix is S(pin,pout) := ∫ [DϕDg]eiS [ϕ,ˆgab] (2.1) where the integration limits are the asymptotic values of the fieldsϕand the metric ˆgab and S = ∫ Ld4x is the full action of the field theory. At the tree level, the classical path 5 dominates and theS matrix is given ...
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Null boundaries A proper treatment of counterterms at null boundaries received little attention over time and was examined in [39, 40] and later by LMPS[27] which we now review. Consider a codimension-1 null hypersurface defined byΦ(xa) := 0 for some scalar function Φ that increases towards the future. The null normal to the hypersurface is na =−µ∇aΦ (2.1...
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BOUNDAR Y TERMS A T NULL INFINITY In this section, we examine the ambiguity in the boundary action at the past and future null infinity of asymptotically flat spacetimes. We show how these ambiguities can be fixed to formulate a boundary action consistent with the memory effect and Weinberg soft theorem. We will work within the conformal spacetime picture...
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Since we are working in the conformal picture atI +, we will keep the affine parameter u intact
Corner term ambiguity As noted by LMPS, there are two types of ambiguities in the boundary action at null boundaries: reparametrization freedom fromλto λ′(λ,xA) and changing the embedding from Φ to Φ′. Since we are working in the conformal picture atI +, we will keep the affine parameter u intact. On the other hand, while we appear to have exhausted the c...
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SUBLEADING SYMMETR Y In this section, we consider the extension of the BMS group to include (holomorphic) superrotations as the asymptotic symmetries onI +. In this scenario, the superrotations are generated by holomorphic vector fields that satisfy the conformal killing equation on the sphere. Unlike meromorphic superrotations (the full extended BMS grou...
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sub n-leading symmetry Consider a ‘generalization’ of the (STF) Geroch tensor TAB→TAB g = ∞∑ n=0 unTAB n+1, T AB 1 :=TAB (4.16) so that25 CAB =σAB + ˆCAB + ∞∑ n=0 un+1TAB n+1 (4.17) Recall that the STF tensorTAB was defined by the condition[13, 50] DATAB + 1 2DBR = 0 (4.18) Equation 4.18 is obeyed at each order inu if we impose that{TAB n ;n≥2}are diverge...
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DISCUSSION We have demonstrated in this paper that the corner ambiguities in the boundary action at the null infinity of asymptotically flat spacetimes can be fixed by demanding that the (exponential of) on-shell boundary action factorizes into the Weinberg soft factor and the hard amplitude. We followed the analysis of Lehner, Myers, Poisson, and Sorkin ...
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The variation of various field quantities under local CKV/Diff(S2) can be found in [41]
Action of superrotations The vector fields generating superrotations atI + is ξY =α∂u + ( YA−1 rDAα+ 1 2r2CABDBα+O(r−3) ) ∂A + ( −r 2DAYA + 1 2D2α+ 1 r ( −1 2CABDADBα−1 4CABDADBα ) +O(r−2) ) ∂r (A.27) where α= u 2DAYA and YA is a local CKV/smooth vector field on the sphere. The variation of various field quantities under local CKV/Diff(S2) can be found in...
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discussion (0)
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