>Magister Scientiae - MSc / We begin by finding a system of differential equations for the background and linearly perturbed variables in the standard, ɅCDM model, using the Einstein Field Equations, and then solving these numerically. Later, we extend this to dynamical dark energy models parameterised by an equation of state, w, and a rest frame speed of sound, cs. We pay special attention to the large-scale behaviour of Δm, the gauge invariant, commoving matter density, since the approximation Δm ≃ δm, where δm is the longitudinal gauge matter density, is more commonly used, but breaks down at large scales. We show how the background is affected by w only, so measurements of perturbations are required to constrain cs. We examine how the accelerated expansion of the universe, caused by dark energy, slows down the growth rate of matter. We then show the matter power spectrum is not in itself useful for constraining dark energy models, but how redshift-space distortions can be used to extract the growth rate from the galaxy power spectrum, and hence how redshift-space power spectra can be used to constrain different dark energy models. We find that on small scales, the growth rate is more dependent on w, while on large scales, it depends more on cs.

>Magister Scientiae - MSc / The growth of structure in the Universe proceeds via the collapse of dark matter and baryons. This process is retarded by dark energy which drives an accelerated expansion of the late Universe. In this thesis we use cosmological perturbation theory to investigate structure formation for a particular class of dark energy models, i.e. interacting dark energy models. In these models there is a non-gravitational interaction between dark energy and dark matter, which alters the standard evolution (with non-interacting dark energy) of the Universe. We consider a simple form of the interaction where the energy exchange in the background is proportional to the dark energy density. We analyse the background dynamics to uncover the e ect of the interaction. Then we develop the perturbation equations that govern the evolution of density perturbations, peculiar velocities and the gravitational potential. We carefully account for the complex nature of the perturbed interaction, in particular for the momentum transfer in the dark sector. This leads to two di erent types of model, where the momentum exchange vanishes either in the dark matter rest-frame or the dark energy rest-frame. The evolution equations for the perturbations are solved numerically, to show how structure formation is altered by the interaction.

