Insupersymmetry,eleven-dimensional supergravity is the theory ofsupergravity in the highest number ofdimensions allowed for a supersymmetric theory. It contains agraviton, agravitino, and a3-form gauge field, with their interactions uniquely fixed by supersymmetry. Discovered in 1978 byEugène Cremmer,Bernard Julia, andJoël Scherk, it quickly became a popular candidate for atheory of everything during the 1980s.^[1] However, interest in it soon faded due to numerous difficulties that arise when trying to construct physically realistic models. It came back to prominence in the mid-1990s when it was found to be thelow energy limit ofM-theory, making it crucial for understanding various aspects ofstring theory.

Supergravity was discovered in 1976 through the construction ofpure four-dimensional supergravity with one gravitino. One important direction in the supergravity program was to try to constructfour-dimensional${\displaystyle \mathcal{N}=8}$ supergravity since this was an attractive candidate for a theory of everything, stemming from the fact that it unifiesparticles of all physically admissiblespins into a singlemultiplet. The theory may additionally be UV finite.Werner Nahm showed in 1978 that supersymmetry with spin less than or equal to two is only possible in eleven dimensions or lower.^[2] Motivated by this, eleven-dimensional supergravity was constructed by Eugène Cremmer, Bernard Julia, and Joël Scherk later the same year,^[1] with the aim ofdimensionally reducing it to four dimensions to acquire the${\displaystyle \mathcal{N}=8}$ theory, which was done in 1979.^[3]

During the 1980s, 11D supergravity was of great interest in its own right as a possible fundamental theory of nature. This began in 1980 whenPeter Freund and Mark Ruben showed that supergravitycompactifies preferentially to four or seven dimensions when using a background where thefield strength tensor is turned on.^[4] Additionally,Edward Witten argued in 1981 that eleven dimensions are also the minimum number of dimensions needed to acquire theStandard Modelgauge group, assuming that this arises as subgroup of theisometry group of thecompactmanifold.^{[5][nb 1]}

The main area of study was understanding how 11D supergravity compactifies down tofour dimensions.^[6] While there are many ways to do this, depending on the choice of the compact manifold, the most popular one was using the7-sphere. However, a number of problems were quickly identified with these approaches which eventually caused the program to be abandoned.^[7] One of the main issues was that many of the well-motivated manifolds could not yield the Standard Model gauge group.^{[nb 2]} Another problem at the time was that standardKaluza–Klein compactification made it hard to acquirechiralfermions needed to build the Standard Model. Additionally, these compactifications generally yielded very large negativecosmological constants which could be hard to remove.^{[nb 3]} Lastly,quantizing the theory gave rise toquantum anomalies which were difficult to eliminate. Some of these problems can be overcome with more modern methods which were unknown at the time.^[8]: 302 For example, chiral fermions can be acquired by usingsingular manifolds, using noncompact manifolds, utilising the end-of-world 9-brane of the theory, or by exploitingstring dualities that relate the 11D theory to chiral string theories. Similarly, the presence ofbranes can also be used to build larger gauge groups.

Due to these issues, 11D supergravity was abandoned in the late 1980s, although it remained an intriguing theory. Indeed, in 1988Michael Green,John Schwartz, and Edward Witten wrote of it that^[9]

It is hard to believe that its existence is just an accident, but it is difficult at the present time to state a compelling conjecture for what its role may be in the scheme of things.

In 1995, Edward Witten discovered M-theory,^[10] whose low-energy limit is 11D supergravity, bringing the theory back into the forefront of physics and giving it an important place in string theory.

In supersymmetry, the maximum number ofrealsupercharges that givesupermultiplets containing particles of spin less than or equal to two, is 32.^[11]: 265 Supercharges with more components result in supermultiplets that necessarily includehigher spin states, making such theories unphysical. Since supercharges arespinors, supersymmetry can only be realized in dimensions that admitspinoral representations with no more than 32 components, which only occurs in eleven or fewer dimensions.^{[nb 4]}

Eleven-dimensional supergravity is uniquely fixed by supersymmetry, with its structure being relatively simple compared tosupergravity theories in other dimensions. The only free parameter is thePlanck mass, setting the scale of the theory. It has a single multiplet consisting of the graviton, aMajorana gravitino, and a 3-form gauge field. The necessity of the 3-form field is seen by noting that it provides the missing 84 bosonicdegrees of freedom needed to complete the multiplet since the graviton has 44 degrees of freedom while the gravitino has 128.

