Calabi–Yau manifold

"Calabi–Yau" redirects here. For the play by Susanna Speier, seeCalabi-Yau (play).

Inalgebraicanddifferential geometry,aCalabi–Yau manifold,also known as aCalabi–Yau space,is a particular type ofmanifoldwhich has certain properties, such asRicci flatness,yielding applications intheoretical physics.Particularly insuperstring theory,the extra dimensions ofspacetimeare sometimes conjectured to take the form of a 6-dimensional Calabi–Yau manifold, which led to the idea ofmirror symmetry.Their name was coined byCandelas et al. (1985),afterEugenio Calabi(1954,1957), who first conjectured that such surfaces might exist, andShing-Tung Yau(1978), who proved theCalabi conjecture.

A 2D slice of a 6D Calabi–Yau quintic manifold.

Calabi–Yau manifolds arecomplex manifoldsthat are generalizations ofK3 surfacesin any number ofcomplex dimensions(i.e. any even number of realdimensions). They were originally defined as compactKähler manifoldswith a vanishing firstChern classand a Ricci-flat metric, though many other similar but inequivalent definitions are sometimes used.

Definitions

The motivational definition given byShing-Tung Yauis of a compactKähler manifoldwith a vanishing first Chern class, that is also Ricci flat.

There are many other definitions of a Calabi–Yau manifold used by different authors, some inequivalent. This section summarizes some of the more common definitions and the relations between them.

A Calabi–Yau$n$-fold or Calabi–Yau manifold of (complex) dimension$n$is sometimes defined as a compact$n$-dimensional Kähler manifold$M$satisfying one of the following equivalent conditions:

• Thecanonical bundleof$M$is trivial.
• $M$has a holomorphic$n$-form that vanishes nowhere.
• Thestructure groupof thetangent bundleof$M$can be reduced from$U(n)$,theunitary group,to$SU(n)$,thespecial unitary group.
• $M$has a Kähler metric with globalholonomycontained in$SU(n)$.

These conditions imply that the first integral Chern class$c_{1}(M)$of$M$vanishes. Nevertheless, the converse is not true. The simplest examples where this happens arehyperelliptic surfaces,finite quotients of a complex torus of complex dimension 2, which have vanishing first integral Chern class but non-trivial canonical bundle.

For a compact$n$-dimensional Kähler manifold$M$the following conditions are equivalent to each other, but are weaker than the conditions above, though they are sometimes used as the definition of a Calabi–Yau manifold:

• $M$has vanishing first real Chern class.
• $M$has a Kähler metric with vanishing Ricci curvature.
• $M$has a Kähler metric with localholonomycontained in$SU(n)$.
• A positive power of thecanonical bundleof$M$is trivial.
• $M$has a finite cover that has trivial canonical bundle.
• $M$has a finite cover that is a product of a torus and asimply connectedmanifold with trivial canonical bundle.

If a compact Kähler manifold is simply connected, then the weak definition above is equivalent to the stronger definition.Enriques surfacesgive examples of complex manifolds that have Ricci-flat metrics, but their canonical bundles are not trivial, so they are Calabi–Yau manifolds according to the second but not the first definition above. On the other hand, their double covers are Calabi–Yau manifolds for both definitions (in fact, K3 surfaces).

By far the hardest part of proving the equivalences between the various properties above is proving the existence of Ricci-flat metrics. This follows from Yau's proof of theCalabi conjecture,which implies that a compact Kähler manifold with a vanishing first real Chern class has a Kähler metric in the same class with vanishing Ricci curvature. (The class of a Kähler metric is the cohomology class of its associated 2-form.) Calabi showed such a metric is unique.

There are many other inequivalent definitions of Calabi–Yau manifolds that are sometimes used, which differ in the following ways (among others):

• The first Chern class may vanish as an integral class or as a real class.
• Most definitions assert that Calabi–Yau manifolds are compact, but some allow them to be non-compact. In the generalization to non-compact manifolds, the difference$(\Omega \wedge {\bar {\Omega }}-\omega ^{n}/n!)$must vanish asymptotically. Here,$\omega$is the Kähler form associated with the Kähler metric,$g$.
• Some definitions put restrictions on thefundamental groupof a Calabi–Yau manifold, such as demanding that it be finite or trivial. Any Calabi–Yau manifold has a finite cover that is the product of a torus and a simply-connected Calabi–Yau manifold.
• Some definitions require that the holonomy be exactly equal to$SU(n)$rather than a subgroup of it, which implies that theHodge numbers$h^{i,0}$vanish for$0<i<\dim(M)$.Abelian surfaces have a Ricci flat metric with holonomy strictly smaller than$SU(2)$(in fact trivial) so are not Calabi–Yau manifolds according to such definitions.
• Most definitions assume that a Calabi–Yau manifold has a Riemannian metric, but some treat them as complex manifolds without a metric.
• Most definitions assume the manifold is non-singular, but some allow mild singularities. While the Chern class fails to be well-defined for singular Calabi–Yau's, the canonical bundle and canonical class may still be defined if all the singularities areGorenstein,and so may be used to extend the definition of a smooth Calabi–Yau manifold to a possibly singular Calabi–Yau variety.

