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Supergravity

Supergravity is a class of supersymmetric gauge theories that extend Einstein's general relativity by incorporating local supersymmetry, unifying the spin-2 graviton with its fermionic partner, the spin-3/2 gravitino, and enabling the consistent inclusion of gravity within a quantum field theory framework.^[1] Developed in 1976 by Sergio Ferrara, Daniel Z. Freedman, and Peter van Nieuwenhuizen through a pragmatic construction of an action invariant under local supersymmetry transformations, supergravity represents the first viable extension of supersymmetry to include gravitational interactions in four spacetime dimensions (N=1 supergravity).^[2] This breakthrough built on earlier global supersymmetry proposals from 1971 by Yuri Golfand, Evgeny Likhtman, and others, addressing the challenges of quantum gravity's non-renormalizability by leveraging bosonic-fermionic cancellations to improve ultraviolet behavior.^[3] Key formulations include on-shell and off-shell versions, with auxiliary fields ensuring the supersymmetry algebra closes off-shell, and extensions to higher dimensions up to 11, such as the maximal N=8 supergravity in four dimensions formulated by Bernard de Wit and Daniel Z. Freedman in 1977^[4] and 11-dimensional supergravity proposed by Cremmer, Julia, and Scherk in 1978.^[5] Supergravity plays a pivotal role in theoretical physics as the low-energy effective theory of string theory and M-theory, facilitating compactifications that embed the Standard Model of particle physics and providing insights into phenomena like black hole entropy, AdS/CFT duality, and cosmic inflation.^[3]

Fundamentals

Definition and Basic Principles

Supergravity (SUGRA) is a supersymmetric extension of general relativity, formulating gravity as a gauge theory of local supersymmetry where the spacetime symmetry group encompasses both bosonic transformations—such as translations and Lorentz rotations—and fermionic supersymmetry transformations. In this framework, the principles of general relativity are unified with supersymmetry, promoting the global supersymmetry of supersymmetric field theories to a local gauge symmetry, analogous to how general relativity gauges the Poincaré group. The core structure revolves around the supergravity multiplet, which minimally includes the graviton, a massless spin-2 boson responsible for gravitational interactions, and the gravitino, a massless spin-3/2 fermion that serves as the superpartner of the graviton. A fundamental distinction lies between global and local supersymmetry: global supersymmetry maintains constant transformation parameters across spacetime, while local supersymmetry—central to supergravity—allows these parameters to vary, necessitating the introduction of auxiliary fields and leading to dynamical gravity via the gauging of the super-Poincaré group. This local gauging ensures diffeomorphism invariance and incorporates the vielbein formalism to handle spinors in curved spacetime, where the metric is expressed in terms of orthonormal frames. The key motivation for supergravity arises from its potential to address ultraviolet divergences in quantum gravity; by pairing bosonic and fermionic degrees of freedom, supersymmetry enforces cancellations in loop diagrams, potentially rendering the theory finite or at least renormalizable at higher orders. At the heart of this is the supersymmetry algebra, characterized by the anticommutator \{Q, \bar{Q}\} \propto P^\mu, where Q denotes the supersymmetry generators and P^\mu the translation generators, linking fermionic transformations directly to spacetime translations. The supersymmetry transformations in supergravity are defined on the fields of the multiplet, with the simplest form for the gravitino in curved spacetime given by

