Dissertations / Theses on the topic 'Cosmology of the early Universe'

Thesis (Ph. D.)--University of Oregon, 2006.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 75-80). Also available for download via the World Wide Web; free to University of Oregon users.

The study of the Early Universe raises some of the most fundamental questions in theoretical physics. This thesis explores three main aspects of early universe cosmology. The first part discusses alternatives to the Big Bang scenario which is the current paradigm of cosmology. Namely, it discusses bouncing universe models where the initial Big Bang singularity is replaced by a finite size universe. After reviewing the necessary cosmology background in the introduction, we show a specific model of a bouncing universe that contains additional "Lee-Wick fields", partners to the standard fields. In particular we prove that a Lee-Wick matter bounce is unstable when one adds radiation to matter. In the second part of this thesis, we consider particle production via parametric resonance during preheating, at the end of cosmological inflation. Specifically, we prove that in the case of a speed-limited inflaton, non-canonical kinetic terms used to described any effective Lagrangian do not enhance particle production. Finally, the last topic involves topological defects during the Quantum Chromodynamics phase transition. Namely, we study cosmic strings coming from pionfields present in the Standard Model of particle physics and find a mechanism to stabilize them. We show how a thermal bath of photons reduces the effective vacuum manifold to a circle and thus allows the presence of topologically stable pionstrings.
L'etude de l'Univers primordial adresse quelques-unes des questions les plus fondamentales de la physique theorique. Cette these a pour objet l'exploration de trois aspects principaux de la cosmologie primordiale. Dans un premier temps, nous discutons d'une alternative au paradigme scientique qu'est le modele du Big Bang. A savoir, nous explorons un model d'univers a rebond qui evite la singularite initiale du Big Bang. Nous commencerons dans l'introduction par revoir les elements de base necessaires a la comprehension de la cosmologie. A la suite de quoi, nous montrerons un modele specifique d'Univers a rebond contenant des champs additionnels particuliers en complements des champs présents habituellement. Ces nouveaux champs proviennent de ce qui s'appelle le modele "Lee-Wick" de la physique des particules. En particulier, nous prouvons qu'un univers a rebond dans ce contexte est instable lorsque l'on ajoute une composante de radiation en plus de la matiere. Dans la seconde partie, nous considérons la production de particules via un phenomene de resonance parametrique durant la phase de "prechauffement", a la fin de l'inflation cosmologique. Plus précisément, nous prouvons que dans le cas ou l'inflaton a une limite de vitesse, les termes cinetiques non-canoniques d'écrivant n'importe quel Lagrangien effectif n'améliorent pas la production de particules. Finalement, le dernier sujet abordé concerne les défauts topologiques pendant la transition de phase de la chromodynamique quantique. A savoir, nous etudions les cordes cosmiques provenant des champs de pions presents dans le modele standard de la physique des particules et trouvons un méchanisme pour les stabiliser. Nous prouvons alors qu'un bain thermique de photons en contact avec ces cordes reduit la variete du vide a un cercle. Cela a pour effet d'autoriser la presence de "cordes pioniques" topologiquement stables.

The very early universe is where we expect the observed primordial perturbations in the cosmic microwave background to have originated. In this thesis we study isocurvature field fluctuations during inflation and ekpyrotic contraction as sources of the primordial curvature perturbations. We start by introducing concepts of modern cosmology followed by an overview of early universe cosmology. After, we introduce perturbation theory and how to compute perturbations from early universe models. After reviewing all fundamental concepts necessary for this thesis, we estimate largescale curvature perturbations from isocurvature fluctuations in the waterfall field during hybrid inflation, in addition to the usual inflaton field perturbations. The tachyonic instability at the end of this inflation model leads to an explosive growth of super-Hubble scale perturbations, but they retain the steep blue spectrum characteristic of vacuum fluctuations in a massive field during inflation. We extend the usual δN formalism to include the essential role of small fluctuations when estimating the large-scale curvature perturbation. The following two chapters study perturbations within the curvaton proposal. Firstly, we consider how non-Gaussianity of the primordial density perturbation and the amplitude of gravitational waves from inflation can be used to determine parameters of the curvaton scenario for the origin of structure. We show that in the simplest quadratic model, where the curvaton evolves as a free scalar field, measurement of the bispectrum relative to the power spectrum, fNL, and the tensor-to-scalar ratio can determine both the expectation value of the curvaton field during inflation and its dimensionless decay rate relative to the curvaton mass. We show how these predictions are altered by the introduction of self-interactions. In the following chapter, we then characterise the primordial perturbations produced due to both inflaton and curvaton fluctuations. We show how observational bounds on non-linearity parameters and the tensor-scalar ratio can be used to constrain curvaton and inflaton parameters. The final research presented in this thesis, considers a simple model of cosmological collapse driven by canonical fields with exponential potentials. We generalise the two-field ekpyrotic collapse to consider non-orthogonal potentials and give the general condition for isocurvature field fluctuations to have a slightly red spectrum of perturbations as required by current observations. However a red spectrum of fluctuations implies that the two-field ekpyrotic phase must have a finite duration and requires a preceding phase which sets the initial conditions for what otherwise appears to be a fine-tuned trajectory in the phase space. We end this thesis with some concluding remarks and comments on possible future work.

Fluctuations in the cosmic microwave background (CMB), the radiation left over from the Big Bang, contain information which has been pivotal in establishing the current cosmological model. CMB data can also be used to test theoretically well-motivated additions to the model, including pre-inflationary relics (signatures of bubble collisions arising in eternal inflation) and topological defects that form after inflation (cosmic strings and textures). These relics typically leave sub-dominant, spatially localised signals, hidden in the “noise” of the primary CMB, the instrumental noise, foreground residuals and other systematics. Standard approaches for searching for such signals involve focusing on statistical anomalies, which carry the danger of extreme a posteriori biases. The self-consistent approach to this problem is Bayesian model comparison; however, the full implementation of this approach is computationally intractable with current CMB datasets, and will only become more difficult with data from the next generation of CMB experiments. I will describe a powerful modular algorithm, capable of coping with the volume of data, which combines a candidate-detection stage (using wavelets or optimal filters) with a full Bayesian parameter-estimation and model-selection stage performed in pixel space within the candidate regions. The algorithm is designed to fully account for the “look-elsewhere” effect, and its use of blind analysis techniques further enhances its robustness to unknown systematics. Finally, I will present the results of applying the algorithm to hunt for the signatures of bubble collisions and cosmic textures in the seven-year data from the Wilkinson Microwave Anisotropy Probe.

