a r X i v :m a t h /
405020v 2 [m a t h .D G ] 8 F e b 2005TORIC SELFDUAL EINSTEIN METRICS ON COMPACT ORBIFOLDS
DAVID M.J.CALDERBANK AND MICHAEL A.SINGER
Abstract.We prove that any compact selfdual Einstein 4-orbifold of positive scalar cur-
vature whose isometry group contains a 2-torus is,up to an orbifold covering,a quaternion
K¨a hler quotient of (k ?1)-dimensional quaternionic projective space by a (k ?2)-torus for
some k 2.We also obtain a topological classi?cation in terms of the intersection form of
the 4-orbifold.
Introduction A selfdual Einstein (SDE)metric is a 4-dimensional Riemannian metric g whose Weyl curvature W is selfdual with respect to the Hodge star operator (W =?W ),and whose Ricci tensor is proportional to the metric (Ric =λg ).The only compact oriented 4-manifolds admitting SDE metrics of positive scalar curvature are S 4and C P 2,with the round metric and Fubini–Study metric respectively (cf.[3,Thm.13.30]).However,if one considers 4-orbifolds,the class of examples is much wider.In [11],K.Galicki and https://www.sodocs.net/doc/f111770539.html,wson constructed SDE 4-orbifolds by taking quaternion K¨a hler quotients of quaternionic projective spaces by tori,and this construction was systematically investigated by Boyer–Galicki–Mann–Rees [6].These examples are all toric ,i.e.,the isometry group of the metric contains a 2-torus,hence they belong to the local classi?cation by H.Pedersen and the ?rst author of toric SDE metrics of nonzero scalar curvature [8],where it was shown that any such metric has an explicit local form determined by an eigenfunction of the Laplacian on the hyperbolic plane H 2.SDE 4-orbifolds have physical relevance as the simplest nontrivial target spaces for nonlin-ear sigma models in N =2supergravity (see [10]).They have also attracted interest recently because of the connection with M-theory and manifolds of G 2-holonomy [7,12]:in particular the results in [8]have been exploited by L.Anguelova and https://www.sodocs.net/doc/f111770539.html,zaroiu [1].The ?rst main theorem of this paper shows that the quaterion K¨a hler quotient is su?cient.Theorem A.Let X be a compact selfdual Einstein 4-orbifold with positive scalar curvature,whose isometry group contains a 2-torus.Then,up to orbifold coverings,X is isometric to a quaternion K¨a hler quotient of quaternionic projective space H P k ?1,for some k 2,by a (k ?2)-dimensional subtorus of Sp(k ).(We remark that the least such k is b 2(X )+2.)This result was already known when the 3-Sasakian 7-orbifold associated to X is smooth,
since toric 3-Sasakian 7-manifolds have been classi?ed by R.Bielawski [4]using analytical techniques.Our methods are quite di?erent,being elementary and entirely explicit.
Before outlining the proof,we recall that it was shown in [8]that the SDE metrics coming from (local)quaternion K¨a hler quotients of H P k ?1as above,are those for which the corre-sponding hyperbolic eigenfunction F is a positive linear superposition of k basic solutions which may be written
(1)F (ρ,η)=k i =1
√Date :February 2008.
1
2DAVID M.J.CALDERBANK AND MICHAEL A.SINGER
Proof.We provide here the main line of the argument,relegating the detail to the body of the paper.First we observe that by Myers’Theorem(which extends easily to orbifolds[5]) the universal orbifold cover?X of X is also a compact toric SDE4-orbifold of positive scalar curvature,so we may assume X=?X.
As we shall explain in section1,compact simply connected4-orbifolds X with an e?ective
action of a2-torus G=T2may be classi?ed by work of Orlik–Raymond[22]and Hae?iger–
Salem[13].The orbit space W=X/G is a polygonal disc whose edges C0,C1,...C k?1,C k=
C0(given in cyclic order)are labelled by orbifold generators v j=(m j,n j)∈Z2,determined up to sign1,with m j n j?1?m j?1n j=0for j=1,...k.The interior of W is the image of the open subset X0of X on which G acts freely,the edges C j are the images of points with
stabilizer G(v j)={(z1,z2)∈G:m j z1+n j z2=0}and cyclic orbifold structure groups of
order gcd(m j,n j),and the corners are the images of the?xed points.A sign choice for v j is
equivalent to an orientation of the corresponding circle orbits.
