Complexified Symplectomorphisms and Geometric Quantization Master
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Complexified Symplectomorphisms and Geometric Quantization Miguel Barbosa Pereira Thesis to obtain the Master of Science Degree in Master in Mathematics and Applications Supervisor(s): Prof. João Luís Pimentel Nunes Examination Committee Chairperson: Prof. Miguel Tribolet de Abreu Supervisor: Prof. João Pimentel Nunes Member of the Committee: Prof. José Manuel Vergueiro Monteiro Cidade Mourão Member of the Committee: Prof. Sílvia Nogueira da Rocha Ravasco dos Anjos November 2017 ii This thesis is dedicated to my parents and to my brother. iii iv Acknowledgments I would like to thank my advisor, Prof. Joao˜ Nunes, for all the time he took helping me, from my first semester as a mathematics student in Riemannian geometry, up to now in my thesis. He was always available for help and was always extremely patient. I also want to express my gratitude to all the teachers of the courses I took during these last two years, for all the knowledge they gave me in class, and for helping me when I had questions about the material outside of the class. This thesis started as a project for the Centre for Mathematical Analysis, Geometry, and Dynamical Systems, that awarded me a scholarship one year and a half ago. So I would like to thank CAMGSD for this opportunity to learn and do math in a way that is closer to the professional level. I also want to thank the members of the Examination Committee, Prof. Miguel Abreu, Prof. Jose´ Mourao,˜ and Prof. S´ılvia Anjos for their time reading and evaluating my thesis, and for their helpful corrections. During the time I was in Lisbon, I had the opportunity to meet new people and spend time with many excellent friends, who enriched my life. They would be too many to list, but I want to thank my friends from aerospace engineering, especially Tiago Silva, Manuel Barjona and Telmo Pires who encouraged me to become a better student, my neighbors from B1 for making it the best floor in RDP, as I do want to thank my friends from my class Z in Braga for the past 9 years. A special mention goes to Andre´ Pereira, for helping me in many occasions. Coming to study to a different city was not an easy experience for me, and I couldn’t have done it without the continuous support of my parents. When I decided to change from engineering to maths, they gave me complete freedom to do so. They have always been there for me. A big thank you, Mom and Dad. I want to thank my brother Pedro for all the memes and for putting up with my explanations of the math I was learning. Pedro, every aspiring physicist should know what a cotangent bundle is. Thank you to all the other members of my family, uncles, and cousins for caring for me. A special thank you to my grandmother Maria, for all the meals she cooked for me, but also for being a caring, strong and inspiring person. Finally, if you are reading this, I want to thank you, and I hope that the text is clear, organized and I hope that it is helpful to your studies. v vi Resumo Nesta tese estudamos geometria complexa, simplectica´ e Kahler.¨ O cap´ıtulo1 e´ introdutorio´ e apresentamos os preliminares teoricos´ relevantes para o resto da tese. No cap´ıtulo2 apresentamos um resumo de [14]. Partindo de uma variedade K ahler,¨ atuamos na sua estrutura complexa via pullback pelo fluxo de um campo vetorial Hamiltoniano. Provamos que as metricas´ Kahler¨ resultantes formam uma curva numa variedade Riemanniana (de dimensao˜ infinita) cujos pontos sao˜ potenciais Kahler,¨ e que esta curva e´ na realidade uma geodesica´ relativamente a` metrica´ (que e´ a metrica´ de Mabuchi). No cap´ıtulo3 estudamos estruturas K ahler¨ no fibrado cotangente de um grupo de Lie G. Explicamos as estruturas Kahler¨ conhecidas de Hall e Kirwin e de Kirwin, Mourao˜ e Nunes, e relacionamos as duas. Em [12] os autores provam que e´ poss´ıvel definir estruturas Kahler¨ em T ∗G usando func¸oes˜ complexificadoras Ad-invariantes. Provamos que a condic¸ao˜ de Ad-invarianciaˆ pode ser substitu´ıda por um conjunto de equac¸oes˜ mais geral. No cap´ıtulo5, damos um exemplo de quantizac¸ ao˜ geometrica´ de T ∗G. Introduzimos um fibrado de ∗ linha L em T G e uma polarizac¸ao˜ Pτ definida por fazer um pullback da polarizac¸ao˜ vertical pelo fluxo complexificado de um campo vetorial Hamiltoniano. A partir destas estruturas, definimos um espac¸o de Hilbert cujos elementos sao˜ as secc¸oes˜ integraveis´ e Pτ -polarizadas de L. Palavras-chave: geometria Kahler,¨ quantizac¸ao˜ geometrica,´ metrica´ de Mabuchi, grupo de Lie, fibrado cotangente, difeomorfismos complexificados vii viii Abstract In this thesis, we study complex, symplectic and Kahler¨ geometry. Chapter1 is introductory and presents the relevant theoretical background for the rest of the thesis. In chapter2 