Certains aspects de l'extraction du signal cosmologique à 21 cm à partir des observations radio, ainsi que le traitement des données interférométriques pour des observations depuis le sol et depuis l'espace ont été étudiés et sont présentés dans cette thèse. J'ai développé un modèle cohérent et à haute résolution du ciel en radio, qui peut fournir une carte complète et précise du ciel, dans la gamme de fréquence 10 MHz à 2,3 GHz, avec une résolution pouvant atteindre une minute d'arc. Le modèle inclut plusieurs sources de rayonnement diffus, en particulier le synchrotron Galactique, les sources radio brillantes du ciel, ainsi qu'un modèle des sources faibles. J'ai également mis au point une méthode pour extraire le signal 21 cm cosmologique, fortement contaminé par les émissions d'avant-plan et le bruit des récepteurs. La méthode utilise une cascade de deux filtres de Wiener, dans l'espace des fréquences d'abord, et ensuite, dans le domaine angulaire. Le premier filtre exploite les variations lentes des émissions d'avant-plan selon la fréquence, tandis que le second filtre exploite les corrélations angulaires du signal cosmologique pour filtrer le bruit des récepteurs, considéré non corrélé entre deux directions différentes. J'ai développé un nouvel algorithme d'imagerie pour les interféromètres en orbite lunaire. Un tel instrument serait idéal pour cartographier le ciel en dessous de 30 MHz; il ne serait en effet pas soumis aux perturbations ionosphériques et serait protégé des interférences électromagnétiques dues aux émissions terrestres. J'ai montré que l'utilisation de la précession du plan orbital du satellite permet de résoudre le problème de la symétrie miroir. La méthode exploite la relation de projection linéaire entre la carte du ciel et les visibilités mesurées, tant dans l'espace angulaire que dans l'espace des harmoniques sphériques pour reconstruire la carte du ciel. L'algorithme d'imagerie gère la complication due à l'ombre de la Lune se déplaçant avec le temps sur le champ de vue des antennes couvrant tout le ciel. Notons que ces effets ne sont pas pris en charge par les algorithmes d'imagerie existants tels que la W-Projection et la WStacking. Enfin, j'ai effectué une première analyse des données de l'interféromètre de transit PAON-4. J'ai évalué la performance globale du réseau en termes de température de bruit et de la réponse des antennes. J'ai pu étalonner avec succès les visibilités, en déterminant à la fois l'amplitude et la phase des termes de gain complexes, tout en corrigeant les décalages de pointage des antennes de PAON4. J'ai ensuite reconstruit la carte du ciel pour une bande de 10 degrés autour de la déclinaison de la source brillante Cygnus A, à partir du flot de données PAON-4 calibré et nettoyé, en appliquant l'algorithme de décomposition en mode m dans l'espace des harmoniques sphériques. / Some aspects of extracting cosmological 21cm signal from radio observations, as well as processing of interferometric data for ground based or space born instruments have been studied and discussed in this dissertation. I have developed a high-resolution self-consistent radio whole sky model, which provides an accurate full sky maps in the frequency range from 10 MHz to 2.3 GHz, with angular resolution up to 1 arcmin. It includes bright and faint radio sources, Galactic synchrotron and Galactic freefree emissions. I have also developed a method to extract the faint cosmological 21-cm signal, heavily contaminated by foreground emissions and receiver noise. The method uses a cascade of two Wiener filters, in frequency domain and then, in angular domain. The first filter exploits the smoothness of the foreground emissions along the frequency, while the second filter exploits the angular correlations of the cosmological signal, due to the receiver noise is considered to be nearly uncorrelated between different directions. I have developed a studied the performance of a new imaging algorithm for lunar orbit interferometers. Such an instrument would be ideal for mapping the radio sky below 30 MHz, as it would be free from ionospheric perturbations, as well as electromagnetic interferences due to terrestrial emissions. I have shown that we make use of the precession of satellite orbital plane to solve the mirror symmetry problem, and exploit the linear mapping between the sky map and the measured visibilities, both in angular space and spherical harmonic space to reconstruct the sky map. The imaging algorithm handles the time-varying Moon's blockage over the whole sky field of view, which are not handled by existing imaging algorithms such as the WProjection and the W-Stacking. Finally, I have carried out a first analysis of the observational visibility data from the PAON-4 transit interferometer. I have evaluated the overall performance of the array in terms of system temperature and antenna response, and successfully calibrated the visibilities, determining both amplitude and phase of the complex gain terms, while correcting PAON-4 antennae pointing offsets. I have then reconstructed the sky map for a 10 degree strip around Cygnus A declination, from the cleaned calibrated PAON-4 data streams, applying the m-mode decomposition map-making algorithm in spherical harmonic space.