The classi?cation result of[8],which we discuss in section2,shows that the interior of W is equipped with a hyperbolic metric g H2and hyperbolic eigenfunction F(with?H2F=3
ρF(ρ,η)has a well-de?ned limit asρ→0,which is a continuous piecewise linear function f0(η)ofη(whose corners are at the corners of W).The half-space coordinates can be chosen so that f0(η)=±(m jη?n j)on C j.
As we shall discuss in section2(cf.[9])a hyperbolic monopole F on H2is determined by its‘boundary value’f0via a‘Poisson formula’
F(ρ,η)=1
ρ2+(η?y)2 3/2.
Integrating twice by parts(in the sense of distributions),we then have
F(ρ,η)=1ρ2+(η?y)2dy
ρ
.
In our case,f0is continuous and piecewise linear,so f′′0is a linear combination of k delta distributions and F is therefore a linear combination of k basic solutions,i.e.,a k-pole solution in the sense of[8].Since the SDE metric has positive scalar curvature,it follows from[8]that the determinant of a certain matrixΦ(ρ,η)associated to F(ρ,η)(see section2)is positive. Using[9](see section2again)we?nd that
(2)detΦ(ρ,η)=1
ρ2+(η?y)2 3/2 ρ2+(η?z)2 3/2dy dz.
Suppose nowηlies in the singular set of f′′0.Then asρ→0,this integral is dominated by the contribution from y nearηand the evaluation at z=ηgiven by the corresponding delta distribution in f′′0.It follows that f0is convex where it is positive and concave where it is negative.Thus,up to an irrelevant sign,f0(η)is positive and convex,hence F(ρ,η)is of the form(1).According to[8],the metric g is therefore locally isometric to a local quaternion K¨a hler quotient of H P k?1by an explicitly de?ned(k?2)-dimensional abelian subgroup of Sp(k),and one easily sees that the integrality conditions v j∈Z2imply that this subgroup is a torus.However,the quaternion K¨a hler quotient of H P k?1by a(k?2)-torus is a compact 4-orbifold[6].Therefore X must be its universal orbifold cover.
TORIC SELFDUAL EINSTEIN METRICS ON COMPACT ORBIFOLDS3 The proof of this theorem shows explicitly how the isotropy data of a toric SDE orbifold X give rise to the hyperbolic eigenfunction de?ning the SDE metric on X and hence to its realization as quaternion K¨a hler quotient.However,it is not yet clear which isotropy data give a toric orbifold admitting an SDE metric.To understand this,we consider the inverse construction.Suppose therefore that X is a quaternion K¨a hler quotient of H P k?1by a(k?2)-
dimensional subtorus H of the standard maximal torus T k=R k/2πZ k in Sp(k).Then the
isometry group of X contains the quotient torus T k/H.If we choose an identi?cation of
T k/H with with R2/2πZ2,then H=h/2πΛis determined by a map from Z k→Z2with kernelΛ,or equivalently by(a i,b i)∈Z2for i=1,...k(the images of the standard basis
elements of Z k).Any two(a i,b i)span Z2?Z Q:otherwise X is a quaternion K¨a hler quotient of H P j?1for some j
Now,by[8],the hyperbolic eigenfunction generating the SDE metric is given by(1)and therefore the boundary value of√
4DAVID M.J.CALDERBANK AND MICHAEL A.SINGER
If(i)–(iii)hold,then X admits a toric selfdual Einstein metric,unique up to homothety and pullback by an equivariant di?eomorphism.