L'énergie sombre est l'un des grands mystères de la cosmologie moderne, responsable de l'actuelle accélération de l'expansion de notre Univers. Son étude est un des axes principaux de ma thèse : une des voies que j'exploite s'appuie sur la structuration de l'Univers à grande échelle à travers un effet observationnel appelé effet Sachs-Wolfe intégré (iSW). Cet effet est théoriquement détectable dans le fond diffus cosmologique (FDC) : avant de nous parvenir cette lumière traverse un grand nombre grandes structures sous-tendues par des potentiels gravitationnels. L'accélération de l'expansion étire et aplatit ces potentiels pendant le passage des photons du FDC, modifiant leur énergie d'une façon dépendante des caractéristiques de l'énergie sombre. L'effet iSW n'a qu'un effet ténu sur le FDC, obligeant l'utilisation de données externes pour le détecter. Une approche classique consiste à corréler le FDC avec un traceur de la distribution de la matière, et donc des potentiels sous-jacents. Maintes fois tentée avec des relevés de galaxies, cette corrélation n'a pas donné à l'heure actuelle de résultat définitif sur la détection de l'effet iSW, la faute à des relevés pas assez profonds et/ou avec une couverture trop faible. Un partie de ma thèse est dédiée à la corrélation du FDC avec un autre fond "diffus" : le fond diffus infrarouge (FDI), qui est constitué de l'émission intégrée des galaxies lointaines non-résolues. J'ai pu montrer qu'il représente un excellent traceur, exempt des défauts des relevés actuels. Les niveaux de signifiance attendus pour la corrélation CIB-CMB excèdent ceux des relevés actuels, et rivalisent avec ceux prédits pour la futur génération de très grands relevés. Dans la suite, ma thèse a porté sur l'empreinte individuelle sur le FDC des plus grandes structures par effet iSW. Mon travail sur le sujet a d'abord consisté à revisiter une étude précédente d'empilement de vignettes de FDC à la position de structures, avec mes propres protocole de mesure et tests statistiques pour vérifier la signifiance de ces résultats, délicate à évaluer et sujette à de possibles biais de sélection. J'ai poursuivi en appliquant cette même méthode de détection à d'autres catalogues de structures disponibles, beaucoup plus conséquents et supposément plus raffinés dans leur algorithme de détection. Les résultats pour un d'eux suggère la présence d'un signal à des échelles et amplitudes compatible avec la théorie, mais à des niveaux modérés de signifiance. Ces résultats empilements font s'interroger concernant le signal attendu : cela m'a amené à travailler sur une prédiction théorique de l'iSW engendré par des structures, par des simulations basées sur la métrique de Lemaître-Tolman-Bondi. Cela m'a permis de prédire l'effet iSW théorique exact de structures existantes : l'amplitude centrale des signaux mesurés est compatible avec la théorie, mais présente des caractéristiques non-reproductibles par ces mêmes prédictions. Une extension aux catalogues étendus permettra de vérifier la signifiance de leurs signaux et leur compatibilité avec la théorie. Un dernier pan de ma thèse porte sur une époque de l'histoire de l'Univers appelée réionisation : son passage d'un état neutre à ionisé par l'arrivée des premières étoiles et autres sources ionisantes. Cette période a une influence importante sur le FDC et ses propriétés statistiques, en particulier sur son spectre de puissance des fluctuations de polarisation. Dans mon cas, je me suis penché sur l'utilisation des mesures de températures du milieu intergalactique, afin d'étudier la contribution possible de la désintégration et annihilation de l'hypothétique matière sombre. A partir d'un travail théorique sur plusieurs modèles et leur comparaison aux observations de température, j'ai pu extraire des contraintes intéressantes et inédites sur les paramètres cruciaux de la matière sombre et des caractéristiques cruciales de la réionisation elle-même. / The dark energy is one of the great mysteries of modern cosmology, responsible for the current acceleration of the expansion of our Universe. Its study is a major focus of my thesis : the way I choose to do so is based on the large-scale structure of the Universe, through a probe called the integrated Sachs-Wolfe effect (iSW). This effect is theoretically detectable in the cosmic microwave background (CMB) : before reaching us this light travelled through large structures underlain by gravitational potentials. The acceleration of the expansion stretches and flattens these potentials during the crossing of photons, changing their energy, in a way that depend on the properties of the dark energy. The iSW effect only has a weak effect on the CMB requiring the use of external data to be detectable. A conventional approach is to correlate the CMB with a tracer of the distribution of matter, and therefore the underlying potentials. This has been attempted numerous times with galaxies surveys but the measured correlation has yet to give a definitive result on the detection of the iSW effect. This is mainly due to the shortcomings of current surveys that are not deep enough and/or have a too low sky coverage. A part of my thesis is devoted to the correlation of FDC with another diffuse background, namely the cosmological infrared background (CIB), which is composed of the integrated emission of the non-resolved distant galaxies. I was able to show that it is an excellent tracer, free from the shortcomings of current surveys. The levels of significance for the expected correlation CIB-CMB exceed those of current surveys, and compete with those predicted for the future generation of very large surveys. In the following, my thesis was focused on the individual imprint in the CMB of the largest structures by iSW effect. My work on the subject first involved revisiting a past study of stacking CMB patches at structures location, using my own protocol, completed and associated with a variety of statistical tests to check the significance of these results. This point proved to be particularly difficult to assess and subject to possible selection bias. I extended the use of this detection method to other available catalogues of structures, more consequent and supposedly more sophisticated in their detection algorithms. The results from one of them suggests the presence of a signal at scales and amplitude consistent with the theory, but with moderate significance. The stacking results raise questions regarding the expected signal : this led me to work on a theoretical prediction of the iSW effect produced by structures, through simulations based on the Lemaître-Tolman-Bondi metric. This allowed me to predict the exact theoretical iSW effect of existing structures. The central amplitude of the measured signals is consistent with the theory, but shows features non-reproducible by my predictions. An extension to the additional catalogues will verify the significance of their signals and their compatibility with the theory. Another part of my thesis focuses on a distant time in the history of the Universe, called reionisation : the transition from a neutral universe to a fully ionised one under the action of the first stars and other ionising sources. This period has a significant influence on the CMB and its statistical properties, in particular the power spectrum of its polarisation fluctuations. In my case, I focused on the use of temperature measurements of the intergalactic medium during the reionisation in order to investigate the possible contribution of the disintegration and annihilation of the hypothetical dark matter. Starting from a theoretical work based on several models of dark matter, I computed and compared predictions to actual measures of the IGM temperature, which allowed me to extract new and interesting constraints on the critical parameters of the dark matter and crucial features of the reionisation itself