Examples

The fundamental fact is that any smoothalgebraic varietyembedded in aprojective spaceis a Kähler manifold, because there is a naturalFubini–Study metricon a projective space which one can restrict to the algebraic variety. By definition, if ω is the Kähler metric on the algebraic variety X and the canonical bundle K_Xis trivial, then X is Calabi–Yau. Moreover, there is unique Kähler metric ω on X such that [ω₀] = [ω] ∈H²(X,R), a fact which was conjectured byEugenio Calabiand proved byShing-Tung Yau(seeCalabi conjecture).

Calabi–Yau algebraic curves

In one complex dimension, the only compact examples aretori,which form a one-parameter family. The Ricci-flat metric on a torus is actually aflat metric,so that theholonomyis the trivial group SU(1). A one-dimensional Calabi–Yau manifold is a complexelliptic curve,and in particular,algebraic.

CY algebraic surfaces

In two complex dimensions, theK3 surfacesfurnish the only compact simply connected Calabi–Yau manifolds. These can be constructed as quartic surfaces in$\mathbb {P} ^{3}$,such as the complex algebraic variety defined by the vanishing locus of

$x_{0}^{4}+x_{1}^{4}+x_{2}^{4}+x_{3}^{4}=0$for$[x_{0}:x_{1}:x_{2}:x_{3}]\in \mathbb {P} ^{3}$

Other examples can be constructed as elliptic fibrations,as quotients of abelian surfaces,or ascomplete intersections.

Non simply-connected examples are given byabelian surfaces,which are real four tori$\mathbb {T} ^{4}$equipped with a complex manifold structure.Enriques surfacesandhyperelliptic surfaceshave first Chern class that vanishes as an element of the real cohomology group, but not as an element of the integral cohomology group, so Yau's theorem about the existence of a Ricci-flat metric still applies to them but they are sometimes not considered to be Calabi–Yau manifolds. Abelian surfaces are sometimes excluded from the classification of being Calabi–Yau, as their holonomy (again the trivial group) is aproper subgroupof SU(2), instead of being isomorphic to SU(2). However, theEnriques surfacesubset do not conform entirely to the SU(2) subgroup in theString theory landscape.

CY threefolds

In three complex dimensions, classification of the possible Calabi–Yau manifolds is an open problem, although Yau suspects that there is a finite number of families (albeit a much bigger number than his estimate from 20 years ago). In turn, it has also been conjectured byMiles Reidthat the number of topological types of Calabi–Yau 3-folds is infinite, and that they can all be transformed continuously ( through certain mild singularizations such asconifolds) one into another—much asRiemann surfacescan.One example of a three-dimensional Calabi–Yau manifold is a non-singularquintic threefoldinCP⁴,which is thealgebraic varietyconsisting of all of the zeros of a homogeneous quinticpolynomialin the homogeneous coordinates of theCP⁴.Another example is a smooth model of theBarth–Nieto quintic.Some discrete quotients of the quintic by variousZ₅actions are also Calabi–Yau and have received a lot of attention in the literature. One of these is related to the original quintic bymirror symmetry.

For every positive integern,thezero set,in the homogeneous coordinates of the complex projective spaceCPⁿ⁺¹,of a non-singular homogeneous degreen+ 2 polynomial inn+ 2 variables is a compact Calabi–Yaun-fold. The casen= 1 describes an elliptic curve, while forn= 2 one obtains a K3 surface.

More generally, Calabi–Yau varieties/orbifolds can be found as weighted complete intersections in aweighted projective space.The main tool for finding such spaces is theadjunction formula.

Allhyper-Kähler manifoldsare Calabi–Yau manifolds.