\delta \psi_\mu = \nabla_\mu \epsilon + \frac{1}{2} \gamma^{\nu\rho} F_{\nu\rho} \epsilon,

where \nabla_\mu is the covariant derivative incorporating the spin connection from the vielbein e^a_\mu, \epsilon is the local supersymmetry parameter (a spinor), \gamma^{\nu\rho} are antisymmetrized Dirac matrices, and F_{\nu\rho} represents the field strength of an associated gauge field (absent in the purest formulation).^[6] This transformation ensures the invariance of the action under local supersymmetry, highlighting the interplay between gravitational and supersymmetric degrees of freedom. Supergravity formulations differ in their treatment of constraints: on-shell versions close the supersymmetry algebra only after imposing the equations of motion, while off-shell formulations include auxiliary fields to close the algebra without such constraints, facilitating quantization and coupling to matter. In four dimensions, the massless graviton contributes 2 on-shell degrees of freedom (corresponding to its two helicity states \pm 2), while the gravitino contributes 4 components in its vector-spinor representation before projections, but effectively 2 on-shell degrees of freedom after accounting for gauge redundancies and the Dirac equation, ensuring balance within the multiplet.^[7] This matching underscores the consistency of supergravity as a unified theory of gravity and supersymmetry.

Supersymmetry in Gravitational Theories

Supersymmetry, when extended to include gravity, requires promoting the global supersymmetry transformations of flat spacetime to local ones that are invariant under diffeomorphisms. This gauging of supersymmetry introduces the gravitino as the gauge field for local SUSY, alongside the metric tensor, but the algebra closes off-shell only with the inclusion of auxiliary fields to match the degrees of freedom between bosons and fermions.^[3]^[8] In four-dimensional minimal supergravity, one formulation, known as new minimal supergravity, has the basic multiplet consisting of the metric g_{\mu\nu} (or equivalently the vielbein e^a_\mu), the gravitino \psi_\mu (a vector-spinor field), an auxiliary vector field A_\mu, and a gauge two-form field B_{\mu\nu}.^[9] This ensures the off-shell degrees of freedom balance at 12 bosonic and 12 fermionic, with the auxiliaries providing 6 bosonic degrees of freedom (3 from the gauge-invariant A_\mu and 3 from the gauge two-form B_{\mu\nu}) to match the off-shell contributions from the vielbein and gravitino.^[9] Extended supergravity multiplets incorporate additional fields, such as vector bosons and scalar fields, to accommodate higher numbers of supersymmetry generators while maintaining the pairing of bosonic and fermionic components.^[3]^[10] The dynamical implications in curved spacetime arise through supersymmetric modifications to the Einstein equations and the introduction of a supercovariant derivative. The supercovariant derivative acts on spinorial fields as D_\mu = \partial_\mu + \frac{1}{2} \omega_\mu^{ab} \Sigma_{ab}, where \omega_\mu^{ab} is the spin connection that includes supersymmetric contributions from the gravitino, ensuring covariance under local Lorentz and SUSY transformations. This leads to supersymmetric Einstein equations where the energy-momentum tensor includes fermionic contributions, altering the geometry to preserve SUSY invariance.^[3] A key supersymmetry constraint in pure supergravity is the vanishing of the supercovariant curvature of the gravitino, R_{\mu\nu}(\psi) = 0, which enforces the consistency of the local SUSY algebra. This condition implies that the connection acquires torsion terms sourced by the gravitino bilinears, such as T_{\mu\nu}^\rho \propto \bar{\psi}_\mu \gamma^\rho \psi_\nu - \bar{\psi}_\nu \gamma^\rho \psi_\mu, distinguishing supergravity from torsion-free general relativity.^[3]^[11] Supersymmetry in gravitational theories provides protection against quantum corrections via non-renormalization theorems, which limit divergences in higher-loop amplitudes due to the extended symmetry. For instance, in maximal supergravity, certain curvature-squared terms like R^4 receive no quantum corrections beyond one loop, improving ultraviolet behavior through bosonic-fermionic cancellations. These theorems ensure that the effective action remains controlled, mitigating the non-renormalizability of pure Einstein gravity.^[12]^[13]