The maximally-extendedalgebra for supersymmetry in eleven dimensions is given by^[11]: 265

${\displaystyle \{Q_{\alpha },Q_{\beta }\}=(C\gamma {)}_{\alpha \beta }^{\mu }{P}_{\mu }+(C\gamma {)}_{\alpha \beta }^{\mu \nu }{Z}_{\mu \nu }+(C\gamma {)}_{\alpha \beta }^{\mu \nu \rho \sigma \gamma }{Z}_{\mu \nu \rho \sigma \gamma },}$

where${\displaystyle C}$ is thecharge conjugation operator which ensures that the combination${\displaystyle C{\gamma }^{{\mu }_1\cdots {\mu }_n}}$ is eithersymmetric orantisymmetric.^{[nb 5]} Since theanticommutator is symmetric, the only admissible entries on the right-hand side are those which are symmetric on their spinor indices, which in eleven dimensions only occurs for one, two, and fivespacetime indices, with the rest being equivalent up toPoincaré duality.^[12]: 253 The corresponding coefficients${\displaystyle Z_{\mu \nu }}$ and${\displaystyle Z_{\mu \nu \rho \sigma \gamma }}$ are known as quasi-central charges. They aren't regularcentral charges in thegroup theoretic sense since they are notLorentz scalars and so do not commute with theLorentz generators, but their interpretation is the same. They indicate that there are extended objects that preserve some amount of supersymmetry, these being theM2-brane and theM5-brane.^[13]: 738 Additionally, there is noR-symmetry group.^[12]: 239

Theaction for eleven-dimensional supergravity is given by^[12]: 209

${\displaystyle S=\frac{1}{2{\kappa }_{11}^2}\int d^{11}xe[R(\omega )-{\overline{\psi }}_{\mu }{\gamma }^{\mu \nu \rho }{D}_{\nu }(\frac{1}{2}(\omega +\hat{\omega })){\psi }_{\rho }-\frac{1}{24}{F}_{\mu \nu \rho \sigma }{F}^{\mu \nu \rho \sigma }}$

${\displaystyle -\frac{\sqrt{2}}{192}({\overline{\psi }}_{\nu }{\gamma }^{\alpha \beta \gamma \delta \nu \rho }{\psi }_{\rho }+12{\overline{\psi }}^{\gamma }{\gamma }^{\alpha \beta }{\psi }^{\delta })(F_{\alpha \beta \gamma \delta }+{\hat{F}}_{\alpha \beta \gamma \delta })}$

${\displaystyle -\frac{2\sqrt{2}}{(144{)}^2}{ϵ}^{\alpha \beta \gamma \delta {\alpha }^{\prime }{\beta }^{\prime }{\gamma }^{\prime }{\delta }^{\prime }\mu \nu \rho }{F}_{\alpha \beta \gamma \delta }{F}_{{\alpha }^{\prime }{\beta }^{\prime }{\gamma }^{\prime }{\delta }^{\prime }}{A}_{\mu \nu \rho }].}$

Heregravity is described using thevielbein formalism${\displaystyle e_{\mu }^a}$ with an eleven-dimensional gravitational coupling constant${\displaystyle {\kappa }_{11}}$^{[nb 6]} and

${\displaystyle {\omega }_{\mu ab}={\omega }_{\mu ab}(e)+K_{\mu ab},}$

${\displaystyle {\hat{\omega }}_{\mu ab}={\omega }_{\mu ab}-\frac{1}{8}{\overline{\psi }}_{\nu }{\gamma }^{\nu \rho }{}_{\mu ab}{\psi }_{\rho },}$