Proof.(i)?(ii).Since the metric is selfdual with positive scalar curvature,the intersection form must be positive de?nite(by Hodge theory for orbifolds and a Bochner argument—cf.[17]).Now we observe that e<χorb(ˉS)follows from a positive scalar curvature analogue of[9,Theorem F].Indeed,the proof in[9,Section6]generalizes to orbifolds,and reversing the sign of the scalar curvature there,we see that any compact,connected,totally geodesic 2-suborbifold of an SDE orbifold of positive scalar curvature must satisfyΣ·Σ<χorb(Σ). (ii)?(iii).The positivity of the intersection form is equivalent to the fact that[v j]∈R P1are in cyclic order(see section1),which proves the?rst part.Because of this,we have?j?1,j>0 for1 j k.Now since[ˉS j]·[ˉS j]<χorb(ˉS j),(4)follows from the following formulae(see section1for the?rst,the second is standard for an orbifold2-sphere):
[ˉS j]·[ˉS j]=?j?1,j+1
?j?1,j?j,j+1
.
(iii)?(i).Under the conditions in(iii)we are still free to cyclicly permute the v j by changing
signs and relabelling.We use this freedom to ensure that m0=?m k 0 under these conditions X admits a toric SDE metric,and the exceptional surfaces,as?xed point sets of a subgroup of the isometry group,are totally geodesic. To establish(a)–(b)we note that(4)may be rewritten as (5)(n j+1?n j)(m j?m j?1)>(m j+1?m j)(n j?n j?1). Since?j?1,j>0,this says that(m j+1,n j+1)lies on the side containing the origin of the line L j joining(m j?1,n j?1)to(m j,n j).We use induction to show that(m j)is increasing.Clearly m1>m0,so suppose m j>m j?1.Then dividing(5)by m j?m j?1,we see that(m j+1,n j+1) is above L j,hence so is the origin.If m j 0,then this,together with the fact that?j,j+1>0, shows that m j+1>m j.Hence the sequence(m j:m j 0)is increasing.A similar induction starting from the fact that m k?1 Thus(a)holds,and dividing(5)by(m j?m j?1)(m j+1?m j)>0,we obtain(b). The proof of(iii)?(i)establishes the existence of a toric SDE metric,which is unique as stated by the proof of Theorem A and the classi?cation of simply connected toric4-orbifolds. Remarks.The second condition in(ii)means equivalently that for any negative toric complex structure on the complement of any?xed point in X,K?1X is nef.Indeed for a toric complex structure on the complement of x∈X,K?1X being nef is equivalent,by the adjunction formula,to[ˉS]·[ˉS]<χorb(ˉS)for allˉS that do not meet x.Now we note that by[16](see also[8]),X admits toric scalar-?at K¨a hler metrics on the complement of any?xed point. Note that X does not admit a global toric complex structure of either orientation unless it is a weighted projective space(or an orbifold quotient thereof).This can be seen by observing that toric complex orbifolds are symplectic and so the sequence[v1],[v2],...[v k]∈R P1must have winding number two(since the v j are the normals to the faces of a convex polytope in g?).It follows easily that the signature is±(k?4),which equals?(k?2)i?k=3. Theorem B shows that not every compact,simply connected toric4-orbifold admits an SDE metric.On the other hand,as was noted in[6],there are SDE toric4-orbifolds with arbitrarily large second Betti number.It is straightforward to construct such4-orbifolds and compute their rational homology using the methods presented here(the integer pairs(a i,b i), i=1,...k,used in formula(1)are constrained only by pairwise linear independence). TORIC SELFDUAL EINSTEIN METRICS ON COMPACT ORBIFOLDS5 Examples.If k=b2(X)+2=2then X is necessarily isometric to an orbifold quotient of S4with the round metric:after an SL(2,Z)transformation we may take v0=(?m,0), v1=(0,?n)and v2=?v0as orbifold data,S4itself being given by mn=±1. When k=3,X is a weighted projective space[11].For instance,orbifold data