L'accélération de l'expansion de l'Univers est l'un des sujets majeurs de la cosmologie actuelle. Elle pourrait être due à une nouvelle composante, appelée énergie noire, qui représenterait 70% du bilan énergétique de l'Univers. Pour étudier sa nature à travers son équation d'état, on mesure une règle étalon fournie par les oscillations baryoniques acoustiques (BAO) à différentes valeurs de décalage vers le rouge. Cette technique a été utilisée avec succès pour la première fois en 2005 par le projet Sloan Digital Sky Survey (SDSS-II). Depuis, l'observation des BAO a été confirmée en 2012 par le projet BOSS (SDSS-III) puis eBOSS (SDSS-IV), à la fois avec des galaxies et des absorbeurs de la raie à 21 cm révélés dans des spectres de quasar. Notre groupe prépare la prochaine génération d'expériences BAO en participant à la construction du spectrographe du nouveau programme Dark Energy Spectroscopic Instrument (DESI). Ce projet va réaliser un sondage 3D de plusieurs dizaines de millions de galaxies et quasars avec le télescope Mayall de 4m en Arizona (USA). J'ai participé à la mise au point du spectrographe de DESI en collaboration avec notre partenaire industriel (WINLIGHT). J'étais aussi en charge de développer un banc optique dans le but de valider l'alignement des capteurs CCD montés dans les enceintes des cryostats. Des matrices de microlentilles sont utilisées pour projeter très précisément des grilles de spots sur les CCD. En fonction de la distortion observée des grilles, nous sommes capables de déterminer la position des CCD. En parallèle, j'ai développé des algorithmes pour la sélection des quasars cibles, les objets les plus distants qui seront observés par DESI, basée sur leurs propriétés photométriques en exploitant des techniques d'apprentissage supervisé. / The accelerating expansion of the universe is one of the main topics of modern cosmology. It may stem from a new component, so-called dark energy, which would make up 70% of the energy content of the universe. To study its nature through its equation of state, one can measure a standard ruler given by baryonic acoustic oscillations (BAO) at various redshifts or for different slices of the universe. This approach was used successfully for the first time in 2005 by the Sloan Digital Sky Survey (SDSS-II) project. Then, the BAO signal was confirmed in 2012 by the BOSS project (SDSS-III) and then by the eBOSS project (SDSS-IV), both with galaxies and HI absorbers revealed in quasar spectra. Our group is preparing the next generation of BAO experiments by taking part in building the spectrograph of the new Dark Energy Spectroscopic Instrument (DESI) program. This project will perform a 3D survey of several tens of millions of galaxies and quasars with the 4-meter Mayall telescope in Arizona (USA). I participated in the adjustments of the spectrograph of DESI in collaboration with our industrial partner (WINLIGHT). I was also in charge of developing an optical bench in order to check the alignment of the CCD sensors mounted within the crysotat vessels. Arrays of microlens are used to project very precisely grids of spots on the CCDs. We are able to infer the position of the CCDs according to the observed distortion of the grids of spots. In parallel, I developped algorithms for the selection of quasar candidates, the more distant objects that will be observed by DESI, based on their photometry properties by making use of machine learning tools.

No description available.

This paper uses the Pantheon+ data set that consists of 1701 light curves of 1550 unique type IaSupernova to find constraints on various cosmological models and compare them with a varying darkenergy model proposed by Chevallier, Polarski and Linder in the early 2000s and analyze it deeper.The results show a tipping point from a decelerating universe to an accelerating one at a redshift ofz = 0.35, and a second tipping point back to a decelerating universe in the future at z = −0.19. The flatChevallier-Polarski-Linder (CPL) model leads to a matter dominated universe with Ωm = 0.52 ± 0.08while the relative dark energy density Ωde = 0.48±0.08. Finally, taking all these models and comparingthem with the results that are found from Taylor expanding the distance relation shows that they aremostly consistent with a deceleration parameter around q0 = −0.28.

No description available.