In this thesis we discuss two key problems: the cosmological constant problem (CCP), an issue that primarily manifests itself in late universe cosmology; and the process of thermalisation during the post-inflationary reheating phase of the early universe. We start by giving a brief review of general relativity, discussing both its successes and failures, in particular, why one might consider modifications of it. We then delve into the aspects of early and late universe cosmology that we aim to address in the research discussed in this thesis. Starting with an overview of the inflationary paradigm, and the need to reheat the universe post-inflation, we give a review of previous research that has been conducted in this area. We then move on to discuss the CCP in detail, in particular, why it is such an issue. After setting the scene for this problem, we proceed to discuss how to approach finding a resolution to it, highlighting certain stumbling blocks that one needs to be mindful of. Having set the scene, we then present a potential solution to the CCP, involving a scalar-tensor modifed theory of gravity, so-called Horndeski theory. Building upon a class of Horndeski theories providing self-tuning solutions to the CCP, we provide a generalisation in which matter interacts with gravity via a disformal coupling to the spacetime metric. We establish the form of the disformally self-tuning Lagrangian on a cosmological Friedmann-Robertson-Walker background, and show that there exist non-trivial self-tuning solutions. In the latter half of this thesis, we move on to review the literature on the non-perturbative description of the early stages of reheating, so-called preheating. With the motivation to study the less well understood thermalisation process that must necessarily take place in this phase, we then present a toy model preheating theory, in which we account for the effects of thermalisation from its onset. Within the density matrix formalism, we derive a (self-consistent) set of quantum Boltzmann equations, which are able to describe the evolution of an ensemble of self-interacting scalar particles that are subject to an oscillating mass term. In particular, we apply this to the preheating scenario in order to study the evolution of scalar particle number densities throughout this process. We then conclude by discussing our numerical analysis of the Boltzmann equations, drawing attention to some important results and features that manifest using this approach, in particular, how the process differs from the standard analysis through the inclusion of thermalisation.

This is a thesis on theoretical cosmology. The first and largest part is a study of cosmic strings, in particular their dynamics and signals in higher dimensional spacetimes. The second part is a study of black holes in a quintessence background. Cosmic strings are predicted by models of the early universe. They were thought to arise, originally, from Grand Unified Theories, and more recently from brane inflationary models based in string theory. In Chapter 3 we find exact solutions for cosmic string loop trajectories in higher dimensions, and find the regions of parameter space for which cusps exist. We find that winding the internal dimensions slows the average velocity of string loops, and conjecture that the periodicity of internal space may contribute to self-intersections. In Chapter 4, we calculate the gravitational wave signal from cosmic string cusps in higher dimensions, and find it is much reduced relative to the 4D case. The main reason for this is the large reduction in the probability of cusps occurring on loops in higher dimensions, as well as a slight reduction in signal from individual cusps. In Chapter 5, we study cosmic string trajectories in warped spacetimes, such as may be found in realistic brane inflation models. We find that contrary to claims in the literature, the warping of the internal space does not prevent the internal motion of strings. The energy associated with the warping of spacetime means that the energy of a loop appears to change over time from our 4D perspective. Finally, in Chapter 6, we find an analytic, general-relativistic solution describing a black hole in a quintessence universe. Quintessence is a model of late-time cosmic acceleration in which expansion is sourced by a scalar field. Our solution shows the interaction between this scalar field and a black hole. The scalar field is shown to continue its cosmological "rolling" behaviour everywhere, including on the black hole event horizon, and the black hole is shown slowly to accrete scalar field. This is a perturbative solution valid throughout all of space but only over a finite period of time.

The subject of this dissertation lies in the region where modern particle physics and cosmology intersect. Broadly speaking, its context is provided by the shortcomings of the 'standard cosmology', and the more recent inflationary scenario. The particular aspects which are tackled - the origin of density fluctuations and the boundary conditions of the universe - are outlined below. A. The Cosmic String Scenario: Phase transitions in the early universe may have produced topological 'knots' or defects, such as monopoles cosmic strings and domain walls. Cosmic strings, in particular, have attracted much interest recently because of the alluring model of galaxy formation they have the potential to produce. The viability of this new scenario, however, rests on untested assumptions about string interaction properties - whether or not they intercommute. A detailed study of global U(1)-strings has demonstrated that under most circumstances they will intercommute, a result expected for other varieties of strings (Part A). Given this firmer foundation for the cosmic string scenario, some of their astrophysical implications are explored. In particular, the large scale peculiar velocities predicted in this model are examined. B. The domain wall problem of the Axion: The axion has attracted considerable interest as a cold dark matter candidate. In part B, we concentrate on what potentially is a cosmological flaw of the axion: The non-trivial vacuum topology of axion models gives rise to cosmic vortex strings and domain walls. The latter are catastrophic unless removed by some mechanism soon after formation. In the dissertation we demonstrate the efficacy of their removal through string/domain wall intercommuting and annihilation. Causality constraints purporting to restrict the rate of this mechanism are found to be circumvented. C. Quantum Cosmology and Recollapse: Regardless of whether all the physical laws of system are known, its boundary conditions must still be given. In a quantum cosmological context, Hartle and Hawking have proposed that these be specified by a path-integral over compact four geometries. In Part C detailed studies of the implications of this proposal are made for two restricted models, the Freidmann-Robertson-Walker universe with a massive scalar field and the anisotropic Kantowski-Sachs cosmology. We evaluate the respective wavefunctions and the trajectories to which they correspond in the classical limit. Attention is focussed on the fact that most classical trajectories will recollapse to a singularity - the difficulty that this presents for the H-H proposal is discussed. The Kantowski-Sachs universes generically evolve from isotropy during expansion to increasing anisotropy during recollapse, ending as a black hole interior. Some comment is made on the arrow of time thereby induced by the Hartle-Hawking proposal.

