Topological Gravity Equivalence Theorem

• TGRET is a framework linking topological gravity with gauge theories via Chern–Simons descents and WZW correspondences.
• It elucidates dualities that manifest through high-energy scattering amplitude cancellations and integrable KdV hierarchies in 2D gravity.
• The theorem underpins holographic dualities and ensemble averages in TQFT, connecting BRST quantization, anomaly inflow, and geometric formulations.

The Topological Gravity Equivalence Theorem (TGRET) establishes deep equivalences between topological gravity theories and gauge-theoretic or topological field theory (TQFT) formulations. In various spacetime dimensions and across diverse mathematical realizations, TGRET provides explicit dualities between apparently distinct actions, partition functions, or scattering amplitudes in gravity and gauge frameworks, often revealing powerful structure in anomalies, holography, and integrability. TGRET appears in contexts ranging from even-dimensional topological gravities, Chern–Simons/WZW correspondences, BRST-quantized topological quantum gravity, AKSZ–Manin perurbations, to scattering amplitudes in topologically massive gravity (TMG), and governs profound cancellation properties and duality flows.

1. Geometric and Field-Theoretic Statement of TGRET

In its original geometric manifestation, TGRET asserts that the action for topological gravity in even dimensions is, up to a normalization, exactly the boundary value (the “descent”) of a one-dimension-higher Chern–Simons theory and is equivalent to a gauged Wess–Zumino–Witten (WZW) model. For $M^{2n}$ an oriented even-dimensional manifold, with vielbein $e^a$ and spin-connection $\omega^{ab}$, the topological gravity action is

$$S_{\rm grav}[e,\omega,\phi] = k \int_{M^{2n}} \epsilon_{a_1\cdots a_{2n+1}} R^{a_1 a_2} \wedge \cdots \wedge R^{a_{2n-1} a_{2n}} \wedge \phi^{a_{2n+1}}$$

where $R^{ab}$ is the curvature, and $\phi^a$ is an internal adjoint scalar (coset coordinate). This action is precisely identified with the gauged WZW action obtained by descending the $(2n+1)$-dimensional Chern–Simons form $Q_{2n+1}(A)$, where $A = e^a P_a + \frac12 \omega^{ab} J_{ab}$ is an ISO($2n,1$) connection. The equivalence is given by

$S_{\rm grav} = S_{\rm WZW}[g,A]$

with $g(x) = \exp(-\phi^a(x) P_a)$ parameterizing the ISO/SO coset (Salgado et al., 2013, Salgado et al., 2014).

Fundamentally, on a manifold $B^{2n+1}$ with boundary $M^{2n}$, the bulk Chern–Simons (or, more generally, transgression) form $Q_{2n+1}(A)$ satisfies

$$\int_{B^{2n+1}} d Q_{2n+1}(A) = \int_{M^{2n}} \omega_{2n}(g^{-1}dg, A)$$

so the TGRET expresses a precise descent from a bulk gauge theory of connections to a boundary topological gravity or WZW-type theory.

2. Algebraic, Homological, and Holographic Structures

TGRET can be phrased in the formalism of principal and associated coframe bundles. Let $P \rightarrow M$ be a principal $G$-bundle ($G$ a Lie group with symmetric coset decomposition $G = S \times C$), equipped with a gauge-invariant topological action $S_{\rm top}[H]$. Given a coframe bundle with Lorentz $S$-structure and fields $(\omega, e)$ on (possibly distinct) manifold $M'$, there exists a bundle morphism

$\Phi: P \rightarrow P'_X$

such that, upon imposing interior derivative constraints $i_X\Omega = i_XT = 0$ for $X$ the coset generators, one has

$$\Phi^* ( S_{\rm grav}[\omega, e] ) = S_{\rm top}[H] + (\text{boundary terms}),$$

establishing an off-shell equivalence. The homological framework encodes this as an isomorphism between interior homologies $$H_q(M', X)_{\rm grav} \longleftrightarrow H_{q+1}(M, X)_{\rm gauge}$$, with the topological invariants in gravity corresponding to cohomology classes in the gauge theory (Assimos et al., 2021).

Holographically, in ensemble-averaged TQFT contexts, TGRET relates the bulk path integral (sum over topologies with fixed boundary) to an ensemble average of boundary conformal partition functions. In 3d Abelian Chern–Simons (or general TQFT), one has

$$Z_{\rm grav}(\Sigma_g) = \sum_{M: \partial M = \Sigma_g} Z_{\rm TQFT}(M) = \langle Z_{\rm CFT}(\Sigma_g) \rangle$$

where the RHS represents a uniform sum over “topological boundary conditions” on the Riemann surface $\Sigma_g$ (Dymarsky et al., 30 May 2024).

3. TGRET in Integrable and Topological Quantum Gravity

In two-dimensional gravity, TGRET underpins a duality between compact and non-compact topological gravity phases at the level of integrable KdV-type hierarchies. Given a polynomial ring framework with dispersionless KdV flows for both compact ($P_c(X)$) and non-compact ($P(Y)$) superpotentials, TGRET identifies the hierarchies, times, and tau-functions via the map $X \leftrightarrow Y^{-1}$, $t^c_i \leftrightarrow t_{-i-2}$, ensuring

$\tau_1(t^c_i; \hbar) = \tau_2(t_{-i-2}; \hbar)$

and hence an isomorphism of all genus-$g$ topological gravity amplitudes in the two phases (Ashok et al., 2018).