Title: Study of dark energy and modified gravity and their influence on the cosmological parameters of the universe Author: Michal Vraštil Institute: Institute of Physics of the Czech Academy of Sciences Supervisor: RNDr. Michael Prouza, Ph.D., Institute of Physics of the Czech Academy of Sciences Abstract: Discovery of the accelerated expansion of the Universe poses a major theoretical puzzle. Although the assumption of a non-zero cosmological constant provides a minimal extension of general relativity that is consistent with observational data, many theories of modified gravity have been suggested as possible alternatives due to serious problems connected with the cosmological constant. Numerical predictions of structure formation for these models in the fully non-linear regime are very expensive and it is difficult, if not impossible, to explore such a huge space of models and parameters using high-resolution N-body simulations. Even in the mildly nonlinear regime, perturbative methods can become extremely complex. We explore whether simplified dynamical approximations, applicable for a certain set of cosmological probes, can be used to investigate models of modified gravity with acceptable accuracy in the latter instance. For the case of chameleon gravity, we found that it is screened away on scales...

Le modèle de concordance de la cosmologie, appelé ΛCDM, est un succès de la physique moderne, car il est capable de reproduire les principales observations cosmologiques avec une grande précision et très peu de paramètres libres. Cependant, il prédit l'existence de matière noire froide et d'énergie sombre sous la forme d'une constante cosmologique, qui n'ont pas encore été détectées directement. Par conséquent, il est important de considérer des modèles allant au-delà de ΛCDM et de les confronter aux observations, afin d'améliorer nos connaissances sur le secteur sombre de l'Univers. Le futur satellite Euclid, de l'Agence Spatiale Européenne, explorera un énorme volume de la structure à grande échelle de l'Univers en utilisant principalement le regroupement des galaxies et la distorsion de leurs images due aux lentilles gravitationnelles. Dans ce travail, nous caractérisons de façon quantitative les performances d'Euclid vis-à-vis des contraintes cosmologiques, à la fois pour le modèle de concordance, mais également pour des extensions phénoménologiques modifiant les deux composantes sombres de l'Univers. En particulier, nous accordons une attention particulière aux corrélations croisées entre les différentes sondes d'Euclid lors de leur combinaison et estimons de façon précise leur impact sur les résultats finaux. D'une part, nous montrons qu'Euclid fournira d'excellentes contraintes sur les modèles cosmologiques qui définitivement illuminera le secteur sombre. D'autre part, nous montrons que les corrélations croisées entre les sondes d'Euclid ne peuvent pas être négligées dans les analyses futures et, plus important encore, que l'ajout de ces corrélations améliore grandement les contraintes sur les paramètres cosmologiques. / The concordance model of cosmology, called ΛCDM, is a success, since it is able to reproduce the main cosmological observations with great accuracy and only few parameters. However, it predicts the existence of cold dark matter and dark energy in the form of a cosmological constant, which have not been directly detected yet. Therefore, it is important to consider models going beyond ΛCDM, and confront them against observations, in order to improve our knowledge on the dark sector of the Universe. The future Euclid satellite from the European Space Agency will probe a huge volume of the large-scale structure of the Universe using mainly the clustering of galaxies and the distortion of their images due to gravitational lensing. In this work, we quantitatively estimate the constraining power of the future Euclid data for the concordance model, as well as for some phenomenological extensions of it, modifying both dark components of the Universe. In particular, we pay special attention to the cross-correlations between the different Euclid probes when combining them, and assess their impact on the final results. On one hand, we show that Euclid will provide exquisite constraints on cosmological models that will definitely shed light on the dark sector. On the other hand, we show that cross-correlations between Euclid probes cannot be neglected in future analyses, and, more importantly, that the addition of these correlations largely improves the constraints on the cosmological parameters.

This thesis examines relativistic fluids. We have used the variational approach to develop tools for studying the dynamics of relativistic fluids to apply this to cosmological modelling. Studies like these go beyond the standard model in cosmology. Researchers believe that such extensions to the standard cosmological model are pivotal to resolving some of the long-standing cosmological problems. An example of such problems is the origin, growth (from quantum electromagnetic fluctuations to large-scale magnetic fields during inflation) and evolution of cosmological magnetic fields that exhibit as large-scale (cosmological) magnetic fields in late time. One other example is the coincidence problem. The standard approach in such studies is to use modelling in the form of the single-fluid formalism. As an alternative one can consider the single-fluid and multi-fluid formalisms that incorporate aspects of electrodynamics and thermodynamics, respectively in the context of the variational approach. This might help us make progress in trying to either resolve some of these problems or at least open up new ways of addressing them. In this regard, we have extended the well-known M¨ueller-Israel-Stewart (hereafter MIS) formalism to allow us to examine the effect on fluid flow in which the components of the multi-species fluids interact thermodynamically. We use the extension to the MIS theory in the context of interacting species to study the growth of dark matter and dark energy, and find that either interaction or entrainment involving dark energy and dark matter suggests a mutual relative modulation of the growth behaviour of the two densities. This may aid in resolving the coincidence problem. Our examination of inflation-generated, large-scale magnetic fields reveals a super-adiabatically evolving mode from the beginning of the radiation-dominated epoch to either much later during the epoch or probably extending far into the era of matter domination which may account for late time, large-scale magnetic fields.

