Dissertations / Theses on the topic 'Axion cosmology'

In this thesis I study astrophysical and cosmological effects of axion-like particles (ALPs). ALPs are pseudo-scalar particles, which are generally very weakly-interacting, with a coupling α/M E · B to electromagnetism. They are predicted by many theories which extend the standard model (SM) of particle physics, most notably string theory. String theory compactifications also predict many scalar fields called moduli which describe the size and shape of the extra, compact dimensions. In string theory models generically the moduli fields are responsible for reheating the universe after inflation. Being gravitationally-coupled, they will also decay to any other particles or sectors of the theory, including any light ALPs, of which there are usually many. The ALPs produced by moduli decay will contribute to dark radiation, additional relativistic energy density. The amount of dark radiation is tightly constrained by observations, this bounds the branching fraction of moduli decays into ALPs, which constrains the string theory model itself. I calculate the amount of dark radiation produced in a model with one light modulus, solely responsible for reheating, called the Large Volume Scenario. I study a minimal version of this model with one ALP and a visible sector comprised of the minimal supersymmetric SM. The dominant visible sector decay mode is to two Higgses, I include radiative corrections to this decay and find that ALP dark radiation is over-produced in this minimal version of the model, effectively ruling it out. The production of ALPs from moduli decay at reheating seems to be a generic feature of string theory models. These ALPs would exist today as a homogeneous cosmic ALP background (CAB). The coupling of ALPs to electromagnetism allows ALPs to convert to photons and vice versa in a magnetic field, leading to potential observable astrophysical signals of this CAB. Observations have shown an excess in soft X-ray emission from many galaxy clusters. I use detailed simulations of galaxy cluster magnetic fields to show that a CAB can explain these observations by conversion of ALPs into X-ray photons. I simulate ALP-photon conversion in four galaxy clusters and compare to soft X-ray observations. I show the excesses (or lack thereof) can be fit consistently across the clusters for a CAB with ALP-photon inverse coupling of M = 6 - 12 x 10¹² GeV, if the CAB spectrum has energy ~ 200 eV. I also study the possibility of using galaxy clusters to search for and constrain the ALP coupling to photons using cluster X-ray emission. Conversion of X-ray photons into ALPs will cause spectral distortions to the thermal X-ray spectrum emitted by galaxy clusters. I show that the non-observation of these distortions is able to produce the strongest constraints to date on the ALP-photon inverse coupling, M ≳ 7 x 10¹¹ GeV.

The ultimate goal of this thesis is to improve our understanding of the cosmology of axions. Axions couple to QCDinstantons and these non-perturbative effects are modeled within the framework of the interacting instanton liquid model (IILM). The thesis describes the significant advances made within the IILM in order to study the quark-gluon plasma in realistic parameter regimes. In particular, a determination of the temperature-dependent axion mass in the IILM lays the foundation for a critical reevaluation and update of present cosmological axion constraints. We develop grand canonical Monte Carlo routines to study topological fluctuations in the quark-gluon plasma. The model is calibrated against the topological susceptibility at zero temperature, in the chiral regime of physical quark masses. A numerical framework to derive interactions among the pseudo-particles is developed that is in principle exact, and is used to cure a pathology in the presently available finite temperature interactions. The IILM reduces field theory to a molecular dynamics description, and we show that, quite generically, the dynamics for non-trivial backgrounds in the presence of light quarks is reminiscent of a strongly associating fluid. To deal with the well-known difficulty in simulating ionic fluids, we develop advanced algorithms based on Biased Monte Carlo techniques. We study the IILM at finite temperature in the quenched and unquenched sector, with due diligence to a consistent thermodynamic limit. Of particular interest is chiral symmetry breaking and the temperature dependence of the topological susceptibility, and we study in detail the effects of instanton--anti-instanton pairs. Our determination of the topological susceptibility provides, for the first time, a well-motivated axion mass for all temperatures. The misalignment mechanism for axion production is studied in detail, solving the evolution equations exactly in a radiation dominated FRW universe with the full temperature dependence of the effective degrees of freedom taken into account. Improved constraints in the classic and anthropic axion window are derived. We generalise the latter to large angle fine-tuning by including in the isocurvature contribution to the cosmic microwave background radiation the full anharmonic axion potential effects. Finally, we reexamine bounds from axion string radiation in the thermal scenario to complete a comprehensive update of all cosmological axion constraints.