For cohomological quantum gravity and Ricci-flow–based models, TGRET equates distinct BRST/gauge-fixed formulations. In particular, the $\mathcal{N}=2$ superspace action with topological deformations and ultralocal diffeomorphism symmetry is cohomologically equivalent (up to $Q$-exact terms) to a standard BRST-fixed theory, with both yielding the same observables and partition functions (Frenkel et al., 2020).

4. TGRET and Scattering Amplitudes in Topologically Massive Gravity

In three-dimensional topologically massive gravity (TMG), TGRET governs the high-energy cancellation properties of $N$-point graviton amplitudes and provides the formal bridge between massive graviton and massless dilaton amplitudes.

In a BRST-quantized covariant TMG, with metric fluctuations $h_{\mu\nu}$ and dilaton $\phi$ (from $g_{\mu\nu} = e^{-\kappa\phi}\bar{g}_{\mu\nu}$), TGRET asserts

$$M[h_P(p_1), \ldots, h_P(p_N)] = \left(\frac12\right)^N C_{\rm mod}\, M[\phi(p_1), \ldots, \phi(p_N)] + \mathcal{O}\left(\frac{m}{E}\right)$$

where $h_P$ is the physical graviton polarization, and $M$ signifies amputated Green's functions or S-matrix elements. This equivalence guarantees that all powers in leading energy scaling cancel between the naive power counting of graviton amplitudes and the dilaton amplitudes, yielding drastic cancellations—e.g., $E^{12} \to E^1$ for the four-graviton amplitude (Liu et al., 11 Dec 2025, Hang et al., 2021, Hang et al., 19 Jun 2024).

The proof combines BRST arguments, gauge-fixing–induced Slavnov–Taylor identities, and LSZ reduction. The correspondence is exact in the high-energy limit and for arbitrary $N$-point functions, and extends naturally through the double-copy structure relating TMYM and TMG amplitudes.

5. Generalizations: AKSZ/Manin Constructions and Integrability

In the AKSZ–Manin framework, for every topological AKSZ model in $d=2,3,4$ dimensions, a suitable Manin (quadratic) deformation produces an action that, upon field redefinitions and integration out of auxiliary fields, becomes exactly the corresponding gravitational theory:

• 2D: Jackiw–Teitelboim/dilaton gravity with cosmological constant and possibly background stress-energy,
• 3D: Einstein–Cartan gravity (AdS/dS/flat) with possible background $T_{\mu\nu}$,
• 4D: MacDowell–Mansouri–Stelle–West Einstein gravity with cosmological constant.

The mapping from AKSZ fields to gravitational variables is linear and explicitly constructed, and the residual gauge/diffeomorphism symmetries are those of the gravitational theory (Borsten et al., 14 Oct 2024).

6. Implications and Applications

TGRET has far-reaching consequences:

• It explains the mathematical origin of auxiliary/coset scalars in even-dimensional gravitational actions.
• It underlies the holographic identification of boundary WZW theories and bulk Chern–Simons/topological gravity, clarifying anomaly inflow and group-cohomology origin of gravitational invariants (Salgado et al., 2013, Salgado et al., 2014).
• In the context of scattering amplitudes, it provides a general mechanism for large energy cancellations and manifests in the double-copy structure extending from gauge to gravity theories (Liu et al., 11 Dec 2025, Hang et al., 19 Jun 2024).
• In the integrable systems and quantum gravity sectors, it enables the explicit transfer of results (tau-functions, correlators, etc.) between compact and non-compact phases, and encodes duality at the level of operator algebras (Ashok et al., 2018).
• It provides a conceptual framework for ensemble holography and the relation between bulk topology sums and randomized boundary theories in TQFT gravity (Dymarsky et al., 30 May 2024).

7. Summary Table: Principal Manifestations

Physical Context	TGRET Statement	Core Reference (arXiv)
Even-dimensional topological gravity	$S_{\rm grav} = S_{\rm WZW}$ (boundary action equals gauged WZW of bulk CS term)	(Salgado et al., 2013, Salgado et al., 2014)
Chern–Simons/topological amplitude	$N$-point physical=transverse amplitude $+$ $\mathcal{O}(m/E)$; double-copy	(Hang et al., 2021, Hang et al., 19 Jun 2024)
Integrable/KdV duality in 2D gravity	Identical tau-functions and correlators under hierarchy/time duality	(Ashok et al., 2018)
TQFT gravity and ensemble holography	Bulk path integral sum$=$boundary CFT ensemble average	(Dymarsky et al., 30 May 2024)
AKSZ–Manin theory	Deformed AKSZ gauge theory $\equiv$ gravity theory	(Borsten et al., 14 Oct 2024)
BRST/cohomological quantum gravity	Superspace and gauge-fixed (BRST) actions are $Q$-cohomologically equivalent	(Frenkel et al., 2020)

TGRET synthesizes structures in topological, geometric, and quantum aspects of gravity, advancing understanding of dualities, anomaly inflow, high-energy amplitude behavior, and holographic correspondences across the modern landscape of mathematical physics.