Na última década, o extraordinário progresso nas observações astronômicas (distâncias com supernovas (SNe Ia), espectros de potência da matéria e da radiação cósmica de fundo (RCF), determinação do brilho de aglomerados de galáxias, etc.) aliado com importantes desenvolvimentos teóricos, transformaram a Cosmologia numa das fronteiras mais excitantes da ciência contemporânea. Nesta tese, diferentes testes observacionais são utilizados para vincular alguns cenários cosmológicos acelerados (com e sem energia escura), todos eles definidos no contexto teórico da Relatividade Geral. Inicialmente, para uma grande classe de modelos com decaimento do vácuo, investigamos os vínculos provenientes da existência de objetos velhos em altos redshifts. No modelo de Chen e Wu generalizado, encontramos que o limite para o parâmetro livre descrevendo a taxa do decaimento do vácuo é 0,21 < n < 0,81. Este resultado descarta o modelo de Chen e Wu original (n=2) e também o modelo de concordância cósmica, LCDM (n=0). Além disso, quando incluímos o fluido bariônico em nossa análise do modelo de Wang e Meng, obtemos para seu parâmetro livre um limite inferior, epsilon > 0,231, um valor em desacordo com estimativas independentes baseadas em SNe Ia, RCF e o brilho de Raios-X de aglomerados. Propusemos também um teste estatístico com base nas idades estimadas para uma amostra de 13 galáxias velhas em altos redshifts. Através de uma análise conjunta envolvendo as idades das galáxias e as oscilações acústicas dos bárions (BAO), vinculamos o valor da constante de Hubble no contexto do modelo LCDM plano. Considerando um tempo de incubação adotado por diferentes autores, obtemos h=0,71±0,04 (1 sigma), um resultado de acordo com observações independentes baseadas em Cefeidas (obtidas com o Hubble Space Telescope) e outras estimativas mais recentes. Outro resultado interessante foi obtido através de uma análise termodinâmica para uma classe de modelos com interação no setor escuro (matéria escura-energia escura). Contrariamente ao que se pensava até então, encontramos que a termodinâmica permite que a matéria escura decaia em energia escura, contanto que ao menos uma das componentes possua um potencial químico não-nulo. Como complemento, mostramos que, para um termo de interação específico, dados de SNe Ia, BAO e RCF favorecem o decaimento da matéria escura com ~ 93% de confiança estatística. Investigamos também o comportamento do redshift de transição em diferentes cosmologias, com e sem energia escura, e mostramos que essa quantidade pode ter uma variação extrema dependendo do modelo cosmológico subjacente. Finalmente, discutimos também um novo modelo cosmológico cuja aceleração em baixos redshifts é determinada pela criação de partículas da matéria escura fria. O modelo representa uma redução do setor escuro, isto é, não tem energia escura, contém apenas um parâmetro livre e satisfaz os vínculos de Supernovas do tipo Ia tão bem quanto o modelo LCDM padrão. / In the last decade, the extraordinary progress of the astronomical observations (distances with supernovas, matter and cosmic background radiation (CBR) power spectrum, X-ray surface brightness of galaxy clusters, etc) associated with important theoretical developments turned Cosmology one of the most exciting frontiers of contemporary science. In this thesis, different observational tests are used to constrain several cosmological accelerating scenarios (with and without dark energy), all of them defined in the theoretical framework of General Relativity. Initially, for a large class of decaying vacuum models, we investigate the constraints provided by the existence of old high redshift objects. In the model proposed by Chen and Wu, we find that the limit for the free parameter describing the decay rate of the vacuum fluid is 0.21 < n < 0.81. This result ruled out the original Chen and Wu model (n = 2) and also the cosmic concordance model, LCDM (n = 0). Further, when we include the baryonic fluid in our analysis of the Wang and Meng model, we find for its free parameter a lower bound, epsilon > 0.231, a value in disagreement with independent estimates based on SNe Ia, CMB (shift parameter) and the X-ray surface brightness of galaxy clusters. We also propose a new cosmological statistical test based on the estimated ages of 13 old high redshift galaxies. By performing a joint analysis involving the ages of the galaxies and the baryon acoustic oscillations (BAO) probe, we constrain the value of the Hubble parameter in the context of the flat LCDM model. For an incubation time adopted by different authors, we find h = 0.71 ± 0.04 (1 sigma), a result in agreement with independent observations based on Cepheids (obtained with the Hubble Space Telescope) and other recent estimations. Another interesting result has been derived from a thermodynamic analysis for a class of models endowed with interaction in the dark sector (dark matter and dark energy). In contrast with some results appearing in the literature, we show that the decaying of cold dark matter into dark energy is not forbidden by thermodynamics, provided that the chemical potential of one component is different from zero. As a complement, we also show (for a specific term describing the interaction) that this kind of decaying is favored by SNe Ia, BAO and CMB data with ~ 93% of statistical confidence. We also investigate in detail the behavior of the transition redshift for different cosmologies (with and without dark energy). It is found that such a quantity may have an extreme variation that depends on the underlying cosmological model. Finally, we also discuss a new cosmological model whose acceleration at low redshifts is determined by the creation of cold dark matter particles. The model represents a reduction of the dark sector, that is, it has no dark energy, contains only one free parameter and satisfies the Supernovae type Ia constraints with the same precision of the standard LCDM model.