Wir bewerten und vergleichen konkurrierende kosmologische Modelle im Hinblick auf theoretische Konsistenz und empirische Kohärenz. Ferner finden wir neue Wege, aktuelle kosmologische Paradigmen des frühen Universums weiter zu entwickeln. Im ersten Teil der Arbeit zeigen wir, dass die empirischen Daten der Planck2013-Satellitenmission für eine spezielle Klasse inflationärer Modelle sprechen, nämlich sog. “plateauartige Modelle mit schmalem Feldbereich”; gleichsam werden die einfachsten inflationären Modelle von den Messdaten nicht gestärkt. Wir formulieren eine neuartige konzeptionelle Schwierigkeit für Plateau-Modelle. Diese besteht darin, dass in einer Energielandschaft, die sowohl plateauartige als auch einfachere Formen der inflationären Potenziale enthält, die plateauartigen weniger Inflation produzieren und es deshalb weniger wahrscheinlich ist, dass sie das observable Universum beschreiben. Wir zeigen ferner, dass dieselben Plateau-Modelle mit einem neuen Multiversumsproblem und einem neuen Anfangswertsproblem behaftet sind. Im zweiten Teil untersuchen wir die Implikationen einer einfachen und experimentell motivierten Zusatzbedingung, Skalenfreiheit. Wir zeigen, dass die uneingeschränkte Palette inflationärer Potenziale sich auf ein wohldefiniertes Bündel inflationärer Modelle reduziert. Dabei verwenden wir eine allgemeine hydrodynamische Beschreibung. Wir klassifizieren und bewerten diese skalenfreien inflationären Modelle im Licht von Planck2013. Anschließend wiederholen wir die Analyse, um ähnliche skalenfreie zyklische Modelle des Universums zu konstruieren. Diese Modelle vergleichen wir mit unseren Ergebnissen, die wir für die skalenfreie inflationäre Theorie gewonnen haben. Im dritten Teil der Arbeit führen wir eine neue Klasse stabiler zyklischer Modelle ein. Wir zeigen, dass diese Modelle weniger Feinabstimmung der Anfangswerte benötigen. Gleichsam generieren sie vernachlässigbare Nicht-Gaussianität in Übereinstimmung mit den Planck2013-Messdaten.
In this thesis we evaluate and compare competing cosmological models for empirical and theoretical consistency and identify new ways of improving current paradigms of early universe cosmology. In the first part, we show that the most recent experimental data from the Planck2013 satellite measuring fluctuations in the cosmic microwave background favors a special class of “small-field plateau-like” models of inflation and disfavors the simplest inflationary potentials. We then identify a new kind of conceptual difficulty for the plateau models that we call the unlikeliness problem – namely, in an energy landscape that includes both plateau-like and simpler potential shapes, the plateau-like produces less inflation and, hence, is less likely to explain our observable universe. In addition, we show that the very same plateau-like models suffer from a new multiverse problem and a new initial conditions problem because they require that inflation starts at energy densities well below the Planck scale. Third, we comment on the impact of these results on the standard view of inflation and more recent versions of the theory invoking the multiverse and complex energy landscapes. In the second part of this thesis, imposing a single, simple, well-motivated constraint – scale-freeness – and using a general hydrodynamic analysis, we show that the unrestricted range of inflationary potentials reduces to a well-defined bundle of inflationary models. We classify and evaluate the scale-free inflationary models in light of Planck2013. We then repeat the construction to produce analogous scale-free bouncing cyclic models of the universe and compare with the inflationary results. In the third part, we introduce a new class of stable ekpyrotic/cyclic models that require less fine-tuning and generate negligible non-Gaussianity consistent with Planck2013 data.

During the epoch when the first collapsed structures formed (6<z<50) our Universe went through an extended period of changes. Some of the radiation from the first stars and accreting black holes in those structures escaped and changed the state of the Intergalactic Medium (IGM). The era of this global phase change in which the state of the IGM was transformed from cold and neutral to warm and ionized, is called the Epoch of Reionization.In this thesis we focus on numerical methods to calculate the effects of this escaping radiation. We start by considering the performance of the cosmological radiative transfer code C2-Ray. We find that although this code efficiently and accurately solves for the changes in the ionized fractions, it can yield inaccurate results for the temperature changes. We introduce two new elements to improve the code. The first element, an adaptive time step algorithm, quickly determines an optimal time step by only considering the computational cells relevant for this determination. The second element, asynchronous evolution, allows different cells to evolve with different time steps. An important constituent of methods to calculate the effects of ionizing radiation is the transport of photons through the computational domain or ``ray-tracing''. We devise a novel ray tracing method called PYRAMID which uses a new geometry - the pyramidal geometry. This geometry shares properties with both the standard Cartesian and spherical geometries. This makes it on the one hand easy to use in conjunction with a Cartesian grid and on the other hand ideally suited to trace radiation from a radially emitting source. A time-dependent photoionization calculation not only requires tracing the path of photons but also solving the coupled set of photoionization and thermal equations. Several different solvers for these equations are in use in cosmological radiative transfer codes. We conduct a detailed and quantitative comparison of four different standard solvers in which we evaluate how their accuracy depends on the choice of the time step. This comparison shows that their performance can be characterized by two simple parameters and that the C2-Ray generally performs best.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted.

The established cosmological theory which describes the history of the Universe since shortly after the “Big Bang” until today is remarkably successful. Thanks to the increasing precision of available observational data, we are now able to considerably constrain the geometry and composition of the Universe - and to glimpse how these will evolve in the near future. However, this success comes at a price: one must assume the Universe “started” in a highly fine-tuned initial condition. Understanding what came before this is therefore one of the main goals of modern cosmology. This thesis attempts to further our understanding of the epoch before this initial condition in three different ways. Firstly, the concept of negative absolute temperatures (NAT) is introduced and its potential relevance for cosmology is investigated. In particular, it is shown that a Universe at a NAT should undergo a period of inflation - although it is unclear whether this would be consistent with current observations. Secondly, work is done on the topic of the evolution of networks of cosmic strings - topological defects which are expected to form in a broad class of phase transitions the Universe may have gone through. A model which takes into account the presence of small-scale structure in strings is used to address questions concerning the existence and stability of scaling regimes for these networks. Finally, it is investigated how future experiments might try to falsify a simple class of canonical single-field slow-roll inflation models by measuring the running and the running of the running of the spectral index of scalar perturbations.

This thesis examines the impact of early twentieth century physics, particularly the sciences of astronomy and cosmology, on the work of W. B. Yeats, James Joyce and Samuel Beckett. I seek to find and make critical use of the traces of Einstein’s cosmic revolution in the aesthetic and philosophical trajectory of modernism. In the chapters that follow, I examine Yeats, Joyce and Beckett as test-cases for modernist aesthetic responses to a universe that had been newly imagined by scientists. In different ways the new cosmology offers a rich source of imaginative as well as narrative and poetic possibilities for these writers. Moreover, although I discuss their work in separate chapters, I have found many connections between their responses, particularly in terms of the new idealist philosophy that came out of popularisations of the new physics. In this sense my approach also offers new ways of talking about Yeats, Joyce and Beckett in relation to each other. The opening chapter begins with a history of relativistic science and its popularisation, then moves on to discuss the reception of relativistic science both within modernism and in the wider contemporary culture, reframing modernism in relation to scientific ideas and discourses. I explore aesthetic responses to this science by authors as different as Thomas Hardy and Ezra Pound, with a view to situating Yeats, Joyce and Beckett within this culture and highlighting their greater receptivity to such ideas. The chapter then moves to a specific consideration of the specialised fields of astronomy and cosmology, explaining the major changes wrought by the Einsteinian revolution and preparing the ground for a discussion of their effect on the works of my authors. The second chapter addresses Yeats’s complex engagement with the new physics and its cosmology, reading against naive critical portrayals of him as entirely anti-scientific. The chapter also offers an account of science in relation to a narrative of Yeats’s whole poetic career, moving from discussions of his longing for an alternative to Newtonian physics in his portrayal of the unpredictable stars in the poems of The Wind Among the Reeds to the strange cosmic, astronomical and occult shapes of A Vision and the later poetry. The third and fourth chapters discuss Joyce’s interest in astronomy and cosmology; in chapter three, I focus on the inspirational power of cosmology in relation to the development of his oeuvre from Portrait to Finnegans Wake. The fourth chapter offers an extended close-reading of a passage from II.1 of the Wake, in which the sudden appearance of the cosmic science of spectroscopy transforms the children’s game of riddles depicted in the chapter into a much more complex problem. In both these chapters, I suggest the salutary aesthetic potential of the difficulty of the new physics when juxtaposed with the difficulty of Joyce texts; the more complex, contested and puzzling universe of contemporary physics suited Joyce much better than the Newtonian science which he sometimes parodied as imperial and monological. Finally, I turn to Beckett’s late modernism in the fifth and sixth chapters. The fifth chapter addresses his novel Murphy in relation to his portrayal of cosmic connections between chaos and absurdity. Beckett’s novel seems increasingly unlike a Newtonian world, as realist frameworks are deliberately undermined by a far more relativistic and chaotic narrative technique. By ‘The Trilogy’, the subject of my sixth and final chapter, which focuses on cosmic and astronomical light in these three novels, Beckett has created a semi-relativistic cosmos in which realist narrative and Newtonian causality are, at first, in Molloy, radically compromised, and finally, in The Unnameable, proved untenable.