Constructed from algebraic curves

For an algebraic curve$C$a quasi-projective Calabi-Yau threefold can be constructedas the total space$V={\text{Tot}}({\mathcal {L}}_{1}\oplus {\mathcal {L}}_{2})$where${\mathcal {L}}_{1}\otimes {\mathcal {L}}_{2}\cong \omega _{C}$.For the canonical projection$p:V\to C$we can find the relative tangent bundle$T_{V/C}$is$p^{*}({\mathcal {L}}_{1}\oplus {\mathcal {L}}_{2})$using the relative tangent sequence

$0\to T_{V/C}\to T_{V}\to p^{*}T_{C}\to 0$

and observing the only tangent vectors in the fiber which are not in the pre-image of$p^{*}T_{C}$are canonically associated with the fibers of the vector bundle. Using this, we can use the relative cotangent sequence

$0\to p^{*}\Omega _{C}\to \Omega _{V}\to \Omega _{V/C}\to 0$

together with the properties of wedge powers that

$\omega _{V}=\bigwedge ^{3}\Omega _{V}\cong f^{*}\omega _{C}\otimes \bigwedge ^{2}\Omega _{V/C}$

and$\Omega _{V/C}\cong {\mathcal {L}}_{1}^{*}\oplus {\mathcal {L}}_{2}^{*}$giving the triviality of$\omega _{V}$.

Constructed from algebraic surfaces

Using a similar argument as for curves, the total space${\text{Tot}}(\omega _{S})$of the canonical sheaf$\omega _{S}$for an algebraic surface$S$forms a Calabi-Yau threefold. A simple example is${\text{Tot}}({\mathcal {O}}_{\mathbb {P} ^{2}}(-3))$over projective space.

Applications in superstring theory

Calabi–Yau manifolds are important insuperstring theory.Essentially, Calabi–Yau manifolds are shapes that satisfy the requirement of space for the six "unseen" spatial dimensions of string theory, which may be smaller than our currently observable lengths as they have not yet been detected. A popular alternative known aslarge extra dimensions,which often occurs inbraneworldmodels, is that the Calabi–Yau is large but we are confined to a small subset on which it intersects aD-brane.Further extensions into higher dimensions are currently being explored with additional ramifications forgeneral relativity.

In the most conventional superstring models, ten conjectural dimensions instring theoryare supposed to come as four of which we are aware, carrying some kind offibrationwith fiber dimension six.Compactificationon Calabi–Yaun-folds are important because they leave some of the originalsupersymmetryunbroken. More precisely, in the absence offluxes,compactification on a Calabi–Yau 3-fold (real dimension 6) leaves one quarter of the original supersymmetry unbroken if theholonomyis the full SU(3).

More generally, a flux-free compactification on ann-manifold with holonomy SU(n) leaves 2¹⁻ⁿof the original supersymmetry unbroken, corresponding to 2⁶⁻ⁿsupercharges in a compactification oftype IIA supergravityor 2⁵⁻ⁿsupercharges in a compactification of type I. When fluxes are included the supersymmetry condition instead implies that the compactification manifold be ageneralized Calabi–Yau,a notion introduced byHitchin (2003).These models are known asflux compactifications.

F-theorycompactifications on various Calabi–Yau four-folds provide physicists with a method to find a large number of classical solutions in the so-calledstring theory landscape.

Connected with each hole in the Calabi–Yau space is a group of low-energy string vibrational patterns. Since string theory states that our familiar elementary particles correspond to low-energy string vibrations, the presence of multiple holes causes the string patterns to fall into multiple groups, orfamilies.Although the following statement has been simplified, it conveys the logic of the argument: if the Calabi–Yau has three holes, then three families of vibrational patterns and thus three families of particles will be observed experimentally.

Logically, since strings vibrate through all the dimensions, the shape of the curled-up ones will affect their vibrations and thus the properties of the elementary particles observed. For example,Andrew StromingerandEdward Wittenhave shown that the masses of particles depend on the manner of the intersection of the various holes in a Calabi–Yau. In other words, the positions of the holes relative to one another and to the substance of the Calabi–Yau space was found by Strominger and Witten to affect the masses of particles in a certain way. This is true of all particle properties.

Calabi-Yau algebra

ACalabi–Yau algebrawas introduced byVictor Ginzburgto transport the geometry of a Calabi–Yau manifold tononcommutative algebraic geometry.

In popular culture

• The Calabi-Yau manifold was the subject of a paper coauthored bySheldon Cooperin the episode 2 of the seventh season inYoung Sheldon.
• Imagery based on Calabi-Yau manifolds was used inepisode 5of the TV series3 Body Problemin order to illustrate the high-dimensional abilities of the San-Ti alien civilization.
• InHalf-Life 2,Dr. Mossman describes teleporters as working via a 'String-based' technology using 'the Calabi-Yau model.'