Models with an axion-like inflaton have received considerable attention since the early 90’s, since pseudo-Nambu Goldstone bosons (pNGBs) have a radiatively stable potential and they are abundant in string theory. In these models, the inflaton can be coupled with a gauge field, leading to a rich phenomenology. The produced gauge quanta source the scalar and tensor components of the metric perturbations, with the latter giving rise to non-vanishing TB and EB correlation functions in the Cosmic Microwave Background (CMB), which can be detected by ongoing and future experiments. In this work, we study the dynamics of axion-inflation models, both analytically and numerically, focusing mainly on chiral gravitational waves that are generated in three different scenarios: natural inflation, axion monodromy and a linear potential with a step-like feature. We find that a signal can be detected by LISA and by advanced LIGO and Einstein Telescope if the step is broad or very steep, respectively, but in these cases problems related to strong backreaction on Friedmann equation might arise. If instead the step is just a small correction to the linear potential, chiral gravitational waves might be detected by LISA in a weak backreaction regime.

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.

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.

Pendant mes trois ans de doctorat j'ai eu le plaisir de travailler sur trois projets très variés ayant un but commun: mieux contraindre certains modèles de nouvelle physique entre cosmolo- gie et le LHC. Le fait que les densités reliques de matière noire et de baryons sont similaires semble indiquer qu'il y a un lien entre les deux. Nous essayons d'expliquer les valeurs observées en reliant un modèle de leptogenèse au miracle des WIMPs, qui produit naturellement la bonne densité relique. Si l'asymétrie baryonique est produit dans des désintégrations hors équilibre à l'échelle électro-faible et si la matière noire est constituée de WIMPs, les deux densités reliques sont con- trôlées par des processus électro-faibles hors équilibre. Je construis un modèle de leptogenèse à l'échelle du TeV en utilisant une extension du type seesaw inverse du modèle standard avec des singlets additionnels. Pour produire suffisamment d'asymétrie baryonique il faut une violation CP ∼ O(1) qui est difficile à obtenir dans mon cadre. Les axions, tout comme les WIMPs sont de bons candidats de matière noire bien motivés. Il serait très utile de pouvoir les distinguer. Sikivie argumente que si des axions sont dans un condensat de Bose-Einstein, alors ils forment des halos galactiques différents des halos de WIMPs. D'après Sikivie ce sont les interactions gravitationnelles qui thermalisent les axions et qui les condensent. La formation d'un condensat nécessite la génération d'entropie qui ne peut pas être fourni par les interactions gravitationnelles au premier ordre. J'étudie la génération d'entropie par les interactions gravitationnelles en estimant une longueur de dissipation dans le fluide d'axions qui vient de la présence d'une pression anisotrope. Je ne peux pas confirmer la thermalisation rapide d'axions causé par leurs interactions gravitationnelles. Des nouveaux bosons de jauges comme le Z' apparaissent dans un grand nombre d'extensions du modèle standard. On les recherche le plus souvent comme une résonance dans le spectre de masse invariante de leurs produits de désintégration. Le Z' doit être produit sur couche de masse dans ces recherches résonantes. Mais la présence d'un Z' peut aussi influencer d'autres observ- ables cinématiques sans être produit directement, ce qu'on peut utiliser dans des recherches non-résonantes. Je compare ces deux types de recherches au LHC et trouve que pour des petits couplages les recherches résonantes sont plus adaptées mais pour de plus grandes masses et couplages les recherches non-résonantes sont plus performantes
During the three years as a PhD student I had the pleasure to work on three major projects which are united in the goal to better constrain new physics models between cosmology and the LHC. The similar values of dark matter and baryon relic abundances raise the question whether there is a link between them. We attempt to explain the observed values by relating leptogenesis to the WIMP miracle which gives naturally the right relic abundance. If the baryon asymmetry is produced in electroweak-scale-out-of-equilibrium decays and dark matter is made of WIMPs, both relic densities are controlled by electroweak scale interactions going out of equilibrium. We construct a TeV-scale leptogenesis model using an inverse-seesaw extension of the SM with additional singlets. To produce a large enough asymmetry we require CP violation ∼ O(1) which is difficult to achieve in our set-up. Axions as well as WIMPs are well motivated dark matter candidates. It would be very useful to be able to tell them apart. Sikivie argues that if axions are in a Bose-Einstein condensate they could form a different galactic dark matter halo than WIMPs and that gravitational interactions drive axions into a Bose-Einstein condensate. However for the formation of such a condensate entropy generation is needed which leading order gravitational interactions do not provide. We explore the entropy generation of gravitational interactions by estimating a dissipation scale in the axion fluid due to the presence of a anisotropic stress. We cannot confirm a fast gravitational thermalisation rate. New neutral gauge bosons like the Z' are generic extensions of the standard model which appear in many different models. Traditionally these particles are searched for in resonant searches at colliders, i.e. by producing the particles on-shell and looking for a resonance in the invariant mass spectrum of their decay products. However the presence of a Z' can also affect other kinematic observables without being actually produced on-shell, i.e. non-resonant searches. We compare compare resonant and non-resonant searches at the LHC and find that while for small couplings resonant searches are more sensitive, for larger couplings non-resonant searches are more efficient