Inflationary cosmology is the leading candidate for explaining the homogeneity, isotropy and spatial flatness of the universe whilst also providing the mechanism for the seeding of large scale structure. The central theme of inflationary dynamics involves the evolution of a scalar field, called the inflaton, such that its potential drives an accelerated expansion. Warm inflation is the dynamical realization in which interactions between the inflaton and other fields can lead to dissipation of inflaton energy to other dynamical degrees of freedom. Heavy fields coupled to the inflaton mediate the transfer of inflaton energy to light degrees of freedom which thermalize and heat the universe. This damps the inflaton’s motion and allows for the potential formation of a thermal bath during the inflationary period. Hybrid inflation models are a natural way in which warm inflation can be realized, with dissipation of inflaton energy mediated by the waterfall fields to fields in the light sector. In this thesis I outline the dynamics and observational predictions of supersymmetric hybrid inflation driven by radiative corrections in the warm regime. As in the standard cold inflationary scenario inflation ends when the effective mass squared of the waterfall field becomes negative, with the tachyonic instability driving the system to a global minimum in a process called the waterfall transition. I present the effect of including thermal mass corrections to the waterfall fields, and SUSY mass splittings on the quantum effective potential and the resulting dissipation coefficient. I show that including dissipative effects can significantly prolong the inflationary period to produce 50-60 e-folds of inflation with an observationally consistent primordial spectrum. Inflation still requires a microphysical description within a fundamental theory of quantum gravity. This has prompted the search for inflaton candidates within the superabundance of scalar fields present in string theory compactifications, with brane-antibrane inflation in particular emerging as a concrete implementation of SUSY hybrid inflation in a UV complete particle physics model. Inflation proceeds in a brane-antibrane system through the movement of a stack of branes towards a stack of antibranes, with the inflaton field being the interbrane distance. Warm inflation can be implemented in a brane-antibrane system with dissipation of inflaton energy mediated by fields corresponding to strings stretched between the brane and antibrane stacks. It has been shown that this dissipation of inflaton energy in warm inflation can greatly alleviate the η-problem in brane-antibrane scenarios. Whilst these strings mediating dissipation have end points fixed on to both the D3 and D3 stacks, the compact nature of the geometry within which the system is constructed allows these strings to have different winding modes. We investigated how strings with increasing winding number can provide an enhancement to the dissipation coefficient, allowing a significant reduction in the number of branes and antibranes in the warm inflation system, whilst also modifying the inflationary dynamics by reducing the speed at which the system evolves. This may go some way to alleviating the η-problem associated with some constructions of brane-antibrane inflation whilst also potentially providing the best way to motivate the large field multiplicities associated with warm inflation models.

It is likely that the early Universe was pervaded by a whole host of scalar fields which are ubiquitous in particle physics models and are employed everywhere from driving periods of accelerated expansion to the spontaneous breaking of gauge symmetries. Just as these scalar fields are important from a particle physics point of view, they can also have serious implications for the evolution of the Universe. In particular in extreme cases their dynamical evolution can lead to the failure of the synthesis of light elements or to exceed the dark matter bound in contrast to observation. These scalar fields are not however isolated systems and interact with the degrees of freedom which comprise their environment. As such two interrelated effects may arise; fluctuations and dissipation. These effects, which are enhanced at finite temperature, give rise to energy transfer between the scalar field and its environment and as such should be taken into account for a complete description of their dynamical evolution. In this thesis we will look at these effects within the inflationary era in a scenario termed warm inflation where amongst other effects, thermal fluctuations can now act as a source of primordial density perturbations. In particular we will show how a model of warm inflation based on a simple quartic potential can be brought back into agreement with Planck data through renormalizable interactions, whilst it is strongly disfavoured in the absence of such effects. Moving beyond inflation, we will consider the effect of fluctuation-dissipation dynamics on other cosmological scalar fields, deriving dissipation coefficients within common particle physics models. We also investigate how dissipation can affect cosmological phase transitions, potentially leading to late time periods of accelerated expansion, as well as presenting a novel model of dissipative leptogenesis.

Cosmology is one of the best tools to understand the physics that governs the universe at high energies. On one hand, inflation is a very robust mechanism to explain the initial conditions of the universe. On the other hand general relativity provides a solid framework for the formation of cosmic structures at cosmological scales. Nevertheless, there are still important issues that remain without a clear answer. For example, inflation still lacks of a concrete microphysical description, and also there is still no satisfactory mechanism to explain the late time acceleration of the universe. This thesis addresses these two topics. In the first part we discuss the effects of heavy degrees of freedom coupled to inflation. This has been an important topic over the years, because the experimental success might make it possible to detect new degrees of freedom in inflation. In chapter two we discuss the case when non relativistic heavy fields are coupled to the inflaton through a non minimal gravitational coupling. Here we find that, for certain geometries, the heavy field can modify the potential for a few e-folds, either stopping inflation, or setting its initial conditions. In chapter 3 we study the dynamics of fluctuations in holographic inspired models of multi-field inflation. We find that the entropy mass $\mu$ (the mass of the fluctuation orthogonal to the trajectory of inflation) satisfies an universal upper bound given by $\mu \leq 3 H / 2$. This bound coincides with the requirement of unitarity of conformal operators living on the boundary of the theory. In the second part of the thesis we study high energy effects on the Cosmic Microwave Background (CMB). In the fourth chapter we study the role of disformal transformation on cosmological backgrounds and its relation to the speed of sound for tensor modes. A speed different from one for tensor modes can arise in several contexts such as Galileons theories, or massive gravity. Nevertheless the speed is very constrained to be one by observations of gravitational wave emission. It has been shown that in inflation a disformal transformation allows the speed for tensor modes, to be set to one without making changes to the curvature power spectrum. We show that on the CMB, after doing the transformation, there is an imprint on the acoustic peaks, and the diffusion damping. This has interesting consequences: for a particular class of theories the transformation can be used to constrain the parameter space in different regimes. In chapter five we study the impact of gravitons with non-vanishing masses on the polarisation of th CMB . We also focus on putative modifications to the speed of the gravitational waves. We find that a change of the graviton speed shifts the acoustic peaks of the B-mode polarization and then could be easily constrained. In all cases when both massless and massive gravitons are present, we find that the B-mode CMB spectrum is characterised by a low $l$ plateau together with a shifted position for the first few peaks compared to a massless graviton spectrum. This shift depends on the mixing between the gravitons in their coupling to matter and could serve as a hint in favour of the existence of multiple gravitons.