Historical Development

Origins in Gauge Supersymmetry

The concept of gauge supersymmetry emerged in the early 1970s as physicists sought to extend the successful framework of Yang-Mills gauge theories, which unify internal symmetries through local transformations, to incorporate supersymmetry—a symmetry relating bosons and fermions. Inspired by the gauging of internal symmetries in non-Abelian gauge theories, researchers began exploring the possibility of "gauging" supersymmetry transformations, treating them as local rather than global symmetries of spacetime. This approach aimed to bridge quantum field theory with gravitational interactions, but initial efforts focused on global supersymmetry formulations before addressing locality.^[2] A pivotal step was the 1971 proposal by Yu. A. Golfand and E. P. Likhtman, who extended the Poincaré algebra to include supersymmetry generators, forming the super-Poincaré algebra and demonstrating its implications for parity violation in field theories. This global supersymmetry framework was further developed in 1974 by Julius Wess and Bruno Zumino, who constructed the first interacting four-dimensional supersymmetric quantum field theory, known as the Wess-Zumino model, featuring a chiral multiplet with scalar, spinor, and auxiliary fields invariant under supersymmetry transformations. Building on this, Wess and Zumino introduced supersymmetric Yang-Mills theories in the same year, extending non-Abelian gauge symmetries to include supersymmetric partners, which provided a crucial bridge toward incorporating gravity by treating supersymmetry as a gauged symmetry akin to the Yang-Mills mechanism.^[14]^[15] The theoretical foundation was solidified in 1975 by the Haag-Łopuszański-Sohnius theorem, which generalized the Coleman-Mandula theorem to superalgebras, proving that the super-Poincaré algebra is the maximal extension of the Poincaré group compatible with S-matrix symmetries in interacting theories, thus justifying the pursuit of gauged supersymmetry. However, global supersymmetry faced fundamental challenges in reconciling with general relativity's requirement of general covariance, as rigid supersymmetry parameters cannot vary locally across curved spacetime without violating the closure of the algebra or introducing inconsistencies in the transformation laws for fields coupled to gravity. This incompatibility—stemming from the fixed nature of global transformations conflicting with diffeomorphism invariance—necessitated the development of a local supersymmetry theory, paving the way for supergravity as a gauge theory of the super-Poincaré group.^[16]^[2]

Formulation of Supergravity Theories

The formulation of supergravity theories marked a pivotal advancement in theoretical physics with the discovery and construction of pure N=1 supergravity in four dimensions in 1976. This breakthrough was achieved through independent but concurrent efforts by Sergio Ferrara, Daniel Z. Freedman, and Peter van Nieuwenhuizen, who developed an action principle invariant under local supersymmetry transformations using only the vierbein field e^a_\mu and the Rarita-Schwinger gravitino field \psi_\mu. Their work demonstrated that general relativity could be extended to include a spin-3/2 gravitino field while preserving gauge invariance under supersymmetry, resolving challenges in coupling higher-spin fields to gravity.^[1] Complementing this, Peter Breitenlohner, Freedman, and van Nieuwenhuizen constructed a consistent variant incorporating complex spin-3/2 gauge fields, ensuring invariance under complex local supersymmetry and addressing potential inconsistencies in the field equations.^[17] These formulations built upon earlier ideas of gauging supersymmetry but provided the first explicit, viable theory of supergravity, establishing it as a candidate for unifying gravity with fermionic matter. The resulting model features a balanced supermultiplet where the first model had 12 bosonic and 12 fermionic degrees of freedom on-shell, reflecting the matching of gravitational and supersymmetric components after gauge fixing. The core of the theory is encapsulated in the supergravity action, expressed as the spacetime integral of a Lagrangian density that combines the Einstein-Hilbert term with supersymmetric corrections:

