Lecture Notes in Mathematics 1947
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Enumerative Invariants in Algebraic Geometry and String Theory
Lectures given at the C.I.M.E. Summer School held in Cetraro, Italy June 6–11, 2005
Editors: Kai Behrend Marco Manetti
ABC Authors and Editors Dan Abramovich Marcos Marino˜ Department of Mathematics Department of Physics Box 1917 Theory Division, CERN Brown University University of Geneva Providence, RI 02912 1211 Geneva USA Switzerland [email protected] [email protected] [email protected] Kai Behrend Department of Mathematics Michael Thaddeus University of British Columbia Department of Mathematics 1984 Mathematics Rd Columbia University Vancouver, BC, V6T 1Z2 2990 Broadway Canada New York, NY 10027 [email protected] USA [email protected] Marco Manetti Department of Mathematics Ravi Vakil “Guido Castelnuovo” Department of Mathematics University of Rome “La Sapienza” Stanford University P.le Aldo Moro 5 Stanford, CA 94305–2125 00185 Rome USA Italy [email protected] [email protected]
ISBN: 978-3-540-79813-2 e-ISBN: 978-3-540-79814-9 DOI: 10.1007/978-3-540-79814-9
Lecture Notes in Mathematics ISSN print edition: 0075-8434 ISSN electronic edition: 1617-9692
Library of Congress Control Number: 2008927358
Mathematics Subject Classification (2000): 14H10, 14H81, 14N35, 53D40, 81T30, 81T45
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9 8 7 6 5 4 3 2 1 springer.com Preface
Starting with the middle of the 1980s, there has been a growing and fruitful interaction between algebraic geometry and certain areas of theoretical high- energy physics, especially the various versions of string theory. In particular, physical heuristics have provided inspiration for new mathematical definitions (such as that of Gromov–Witten invariants) leading in turn to the solution of (sometimes classical) problems in enumerative geometry. Conversely, the avail- ability of mathematically rigorous definitions and theorems has benefitted the physics research by providing needed evidence, in fields where experimental testing seems still very far in the future. This process is still ongoing in the present day, and actually expanding. A partial reflection of it can be found in the courses of the CIME session Enumerative invariants in algebraic geometry and string theory. The session took place in Cetraro from June 6 to June 11, 2005 with the following courses: • Dan Abramovich (Brown University): Gromov–Witten Invariants for Orb- ifolds. (5 h) • Marcos Mari˜no (CERN): Open Strings. (5 h) • Michael Thaddeus (Columbia University): Moduli of Sheaves. (5 h) • Ravi Vakil (Stanford University): Gromov–Witten Theory and the Moduli Space of Curves. (5 h) Moreover, the following two talks were given as complementary material to the course of Abramovich. • Jim Bryan (University of British Columbia): Quantum cohomology of orb- ifolds and their crepant resolutions. • Barbara Fantechi (Sissa): Virtual fundamental class.
Orbifolds are a natural generalization of complex manifolds, where local charts are given not by open subsets of a complex vector space but by their quotients by finite groups. There are two natural descriptions, one in terms of charts (which actually works in symplectic geometry) and one in algebraic VI Preface geometry as smooth complex Deligne–Mumford stacks. Physicists have long suggested treating orbifolds analogously to manifolds; this has led to the de- velopment of an orbifold Euler characteristic, then of orbifold Hodge numbers, and finally (due to Chen–Ruan, in 2000) of an orbifold cohomology, induced by a full theory of Gromov Witten invariants for orbifolds. The course of Dan Abramovich has presented the foundations, laid down in a series of papers by Abramovich, Vistoli and others, of the definition of Gromov–Witten invariants for orbifolds in the algebraic setting. The course of Marcos Mari˜no presented explicit enumerative computations by manipulation of formal power series, based on the physical idea of trans- forming an open string theory to a closed string theory. The aim was to de- rive explicit relationships between Gromov–Witten, Donaldson–Thomas and Chern Simons invariants. In particular, the technique of the topological vertex was explained, which allows a cut-and-paste approach to the determination of such invariants. These methods have only partially received mathematical proofs; they are therefore an important source of conjectures and methods for further developments. Enumerative geometry computations on moduli spaces of sheaves have long been extremely useful both in physics and in mathematics; for instance we may recall the definition of Donaldson and (more recently) Donaldson– Thomas invariants, for surfaces and (some) threefolds respectively. The course by Michael Thaddeus has been a very broad overview of this kind of tech- niques, with a particular accent on the definition of the Donaldson–Thomas invariants and the recent conjectures that relate them to Gromov–Witten in- variants for Calabi Yau threefolds; evidence for the conjectures and examples illustrating their significance have also been included. One of the more established parts of the algebraic geometry – high energy physics interaction has been the rigorous definition and the computation of Gromov–Witten invariants for smooth projective varieties. At the basis of the very definition there is the existence and properness of the moduli stack M g,n of stable curves. Surprisingly, in recent years it has been possible to deduce theorems about M g,n using the results of the Gromov–Witten theory. The course of Ravi Vakil gave a general introduction to this area of research, starting at a comparatively elementary level and then reaching proofs of some conjectures of C. Faber on the tautological cohomology ring of M g,n. We acknowledge the COFIN 2003 “Spazi di moduli e teoria di Lie” for the partial financial support given to this C.I.M.E. session. We express our deep gratitude to Barbara Fantechi, for her very active role in the organization, for help and for precious scientific advices to the second editor.