${\displaystyle K_{\mu ab}=-\frac{1}{4}({\overline{\psi }}_{\mu }{\gamma }_b{\psi }_a-{\overline{\psi }}_a{\gamma }_{\mu }{\psi }_b+{\overline{\psi }}_b{\gamma }_a{\psi }_{\mu })+\frac{1}{8}{\overline{\psi }}_{\nu }{\gamma }^{\nu \rho }{}_{\mu ab}{\psi }_{\rho },}$

${\displaystyle {\hat{F}}_{\mu \nu \rho \sigma }=F_{\mu \nu \rho \sigma }+\frac{3}{2}\sqrt{2}{\overline{\psi }}_{[\mu }{\gamma }_{\nu \rho }{\psi }_{\sigma ]}.}$

Thetorsion-free connection is given by${\displaystyle {\omega }_{\mu ab}(e)}$, while${\displaystyle K_{\mu ab}}$ is thecontorsion tensor. Meanwhile,${\displaystyle D_{\nu }(\omega )}$ is thecovariant derivative with aspin connection${\displaystyle \omega }$, which acting on spinors takes the form

${\displaystyle D_{\mu }(\omega ){\psi }_{\nu }={\mathrm{\partial }}_{\mu }{\psi }_{\nu }+\frac{1}{4}{\omega }_{\mu }^{ab}{\gamma }_{ab}{\psi }_{\nu },}$

where${\displaystyle {\gamma }_{ab}={\gamma }_{[a}{\gamma }_{b]}}$. The regulargamma matrices satisfying theDirac algebra are denote by${\displaystyle {\gamma }_a}$, while${\displaystyle {\gamma }_{\mu }=e_{\mu }^a{\gamma }_a}$ are position-dependentfields. The first line in the action contains the covariantizedkinetic terms given by theEinstein–Hilbert action, theRarita–Schwinger equation, and thegauge kinetic action. The second line corresponds to cubic graviton-gauge field terms along with somequartic gravitino terms. The last line in theLagrangian is aChern–Simons term.^{[nb 7]}

The supersymmetry transformation rules are given by^[11]: 267

${\displaystyle {\delta }_s{e}_{\mu }^a=\frac{1}{2}\overline{ϵ}{\gamma }^a{\psi }_{\mu },}$

${\displaystyle {\delta }_s{\psi }_{\mu }=D_{\mu }(\hat{\omega })ϵ+\frac{\sqrt{2}}{288}({\gamma }^{\alpha \beta \gamma \delta }{}_{\mu }-8{\gamma }^{\beta \gamma \delta }{\delta }_{\mu }^{\alpha }){\hat{F}}_{\alpha \beta \gamma \delta }ϵ,}$

${\displaystyle {\delta }_s{A}_{\mu \nu \rho }=-\frac{3\sqrt{2}}{4}\overline{ϵ}{\gamma }_{[\mu \nu }{\psi }_{\rho ]},}$

where${\displaystyle ϵ}$ is the supersymmetry Majorana gauge parameter. All hatted variables are supercovariant in the sense that they do not depend on the derivative of the supersymmetry parameter${\displaystyle {\mathrm{\partial }}_{\mu }ϵ}$. The action is additionally invariant underparity, with the gauge field transforming as apseudotensor${\displaystyle A\to -A}$. Theequations of motion for this supergravity also have a rigid symmetry known as the trombone symmetry under which${\displaystyle g_{\mu \nu }\to {\alpha }^2{g}_{\mu \nu }}$ and${\displaystyle A_{\mu \nu \rho }\to {\alpha }^3{A}_{\mu \nu \rho }}$.^[14]

There are a number of special solutions in 11D supergravity, with the most notable ones being thepp-wave, M2-branes, M5-branes, KK-monopoles, and the M9-brane. Brane solutions aresolitonic objects within supergravity that are the low-energy limit of the corresponding M-theory branes. The 3-form gauge fieldcouples electrically to M2-branes and magnetically to M5-branes.^[8]: 307 Explicit supergravity solitonic solutions for the M2-branes and M5-branes are known.