\mathcal{L} = \frac{e R}{2\kappa^2} - i \bar{\psi}_\mu \gamma^5 \gamma^\nu D_\nu \psi^\mu + \cdots

where e = \det(e^a_\mu) is the vierbein determinant, R is the Ricci scalar (modified by supersymmetric torsion), \kappa is the gravitational coupling constant, \psi_\mu is the Majorana gravitino field, \gamma^\mu are Dirac matrices, D_\nu is the covariant derivative incorporating the spin connection, and the ellipsis denotes additional terms involving gravitino torsion and supercovariant curvature contributions essential for local supersymmetry invariance.^[1] More broadly, the action takes the form of an integral over R - \bar{\psi} \gamma^{\mu\nu\rho} D_\nu \psi plus auxiliary field terms to close the algebra off-shell. Off-shell extensions, such as the rheonomic approach, further incorporate auxiliary fields to maintain supersymmetry without on-shell constraints, facilitating couplings to matter fields. This 4D N=1 supergravity stands as the simplest non-trivial supergravity theory, as it represents the minimal gauging of supersymmetry in four spacetime dimensions where Dirac spinors allow a non-trivial extension of the Poincaré algebra to the super-Poincaré algebra without introducing redundant or vanishing representations.^[1]

Minimal and Maximal Supergravities

Parallel to higher-dimensional developments, the maximal N=8 supergravity in four dimensions was formulated in 1978 by Bernard de Wit and others, but primarily by Eugène Cremmer and Bernard Julia, featuring eight gravitini and a spectrum including 28 vector fields and 70 scalars, saturating the supersymmetry representations.^[5] Minimal supergravity (mSUGRA) emerged in the early 1980s as a phenomenological framework that couples N=1 supergravity in four dimensions to the minimal supersymmetric extension of the Standard Model, incorporating chiral matter multiplets to describe particle interactions while addressing the hierarchy problem through supersymmetry breaking mechanisms. This model posits a hidden sector where supersymmetry is spontaneously broken at a high energy scale, typically via supergravity effects, with the breaking transmitted to the observable sector through gravitational interactions, leading to soft supersymmetry-breaking terms in the effective Lagrangian. The construction assumes universal boundary conditions at the grand unification scale, simplifying the parameter space and facilitating predictions for superpartner masses and couplings that align with grand unified theories. A hallmark of mSUGRA is its reliance on five key parameters defined at the grand unification scale: the universal scalar mass m_0, the universal gaugino mass m_{1/2}, the universal trilinear coupling A_0, the ratio of Higgs vacuum expectation values \tan \beta, and the sign of the Higgsino mass parameter \operatorname{sign}(\mu). These parameters determine the entire spectrum of superpartners and Higgs bosons through renormalization group evolution down to the electroweak scale, enabling testable predictions for collider experiments and cosmology, such as neutralino dark matter abundance. By assuming flavor universality and minimal field content, mSUGRA provides a constrained yet viable extension of the Standard Model, influential in early supersymmetry phenomenology despite later extensions incorporating non-universality. In contrast, maximal supergravity refers to the unique N=1 supergravity theory formulated in eleven dimensions, constructed in 1978 as the highest-dimensional supersymmetric gravity theory free from quantum anomalies and tachyon instabilities. This theory, developed by Cremmer, Julia, and Scherk, features a single supersymmetry multiplet comprising a graviton, a gravitino, and a 3-form gauge field, with on-shell degrees of freedom balancing at 128 for bosons (44 from the graviton and 84 from the 3-form) and 128 for the gravitino, ensuring supersymmetric closure. The eleven-dimensional formulation arises naturally from dimensional enhancement of lower-dimensional theories, offering a unified description of gravity and other forces without requiring additional matter fields, and it sets a benchmark for higher-dimensional supergravities by saturating the bound imposed by supersymmetry representations in spacetime. The bosonic sector of eleven-dimensional supergravity is governed by the action

S = \int d^{11}x \sqrt{-g} \left[ R - \frac{1}{2} |F_4|^2 \right] + \frac{1}{48} \int B_2 \wedge F_4 \wedge F_4,

where R is the Ricci scalar, F_4 = dB_3 is the 4-form field strength (with B_3 the 3-form potential, sometimes denoted differently), and the Chern-Simons term ensures invariance under supersymmetry transformations and gauge symmetries. This structure highlights the theory's elegance, with the Chern-Simons contribution playing a crucial role in anomaly cancellation and compactification possibilities, though the full fermionic terms complete the supersymmetric invariance. As the maximal extension, it represents a pinnacle of pure supergravity constructions before the integration of matter sectors became prominent.