Despite the many successes of the current standard model of cosmology on the largest physical scales, it relies on two phenomenologically motivated constituents, cold dark matter and dark energy, which account for approximately 95% of the energy-matter content of the universe. From a more fundamental point of view, however, the introduction of a dark energy (DE) component is theoretically challenging and extremely fine-tuned, despite the many proposals for its dynamics. On the other hand, the concept of cold dark matter (CDM) also suffers from several issues such as the lack of direct experimental detection, the question of its cosmological abundance and problems related to the formation of structure on small scales. A perhaps more natural solution might be that the gravitational interaction genuinely differs from that of general relativity, which expresses itself as either one or even both of the above dark components. Here we consider different possibilities on how to constrain hypothetical modifications to the gravitational sector, focusing on the subset of tensor-vector-scalar (TeVeS) theory as an alternative to CDM on galactic scales and a particular class of chameleon models which aim at explaining the coincidences of DE. Developing an analytic model for nonspherical lenses, we begin our analysis with testing TeVeS against observations of multiple-image systems. We then approach the role of low-density objects such as cosmic filaments in this framework and discuss potentially observable signatures. Along these lines, we also consider the possibility of massive neutrinos in TeVeS theory and outline a general approach for constraining this hypothesis with the help of cluster lenses. This approach is then demonstrated using the cluster lens A2390 with its remarkable straight arc. Presenting a general framework to explore the nonlinear clustering of density perturbations in coupled scalar field models, we then consider a particular chameleon model and highlight the possibility of measurable effects on intermediate scales, i.e. those relevant for galaxy clusters. Finally, we discuss the prospects of applying similar methods in the context of TeVeS and present an ansatz which allows to cast the linear perturbation equations into a more convenient form.

This thesis is divided in two parts: the first part is dedicated to the study of black hole solutions in a theory of modified gravity, called massive gravity, that may be able to explain the actual stage of accelerated expansion of the Universe, while in the second part we focus on constraining primordial black holes as dark matter candidates.<p><p>In particular, during the first part we study the thermodynamical properties of specific black hole solutions in massive gravity. We conclude that such black hole solutions do not follow the second and third of law of thermodynamics, which may signal a problem in the model. For instance, a naked singularity may be created as a result of the evolution of a singularity-free state.<p><p>In the second part, we constrain primordial black holes as dark matter candidates. To do that, we consider the effect of primordial black holes when they interact with compact objects, such as neutron stars and white dwarfs. The idea is as follows: if a primordial black hole is captured by a compact object, then the accretion of the neutron star or white dwarf’s material into the hole is so fast that the black hole destroys the star in a very short time. Therefore, observations of long-lived compact objects impose constraints on the fraction of primordial black holes. Considering both direct capture and capture through star formation of primordial black holes by compact objects, we are able to rule out primordial black holes as the main component of dark matter under certain assumptions that are discussed.<p><p>To better understand the relevance of these subjects in modern cosmology, we begin the thesis by introducing the standard model of cosmology and its problems. We give particular emphasis to modifications of gravity, such as massive gravity, and black holes in our discussion of the dark sector of the Universe./<p>Cette thèse est divisée en deux parties :la première partie est consacrée à l’étude de certaines solutions de trous noirs dans une théorie modifiée de la gravité, appelée la gravité massive, qui peut être en mesure d’expliquer l’expansion accélérée de l’Univers; tandis que dans la seconde partie, nous nous concentrons sur des contraintes sur les trous noirs primordiaux comme candidats de matière noire.<p><p>En particulier, au cours de la première partie, nous étudions les propriétés thermodynamiques de solutions spécifiques de trous noirs en gravité massive. Nous en concluons que ces solutions de trous noirs ne suivent ni la deuxième, ni la troisième loi de la thermodynamique, ce qui semble indiquer une inconsistance dans le modèle. Par exemple, une singularité nue peut être créée à la suite de l’évolution d’un état sans aucune singularité.<p><p>Dans la deuxième partie, nous mettons des contraintes sur les trous noirs primordiaux en tant que candidats de matière noire. Pour ce faire, nous considérons l’effet des trous noirs primordiaux lorsqu’ils interagissent avec des objets compacts, tels que les étoiles à neutrons et les naines blanches. L’idée est comme suit :si un trou noir primordial est capturé par un objet compact, alors l’accrétion du matériel constituant l’étoile à neutrons ou la naine blanche est si rapide que le trou noir détruit l’étoile en un temps très court. Par conséquent, les observations d’objets compacts imposent des contraintes sur la fraction de trous noirs primordiaux. Considérant à la fois la capture directe des trous noirs primordiaux par les objets compacts et la capture au travers de la formation stellaire, nous sommes en mesure d’exclure les trous noirs primordiaux comme la composante principale de matière noire sous certaines hypothèses qui sont discutées.<p><p>Pour mieux comprendre la pertinence de ces sujets dans la cosmologie moderne, nous commençons la thèse par l’introduction du modèle standard de la cosmologie et de ses problèmes. Nous donnons une importance particulière aux modifications de la gravité, telles que la gravité massive, et aux trous noirs dans notre discussion sur le secteur sombre de l’Univers. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Master of Science / Department of Physics / Bharat Ratra / In this report, we discuss the four standard energy conditions of General Relativity (null, weak, dominant, and strong) and investigate their cosmological consequences. We note that these energy conditions can be compatible with cosmic acceleration provided that a repulsive cosmological constant exists and the acceleration stays within certain bounds. Scalar fields and dark energy, and their relationships to the energy conditions, are also discussed. Special attention is paid to the 1988 Ratra-Peebles scalar field model, which is notable in that it provides a physical self-consistent framework for the phenomenology of dark energy. Appendix B, which is part of joint-research with Anatoly Pavlov, Khaled Saaidi, and Bharat Ratra, reports on the existence of the Ratra-Peebles scalar field tracker solution in a curvature-dominated universe, and discusses the problem of investigating the evolution of long-wavelength inhomogeneities in this solution while taking into account the gravitational back-reaction (in the linear perturbative approximation).

