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Thesis Reference Thesis Aspects of non standard cosmology VONLANTHEN, Marc Abstract Divers aspects de la cosmologie au-delà du modèle standard ont été explorés durant cette thèse. Le modèle standard fait face à de nombreux problèmes (énergie noire...), à tel point qu'un consensus s'est établi dans la communauté scientifique portant sur la nécessité de dépasser ce modèle. Nous avons successivement étudier des modèles à dimensions spatiales additionnelles, analyser des données cosmologiques de façon indépendante du modèle standard, étudier les conséquences d'une expansion anisotrope causée par un champ magnétique en présence de neutrinos et finalement, nous avons remis en question le principe d'homogénéité du modèle standard au travers d'un modèle comportant une densité d'énergie dépendant des coordonnées spatiales. Reference VONLANTHEN, Marc. Aspects of non standard cosmology. Thèse de doctorat : Univ. Genève, 2012, no. Sc. 4428 URN : urn:nbn:ch:unige-217895 DOI : 10.13097/archive-ouverte/unige:21789 Available at: http://archive-ouverte.unige.ch/unige:21789 Disclaimer: layout of this document may differ from the published version. 1 / 1 UNIVERSITE´ DE GENEVE` FACULTE´ DES SCIENCES D´epartement de physique th´eorique Prof. Ruth DURRER Aspects of non standard cosmology THESE` pr´esent´ee `ala Facult´edes Sciences de l’Universit´ede Gen`eve pour obtenir le grade de Docteur `es sciences, mention physique par Marc Vonlanthen de Romont (FR) Th`ese N◦ XXXXXX GENEVE` Atelier de reproduction de la Section de physique 2012 3 Remerciements En premier lieu, je tiens `asinc`erement remercier ma directrice de th`ese Ruth Durrer. Tout au long de ces quatre ann´ees de th`ese, Ruth Durrer a su r´eunir et mettre `ama disposition une intelligence et des qualit´es qui vont largement au-del`ade ses comp´etences purement scientifiques unaniment reconnues. Elle est encore parvenue `amettre en place et `amain- tenir un climat de travail serein bas´esur la confiance, qui m’a largement permis de mener `aterme ce travail, tout en le conjuguant avec les exigences de mon heureuse paternit´eet de mes autres engagements. Davantage qu’une simple et banale formation scientifique, c’est une mani`ere de former et de guider l’´etudiant, de lui montrer un chemin en y investissant de sa personne, tout en le laissant libre de faire ses propres d´etours, dont j’ai eu la chance de b´en´eficier. Et c’est avant tout ceci que je retiendrai de Ruth. Je remercie ensuite celles et ceux qui m’ont apport´eune aide quelconque ou avec qui j’ai eu la chance de discuter ou de collaborer scientifiquement: Marcus Ruser, Peter Wittwer, Syksy R¨as¨anen, Domenico Sapone, Julian Adamek, Elisa Fenu, Enea Didio, Martin Kunz et Andreas Malaspinas. Pour leur travail et leur gentillesse, je remercie encore les deux secr´etaires du d´epartement, Francine Gennai-Nicole et C´ecile Jaggi. Tout aussi important que la dimension scientifique, et sans doute mˆeme davantage, je remercie celles et ceux avec qui, au fil des doutes, des interrogations, des discussions et des joies, j’ai eu le privil`ege de cheminer durant ces ann´ees pass´ees. Parmi ces derni`eres et derniers, je ne peux manquer de citer mes amies et amis, compagnons et compagnes de groupe ou de bureau, G´eraldine Haack, Elisa Fenu, Mona Frommert, Lukas Hollenstein, Umberto Canella et Enea Didio. Finalement, mais `ace niveau-l`ad’engagement, ce ne sont plus des remerciements qu’elle m´erite, mais ma plus profonde reconnaissance `aJulia, ma compagne de coeur et de route, dont la patience, la tendresse, la confiance et les encouragements continuels ont ´et´ela clef de voˆute de ce travail. Julia m’a de plus donn´ecelle dont les yeux rayonnent `aen faire pˆalir de jalousie les feux c´elestes les plus intenses, Jeanne ma fille, que je remercie simplement de me rappeler tous les jours, sans mots, ce qui est essentiel pour moi. Examinateurs Le jury de cette th`ese se compose de Prof. Dr. Ruth Durrer, D´epartement de Physique Th´eorique de l’Universit´ede • Gen`eve (Suisse) Dr. Chiara Caprini, CEA-Saclay (France) • Dr. Martin Kunz, D´epartement de Physique Th´eorique de l’Universit´ede Gen`eve • (Suisse) Dr. Julien Lesgourgues, CERN (Gen`eve, Suisse) et Ecole´ Polytechnique F´ed´erale de • Lausanne (EPFL) Je les remercie chaleureusement d’avoir accept´ede faire partie du jury, ainsi que d’avoir consacr´edu temps `ala lecture de cette th`ese. 4 5 Pr´eliminaires Une science se d´efinit par son objet. Celui de la cosmologie est l’univers, le cosmos pour en r´ef´erer `al’´ethymologie grecque du mot. L’extension du concept de cosmos n’est pas demeur´ee fig´ee depuis l’av´enement de la longue chaˆıne de penseurs dont les r´eflexions ont ´et´enourries de l’insatiable curiosit´e`acomprendre l’infini qui nous entoure. De mani`ere simple, l’´evolution de la cosmologie depuis les philosophes pr´esocratiques, Thal`es (VIIe si`ecle av. J.