Challenges and the End of the Pure SUGRA Era

Despite the promising structure of supergravity theories, significant theoretical challenges emerged in their quantum treatment during the late 1970s and 1980s, particularly concerning renormalizability. Pure supergravity in four dimensions is renormalizable at one loop due to supersymmetric cancellations, but beyond this order, it exhibits non-renormalizable divergences, requiring an infinite number of counterterms to absorb ultraviolet infinities in higher-loop calculations.^[18] This issue is exacerbated in higher dimensions; for instance, the maximal 11-dimensional supergravity is non-renormalizable starting at two loops, as demonstrated by explicit computation of the two-loop counterterm involving the square of the Riemann tensor contracted with the four-form field strength. A key realization in the 1980s was that no pure supergravity theory could serve as a finite quantum theory of gravity without a ultraviolet completion. For the maximal N=8 supergravity in four dimensions, early calculations confirmed finiteness up to three loops, and subsequent higher-loop computations (up to four loops) have continued to support UV finiteness, with theoretical arguments indicating perturbative finiteness to all orders.^[19] Similarly, in dimensions greater than 11, pure supergravity suffers from uncanceled gravitational anomalies, as the spectrum lacks the necessary matter fields to achieve anomaly cancellation, rendering consistent formulations impossible without additional exotics. The culmination of these obstacles marked the end of the pure supergravity era by the mid-1980s. Challenges in achieving realistic supersymmetry breaking in extended supergravity models, such as difficulties in partial breaking without additional fields or hidden sectors, combined with the mounting evidence of quantum inconsistencies, led to a decline in interest in standalone supergravity approaches just prior to advances in string theory, shifting focus away from pure SUGRA as a fundamental theory.^[20]

Connections to String Theory

Supergravity as Effective Theory

Supergravity serves as the low-energy effective field theory for superstring theories, obtained in the limit where the string tension parameter, related to the Regge slope α', tends to zero, thereby capturing the dynamics of gravity coupled to supersymmetry at energy scales well below the fundamental string scale. In this approximation, the infinite tower of massive string modes decouples, leaving a finite spectrum of massless and light fields described by a supersymmetric extension of Einstein gravity.^[21] This framework provides a controlled description of phenomena where stringy effects are negligible, yet it inherits the consistency of the underlying string theory. A key example is Type IIA supergravity in ten dimensions, which emerges as the effective theory of the Type IIA superstring. Similarly, eleven-dimensional supergravity arises as the low-energy limit of M-theory, unifying various string theories at strong coupling. These higher-dimensional supergravities encode the essential interactions, including the graviton, gravitino, and form fields, while higher-order α' corrections can be systematically included to probe deviations from the field theory limit.^[21] To connect to four-dimensional physics, dimensional reduction via compactification of the extra dimensions on manifolds like Calabi-Yau spaces is employed, preserving supersymmetry and yielding an effective theory with the Einstein-Hilbert term plus contributions from moduli fields.^[22] The resulting action takes the schematic form

S_{4D} = \int d^4 x \, \sqrt{-g} \left( R - \frac{1}{2} (\partial \phi)^2 - V(\phi) + \cdots \right),

where R is the Ricci scalar, \phi denotes scalar fields such as the dilaton and Kähler moduli, and V(\phi) is a potential stabilizing these fields, with the ellipsis representing fermionic and higher-form terms. This reduction from ten or eleven dimensions introduces a rich scalar sector governing the geometry of the internal space. The primary advantages of viewing supergravity in this light include its ability to address strong-coupling dynamics non-perturbatively through the parent string or M-theory, and its resolution of ultraviolet divergences that plague standalone supergravity formulations by embedding them in a finite quantum theory. This perspective reframes earlier challenges in pure supergravity, such as non-renormalizability, as artifacts of ignoring the higher-energy stringy completion.^[21]