for C P2 can be taken to be(?2,?1),(?1,?1),(1,0),(2,1). An example with k=4and only one orbifold singularity is given by the data(?2,?3), (?1,?1),(0,?1),(1,0),(2,3).More generally by taking the n j su?ciently negative,one can construct in?nitely many examples with b2(X)+2=k for any k 2|m0|.The graph of z=f0(y)is a union of line segments with integer slopes in the region{(y,z):z |m0y?n0|} as shown in Figure1. Figure1.Graph of a typical boundary value for a compact SDE orbifold. The body of the paper is really a series of appendices.In section1,we review the classi-?cation of compact toric4-orbifolds,following[22,13].In fact,we present the classi?cation of simply connected compact n-orbifolds with a cohomogeneity two torus action,since this is the most natural context and the fundamental paper of Hae?iger and Salem[13]rather understates the power of their theory in proving results such as this. In section2,we review the material from[8,9]that we use.Then,in section3,we present the main technical arguments that we skipped in the proof of Theorem A. We assume throughout that the reader is familiar with the theory of orbifolds. Acknowledgements.We thank W.Ziller for encouraging us to write this paper.The authors are grateful to EPSRC and the Leverhulme Trust for?nancial support.This paper was partly written while the?rst author was visiting ESI and the second author was on leave at MIT.We are grateful to these institutions for hospitality and?nancial support. 1.Torus actions on orbifolds In this section we summarize the description of compact orbifolds with torus actions due to Orlik–Raymond[22]and Hae?iger–Salem[13](see also[14,18]). 1.1.Lie groups and tori acting on orbifolds.Let X be an oriented n-orbifold with a smooth e?ective action of a compact Lie group G.Fix x∈X with stabilizer H G and orbifold structure groupγ.Letφ:?U→?U/γ=U be an H-invariant uniformizing chart about x∈U and let?H be the group of di?eomorphisms of?U which project to di?eomorphisms induced by elements of H.Thusγis a normal subgroup of?H and H=?H/γ. Elements of the Lie algebra g of G induceγ-invariant vector?elds on?U and the integral submanifold throughφ?1(x)isφ?1(G·x∩U).Let W=?T x X/?T x(G·x)be the quotient of the uniformized tangent spaces to X and G·x at x.Since?H preserves?T x(G·x),it acts linearly on W and this induces an action of H on W/γ.Hence by the di?erentiable slice theorem: there is a G-invariant neighbourhood of the orbit G·x that is G-equivariantly di?eomorphic to G×H(B/γ),where B is a?H-invariant ball in W. 6DAVID M.J.CALDERBANK AND MICHAEL A.SINGER Now suppose that G =g /2πΛis an m -torus (m n ),where Λis a lattice in g .Then we can improve on the above as follows.Let U now be a G -invariant tubular neighbourhood of G ·x with orbifold fundamental group Γ.Since π2(G ·x )=0,the universal orbifold cover π:?U →?U/Γ=U is smooth [13].Let ?G be the group of di?eomorphisms of ?U that project to di?eomorphisms of U induced by elements of G ,and let ?H be the stabilizer of a point ?x in π?1(x ),so that Γis normal in ?G and G =?G/Γ.Then by the di?erentiable slice theorem:there is a G -invariant neighbourhood of the orbit G ·x that is G -equivariantly di?eomorphic to (?G ×?H B )/Γ,where B is a ?H -invariant ball in W .Observe that ?H ∩Γ=γ,so that (?G ×?H B )/Γ=(?G/Γ)×?H/γ (B/γ)=G ×H (B/γ)as before.Since ?U is 1-connected,?G/?H is the universal cover of G/H ,namely g /h .Thus ?G/?G 0=?H/?H 0is a ?nite group D ,where ?G 0=g /2πΛ=G and ?H 0=h /2πΛ0denote the identity components,Λ0being a subgroup of Λ.Since ?H is the (unique)maximal compact subgroup of ?G we have the following.1.2.Proposition.