we present an overview of [14]. Starting with a K ahler¨ manifold, we act on its complex structure by pulling back along the complexified flow of an Hamiltonian vector field. We prove that the resulting Kahler¨ metrics form a curve in an (infinite dimensional) Riemannian manifold whose points are Kahler¨ potentials and that this curve is actually a geodesic with respect to the metric (which is the Mabuchi metric). In chapter3 we study K ahler¨ structures on the cotangent bundle of a Lie group G. We explain the known Kahler¨ structures of Hall and Kirwin and of Kirwin, Mourao,˜ and Nunes and relate the two. In [12], the authors prove that one can define Kahler¨ structures on T ∗G using Ad-invariant complexifier functions. We prove that the condition of Ad-invariance can be replaced by a more general set of equations. In chapter5, we give an example of geometric quantization of T ∗G. We introduce a line bundle L on ∗ T G and a polarization Pτ defined by pulling back the vertical polarization along the complexified flow of an Hamiltonian vector field. From these structures, we define a Hilbert space whose elements are integrable Pτ -polarized sections of L. Keywords: Kahler¨ geometry, geometric quantization, Mabuchi metric, Lie group, cotangent bundle, complexified diffeomorphisms ix x Contents Acknowledgments...........................................v Resumo................................................. vii Abstract................................................. ix List of Tables.............................................. xiii Nomenclature.............................................. xv Glossary................................................1 1 Theoretical Background1 1.1 Symplectic Manifolds.......................................1 1.2 Moser’s Theorem.........................................3 1.3 Complex Geometry........................................3 1.3.1 Almost complex manifolds. Complex manifolds....................3 1.3.2 Splittings and forms on complex manifolds.......................5 1.3.3 When is an almost complex manifold a complex manifold?..............6 1.3.4 Compatible structures. Kahler¨ manifolds........................7 1.3.5 Cohomology results on Kahler¨ manifolds.......................8 1.4 Lie groups and Lie algebras................................... 11 1.4.1 Representations..................................... 11 1.4.2 Adjoint representations................................. 12 1.5 Structure functions and structure constants.......................... 13 2 Geodesics on the Space of Kahler¨ Metrics of a Manifold 15 2.1 Lie Series and complexified flows................................ 15 2.2 Action of a complexified analytic flow on a complex structure................ 18 2.3 Restriction to Hamiltonian flows................................. 19 2.4 Action on Kahler¨ structures................................... 19 2.5 Geodesics on the Space of Kahler¨ metrics........................... 24 2.6 Symplectic picture and complex picture............................ 25 2.7 The path of Kahler¨ metrics (generated by a complex flow of an Hamiltonian vector field) is a geodesic............................................ 26 xi 3 Kahler¨ structures on the Cotangent Bundle of a Lie Group 31 3.1 T ∗G is diffeomorphic to G × g .................................. 31 3.2 Kahler¨ structures on T ∗G .................................... 32 3.2.1 Standard Kahler¨ structure on T ∗G ........................... 32 3.2.2 The Kahler¨ structure of Hall and Kirwin........................ 33 3.2.3 The Kahler¨ structure of Kirwin, Mourao˜ and Nunes.................. 34 3.2.4 Problems to study.................................... 35 3.3 Useful coordinates based on the diffeomorphism T ∗G =∼ G × g ............... 35 3.4 Relation between theorems 3.2.2 and 3.2.4.......................... 41 3.5 The theorem of Hall and Kirwin in the case of a Lie group.................. 43 3.5.1 Attempt at proving conjecture 3.5.1........................... 43 3.5.2 !β as a pullback of !ST by a diffeomorphism..................... 45 3.6 The theorem of Kirwin, Mourao˜ and Nunes in the case of non Ad-invariant h ........ 47 τ 3.6.1 'h is a diffeomorphism. The complex structure J .................. 47 ∗ 3.6.2 A basis for T1;0(T G) .................................. 48 3.6.3 Equations for T ∗G being Kahler¨ in terms of h ..................... 49 4 A Short Overview of Geometric Quantization 53 4.1 Introduction and motivation................................... 53 4.2 General set up.......................................... 54 5 An Example of Quantization of the Cotangent Bundle of a Lie Group 55 5.1 General setup........................................... 55 5.2 A prequantum line bundle for T ∗G ............................... 56 5.3 A polarization for T ∗G ...................................... 57 5.4 T ∗G is foliated