Integration with Superstring Revolutions

The first superstring revolution of 1984–1985 marked a pivotal integration of supergravity into string theory, particularly through the formulation of heterotic string theories, which relied on ten-dimensional supergravity as their low-energy effective description for compactifications to four dimensions.^[22] These compactifications, often on Calabi–Yau manifolds, preserved supersymmetry and yielded realistic phenomenological models with grand unified gauge groups like E_8 \times E_8. A cornerstone of this revolution was the anomaly cancellation mechanism uncovered by Green and Schwarz, which demonstrated the quantum consistency of supersymmetric ten-dimensional gauge theories underlying the heterotic strings, thereby elevating supergravity from a standalone framework to an essential tool in consistent string constructions.^[22] The second superstring revolution, beginning in 1995, profoundly transformed supergravity's role by revealing its centrality in non-perturbative string dualities and higher-dimensional unification. Key insights from Duff, Hull, and collaborators highlighted how eleven-dimensional maximal supergravity emerges as the strong-coupling limit of type IIA superstring theory, where the extra dimension decompactifies as the string coupling grows, suggesting an underlying eleven-dimensional "M-theory" whose low-energy limit is precisely this supergravity.^[23] This duality resolved apparent inconsistencies among the five superstring theories, positioning supergravity as a bridge across perturbative regimes. Throughout the 1990s, discoveries of T-duality and S-duality further intertwined supergravity with string theory, mapping gravitational backgrounds and field configurations between different string theories while preserving the supergravity structure. For instance, T-duality relates type IIA and type IIB theories by inverting compactification radii, while S-duality in type IIB exchanges weak and strong coupling, acting as a self-duality on the supergravity sector. The 1997 advent of the AdS/CFT correspondence by Maldacena established supergravity on anti-de Sitter spaces as a classical dual to strongly coupled conformal field theories, revitalizing supergravity as a holographic tool for quantum gravity studies. These revolutions dispelled the earlier notion that the pure supergravity era had concluded due to perturbative inconsistencies, instead affirming its enduring relevance as the non-perturbative backbone of string theory and holography.

Specific Formulations

4D N=1 Supergravity

Four-dimensional N=1 supergravity represents the minimal extension of general relativity incorporating a single Majorana spinor gravitino, unifying the graviton and gravitino into a supergravity multiplet while allowing couplings to matter fields. This theory is formulated by coupling the pure supergravity sector to chiral superfields, which describe scalar fields, their fermionic partners, and auxiliary fields. The interactions are governed by two key functions: the Kähler potential K(\phi^i, \bar{\phi}^{\bar{j}}), which defines the metric on the scalar field space K_{i\bar{j}} = \partial_i \partial_{\bar{j}} K, and the holomorphic superpotential W(\phi^i), which encodes the Yukawa couplings and mass terms. These functions ensure the preservation of local supersymmetry transformations, with the scalar kinetic terms arising from the Kähler metric and the superpotential contributing to the F-term potential. The Lagrangian of 4D N=1 supergravity is most elegantly expressed in the superconformal framework, where supersymmetry is embedded within a larger superconformal group before breaking to Poincaré supergravity via gauge fixing. In this formulation, the action includes the Einstein-Hilbert term coupled to matter and takes the form

S = \int E \left[ \frac{R}{2\kappa^2} - 3 e^{-K/3} |W|^2 + \cdots \right],

where E is the superdeterminant of the vielbein in superspace, R is the supercurvature scalar, \kappa^2 = 8\pi G, and the ellipsis denotes contributions from chiral and vector superfield kinetic terms. The off-shell version of this formulation relies on the superconformal tensor calculus developed by Wess and Bagger in 1982, which provides a systematic way to construct invariant actions by treating supersymmetry as a gauge symmetry and introducing compensator fields to fix the conformal gauge.^[24] A central feature is the scalar potential derived from the F-terms of the superpotential, given by