[13]Let G =g /2πΛbe an m -torus acting e?ectively on an oriented n -manifold X and let G ·x be an orbit with k -dimensional stabilizer H .Then there is ?a rank k sublattice Λ0of Λ, ?a ?nite group D with a central extension 0→Λ/Λ0→Γ→D →1,?and a faithful representation ?H →SO (n ?m +k ),where ?H is the maximal compact subgroup of the pushout extension ?G =Γ×Λ/Λ0 g /2πΛ0,such that a G -invariant tubular neighbourhood U of G ·x is G -equivariantly di?eomorphic to (?G ×?H B )/Γfor a ball B ?R n ?m +k .These data classify tubular neighbourhoods of orbits up to G -equivariant di?eomorphism. This result is easy to apply when k =n ?m or k =n ?m ?1,when ?H 0=h /2πΛis a maximal torus in SO (2(n ?m ))or SO (2(n ?m ?1)+1).Then ?H =?H 0(since ?H is in the centralizer of ?H 0),so D =1and Γ=Λ/Λ0.Hence a tubular neighbourhood U of such an orbit is classi?ed by a subgroup Λ0of Λsuch that ?H =h /2πΛ0.?When k =n ?m ,U/G is homeomorphic to [0,1)n ?m and Λ0= n ?m j =1Λj 0,where Λj 0are linearly independent rank one sublattices of Λsuch that (Λj 0?Z R )/2πΛj 0is the stabilizer the orbits over the j th face of U/G .?When k =n ?m ?1,U/G is homeomorphic to [0,1)n ?m ?1×(?1,1)and Λ0= n ?m ?1j =1Λj 0,with Λj 0as before. To obtain a global classi?cation,one must patch together such local tubes.This is conve-niently encoded by the ˇCech cohomology of W =X/G with values in Λ.1.3.Proposition.[13]Suppose W = i W i is a union of open sets and (X i ,πi :X i →W i )are G -orbifolds with orbit maps πi .Then there is a G -orbifold (X,π:X →W )with π?1(W i )G -equivariantly di?eomorphic to X i if and only if a ˇCech cohomology class in H 3(W,Λ)associated to {(X i ,πi )}vanishes.If this is the case then the set of such G -orbifolds (X,π)is an a?ne space modelled on H 2(W,Λ). 1.4.Cohomogeneity two torus actions on orbifolds.Let us now specialize to the case dim W =2(i.e.,n =m +2).The union X 0of the m -dimensional orbits is the dense open subset on which the action of G is locally free,hence W 0=X 0/G is a 2-orbifold.The remaining orbits have dimension m ?1or m ?2,i.e.,stabilizers of dimension k =n ?m ?1=1or k =n ?m = 2.Hence we can obtain a global classi?cation in this case.(Similar arguments give a global classi?cation when dim W =1.) TORIC SELFDUAL EINSTEIN METRICS ON COMPACT ORBIFOLDS 7 1.5.Theorem.[22,13](i)Let X be a compact connected oriented (m +2)-orbifold with a smooth e?ective action of an m -torus G =g /2πΛ.Then W =X/G is a compact connected oriented 2-orbifold with boundary and corners,equipped with a labelling Λj 0of the edges of W (the connected components of the smooth part of the boundary )by rank 1sublattices of Λsuch that at each corner of W the corresponding two lattices are linearly independent. (ii)For any such data on W ,there is a G -orbifold X inducing these data,and if H 2(W,Λ)=0then X is uniquely determined up to G -equivariant di?eomorphism. (iii)The orbifold fundamental group of X is determined by the long exact sequence:πorb 2(W 0)→Λ/ j Λj 0→πorb 1(X )→πorb 1(W 0)→1. In particular X is simply connected if and only if either W 0is a smooth open disc and the lattices Λj 0generate Λ,or W 0=W is a simply connected orbifold 2-sphere (so that πorb 2(W )=Z )and πorb 2 (W 0)→Λis an isomorphism (so m =1)https://www.sodocs.net/doc/f111770539.html,pact toric 4-orbifolds.We now apply the preceding result when m =2and X is a simply connected 4-orbifold.Then W is a smooth polygonal disc with rank 1sublattices Λj 0?Λ~=Z 2(j =1,...k )labelling the edges C j of W ,which we order cyclicly.Λj 0is determined by one of its generators v j =(m j ,n j )∈Z 2,which is unique up to a sign.The corner conditions mean that v j ?1,v j are linearly independent,or equivalently ?j ?1,j =0,for j =1,...k (where v 0=?v k and ?i,j :=m i n j ?m j n i ).The simple connectivity of X means that {v j :j =1,...k }spans Z 2.Since H 2(W,Z 2)=0,X is uniquely determined by these data.