See also

• Quintic threefold
• G2 manifold

References

• Calabi, Eugenio(1954),"The space of Kähler metrics",Proc. Internat. Congress Math. Amsterdam,vol. 2, pp.206–207, archived fromthe originalon 2011-07-17
• Calabi, Eugenio (1957), "On Kähler manifolds with vanishing canonical class", inFox, Ralph H.;Spencer, Donald C.;Tucker, Albert W.(eds.),Algebraic geometry and topology. A symposium in honor of S. Lefschetz,Princeton Mathematical Series, vol. 12,Princeton University Press,pp.78–89,ISBN9780691079073,MR0085583
• Candelas, Philip; Horowitz, Gary; Strominger, Andrew; Witten, Edward (1985),"Vacuum configurations for superstrings",Nuclear Physics B,258:46–74,Bibcode:1985NuPhB.258...46C,doi:10.1016/0550-3213(85)90602-9,archived fromthe originalon December 20, 2012
• Hitchin, Nigel(2003), "Generalized Calabi–Yau manifolds",The Quarterly Journal of Mathematics,54(3):281–308,arXiv:math.DG/0209099,CiteSeerX10.1.1.237.8935,doi:10.1093/qmath/hag025,MR2013140
• Tian, Gang; Yau, Shing-Tung (1991), "Complete Kähler manifolds with zero Ricci curvature, II",Invent. Math.,106(1):27–60,Bibcode:1991InMat.106...27T,doi:10.1007/BF01243902,S2CID122638262
• Yau, Shing Tung (1978), "On the Ricci curvature of a compact Kähler manifold and the complex Monge-Ampère equation. I",Communications on Pure and Applied Mathematics,31(3):339–411,doi:10.1002/cpa.3160310304,MR0480350
• Yau, Shing-Tung (2009a), "A survey of Calabi–Yau manifolds",Geometry, analysis, and algebraic geometry: forty years of the Journal of Differential Geometry,Surveys in Differential Geometry, vol. 13, Somerville, Massachusetts: Int. Press, pp.277–318,doi:10.4310/SDG.2008.v13.n1.a9,MR2537089
• Yau, Shing-Tung; Nadis, Steve (2010),The Shape of Inner Space,Basic Books,ISBN978-0-465-02023-2

Further reading

• Besse, Arthur L.(1987),Einstein manifolds,Ergebnisse der Mathematik und ihrer Grenzgebiete (3), vol. 10, Berlin, New York:Springer-Verlag,ISBN978-3-540-15279-8,OCLC13793300
• Bini; Iacono (2016),Diffeomorphism Classes of Calabi–Yau Varieties(PDF),arXiv:1612.04311,Bibcode:2016arXiv161204311B
• Chan, Yat-Ming (2004),Desingularizations of Calabi-Yau 3-folds with a conical singularity,arXiv:math/0410260,Bibcode:2004math.....10260C
• Greene, Brian(1997),String theory on Calabi–Yau manifolds,Fields, strings and duality (Boulder, CO, 1996), River Edge, NJ: World Sci. Publ., pp.543–726,arXiv:hep-th/9702155v1,Bibcode:1997hep.th....2155G,MR1479700
• Gross, M.; Huybrechts, D.;Joyce, Dominic(2003),Calabi–Yau manifolds and related geometries,Universitext, Berlin, New York:Springer-Verlag,doi:10.1007/978-3-642-19004-9,ISBN978-3-540-44059-8,MR1963559,OCLC50695398
• He, Yang-Hu (2021),The Calabi–Yau Landscape: From Geometry, to Physics, to Machine Learning,Switzerland: Springer International Publishing,ISBN978-3-030-77562-9
• Hübsch, Tristan (1994),Calabi–Yau Manifolds: a Bestiary for Physicists,Singapore, New York:World Scientific,ISBN978-981-02-1927-7,OCLC34989218,archived fromthe originalon 2010-01-13,retrieved2009-02-04
• Joyce, Dominic(2000),Compact Manifolds with Special Holonomy,Oxford University Press,ISBN978-0-19-850601-0,OCLC43864470
• Tian, Gang; Yau, Shing-Tung (1990), "Complete Kähler manifolds with zero Ricci curvature, I",J. Amer. Math. Soc.,3(3):579–609,doi:10.2307/1990928,JSTOR1990928
• Yau, S. T. (2009b), "Calabi–Yau manifold",Scholarpedia,4(8): 6524,Bibcode:2009SchpJ...4.6524Y,doi:10.4249/scholarpedia.6524(similar to (Yau 2009a))

External links

• Calabi–Yau Homepageis an interactive reference which describes many examples and classes of Calabi–Yau manifolds and also the physical theories in which they appear.
• Spinning Calabi–Yau Space video.
• Calabi–Yau Spaceby Andrew J. Hanson with additional contributions by Jeff Bryant,Wolfram Demonstrations Project.
• Weisstein, Eric W."Calabi–Yau Space".MathWorld.

Beginner articles

• An overview of Calabi-Yau Elliptic fibrations
• Lectures on theCalabi-Yau Landscape
• Fibrations in CICY Threefolds- (complete intersectionCalabi-Yau)