V = e^K \left( K^{i\bar{j}} D_i W D_{\bar{j}} \bar{W} - 3 |W|^2 \right),

where D_i W = \partial_i W + (\partial_i K) W is the Kähler covariant derivative, and K^{i\bar{j}} is the inverse Kähler metric (with Planck mass units set to 1). This potential combines positive contributions from supersymmetry breaking F-terms with a negative term from the superpotential, allowing for de Sitter vacua relevant to cosmology. In applications, 4D N=1 supergravity provides the framework for gravity-mediated supersymmetry breaking, where a hidden sector F-term \langle F \rangle induces soft breaking terms of order the gravitino mass m_{3/2} \sim \langle F \rangle / M_{\rm Pl}, parameterizing the minimal supersymmetric standard model (MSSM) Lagrangian. These soft terms, including gaugino masses, scalar masses, and trilinear couplings, influence LHC phenomenology by predicting sparticle spectra testable through missing energy signatures and jet multiplicities. As an effective theory from string compactifications, such as type IIB on Calabi-Yau manifolds, 4D N=1 supergravity captures the low-energy dynamics with moduli-stabilized Kähler potentials and non-perturbative superpotentials.

4D N=8 Supergravity

The four-dimensional N=8 supergravity theory, the most supersymmetric extension of general relativity in four dimensions, was constructed in 1978 by Bernard Julia and Eugène Cremmer through a systematic dimensional reduction of eleven-dimensional supergravity on a seven-sphere.^[25] This maximal supergravity features a single supermultiplet containing one graviton, eight gravitini, twenty-eight vector fields, forty-eight Majorana spin-1/2 fermions, and seventy real scalars, with the on-shell degrees of freedom balancing at 128 for bosons and 128 for fermions to preserve supersymmetry.^[26] The seventy scalars parametrize the coset manifold E_{7(7)}/SU(8), realizing the exceptional symmetry group E_{7(7)} nonlinearly, while the vectors transform in the fundamental 56 representation, and the full bosonic sector exhibits an SU(8) R-symmetry.^[27] A hallmark of N=8 supergravity is its remarkable ultraviolet finiteness properties, which have been verified through explicit amplitude calculations up to five loops, showing no divergences in four dimensions. Early computations confirmed finiteness at one and two loops in the 1970s and 1980s, followed by three-loop finiteness in 2007 using unitarity methods.^[19] Four-loop and five-loop amplitudes, computed in 2009 and 2018 respectively, also exhibit ultraviolet finiteness, surpassing naive power-counting expectations due to enhanced cancellations from supersymmetry and the E_{7(7)} symmetry.^[28]^[29] The first potential counterterm, D^8 R^4, compatible with all symmetries, would imply a divergence at seven loops, but ongoing evidence suggests possible further cancellations that might render the theory finite to all orders. The Lagrangian of N=8 supergravity includes a nonlinear sigma model for the scalar sector, capturing the E_{7(7)} invariance. The kinetic term for the scalars is given by

\mathcal{L}_{\text{scalar}} = -\frac{1}{2} g_{ij}(\phi) \partial_\mu \phi^i \partial^\mu \phi^j,

where g_{ij}(\phi) is the E_{7(7)}-invariant metric on the coset (E_{7(7)}/SU(8), and the \phi^i$ are the seventy scalar fields.^[27] This term couples to the gravitational and fermionic sectors, ensuring the full action respects the extended supersymmetry. In the context of the AdS/CFT correspondence, ungauged N=8 supergravity in four dimensions provides a holographic dual to certain observables in \mathcal{N}=4 super Yang-Mills theory on the boundary, particularly through Kaluza-Klein reductions of the type IIB supergravity on AdS_5 \times S^5. Current research focuses on higher-loop scattering amplitudes using modern techniques like the unitarity method and double-copy relations, aiming to uncover deeper insights into quantum gravity, including potential ultraviolet finiteness and connections to string theory.^[30] These efforts, exemplified by recent three-loop integrand reconstructions, continue to probe the theory's behavior beyond seven loops.