Kai Behrend Marco Manetti Contents
Preface ...... V Lectures on Gromov–Witten Invariants of Orbifolds D. Abramovich ...... 1 1 Introduction ...... 1 1.1 WhatThisIs...... 1 1.2 Introspection ...... 1 1.3 Where Does All This Come From? ...... 2 1.4 Acknowledgements ...... 2 2 Gromov–Witten Theory ...... 2 2.1 Kontsevich’sFormula...... 2 2.2 Set-Up for a Streamlined Proof ...... 3 2.3 TheSpaceofStableMaps...... 7 2.4 NaturalMaps...... 8 2.5 Boundary of Moduli ...... 9 2.6 Gromov–Witten Classes ...... 10 2.7 The WDVV Equations ...... 11 2.8 ProofofWDVV...... 12 2.9 About the General Case ...... 15 3 Orbifolds/Stacks ...... 16 3.1 GeometricOrbifolds...... 16 3.2 ModuliStacks ...... 17 3.3 Where Do Stacks Come Up? ...... 19 3.4 AttributesofOrbifolds...... 19 3.5 Etale´ Gerbes ...... 20 4 Twisted Stable Maps ...... 21 4.1 StableMapstoaStack ...... 21 4.2 Twisted Curves ...... 22 4.3 Twisted Stable Maps ...... 23 4.4 Transparency 25: The Stack of Twisted Stable Maps ...... 24 4.5 Twisted Curves and Roots ...... 25 4.6 Valuative Criterion for Properness ...... 27 VIII Contents
5 Gromov–Witten Classes ...... 29 5.1 Contractions ...... 29 5.2 Gluing and Rigidified Inertia...... 29 5.3 Evaluation Maps ...... 31 5.4 The Boundary of Moduli ...... 32 5.5 Orbifold Gromov–Witten Classes ...... 32 5.6 FundamentalClasses ...... 34 6 WDVV, Grading and Computations ...... 35 6.1 TheFormula...... 35 6.2 Quantum Cohomology and Its Grading ...... 36 6.3 GradingtheRings...... 38 6.4 Examples ...... 38 6.5 OtherWork ...... 41 6.6 Mirror Symmetry and the Crepant Resolution Conjecture ...... 42 A The Legend of String Cohomology: Two Letters of Maxim KontsevichtoLevBorisov...... 43 A.1 The Legend of String Cohomology ...... 43 A.2 The Archaeological Letters ...... 44 References ...... 46 Lectures on the Topological Vertex M. Mari˜no ...... 49 1 Introduction and Overview ...... 49 2 Chern–Simons Theory ...... 51 2.1 BasicIngredients...... 51 2.2 Perturbative Approach ...... 55 2.3 Non-Perturbative Solution ...... 61 2.4 Framing Dependence ...... 68 2.5 The 1/N Expansion in Chern–Simons Theory ...... 70 3 Topological Strings ...... 73 3.1 Topological Strings and Gromov–Witten Invariants ...... 74 3.2 Integrality Properties and Gopakumar–Vafa Invariants ...... 76 3.3 Open Topological Strings ...... 77 4 Toric Geometry and Calabi–Yau Threefolds ...... 79 4.1 Non-Compact Calabi–Yau Geometries: An Introduction ...... 79 4.2 Constructing Toric Calabi–Yau Manifolds ...... 81 4.3 Examples of Closed String Amplitudes ...... 87 5 The Topological Vertex ...... 89 5.1 The Gopakumar–Vafa Duality ...... 89 5.2 Framing of Topological Open String Amplitudes...... 89 5.3 Definition of the Topological Vertex ...... 91 5.4 GluingRules ...... 93 5.5 Explicit Expression for the Topological Vertex ...... 95 5.6 Applications ...... 96 A Symmetric Polynomials ...... 99 References ...... 100 Contents IX