Dans les dernières décennies, un modèlecosmologique, dénommé “modèle ACDM” ,a émergé et plusieurs observations ont montréqu’il est consistant avec les données. Toutefois, ilsexistent des problèmes encore ouverts ; en particuliernous ne connaissons pas la cause de l’accélérationobservée de l’expansion de l’Univers. De nombreuxmodèles alternatifs ont été proposés.Cette thèse traite d’une approche pour contraindreles alternatives au modèle ACDM dénommée “EffectiveTheory of Dark Energy”. Elle s’applique àtoutes les théories où un seul degré de liberté scalaireest ajouté aux deux degrés de liberté de la RelativitéGénérale. Ces théories sont appelées “théoriestenseur-scalaire”.Le chapitre 1 résume la plus générale classe de théoriestenseur-scalaire viables actuellement connue.Le chapitre 2 introduit la théorie efficace de l’énergienoire. L’inclusion dans la théorie efficace descouplages généraux entre le secteur gravitationnelle(métrique et champ scalaire) et la matière est décritdans le chapitre 3. Ceci inclut aussi la possibilitéque différentes espèces interagissent de façondifférente avec la gravité. La possibilité d’analyseren détail la stabilité de la théorie est explicité dansle chapitre 4.Un objectif de cette thèse est aussi l’investigationdes effets observables des déviations du modèleLCDM. Le chapitre 5 considère la possibilité d’uneinteraction entre la matière noire et l’énergie noire.Ce chapitre étudie les effets d’une telle interactionet, plus généralement, analyse la capacité des observationsfutures à imposer des contraintes sur lesparamètres libres de la théorie efficace. Le chapitre 6 se concentre sur les effets observationnels de lathéorie “ beyond-Horndeski” , en démontrant quedans ce théorie il existe un melange cinétique entrela matière et le champ scalaire. Ce melange donnelieu à un effets particuliers: l’affaiblissement de lagravité aux échelles des grands structures. / In the last decades, a cosmological model that fits observations through a vast range of scales emerged. It goes under the name of ACDM. However, there are still challenging questions that remain unanswered by this model, such as what causes the observed accelerated expansion of the universe. Hence, many alternative models have been proposed. This thesis concerns an approach to test such models known as “Effective Theory of Dark Energy” . It applies to all models where general relativity is modified by adding a single scalar degree of freedom, called “ scalar-tensor theories”. In Chapter 1 I introduce the most general classof such theories currently known. In Chapter 2, I resume the Effective theory of dark energy. The inclusion of a general coupling between matter and the gravitational sector is the subject of Chapter 3. Chapter 4 analyses in details the stability of different classes of theories.Another goal of the thesis is to study the observable effects of deviations from ACDM. In Chapter5, I consider the possibility of an interaction between dark matter and dark energy and I analyse the constraining power of future surveys on the free parameters of the theory. Chapter 6 focuses on the observational effects of theories where a kinetic mixing between matter and the scalar field exists.This gives a peculiar effect, namely the weakening of gravity at large scale structure scales.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22028号 / 理博第4532号 / 新制||理||1651(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 向山 信治, 准教授 Antonio De Felice, 教授 鶴 剛 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Les résultats des observations cosmologiques réalisées à la charnière du siècle (SN1A, CMB, BAO) montrent que contrairement aux prévisions du modèle standard, l'expansion de l'Univers est actuellement en train de s'accélérer. Pour rendre compte de ce phénomène, un composant inconnu dénommé "énergie noire" (Dark Energy) a été introduit soit directement comme un fluide de pression négative, soit indirectement en modifiant la Relativité générale. Après avoir présenté le cadre général de la description de l'Univers, ainsi que le modèle cosmologique standard actuellement accepté, la présente thèse étudie les interactions possibles entre l'énergie noire et une éventuelle courbure de l'espace, en s'intéressant plus particulièrement aux cas où l'incertitude sur la courbure peut falsifier la nature "fantôme" de cette énergie noire. Dans un deuxième temps, la possibilité d'obtenir un comportement de type énergie noire au moyen d'une modification de la Relativité générale est abordée en faisant appel aux théories scalaire-tenseur. Les conditions générales de viabilité de ces théories sont présentées, ainsi que les conditions d'existence d'énergie noire, normale et fantôme. Enfin la possibilité de mettre en évidence cette énergie noire d'origine scalaire-tenseur par des mesures dans le Système solaire est étudiée en utilisant le formalisme de l'analyse post-newtonienne paramétrée. / The results of the cosmological observations at the turn of the century (SN1a, CMB, BAO) show that, in contrast to the predictions of the standard model, the Universe expansion is presently accelerating. To account for this fact, an unknown component dubbed "dark energy" was introduced either directly as a fluid with negative pressure, or indirectly as a modification of General Relativity.After the presentation of the general frame of the Universe description, and of the presently accepted cosmological standard model, we study the interactions between dark energy and a possible spatial curvature, with special attention to the cases where the curvature uncertainty may falsify the phantom nature of dark energy. In a second step we consider a modification of General Relativity, the Scalar-Tensor theories, as a way to generate dark energy. The general viability conditions for these theories are presented, as well as the conditions for the presence of normal and phantom dark energy. In particular we study the possibility to detect this Scalar-Tensor dark energy with measurements within the Solar System using the Parametrised Post-Newtonian formalism.