In this thesis, I study the relativistic effects of matter inhomogeneities on the accelerating expansion of the Universe. The acceleration is often taken to be caused by the presence of an exotic fluid called Dark Energy, or else a non-zero 'cosmological constant' term in the field equations of General Relativity. I consider whether this result could instead be an artefact caused by using an incorrect model to interpret observations. The standard 'concordance' cosmological model assumes the Cosmological Principle, which states that the matter distribution on large scales is homogeneous. One possibility is that correction terms appear in the field equations when small-scale inhomogeneities are smoothed over to produce this homogeneous model. These 'backreaction' effects could affect the dynamics of the spacetime, causing an apparent acceleration. I clarify the relationship between acceleration of the averaged spacetime and acceleration inferred from observable quantities, and show that they are closely related in statistically-homogeneous spacetimes. Another possibility is that the Universe could be inhomogeneous on large scales. If there was a large ‘void’, with us at the centre, the lensing of light by the void could reproduce the observations that imply cosmic acceleration. I show that a popular class of void models, based on spherically-symmetric Lemaitre-Tolman-Bondi spacetimes, are unable to simultaneously fit a selection of observational data, thus effectively ruling-out this possibility. These data include the Kinematic Sunyaev-Zel'dovich (KSZ) effect, which is a distortion/shift of the Cosmic Microwave Background (CMB) frequency spectrum caused by the Compton scattering of photons by hot gas in galaxy clusters. This, and other distortions of the CMB frequency spectrum, are sensitive to the degree of anisotropy in the CMB about a scattering cluster. I suggest tests involving these observables that exploit the strong link between isotropy and homogeneity to (a) distinguish between different causes of a deviation from spatial flatness on the horizon scale, and (b) potentially confirm the Cosmological Principle using observations. Finally, I describe a novel Bayesian CMB component separation method for extracting the Sunyaev-Zel'dovich signal of clusters from CMB sky maps.

Black hole jets and accretion discs are the most extreme objects in modern astrophysics whilst dark energy is undoubtedly the most mysterious. This thesis focuses on understanding these three topics. The majority of this thesis is dedicated to investigating the structure and properties of black hole jets by modelling their emission. I develop an inhomogeneous jet model with a magnetically dominated parabolic accelerating base, transitioning to a slowly decelerating conical jet, with a geometry set by radio observations of M87. This model is able to reproduce the simultaneous multiwavelength spectra of all 38 Fermi blazars with redshifts in unprecendented detail across all wavelengths. I constrain the synchrotron bright region of the jet to occur outside the BLR and dusty torus for FSRQs using the optically thick to thin synchrotron break. At these large distances their inverse-Compton emission originates from scattering CMB photons. I find an approximately linear relation between the jet power and the transition region radius where the jet first comes into equipartition, transitions from parabolic to conical and stops accelerating. The decreasing magnetic field strength and increasing bulk Lorentz factor with jet power are the physical reasons behind the blazar sequence. I calculate the conditions for instability in a thin accretion disc with an α parameter which depends on the magnetic Prandtl number, as suggested by MHD simulations. The global behaviour of the instability induces cyclic flaring in the inner regions of the disc, for parameters appropriate for X-ray binary systems, thereby offering a potential solution to a long standing problem. Finally, I calculate the effect of an interacting quintessence model of dark energy on cosmological observables. I find that a scalar-tensor type interaction in the dark sector results in an observable increase in the matter power spectrum and integrated Sachs-Wolfe effect at horizon scales.