Floer Cohomology with Gerbes M. Thaddeus ...... 105 1 Floer Cohomology ...... 106 1.1 Newton’s Second Law ...... 106 1.2 The Hamiltonian Formalism ...... 107 1.3 TheArnoldConjecture ...... 108 1.4 Floer’sProof ...... 108 1.5 MorseTheory...... 109 1.6 Bott–Morse Theory ...... 110 1.7 MorseTheoryontheLoopSpace...... 110 1.8 Re-Interpretation #1: Sections of the Symplectic Mapping Torus 112 1.9 Re-Interpretation #2: Two Lagrangian Submanifolds ...... 113 1.10 Product Structures ...... 114 1.11 The Finite-Order Case ...... 115 1.12 Givental’s Philosophy ...... 115 2 Gerbes...... 117 2.1 Definition of Stacks ...... 117 2.2 ExamplesofStacks...... 118 2.3 Morphismsand2-Morphisms ...... 118 2.4 Definition of Gerbes ...... 120 2.5 TheGerbeofLiftings...... 121 2.6 TheLienofaGerbe...... 122 2.7 Classification of Gerbes ...... 123 2.8 AllowingtheBaseSpacetoBeaStack...... 123 2.9 Definition of Orbifolds ...... 124 2.10 Twisted Vector Bundles ...... 124 2.11 Strominger–Yau–Zaslow ...... 125 3 Orbifold Cohomology and Its Relatives ...... 126 3.1 Cohomology of Sheaves on Stacks ...... 126 3.2 TheInertiaStack ...... 127 3.3 Orbifold Cohomology ...... 128 3.4 Twisted Orbifold Cohomology ...... 129 3.5 TheCaseofDiscreteTorsion ...... 129 3.6 The Fantechi–G¨ottsche Ring ...... 130 3.7 Twisting the Fantechi–G¨ottsche Ring with Discrete Torsion . . . . . 131 3.8 Twisting It with an Arbitrary Flat Unitary Gerbe ...... 131 3.9 TheLoopSpaceofanOrbifold...... 132 3.10 Addition of the Gerbe ...... 134 3.11 The Non-Orbifold Case ...... 135 3.12 The Equivariant Case ...... 135 3.13 A Concluding Puzzle ...... 136 4 NotesontheLiterature...... 137 4.1 NotestoLecture1...... 137 4.2 NotestoLecture2...... 139 4.3 NotestoLecture3...... 140 X Contents
The Moduli Space of Curves and Gromov–Witten Theory R. Vakil ...... 143 1 Introduction ...... 143 2 TheModuliSpaceofCurves...... 145 3 Tautological Cohomology Classes on Moduli Spaces ofCurves,andTheirStructure...... 154 4 A Blunt Tool: Theorem and Consequences ...... 173 5 Stable Relative Maps to P1 and Relative Virtual Localization ...... 177 6 Applications of Relative Virtual Localization ...... 186 7 Towards Faber’s Intersection Number Conjecture 3.23 via Relative Virtual Localization ...... 190 8 Conclusion ...... 194 References ...... 194
List of Participants ...... 199 Lectures on Gromov–Witten Invariants of Orbifolds
D. Abramovich
Department of Mathematics, Box 1917, Brown University Providence, RI 02912, USA [email protected]
1 Introduction
1.1 What This Is
This text came out of my CIME minicourse at Cetraro, June 6–11, 2005. I kept the text relatively close to what actually happened in the course. In particular, because of last minutes changes in schedule, the lectures on usual Gromov–Witten theory started after I gave two lectures, so I decided to give a sort of introduction to non-orbifold Gromov–Witten theory, including an exposition of Kontsevich’s formula for rational plane curves. From here the gradient of difficulty is relatively high, but I still hope different readers of rather spread-out backgrounds will get something out of it. I gave few com- putational examples at the end, partly because of lack of time. An additional lecture was given by Jim Bryan on his work on the crepant resolution con- jecture with Graber and with Pandharipande, with what I find very exciting computations, and I make some comments on this in the last lecture. In a way, this is the original reason for the existence of orbifold Gromov–Witten theory.