Galaxy redshift surveys are powerful probes of cosmology. Yet, in order to fully exploit the information contained in galaxy surveys, we need to improve upon our understanding of the structure formation in the Universe. Galaxies are formed/observed at late times when the density field is no longer linear so that understanding non-linearities is essential. In this thesis, we show that, at high redshifts, we can accurately model the galaxy power spectrum in redshift space by using the standard cosmological perturbation theory. Going beyond the power spectrum, we can use the three-point function, or the bispectrum, to gain important information on the early universe as well as on the galaxy formation via measurements of primordial non-Gaussianity and galaxy bias. We show that the galaxy bispectrum is more sensitive to primordial non-Gaussianities than previously recognized, making high-redshift galaxy surveys a particularly potent probe of the physics of inflation. Weak lensing offers yet another way of probing cosmology. By cross correlating the angular position of galaxies with the shear measurement from galaxy lensing or CMB lensing, we also show that one can obtain the information on cosmological distance scale, the galaxy bias, and the primordial non Gaussianity from weak lensing method. / text

The nature of dark matter and dark energy are currently two of the most important questions in cosmology. In this thesis, we consider studying the dark universe with the redshifts and peculiar velocities of galaxies. In the first half of the thesis, we analyse current peculiar velocity measurements of the bulk flow of our local volume to estimate the underlying dark matter power spectrum. In the second half of the thesis, we consider the prospects for measuring dark matter and dark energy with future galaxy redshift surveys, particularly via redshift space distortions. Fundamentally, bulk flow measurements and redshift space distortions are both sensitive probes of the power spectrum and growth rate of cosmic structure. In the final chapter, we directly compare power spectrum measurements with both methods.

Doctor of Philosophy / Department of Physics / Bharat Ratra / The expansion rate of the Universe changes with time, initially slowing (decelerating) when the universe was matter dominated, because of the mutual gravitational attraction of all the matter in it, and more recently speeding up (accelerating). A number of cosmological observations now strongly support the idea that the Universe is spatially flat (provided the dark energy density is at least approximately time independent) and is currently undergoing an accelerated cosmological expansion. A majority of cosmologists consider ``dark energy" to be the cause of this observed accelerated cosmological expansion. The ``standard" model of cosmology is the spatially-flat $\Lambda$CDM model. Although most predictions of the $\Lambda$CDM model are reasonably consistent with measurements, the $\Lambda$CDM model has some curious features. To overcome these difficulties, different Dark Energy models have been proposed. Two of these models, the XCDM parametrization and the slow rolling scalar field model $\phi$CDM, along with ``standard" $\Lambda$CDM, with the generalization of XCDM and $\phi$CDM in non-flat spatial geometries are considered here and observational data are used to constrain their parameter sets. In this thesis, we start with a overview of the general theory of relativity, Friedmann's equations, and distance measures in cosmology. In the following chapters we explain how we can constrain the three above mentioned cosmological models using three data sets: measurements of the Hubble parameter $H(z)$, Supernova (SN) apparent magnitudes, and the baryonic acoustic oscillations (BAO) peak length scale, as functions of redshift $z$. We then discuss constraints on the deceleration-acceleration transition redshift $z_{\rm da}$ using unbinned and binned $H(z)$ data. Finally, we incorporate the spatial curvature in the XCDM and $\phi$CDM model and determine observational constraints on the parameters of these expanded models.