LABORATOIRE DE PHYSIQUE THEORIQUE´ ECOLE´ NORMALE SUPERIEURE´ THESE` DE DOCTORAT DE L'UNIVERSITE´ PARIS VI Specialit´e: Physique Th´eorique Present´eepar : Ioannis G. Florakis 1 Pour obtenir le grade de Docteur de l'Universit´eParis VI Titre : Th´eoriede Cordes et Applications Ph´enom´enologiques et Cosmologiques tel-00607408, version 1 - 8 Jul 2011 7 Juillet 2011 Jury Costas Bachas Massimo Bianchi (Rapporteur) Pierre Binetruy Costas Kounnas (Directeur de th`ese) Augusto Sagnotti Gabriele Veneziano (Rapporteur) 1. fl[email protected] , fl[email protected] Remerciements Now that this manuscript has been completed and the four-year journey has reached its end, I would like to take this opportunity to thank the people who have been instrumental for the accomplishment of this task. First and foremost, I would like to thank my thesis advisor, Costas Kounnas, for in- troducing me to the world of research and for playing a central role in the formation of my `research personality'. His unparalleled enthusiasm and love for theoretical physics have been a unique source of inspiration for me. His teachings have, at times, surpassed the mere form of equations on his blackboard 2 and have been influential and beneficial to me in ways that I would never be able to adequately describe in words, shaping my perception of scientific integrity. Working with him for four years has been an adventure, a journey into the realm of theoretical physics. For this and many other things, I would like to thank him here. I would also like to thank Ioannis Iliopoulos for his enormous support, in particular, during my first years in France. It is true that without his influence and guidance, this work would never have been possible. Similarly, I would like to thank Marumi Kado, for his warm support and advice at that early stage of my doctoral studies. My sincere thanks extend to the former director of the Laboratoire de Physique Th´eorique de l'Ecole Normale Sup´erieure,Bernard Julia, for welcoming me to the laboratory, as well as to the secretaries, Nicole Ribet, Mireille Jouet, Beatrice Fixois and Christine Trecul for their constant administrative support. At the same time I would like to acknowledge the help of Marc-Thierry Jeakel and Bernard Massot for their assistance with technical matters. I would like to express my gratitude to Augusto Sagnotti, Costas Bachas, Massimo Bian- chi, Pierre Binetruy and Gabriele Veneziano for agreeing to become members of my jury. In addition, throughout my years at ENS, I have greatly benefitted from discussions with Costas Bachas and Jan Troost. I would like to thank them both. During my doctoral work I have had the pleasure to collaborate with Nicolas Toumbas, tel-00607408, version 1 - 8 Jul 2011 Herv´ePartouche, Thomas Mohaupt, Mirian Tsulaia, John Rizos and Alon Faraggi. It is difficult to underestimate the influence they have had on my perception of string theory and I have benefitted a great deal by working with them. In addition, I would like to thank Nicolas Toumbas for inviting me to the University of Cyprus several times during our collaboration. The discussions we shared during those visits have been characterized by his deep physical intuition and working with him has been a unique creative experience as well as a true pleasure. In addition, I would like to thank Dieter L¨ust,Carlo Angelantonj, Costas Skenderis, Massimo Bianchi, Chris Hull, Alon Faraggi, for inviting me to present my work at the string theory groups of the Universities of M¨unchen, Torino, Amsterdam, Roma \Tor Vergata", Imperial College in London and Liverpool, respectively, and to Emmanuel Floratos and George Savvidis for inviting me to present a talk to the National Centre of Scientific Research `Demokritos' in Athens. Their questions and comments during the talks and subsequent 2. Which has become, by now, legendary to all those that have discussed physics with him. 2 discussions have been invaluable to me and have lead to a deeper understanding of certain notions. I would, further, like to thank George Zoupanos and the organizers of the Corfu Summer Institute for inviting me to present my work in September 2009 and 2010 and especially Ifigeneia Moraiti for her outstanding logistic support. I am indebted to Ignatios Antoniadis for welcoming me to the CERN Theory Division as an associate, during the last six months of my doctorate. My stay at CERN has been an invaluable experience for me, placing me in the heart of the european high energy physics community, in an environment fostering strong ties between theory and experiment. I have greatly benefitted from my stay there and from the discussions with staff members, postdocs and visitors. I am grateful especially to Michelle Connor, Elena Gianolio, Nanie Perrin and Jeanne Rostant for their exceptional exceptional administrative support. During those last four years in Paris and my semester at CERN, I was lucky enough to have met and interacted with many young researchers and fellow