Les travaux présentés dans cette thèse contraignent les paramètres d'un modèle de cosmologie, le modèle du galileon, et d'un modèle de physique des particules, le modèle du branon, qui sont des extensions des modèles standards. Ces modèles, qui supposent l'existence d'un champ scalaire additionnel et peuvent trouver leurs origines dans les théories à dimensions supplémentaires, offrent des explications élégantes aux questions de la nature de l'énergie noire et de la matière noire respectivement. La première partie de cette thèse présente les prédictions du modèle du galileon et les résultats obtenus par leur comparaison aux observations cosmologiques et astrophysiques récentes. Cette étude montre que le modèle du galileon a de sérieuses difficultés à reproduire ces observations, qui regroupent le fond diffus cosmologique, les mesures liées aux distances cosmologiques, et l'observation d'un évènement par ondes gravitationnelles et contrepartie électromagnétique. La seconde partie décrit la recherche de branons dans les collisions proton-proton enregistrées en 2016 par le Solénoïde Compact à Muons au Grand Collisionneur de Hadrons, à une énergie dans le centre de masse de 13 TeV. Des évènements qui présentent un jet de haute énergie, produit par un quark ou gluon ou bien par désintégration hadronique d'un boson vecteur, et de l'énergie transverse manquante dans l'état final sont sélectionnés et comparés aux estimations pour les évènements de bruits de fond. Aucun excès d'évènements n'est observé, ce qui permet de poser des contraintes sur les valeurs possibles des paramètres du modèle du branon. / This PhD thesis presents constraints on the parameters of a cosmological model, the galileon model, and a particle physics model, the branon model. Both are extensions of the standard models that include an additional scalar field and that can be built from extra dimensions theories. The galileon model propose an alternative to the cosmological constant as the nature of dark energy, and the branon model give a dark matter particle candidate. The first part of this thesis shows the predictions of the galileon model and the results obtained from their comparison with recent cosmological and astrophysical observations. The set of observations used contains the cosmic microwave background, cosmological distances measurements and the detection of gravitational waves along with their electromagnetic counterpart from the merger of a binary star system. The study shows that the galileon model has serious difficulties to reproduce these observations. The second part describe the search for branons in proton-proton collisions data at 13 TeV collected in 2016 with the Compact Muon Solenoid at the Large Hadron Collider. Events with high energy jets, produced by a quark, a gluon or a vector boson decaying hadronically, and missing transverse energy in the final state are selected and compared to background estimations. No excess of event has been found allowing for experimental constraints to be put in the parameter space of the branon model.

Until now the quantum field theory (QFT) that successfully describes the electric, weak and strong interactions (three out of the four fundamental forces) between particles is the Standard Model, but it omits gravity. The prime candidate for a quantum theory of gravity is string theory. However, recent developments in string theory suggest that a portion of the alternative quantum field theories that are being considered, are incompatible with gravity. In 2005, this led string theorists to outline the conditions an effective field theory (EFT) should satisfy in order to be consistent with a quantum theory of gravity. These conditions are the ones that separate the so-called landscape from the swampland. An EFT that satisfies these conditions is said to reside in the landscape, while EFTs that do not satisfy these conditions belong to the swampland. This mapping out of EFTs to the swampland gives rise to a number of predictions that are related to the physics of the Early universe and the nature of dark energy. The de Sitter conjecture and the Trans-Planckian censorship conjecture are some of these conditions and will be the main focus of this thesis. The main purpose behind this work is to gain a deep understanding of each criterium, as well as unravel their implications and predictions related to the dynamics of the Early Universe. We do this by writing a pedagogical introduction of the topic and by introducing some possible alternative to the inflationary scenario, cosmologies that seem to be consistent with the aforementioned constraints.

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