This Thesis is written in three parts. The first part describes the analytic calculation of the unequal-time correlator of cosmic strings and superstrings. The first efficient constraint analysis of all string and superstring network parameters is performed. By studying the effect of cosmic strings on the cosmic microwave background (CMB) radiation it is discovered that cosmic strings must make up a vanishingly small proportion of the energy density of the universe. The constraints on string network parameters are all skewed toward reducing the magnitude of energy density arising from strings. Also in this Part, a better comprehension of the unconnected segment model (USM) was gained. In particular, a greater understanding of the string scaling parameter $L_f$ was garnered, as well as finding the reason why the USM tends to provide greater power than simulations of Nambu-Goto cosmic strings. The second part contains a detailed description of statistical cosmology and how differences between parameter constraints from different data sets can lead to misleading quantification of discordance. The majority of this part describes different methods of quantifying differences between probability distributions and how these can be interpreted. In particular, using the most up-to-date data possible, differences between parameter constraints using the CMB and probes of large scale structure (LSS) in the universe can be measured. With current data the discordance can be interpreted as a low level of disagreement, but the application of prior ranges on well known parameters can force the tension to be greater. Using data from earlier work, this issue is considered in greater detail, with extensions to the accepted LCDM model added to test if the discordance can be alleviated. These extensions include the addition of active or sterile neutrinos and even ad-hoc changes to the primordial power spectrum. Although there are slight hints that these may help, when considering only the new data it might be unwise to believe that the discordance between parameter distributions from different data sets exists to a degree where the modifications are necessary. Finally, application of deep learning to astrophysical observations is discussed. Using neural networks to learn about specific problems is de rigueur and their use in astronomy and cosmology is a promising field of study. In particular, applying raw data to neural networks can often outperform, or add enhanced features, to what is possible with current, non-empirical feature detection. The classification of supernovae from their light curves can be achieved using a specific machine learning architecture called a recurrent neural network (RNN). Using the raw data from supernova light curves, the RNN is able to learn about features in sequences which can be used to classify types of supernova. Although a large training set is needed to perform as well as current techniques, one major advantage the RNN method has is the possibility of early detection. Rather than needing the entire light curve to perform statistical fits to categorise the supernova type, relatively little information from the early observation data is needed to classify using the RNN. Installing RNN on machinery for observation would save a vast amount of time by early classification since only supernovae of interest can be concentrated on.

Light scalar fields such as axions and string moduli can play an important role in early-universe cosmology. However, many factors can significantly impact their late-time cosmological abundances. For example, in cases where the potentials for these fields are generated dynamically --- such as during cosmological mass-generating phase transitions --- the duration of the time interval required for these potentials to fully develop can have significant repercussions. Likewise, in scenarios with multiple scalars, mixing amongst the fields can also give rise to an effective timescale that modifies the resulting late-time abundances. Previous studies have focused on the effects of either the first or the second timescale in isolation. In this thesis, by contrast, we examine the new features that arise from the interplay between these two timescales when both mixing and time-dependent phase transitions are introduced together. First, we find that the effects of these timescales can conspire to alter not only the total late-time abundance of the system --- often by many orders of magnitude --- but also its distribution across the different fields. Second, we find that these effects can produce large parametric resonances which render the energy densities of the fields highly sensitive to the degree of mixing as well as the duration of the time interval over which the phase transition unfolds. Finally, we find that these effects can even give rise to a "re-overdamping" phenomenon which causes the total energy density of the system to behave in novel ways that differ from those exhibited by pure dark matter or vacuum energy. All of these features therefore give rise to new possibilities for early-universe phenomenology and cosmological evolution. They also highlight the importance of taking into account the time dependence associated with phase transitions in cosmological settings. In the second part of this thesis, we proceed to study the early-universe cosmology of a Kaluza-Klein (KK) tower of scalar fields in the presence of a mass-generating phase transition, focusing on the time-development of the total tower energy density (or relic abundance) as well as its distribution across the different KK modes. We find that both of these features are extremely sensitive to the details of the phase transition and can behave in a variety of ways significant for late-time cosmology. In particular, we find that the interplay between the temporal properties of the phase transition and the mixing it generates are responsible for both enhancements and suppressions in the late-time abundances, sometimes by many orders of magnitude. We map out the complete model parameter space and determine where traditional analytical approximations are valid and where they fail. In the latter cases we also provide new analytical approximations which successfully model our results. Finally, we apply this machinery to the example of an axion-like field in the bulk, mapping these phenomena over an enlarged axion parameter space that extends beyond those accessible to standard treatments. An important by-product of our analysis is the development of an alternate "UV-based" effective truncation of KK theories which has a number of interesting theoretical properties that distinguish it from the more traditional "IR-based" truncation typically used in the literature.

Contemporary cosmology is a vibrant field, with data and observations increasing rapidly. This allows for accurate estimation of the parameters describing our cosmological model. In this thesis we present new research based on two different types of cosmological observations, which probe the universe at multiple epochs. We begin by reviewing the current concordance cosmological paradigm, and the statistical tools used to perform parameter estimation from cosmological data. We highlight the initial conditions in the universe and how they are detectable using the Cosmic Microwave Background radiation. We present the angular power spectrum data from temperature observations made with the Atacama Cosmology Telescope (ACT) and the methods used to estimate the power spectrum from temperature maps of the sky. We then present a cosmological analysis using the ACT data in combination with observations from the Wilkinson Microwave Anisotropy Probe to constrain parameters such as the effective number of relativistic species and the spectral index of the primordial power spectrum, which we constrain to deviate from scale invariance at the 99% confidence limit. We then use this combined dataset to constrain the primordial power spectrum in a minimally parametric framework, finding no evidence for deviation from a power-law spectrum. Finally we present Bayesian Estimation Applied to Multiple Species, a parameter estimation technique using photometric Type Ia Supernova data to estimate cosmological parameters in the presence of contaminated data. We apply this algorithm to the full season of the Sloan Digital Sky Survey II Supernova Search, and find that the constraints are improved by a factor of three relative to the case where one uses a smaller, spectroscopically confirmed subset of supernovae.