1.2 Introspection
One of the organizers’ not-so-secret reasons for inviting me to give these lec- tures was to push me and my collaborators to finish the paper [3]. The or- ganizers were only partially successful: the paper, already years overdue, was only being circulated in rough form upon demand at the time of these lectures. Hopefully it will be made fully available by the time these notes are published. Whether they meant it or not, one of the outcomes of this intention of the organizers is that these lectures are centered completely around the paper [3]. I am not sure I did wisely by focussing so much on our work and not bringing in approaches and beautiful applications so many others have contributed, but this is the outcome and I hope it serves well enough.
K. Behrend, M. Manetti (eds.), Enumerative Invariants in Algebraic Geometry 1 and String Theory. Lecture Notes in Mathematics 1947, c Springer-Verlag Berlin Heidelberg 2008 2 D. Abramovich
1.3 Where Does All This Come From?
Gromov–Witten invariants of orbifolds first appeared, in response to string theory – in particular Zaslow’s pioneering [41] – in the inspiring work of W. Chen and Y. Ruan (see, e.g. [15]). The special case of orbifold cohomology of finite quotient orbifolds was treated in a paper of Fantechi-G¨ottsche [20], and the special case of that for symmetric product orbifolds was also dis- covered by Uribe [39]. In the algebraic setting, the underlying construction of moduli spaces was undertaken in [4], [5], and the algebraic analogue of the work of Chen-Ruan was worked out in [2] and the forthcoming [3]. The case of finite quotient orbifolds was developed in work of Jarvis–Kauffmann– Kimura [25]. Among the many applications I note the work of Cadman [11], which has a theoretical importance later in these lectures. In an amusing twist of events, Gromov–Witten invariants of orbifolds were destined to be introduced by Kontsevich but failed to do so – I tell the story in the appendix.
1.4 Acknowledgements
First I’d like to thank my collaborators Tom Graber and Angelo Vistoli, whose joint work is presented here, hopefully adequately. I thank the organizers of the summer school – Kai Behrend, Barbara Fantechi and Marco Manetti – for inviting me to give these lectures. Thanks to Barbara Fantechi and Damiano Fulghesu whose notes taken at Cetraro were of great value for the preparation of this text. Thanks are due to Maxim Kontsevich and Lev Borisov who gave permission to include their correspondence. Finally, it is a pleasure to acknowledge that this particular project was inspired by the aforementioned work of W. Chen and Ruan.
2 Gromov–Witten Theory
2.1 Kontsevich’s Formula
Before talking of orbifolds, let us step back to the story of Gromov–Witten invariants. Of course these first came to be famous due to their role in mirror symmetry. But this failed to excite me, a narrow-minded algebraic geometer such as I am, until Kontsevich [28] gave his formula for the number of rational plane curves. This is a piece of magic which I will not resist describing.
Setup
Fix an integer d>0. Fix points p1,...p3d−1 in general position in the plane. Look at the following number: Lectures on Gromov–Witten Invariants of Orbifolds 3
Definition 2.1.1. ⎧ ⎫ ⎨ C ⊂ P2 a rational curve, ⎬ Nd = # ⎩ deg C = d, and ⎭ . p1,...p3d−1 ∈ C
Remark 2.1.1. One sees that 3d − 1 is the right number of points using an elementary dimension count: a degree d map of P1 to the plane is parametrized by three forms of degree d (with 3(d+1) parameters). Rescaling the forms (one parameter) and automorphisms of P1 (three parameters) should be crossed out, giving 3(d +1)− 1 − 3=3d − 1.
Statement
Theorem 2.1.1 (Kontsevich). For d>1 we have − − 2 2 3d 4 − 3 3d 4 Nd = Nd1 Nd2 d1d2 d1d2 . 3d1 − 2 3d1 − 1 d = d1 + d2 d1,d2 > 0
Remark 2.1.2. The first few numbers are
N1 =1,N2 =1,N3 =12,N4 = 620,N5 = 87304.