Higher-Dimensional Supergravities

Supergravity theories in dimensions greater than four spacetime dimensions extend the framework of supersymmetry and general relativity to higher-dimensional manifolds, playing a crucial role in unifying gravity with other forces and providing low-energy approximations to string theory. These theories are constrained by supersymmetry, which requires the spacetime dimension D to satisfy 4 ≤ D ≤ 11 for consistent formulations without anomalies, with the maximum number of supersymmetries decreasing as D increases beyond four. For instance, the maximal supergravity in D=11 preserves 32 supercharges and is unique up to field redefinitions, while in D=10, maximal theories include Type IIA and Type IIB with 32 supercharges, the latter featuring an SL(2,ℤ) duality symmetry acting on the axion-dilaton sector. The general action for a D-dimensional supergravity theory typically takes the form

S = \int d^D x \sqrt{-g} \left[ R - \frac{1}{2} \sum_p |F_p|^2 \right] + \int \text{Chern-Simons terms},

where R is the Ricci scalar, F_p are p-form field strengths for the gauge fields, and the Chern-Simons terms ensure consistency with supersymmetry and dualities; this structure accommodates the metric, fermions, and p-form potentials inherent to these theories. In D=10, key examples include the Type IIA supergravity, which contains a Neveu-Schwarz three-form and Ramond-Ramond forms of odd rank, and Type IIB with even-rank Ramond-Ramond forms, both arising as the low-energy limits of corresponding superstring theories; the heterotic SO(32) and E_8 × E_8 supergravities in D=10 incorporate Yang-Mills gauge fields with anomaly cancellation via Green-Schwarz mechanisms. A landmark development occurred in 1978 with the discovery of eleven-dimensional supergravity, formulated as a unique theory with a graviton, gravitino, and a three-form gauge field, unifying earlier attempts and suggesting a fundamental role for M-theory.^[31] In lower dimensions like D=5, the N=2 gauged supergravity provides a framework for studying AdS black hole solutions and their thermodynamics, often used to model holographic duals in condensed matter physics. Additionally, D=5 N=2 supergravity supports extremal brane solutions, such as those wrapping cycles in Calabi-Yau compactifications, which are essential for understanding non-perturbative effects in string theory. Higher-dimensional supergravities are incomplete as standalone quantum theories due to non-renormalizability, but recent advances, such as the swampland distance conjecture proposed around 2018, imply that large-field excursions in moduli space destabilize these theories, affecting mechanisms for moduli stabilization in compactifications to four dimensions. This conjecture highlights tensions between supergravity approximations and quantum gravity constraints, influencing ongoing research into consistent higher-dimensional embeddings.

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Oct 19, 1982 · We have only to follow exactly the same procedure as was employed in deducing the usual Poincaré tensor calculus from the superconformal one.
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The SO(8) supergravity action is constructed in closed form. A local SU(8) group as well as the exceptional group E 7 are invariances of the equations of ...
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Feb 28, 2008 · We present an explicit, exact to all orders in gravitational coupling E7(7) symmetry transformations of on-shell N=8 supergravity fields in the ...Missing: sigma metric
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We describe the construction of the complete four-loop four-particle amplitude of N=8 supergravity. The amplitude is ultraviolet finite, not only in four ...
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Relating gauge and gravity predictions, directly graph by graph, clarifies aspects of the tree-level relation between gravitational scattering amplitudes and ...Missing: ongoing | Show results with:ongoing