L'objectif de cette thèse est d'identifier et d'étudier la population à haut décalage spectral. J'ai utilisé des données dans le proches infrarouge venant du sondage UltraVista associé à des données multi-longueur d'onde disponible dans le champ COSMOS ainsi que le sondage ultra profond de VIMOS utilisé comme un échantillon de contrôle pour la sélection des candidats à grand décalage spectrale. Cette analyse m'a amené à sélectionner des galaxies à z>4.5 en utilisant les décalages spectraux photométriques estimés à partir de la distribution spectrale d'énergie complète ainsi que des limites en magnitudes basés sur la profondeur des données dans chaque bande. Cette sélection a amené à la production d'un catalogue unique de 2036 galaxies dans l'intervalle z~5 et de 330 galaxies dans l'intervalle z~6 faisant de ce catalogue le catalogue le plus grand et le plus complet à ce jour. J'ai trouvé que la fonction de luminosité à z~5 est bien reproduite par une fonction de Schechter. A z~6, j'ai observé que le fin lumineuse de la fonction de luminosité semble être plus peuplée qu'une fonction de Schechter le laisse présager, en accord avec les résultats d'autres études Ceci étant une indication que les processus d'assemblage de la masse ont évolué rapidement. Finalement, j'ai intégré la fonction de luminosité pour en déduire la densité de luminosité et dérivé la densité de formation stellaire entre z=4.5 et z=6.5. Mes résultats montrent une densité de formation stellaire importante, en comparaison des derniers résultats avec les données du télescope Hubble, ainsi qu'une précision plus grande liée aux meilleures contraintes sur la fin lumineuse de la fonction de luminosité
The main purpose of this THESIS is to identify and study the population of high redshift galaxies in the redshift range (4.5 < z < 6.5). I use the near infrared data from the UltraVista survey conducted with the Vista telescope in combination with multi-wavelength data available in the COSMOS field and use The VIMOS Ultra Deep spectroscopic redshift survey (VUDS) as a control sample for the selection of high redshift candidates. I made a analysis leads me to select galaxies at z ≥ 4.5 using photometric redshifts computed from the full spectral energy distribution (SED) combined with well tuned magnitude limits based on the depth of the data in each band. At the end of this process I produce a unique catalogue of 2036 galaxies with 4.5 ≤ z ≤ 5.5 and 330 galaxies with 5.5 ≤ z ≤ 6.5, the largest and most complete catalogue of sources at these redshifts existing today. I find that the LF at z ∼ 5 is well fit by a Schechter function. At z ∼ 6 I find that the bright end might be more populated than expected from a Schechter function, in line with results from other authors, an indication that the mass assembly processes have evolved quickly in a short 0.5-1 Gyr timescale. Finally I integrate the luminosity functions to compute the luminosity density and derive the star formation rate density (SFRD) in 4.5 ≤ z ≤ 6.5. My results show a high SFRD comparable to the latest results derived from the HST data, with an improved accuracy linked to the better constraints at the bright end of the LF

In recent years, the increasingly precise constraints on the value of the Hubble constant, H0, have highlighted a discrepancy between the results arising from early-time and late-time measurements. A potential solution to this so-called Hubble tension is the hypothesis that we reside in a cosmic void, i.e. an underdense cosmic neighbourhood characterized by a faster local expansion rate. In this thesis, we model this scenario through the Lemaître-Tolman-Bondi formalism for an isotropic but inhomogeneous universe containing matter, curvature and a cosmological constant, which we denote by ΛLTB. We numerically implement this framework with two different formulations for the local matter density profile, respectively based upon a more realistic Gaussian ansatz and the idealized scenario of the so-called Oppenheimer-Snyder model. We then constrain the background cosmology and the void parameters involved in each case through a Markov Chain Monte Carlo analysis with a combination of recent data sets: the Pantheon Sample of type Ia supernovae, a collection of baryon acoustic oscillations data points from different galaxy surveys and the distance priors extracted from the latest Planck data release. For both models, the resulting bounds on the investigated parameter space suggest a preference for a -13% density drop with a size of approximately 300 Mpc, interestingly matching the prediction for the so-called KBC void already identified on the basis of independent analyses using galaxy distributions. We quantify the level of improvement on the Hubble tension by analyzing the ΛLTB constraints on the B-band absolute magnitude of the supernovae, which provides the calibration for the local measurements of H0. Since no significant difference is observed with respect to an analogous fit performed with the standard ΛCDM model, we conclude that the potential presence of a local void does not resolve the tension.