-C.), Anaximandre (VIe si`ecle av. J.-C.), Anaxim`ene (VIe si`ecle av. J.-C.) et consorts, peut se comprendre comme un mouvement double ayant des composantes verticale de d´emythologisation du cosmos et plus largement de la nature s’accompa- • gnant d’une dramatique r´eduction de la densit´eontologique de l’univers `asa seule r´ealit´ephysique et horizontale d’hyper-scientifisation et de m´ecanisation irr´esistiblement conduites par • la math´ematisation de cette r´ealit´ephysique et des succ`es cons´ecutifs en termes de pr´edictions. Pour illustrer correctement ce mouvement double, il conviendrait de dresser un historique de l’id´ee de nature lors des deux derniers mill´enaires. Un tel travail d´epasse le cadre de cette introduction, et a de plus d´ej`a´et´emen´epar le philosophe Pierre Hadot1. Il est n´eanmoins int´eressant d’en relever quelques caract´eristiques permettant d’en appr´ehender le contenu essentiel. Dans les grands syst`emes cosmologiques antiques, tels celui d´ecrit par Platon (428-347 av. J.-C.) dans le Tim´ee, par Aristote (384-322 av. J.-C.) dans Du Ciel ou par Plotin (205- 270 ap. J.-C.) dans les Enn´eades, le r´e´el d´efinit une hi´erarchie de mondes dont le monde sensible, objet exclusif de la cosmologie moderne, repr´esente le niveau le plus grossier, celui dans lequels les objets naissent et meurent, croissent et d´ecroissent, sont g´en´er´es et cor- rompus. Le monde sensible est le monde du devenir. A ce titre, la connaissance du monde sensible poss`ede une valeur toute relative, car elle ne porte pas sur des lois ou principes immuables, mais bien sur des corps appel´es `adisparaˆıtre. C’est en ce sens que pour Platon, la science du monde physique n’est pas une connaissance `aproprement parler, mais une opinion incertaine. N´eanmoins, l’int´erˆet de la connaissance de la r´ealit´ephysique tient au fait qu’elle manifeste des r´ealit´es sup´erieures dans la hi´erarchie des mondes, dont elle est en quelque sorte le signe qui invite `ala connaissance de principes plus fondamentaux, lesquels sont `aleur tour eux-mˆemes signes de principes sup´erieurs. Il s’agit donc, dans ces syst`emes antiques, d’une conception du cosmos en pyramide, dont la base est constitu´ee par le monde sensible, les ´etages interm´ediaires par des principes immuables d´ependants du syst`eme en question (les Id´ees platoniciennes, les diff´erentes Amesˆ plotiniennes...) et le sommet par un principe premier qui produit et met en mouvement l’ensemble du r´eel. Ce premier principe est nomm´eD´emiurge dans le syst`eme de Platon, Moteur Immobile chez Aristote, l’Un chez Plotin, ou encore, plus tard, Dieu dans la vision cosmologique du christianisme et des autres monoth´eismes. Il convient toutefois de se garder d’identifier ces diff´erentes d´enominations, chacune ayant des sp´ecificit´es propres au corpus philosophique dont elle est 1P. Hadot, Le Voile d’Isis, 2004, Gallimard 6 issue. Nous avons d´ej`amentionn´eque du point de vue ´epist´emologique, la connaissance du monde physique n’a pas de valeur en soi. Ceci demande `aˆetre compl´et´een ajoutant que le type de connaissance pr´esente un degr´ede certitude d’autant plus ´elev´eet ferme que son objet affiche un caract`ere principiel ou fondamental. Ainsi, le degr´esuprˆeme de la connaissance est la connaissance du principe premier ou divin, et n’est accessible que par la raison (Aristote, Thomas d’Acquin) ou par l’extase mystique (Platon, Plotin, Augustin). L’impulsion cruciale au mouvement double bri`evement d´ecrit ci-dessus se situe au tour- nant de la Renaissance (XVIe-XVIIe si`ecles). Outre un contexte socio-culturel favor- able (apparition des universit´es, d´emocratisation du savoir), cette impulsion est due `a une poign´ee de philosophes et scientifiques dont les id´ees novatrices ne tarderont pas `a s’imposer. Galil´ee (1564-1642) compte bien ´evidemment parmi ces penseurs: ”La philosophie est ´ecrite dans ce livre immense qui se tient ouvert sous nos yeux - l’univers - et qui ne peut se comprendre si l’on n’a pr´ealablement appris `aen comprendre la langue et `aconnaˆıtre les caract`eres employ´es pour l’´ecrire. Ce livre est ´ecrit dans la langue math´ematique; ses caract`eres sont des triangles, des cercles et d’autres figures g´eom´etriques, sans l’interm´ediaire desquels il est impossible d’en comprendre humainement un seul mot.” 2 Clairement, cette citation illustre la r´eduction verticale du cosmos antique et de sa hi´erar- chie des mondes `asa dimension la plus superficielle, celle que l’homme appr´ehende par ses sens, la dimension physique.
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