(The classi?cation for manifolds is more subtle,since then we must have ?j ?1,j =±1,i.e.,adjacent pairs of labels form a Z -basis,which leads to combinatorial problems.) We end by discussing the rational homology of such a toric 4-orbifold X .Since X is oriented and simply connected,this amounts to describing H 2(X,Q )and its intersection form.We have already remarked that b 2(X )=k ?2(and this is easy to establish by a spectral sequence argument):in fact the closures ˉS j of the exceptional surfaces S j =π?1(C j ),once oriented,de?ne rational homology classes generating H 2(X,Q ).Obviously the only nontrivial intersections are the self-intersections and the intersections of adjacent ˉS j .For the latter,we note that in the orbifold uniformizing chart of order |?j,j +1|about ˉS j ∩ˉS j +1,the intersection number is ±1and hence [ˉS j ]·[ˉS j +1]=±1/?j,j +1.Similarly,by considering the link of S j (which is an orbifold lens space),we ?nd that [ˉS j ]·[ˉS j ]=±?j ?1,j +1/(?j ?1,j ?j,j +1).In fact our orientation conventions give (1.1)[ˉS j ]·[ˉS j ]=?j ?1,j +1/(?j ?1,j ?j,j +1),[ˉS j ]·[ˉS j +1]=[ˉS j +1]·[ˉS j ]=?1/?j,j +1.We notice that k j =1m j [ˉS j ]and k j =1n j [ˉS j ]have trivial intersection with any [ˉS i ].Since the latter classes span the rational homology,and the intersection form is nondegenerate,we have k j =1m j [ˉS j ]=0= k j =1n j [ˉS j ].Since b 2(X )=k ?2,these span the relations amongst the rational classes [ˉS j ].For Theorem B we need a formula for the signature of X (i.e.,of the intersection form on H 2(X,Q ))which was given by Joyce [16]in the manifold case and by Hattori–Masuda [14]in general.For this formula,choose an arbitrary vector v =(m,n )∈Z 2which is not a multiple of any v j and de?ne ?j =mn j ?nm j .Then σ(X )=k j =1sign (?j ?1?j ?1,j ?j ). This is evidently independent of the sign choices for the v j ,and it is independent of the choice of v ,since if we move v so that one ?j changes sign then only two terms in the above sum change sign,but they have the opposite sign.Let us choose the signs of the v j so that ?j >0 8DAVID M.J.CALDERBANK AND MICHAEL A.SINGER for i=1,...k(so that the v j lie in a half-space bounded by the span of v);then?0<0.It follows that|σ(X)|=k?2=b2(X)i??j?1,j all have the same sign for j=1,...k,i.e.,i? [v1],...[v k]are in cyclic order in R P1.Thus{±v j:j=1,...k}are the normals to a compact convex polytope in g?symmetric under v→?v,as in Anguelova–Lazaroiu[2]. 2.Toric selfdual Einstein metrics In this section we give a brief account of the relevant results of[8]and[9]. 2.1.Local classi?cation.Let H2denote the hyperbolic plane,which we regard as the positive de?nite sheet of the hyperboloid{a∈S2R2:det a=1}in the space S2R2of symmetric2×2matrices,with the induced metric(?det is the quadratic form of a Minkowski metric on this vector space). Let G denote the standard2-torus R2/2πZ2,with linear coordinates z=(z1,z2).Consider the metric g F constructed on(open subsets of)H2×G through the formula (2.1)g F=|detΦ| 4 F, dz=(dz1,dz2),A is the tautological S2R2-valued function on H2(with A a=a),and Φ=1 4F2?|dF|2.(The metric has singularities where F=0or detΦ=0.)Furthermore,any SDE metric with nonzero scalar curvature and a2-torus in its isometry group is obtained locally from this construction. A more explicit form of the metric can be obtained by introducing half-space coordinates (ρ(a)>0,η(a))on H2.The standard basis of R2leads to a preferred choice A(ρ,η)= 1 ρF(ρ,η),v1=(fρ,ηfρ?ρfη),v2=(fη,ρfρ+ηfη?f), so thatΦ=λ1?v1+λ2?v2,whereλ1=(√ρ,1/√ f2 dρ2+dη2ε(v1,v2)2 , whereε(·,·)denotes the standard symplectic form on R2.