Cosmological observations suggest that the universe is homogeneous and isotropic on large scales and that the temperature fluctuations are Gaussian. This has been confirmed by Planck, that measured a level of non-Gaussianity compatible with zero at 68% CL for the primordial local, equilateral and orthogonal bispectrum amplitude . All these observational evidences seem to be in accordance with a scalar-driven inflation epoch in which a scalar field, the inflaton, drives a quasi de Sitter exponential phase of expansion. Nevertheless, Planck measures a nearly scale-invariant spectrum of fluctuations . This nearly scale-invariance suggests that the time-traslational symmetry is slightly broken during inflation. So it becomes natural to ask if other symmetries are also broken and what are the observational consequences. Furthermore, the evidence of some ‘anomalies’, previously observed in the WMAP data and now confirmed (at similar level of significance) by Planck, suggests a possible violation of some symmetries at some point in the evolution of the universe, possibly at very early times. Different anomalies have been observed: a quadrupole-octupole alignment, a dipolar power asymmetry and also an hemispherical asymmetry in power between the northern and southern hemisphere. These features suggest a possible violation of statistical isotropy and/or of parity invariance. Invariance under spatial rotations and parity transformations remains unbroken in the usual inflation models based on scalar fields, so it is necessary to modify the matter content of primordial universe introducing new field(s) or assuming new configuration pattern for the background field that differs from the usual time-dependent background scalar field one. Motivated by these observations, theoretical models that can sustain anisotropic phase of expansion can have an active role and generate statistical anisotropy in primordial fluctuations. This can be realized by introducing gauge field coupled with scalar and/or pseudoscalar fields or by considering three scalar fields in anisotropic background with an unusual breaking pattern of spacetime symmetries that does not involve breaking of time translations. Breaking of rotational symmetry implies that the correlation functions exhibit a direction dependence and, in particular, the two-point correlation function in Fourier space (power spectrum) of primordial curvature perturbations defined by $\langle\zeta_{k_{1}} \zeta_{k_{2}}\rangle=\left(2\pi\right)^3 \delta^{(3)}\left(\textbf{k}_{1}+\textbf{k}_{2}\right)P_{\zeta}\left(\textbf{k}_{1}\right)$ is modified as Pζ(k) =Piso (k) [1+ g* (k)( k°n)] where Piso (k) is the isotropic power spectrum, n is a space preferred direction and g* is a parameter characterizing the amplitude of violation of rotational symmetry. Within the context of primordial anisotropic models we have developed this Ph.D thesis and in particular we have analyzed a model in which a suitable coupling of the inflaton ᶲ to a vector kinetic term F2 generates an anisotropic power spectrum and a bispectrum with a non-trivial angular dependence in the squeezed limit. In particular we have found that an anisotropy amplitude g* of order 1% (10%) is possible if inflation lasted ~5 (~50) e-folds more than the usual 60 required to produce the CMB modes. One of the most important results found in this analysis concerns the presence of infrared modes of the perturbations of the gauge field. These infrared modes determine a classical vector field that tends to raise the level of statistical anisotropy to levels very close to the observational limits. Peculiar predictions of this model are TB and EB mixing between temperature and polarizations modes in the CMB due to the anisotropy and a correlation between the anisotropy in power spectrum g* and the amplitude of the bispectrum fNL that can be considered a consistency relation for all these kind of models that break the rotational invariance. Always in the aim of isotropy violation, but with a completely different approach that involves a scalar fields model, later we have shown, for the first time, how with standard gravity and scalar fields only, is possible to evades the conditions of the cosmic no-hair conjecture. In this model, dubbed solid / elastic model, inflation is driven by a solid. A prolonged slow-roll period of acceleration is guaranteed by the extreme insensibility of the solid to the spatial expansion. We point out that, because of this property, the solid is also rather inefficient in erasing anisotropic deformations of the geometry. This allows for a prolonged inflationary anisotropic solution and for a generation of a non-negligible amount of anisotropy g* in the power spectrum. Finally we have investigated parity-violating signatures of temperature and polarization bispectra of the cosmic microwave background (CMB) in an inflationary model where a rolling pseudoscalar, coupled with a vector field, produces large equilateral tensor non-Gaussianity. We have shown that the possibility to use polarization information and the parity-even and parity-odd l-space improves of many order of magnitude the detectability of such bispectra with respect to an analysis with only temperature. Considering the progressive improvements in accuracy of the next cosmological surveys it is useful to introduce and analyze particular tools, like statistical anisotropy, parity violation, new shapes of non-Gaussianity, that can help to discriminate between the plethora of primordial inflationary models.
Le osservazioni cosmologiche suggeriscono che l'universo è omogeneo e isotropo su grandi scale e che le fluttuazioni di temperatura sono Gaussiane. Questo è stato confermato da Planck che ha misurato un livello di non-Gaussianità compatibile con zero con un livello di significatività del 68% per l'ampiezza del bispettro primordiale nelle configurazioni locale, equilatera e ortogonale. Tutte queste evidenze osservative sembrano essere in accordo con un'epoca inflazionaria guidata da un campo scalare dove questo campo, l'inflatone, guida una fase di espansione esponenziale quasi de Sitter. Tuttavia Planck misura uno spettro di potenza quasi invariante di scala. Questa quasi invarianza suggerisce che la simmetria per traslazioni temporali sia leggermente rotta durante l'inflazione. Quindi viene naturale chiedersi se altre simmetrie siano rotte e quali siano le conseguenze osservative. Inoltre, l'evidenza di alcune anomalie, precedentemente osservate nei dati di WMAP, e ora confermate (con un simile livello di significatività) da Planck, suggerisce una possibile violazione di alcune simmetrie ad un certo punto durante l'evoluzione dell'universo, possibilmente a tempi molto primordiali. Diverse anomalie sono state osservate: un allineamento tra il quadrupolo e l'ottupolo, un'asimetria dipolare in potenza e un'asimmetria emisferica in potenza tra l'emisfero galattico nord e l'emisfero galattico sud. Queste peculiarità suggeriscono una possibile violazione dell'isotropia statistica e/o dell'invarianza per parità. L'invarianza per rotazioni spaziali e trasformazioni di parità rimane conservata nei tipici modelli inflazionari basati su campi scalari, quindi è necessario modificare il contenuto della materia dell'universo primordiale introducendo nuovi campi o assumendo nuove configurazioni per il campo di background che differiscano dal background dipendente dal tempo che si ha nel caso dei tipici modelli scalari. Motivati da queste osservazioni, modelli teorici che possono sostenere una fase di espansione anisotropa possono avere un ruolo attivo e generare anisotropia statistica nelle fluttuazioni primordiali. Questo può essere realizzato introducendo campi di gauge accoppiati con campi scalari e/o pseudoscalari o considerando tre campi scalari in un background anisotropo con una configurazione non-standard per le simmetrie spazio-temporali di background, che non sfrutta la rottura per traslazioni temporali. La rottura di simmetria per rotazione implica che le funzioni di correlazione esibiscono una dipendenza dalla direzione e, in particolare, la funzione di correlazione a due punti nello spazio di Fourier (spettro di potenza) delle perturbazioni primordiali di curvatura definita da $\langle\zeta_{k_{1}} \zeta_{k_{2}}\rangle=\left(2\pi\right)^3 \delta^{(3)}\left(\textbf{k}_{1}+\textbf{k}_{2}\right)P_{\zeta}\left(\textbf{k}_{1}\right)$ si modifichi in Pζ(k) =Piso (k) [1+ g* (k)( k°n)] dove Piso (k) rappresenta lo spettro di potenza isotropo, n è una direzione spaziale privilegiata e g* un parametro che caratterizza l'ampiezza della violazione di simmetria per rotazione. Nel contesto di modelli primordiali anisotropi abbiamo sviluppato questo lavoro di tesi di dottorato e in particolare abbiamo analizzato un modello in cui un opportuno accoppiamento tra l'inflatone ᶲ e il termine cinetico vettoriale F2 genera uno spettro di potenza anisotropo e un bispettro con una dipendenza angolare non banale nella configurazione "squeezed''. In particolare abbiamo trovato che un'ampiezza dell'anisotropia g* dell'ordine del 1% (10%) è possibile se l'inflazione dura ~ 5 (~ 50) e-folds in più dei soliti 60 richiesti per generare i modi della radiazione di fondo cosmico di microonde. Uno dei risultati più importanti trovati in questa analisi riguarda la presenza di modi infrarossi delle perturbazioni del campo di gauge. Tali modi infrarossi determinano un campo vettoriale classico che in genere tende ad innalzare il livello di anisotropia statistica a livelli molto vicini ai limiti osservativi. Predizioni caratterizzanti per questo modello è il mixing tra i modi TB e EB, tra polarizzazione e temperatura, causati dall'anisotropia, e una correlazione tra l'anisotropia nello spettro di potenza g* e l'ampiezza del bispettro fNL che può essere considerata una relazione di consistenza per tutti i tipi di modelli che rompono l'invarianza per rotazione. Sempre nell'ottica della violazione di isotropia, ma con un approccio completamente differente che coinvolge campi scalari, abbiamo poi mostrato, per la prima volta, come con gravità standard e campi scalari, è possibile violare le condizioni del teorema di Wald. In questo modello, chiamato modello solido/elastico, l'inflazione è guidata da un solido. Un prolungato periodo di accelerazione con lento rotolamento è garantito dall'estrema insensibilità del solido all'espansione spaziale. Noi abbiamo dimostrato che, a causa di questa proprietà, il solido è anche piuttosto inefficiente nel diluire deformazioni anisotrope della geometria. Questo permette una soluzione inflazionaria anisotropa prolungata e la generazione di un contributo anisotropo non trascurabile g* allo spettro di potenza. Infine abbiamo investigato i segnali di violazione di parità nel bispettro del fondo cosmico di microonde per temperatura e polarizzazione in un modello dove un campo pseudoscalare che rotola lentamente, accoppiato ad un campo vettoriale, produce elevata non-Gaussianità nella configurazione equilatera. Abbiamo mostrato che la possibilità di usare la polarizzazione con segnale non nullo sia nello spazio delle configurazioni delle l-pari che dispari accresce di diversi ordini di grandezza la rilevabilità di tali bispettri rispetto ad un'analisi con solo temperatura. Considerando i progressivi miglioramenti in accuratezza delle prossime missioni spaziali è utile introdurre e analizzare mezzi particolari, come l'anisotropia statistica, la violazione di parità e nuove configurazioni per la non-Gaussianità, che possano essere utili per discriminare tra la pletora di modelli inflazionari primordiali.