The first two are elementary, the third is classical, but N4 and N5 are non- trivial.
Remark 2.1.3. The first nontrivial analogous number in P3 is the number of lined meeting four other lines in general position (the answer is 2, which is the beginning of Schubert calculus).
2.2 Set-Up for a Streamlined Proof
M0,4
We need one elementary moduli space: the compactified space of ordered four-tuples of points on a line, which we describe in the following unorthodox manner :
L P1 M0,4 = p1,p2,q,r ∈ L p1,p2,q,r distinct
1 TheopensetM0,4 indicated in the braces is isomorphic to P {0, 1, ∞},the coordinate corresponding to the cross ratio 4 D. Abramovich
p1 − p2 q − r CR(p1,p2,q,r)= . p1 − r q − p2 The three points in the compactification, denoted
0=(p1,p2 | q,r),
1=(p1,q | p2,r), and
∞ =(p1,r | p2,q),
p2 r r q q p2 r p2 q p1 p1 p1
01 ∞ describe the three different ways to split the four points in two pairs and position them on a nodal curve with two rational components.
A One-Parameter Family
We now look at our points p1,...p3d−1 in the plane. We pass two lines 1, 2 with general slope through the last point p3d−1 and consider the following family of rational plane curves in C → B parametrized by a curve B:
• Each curve Cb contains p1,...p3d−2 (but not necessarily p3d−1). • One point q ∈ Cb ∩ 1 is marked. • One point r ∈ Cb ∩ 2 is marked. In fact, we have a family of rational curves C → B parametrized by B, most of them smooth, but finitely many have a single node, and a morphism f : C → P2 immersing the fibers in the plane. Lectures on Gromov–Witten Invariants of Orbifolds 5
q 2
r
p3d−2
p1 p2 1
The Geometric Equation
We have a cross-ratio map
λ B −→ M0,4
C → CR(p1,p2,q,r)
Since points on P1 are homologically equivalent we get
−1 | −1 | degB λ (p1,p2 q,r) = degB λ (p1,q p2,r).
The Right-Hand Side
−1 | Now, each curve counted in degB λ (p1,q p2,r) is of the following form:
• It has two components C1,C2 of respective degrees d1,d2 satisfying d1 + d2 = d. • We have p1 ∈ C1 as well as 3d1 − 2 other points among the 3d − 4 points p3,...p3d−2. • We have p2 ∈ C2 as well as the remaining 3d2 −2 points from p3,...p3d−2. • We select one point z ∈ C1∩C2 where the two abstract curves are attached. • We mark one point q ∈ C1 ∩ 1 and one point r ∈ C2 ∩ 2. 6 D. Abramovich
q 2
r z C1
C2 p1 p2 1 3d−4 For every choice of splitting d1 + d2 = d we have ways to choose 3d1−2 the set of 3d1 − 2 points on C1 from the 3d − 4 points p3,...p3d−2.Wehave · Nd1 choices for the curve C1 and Nd2 choices for C2.Wehaved1 d2 choices for z, d choices for q and d for r. This gives the term 1 2 − −1 | 3d 4 · · · · degB λ (p1,q p2,r)= Nd1 Nd2 d1d2 d1 d2. 3d1 − 2 d = d1 + d2 d1,d2 > 0 A simple computation in deformation theory shows that each of these curves actually occurs in a fiber of the family C → B, and it occurs exactly once with multiplicity 1.
The Left-Hand Side −1 | Curves counted in degB λ (p1,p2 q,r) come in two flavors: there are irre- ducible curves passing through q = r = 1 ∩ 2. This is precisely Nd.
q = r = p3d−1 2
p2 p1 1 Lectures on Gromov–Witten Invariants of Orbifolds 7
−1 | Now, each reducible curve counted in degB λ (p1,p2 q,r) is of the follow- ing form:
• It has two components C1,C2 of respective degrees d1,d2 satisfying d1 + d2 = d. • We have 3d1 − 1 points among the 3d − 4 points p3,...p3d−2 are on C1. • We have p1,p2 ∈ C2 as well as the remaining 3d2 − 2 points from p3,...p3d−2. • We select one point z ∈ C1 ∩ C2. where the two abstract curves are at- tached. • We mark one point q ∈ C1 ∩ 1 and one point r ∈ C1 ∩ 2.
q 2
r
z C1
C2
p1 p2