M2-branes and M5-branes have a regularnon-degenerateevent horizon whose constanttimecross-sections aretopologically 7-spheres and 4-spheres, respectively.^{[13]: 737–740} Thenear-horizon limit of theextreme M2-brane is given by an${\displaystyle Ad{S}_4\times S^7}$geometry while for the extreme M5-brane it is given by${\displaystyle Ad{S}_7\times S^4}$. These extreme-limit solutions preserve half of the supersymmetry of thevacuum solution, meaning that both the extreme M2-branes and the M5-branes can be seen as solitons interpolating between two maximally supersymmetricMinkowski vacua at infinity, with an${\displaystyle Ad{S}_4\times S^7}$ or${\displaystyle Ad{S}_6\times S^4}$ horizon, respectively.

TheFreund–Rubin compactification of 11D supergravity shows that it preferentially compactifies to seven and four dimensions, the latter of which led to it being extensively studied throughout the 1980s.^[4] This compactification is most easily achieved by demanding that the compact and noncompact manifolds have aRicci tensor that is proportional to themetric, meaning that they areEinstein manifolds. One additionally demands that the solution isstable against fluctuations, which inanti-de Sitter spacetimes requires that the Bretenlohner–Freedman bound is satisfied. Stability is guaranteed if there is some unbroken supersymmetry, although there also exist classically stable solutions that fully break supersymmetry.

One of the main compactification manifolds studied was the 7-sphere.^[6] The manifold has 8Killing spinors, meaning that the resulting four dimensional theory has${\displaystyle \mathcal{N}=8}$ supersymmetry. Additionally, it also results in an${\displaystyle \text{SO}(8)}$ gauge group, corresponding to the isometry group of the sphere. A similar widely studied compactification was using a squashed 7-sphere, which can be acquired byembedding the 7-sphere in aquaternionic projective space, with this giving a gauge group of${\displaystyle \text{SO}(5)\times \text{SU}(2)}$.

A key property of 7-sphere Kaluza-Klein compactifications is that their truncation is consistent, which is not necessarily the case for other Einstein manifolds besides the7-torus. An inconsistent truncation means that the resulting four dimensional theory is not consistent with thehigher dimensional field equations. Physically this needs not be a problem in compactifications to Minkowski spacetimes as the inconsistent truncation merely results in additionalirrelevant operators in the action. However, most Einstein manifold compactifications are to anti-de Sitter spacetimes which have a relatively large cosmological constant. In this case irrelevant operators can be converted to relevant ones through the equation of motion.^{[nb 8]}

While eleven-dimensional supergravity is the unique supergravity in eleven dimensions at the level of an action, a related theory can be acquired at the level of the equations of motion, known as modified 11D supergravity.^{[nb 9]} This is done by replacing the spin connection by one that isconformally related to the original.^[14] Such a theory is inequivalent to standard 11D supergravity only in spaces that are notsimply connected. An action for amassive 11D theory can also be acquired by introducing anauxiliary nondynamicalKilling vector field, with this theory reducing tomassive type IIA supergravity upon dimensional reduction.^[15] This is not a proper eleven-dimensional theory since the fields explicitly do not depend on one of the coordinates, but it is nonetheless useful for studying massive branes.

Dimensionally reducing 11D supergravity to ten dimensions gives rise totype IIA supergravity, while dimensionally reducing it to four dimensions can give${\displaystyle \mathcal{N}=8}$ supergravity, which was one of the original motivations for constructing the theory.^[16] While eleven-dimensional supergravity is not UV finite, it is the low energy limit of M-theory. The supergravity also receives corrections at thequantum level, where these corrections sometimes playing an important role in various compactification mechanisms.^{[8]: 469–471}

Unlike for supergravity in other dimensions, an extension to eleven dimensional anti-de Sitter spacetime does not exist.^[17] While the theory is the supersymmetric theory in the highest number of dimensions, the caveat is that this only holds for spacetime signatures with one temporal dimension. If arbitrary spacetime signatures are allowed, then there also exists a supergravity in twelve dimensions withtwo temporal dimensions.

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