The prevailing model of modern cosmology stipulates the existence of exotic substances such as dark matter and dark energy and events such as inflation. However, their underlying nature is not currently known. In this thesis, we explore new models and measurement techniques that may be used to characterize their cosmological effects and shed light on their inner workings. A model of inflation driven by a substance that may be described macroscopically as a cosmological elastic solid is studied. The proper techniques for the quantization of perturbations within the elastic solid are presented. We find that a sufficiently rigid elastic solid with slowly varying sound speeds can produce an inflationary period. Interestingly, we find models where the elastic solid has an equation of state significantly greater than -1 that nevertheless produces nearly scale-invariant scalar and tensor spectra. The remaining chapters of this thesis concern the use of 21-cm radiation as a probe of the physics of dark matter and dark energy. The effects of warm dark matter on the highly-redshifted 21-cm signal is examined. If dark matter is warm instead of cold, its non-negligible velocities may inhibit the formation of low-mass halos, thereby delaying star-formation, which may delay the emission and absorption signals expected in the mean 21-cm signal. The effects of warm dark matter on both the mean 21-cm signal, as well as on its power spectrum, are described and degeneracies between the effects of warm dark matter and other astrophysical parameters are quantified. One of the primary goals of 21-cm radiation intensity mapping is to measure baryon acoustic oscillations over a wide range of redshifts to constrain the properties of dark energy from the expansion history of the late-time Universe. We forecast the constraining power of the CHIME radio telescope on the matter power spectrum and dark energy parameters. Lastly, we devise new calibration algorithms for the gains of an interferometric radio telescope such as CHIME.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

We study the early-universe cosmology of a Kaluza-Klein (KK) tower of scalar fields in the presence of a mass-generating phase transition, focusing on the time development of the total tower energy density (or relic abundance) as well as its distribution across the different KK modes. We find that both of these features are extremely sensitive to the details of the phase transition and can behave in a variety of ways significant for late-time cosmology. In particular, we find that the interplay between the temporal properties of the phase transition and the mixing it generates are responsible for both enhancements and suppressions in the late-time abundances, sometimes by many orders of magnitude. We map out the complete model parameter space and determine where traditional analytical approximations are valid and where they fail. In the latter cases we also provide new analytical approximations which successfully model our results. Finally, we apply this machinery to the example of an axion-like field in the bulk, mapping these phenomena over an enlarged axion parameter space that extends beyond that accessible to standard treatments. An important by-product of our analysis is the development of an alternate "UV-based" effective truncation of KK theories which has a number of interesting theoretical properties that distinguish it from the more traditional "IR-based" truncation typically used in the extra-dimension literature.

This thesis uses theoretical studies, and numerical simulations to provide results of the experimental reach to detect the QCD axion as dark matter in a Non-standard cosmological background. Assuming that the QCD axion constitutes the full CDM abundance of the universe, this thesis elaborates on its potential detection from experimental setups for the mass window of the axion. The set of results that is obtained here are the relic CDM energy density of axions produced by the vacuum realignment mechanism and the CDM energy density of axions produced from the decay of a network of cosmic strings. This thesis provides results regarding the possibility to detect a primordial gravitational wave relic, which is possible within some favorable cosmological scenarios for the background.

Warm inflation is an implementation of exponential early-universe expansion that incorporates interactions between the inflaton field and its environment. These interactions allow the inflaton to dissipate some of its energy into other fields, which may then thermalise and form a radiation bath. A radiation bath present throughout inflation changes the inflaton dynamics and introduces thermal fluctuations that enhance the spectrum of primordial density perturbations. In the models we consider, the inflaton decays into the light particles of the radiation bath via heavy mediator particles. Warm inflation is subject to a complicated set of constraints which typically requires a large number of such mediator fields to be included in the model. The motivation for this work was to use the parametric dependence of the full low-temperature dissipation coefficient to uncover regimes where this number can be reduced. Previous studies have examined primarily the low-momentum regime of the dissipation coefficient, where inflaton dissipation occurs via off-shell mediator particles. In the low-temperature regime, the production of on-shell mediators in the so-called pole regime suffers from Boltzmann suppression and was therefore thought to be negligible. It has been found, however, that the exponential suppression can be compensated by a sufficiently small effective coupling between the mediator fields and the light fields. In this thesis, we present a numerical code that scans the parameter space of warm-inflation models including both the low-momentum and the pole contribution to the dissipation coefficient. We generate random values for the parameters of the model and the initial conditions of the field and the radiation density; we then solve the full equations of motion for the radiation density and the inflaton field using the general low-temperature dissipation coefficient. Our search includes chaotic, hybrid, and hilltop models, each of which inhabits different regions of warm-inflation parameter space. Our main finding is that the pole contribution to inflaton dissipation significantly extends the parameter ranges accessible to warm inflation. Specifically, we can achieve 50 e-folds of inflation and a spectral index compatible with Planck data with fewer mediator fields and smaller coupling constants. For instance, while low-momentum-dominated dissipation typically requires O(10⁶) mediator fields, we find pole-dominated solutions with as few as O(10⁴) for the quadratic hilltop potential. It is clear that the inclusion of the pole contribution opens up interesting model-building possibilities and that the parametric dependence of the full dissipation coefficient holds promise for achieving even greater reductions of the field content.

Many physical theories beyond the Standard Model predict time variations of basic physics parameters. Direct measurement of the time variations of these parameters is very difficult or impossible to achieve. By contrast, measurements of fundamental constants are relatively easy to achieve, both in the laboratory and by astronomical spectra of atoms and molecules in the early universe. In this work, measurements of the proton to electron mass ratio mu and the fine structure constant alpha are combined to place mildly model-dependent limits on the fractional variation of the quantum chromodynamic scale and the sum of the fractional variations of the Higgs vacuum expectation value (VEV) and the Yukawa couplings on time-scales of more than half the age of the universe. The addition of another model parameter allows the fractional variation of the Higgs VEV and the Yukawa couplings to be computed separately. Limits on their variation are found at the level of less than 5 x 10(-5) over the past 7 Gyr. A model-dependent relation between the expected fractional variation of a relative to mu tightens the limits to 10(-7) over the same time span. Limits on the present day rate of change of the constants and parameters are then calculated using slow roll quintessence. A primary result of this work is that studies of the dimensionless fundamental constants such as a and mu, whose values depend on the values of the physics parameters, are excellent monitors of the limits on the time variation of these parameters.

In this Thesis work, we analyse 21cm line observations taken with the Precision Array to Probe the Epoch of Reionization (PAPER) in the 120-180MHz range (6