students. Discussing physics with them has been a real pleasure, giving me the opportunity to gain insight into a variety of different subjects, from string theory to phenomenology and cosmology. To this end, I would like to thank my colleagues and friends at the LPT-ENS and CPHT, Fran¸cois Bourliot, Sebastien Leurent, Tristan Catelin-Jullien, Vitor Sessak and especially to Cezar Condeescu and Konstantinos Siampos. Among the many people I have had the pleasure to discuss with and learn from at CERN, I will mention, in particular, Ahmad Zein Assi, Андрей Khmelnitski (and his precious HP computer for the lovely `jet' sound it would make in the office we shared at CERN), Emanuele Castorina, Sandeepan Gupta and Marius Wiesemann. Going back to my undergraduate years, it is only fair for me to acknowledge the ins- trumental influence of Fokion Hadjioannou, Harris Apostolatos and Petros Ioannou. These were the people that first sparkled my interest in theoretical physics during my `young and innocent' years at the University of Athens. This thesis would never have been written were it not for them. Furthermore, I would like to thank Konstantinos Sfetsos, for his mysteriously motivating remarks during some of our discussions, which have been highly beneficial to me in the longterm. I would also like to thank `Max' for invaluable lessons of life, which this margin would tel-00607408, version 1 - 8 Jul 2011 be too small to contain... I am grateful to Mouly and Angelica for generously offering the most precious of smiles, and to Evi and Yanni for the relaxing music compilations that kept me company during those endless nights in Geneva, while I was typing this thesis. Finally, a big thank you goes to Ioanna, Michelle, Анастасия, Татьяна and Ольга for adding some colour charge into my life spectrum... Last but certainly not least, I would like to thank my parents and sister, for their support and understanding for all these years, without which this endeavor would never have been completed. This thesis is dedicated to them. 3 Abstract This thesis treats applications of String Theory to problems of cosmology and high energy phenomenology. In particular, we investigate problems related to the description of the ini- tial state of the universe, using the methods of perturbative String Theory. After a review of the string-theoretic tools that will be employed, we discuss a novel degeneracy symme- try between the bosonic and fermionic massive towers of states (MSDS symmetry), living at particular points of moduli space. We study the marginal deformations of MSDS vacua and exhibit their natural thermal interpretation, in connection with the resolution of the Hagedorn divergences of string thermodynamics. The cosmological evolution of a special, 2d thermal `Hybrid' model is presented and the correct implementation of the full stringy degrees of freedom leads to the absence of gravitational singularities, within a fully pertur- bative treatment. Keywords : String Theory, Conformal Field Theory, Orbifold Compactifications, String Thermodynamics, Hagedorn Phase Transition, Non-Singular String Cosmology. R´esum´e Cette th`esetraite des applications de la Th´eoriedes Cordes aux probl`emesde la cosmo- tel-00607408, version 1 - 8 Jul 2011 logie et de la ph´enom´enologie.En particulier, nous ´etudionsdes probl`emesli´es`ala descrip- tion de l'´etatinitial de l'Univers, en utilisant les m´ethodes perturbatives de la Th´eoriedes Cordes. Apr`esune pr´esentation des outils n´ecessaires, nous pr´esentons une nouvelle sym´etrie de d´eg´en´erescencespectrale entre les ´etatsmassifs bosoniques et fermioniques (appel´ee sym´etrieMSDS), se trouvant aux points particuliers de l'espace des modules. Nous ´etudions les d´eformationsmarginales des vides MSDS et mettons en ´evidenceleur interpr´etationther- mique, et leur lien avec la r´esolutiondes divergences de Hagedorn de la thermodynamique des cordes. L'´evolution cosmologique d'un vide thermique bidimensionnel est pr´esent´ee.On d´emontre que la prise en compte des tous les degr´esde libert´eau niveau des cordes m`ene`a l'absence des singularit´esgravitationnelles, dans un traitement enti`erement perturbatif. Mots-cl´es: Th´eoriedes Cordes, Th´eorieConforme des Champs, Compactification sur des Orbifolds, Thermodynamique des Cordes, Transition de Phase de Hagedorn, Cosmologie des Cordes sans Singularit´e. 4 tel-00607408, version 1 - 8 Jul 2011 5 R´esum´eD´etaill´e Cosmologie des Cordes et S´electiondu Vide Initial Certains des probl`emesles plus difficiles ouverts de la cosmologie moderne sont directe- ment li´es`al'´ereprimordiale de notre Univers, o`ules notions de la th´eoriedes champs ne sont plus valides, `acause des effets quantiques de la gravit´e,qui dominent la phase chaude et fortement courb´eede l'Univers. Actuellement, la th´eoriedes cordes ainsi que son extension non-perturbative, la th´eorie-M,sont les candidates les plus prometteuses pour formuler une th´eoriecoh´erente de la gravitation quantique.