This thesis explores the expansive world of General Relativity, and its role to play in modern cosmology and quantum field theory. We begin with a pedagogical approach to relativity, in particular, highlighting upon the ambiguity that arises with the conventions used in different textbooks. A brief introduction to tensor calculus has also been provided in the appendix. The preliminary chapters are also complimented with examples of numerical relativity via simulation. We then move on to discuss examples of non-linear systems, and their exact solutions. Such systems will be analogous to those we shall encounter later, upon considering scalar field theories as a means of modelling dark energy. We shall introduce the axion as our highly motivated dark matter candidate, since this will ultimately determine the behaviour of the scalar field. Coupled to a scaling factor across the spatial domain, it is found that this scalar field will ultimately determine the evolution of our universe. The key result of this thesis has been the possibility to screen both the cosmological constant, and flatness of the universe, to within observable parameters. These results will be explicitly derived from first principles. Also included is a tentative approach to holographic theory, in which strongly correlated systems may be modelled within the asymptotic domain of Anti-de Sitter (AdS) space. Ultimately, our aspirations are to bridge the gap with condensed matter theory, in particular with the publications included within the latter appendices. These publications discuss graphene as a revolutionary new material, for inclusion in both transistor-based and optoelectronic devices.

Pendant mes trois ans de doctorat j'ai eu le plaisir de travailler sur trois projets très variés ayant un but commun: mieux contraindre certains modèles de nouvelle physique entre cosmologie et le LHC. Le fait que les densités reliques de matière noire et de baryons sont similaires semble indiquer qu'il y a un lien entre les deux. Nous essayons d'expliquer les valeurs observées en reliant un modèle de leptogenèse au miracle des WIMPs, qui produit naturellement la bonne densité relique. Si l'asymétrie baryonique est produit dans des désintégrations hors équilibre à l'échelle électro-faible et si la matière noire est constituée de WIMPs, les deux densités reliques sont contrôlées par des processus électro-faibles hors équilibre. Je construis un modèle de leptogenèse à l'échelle du TeV en utilisant une extension du type seesaw inverse du modèle standard avec des singlets additionnels. Pour produire suffisamment d'asymétrie baryonique il faut une violation CP ~ O(1) qui est difficile à obtenir dans mon cadre. Les axions, tout comme les WIMPs sont de bons candidats de matière noire bien motivés. Il serait très utile de pouvoir les distinguer. Sikivie argumente que si des axions sont dans un condensat de Bose-Einstein, alors ils forment des halos galactiques différents des halos de WIMPs. D'après Sikivie ce sont les interactions gravitationnelles qui thermalisent les axions et qui les condensent. La formation d'un condensat nécessite la génération d'entropie qui ne peut pas être fourni par les interactions gravitationnelles au premier ordre. J'étudie la génération d'entropie par les interactions gravitationnelles en estimant une longueur de dissipation dans le fluide d'axions qui vient de la présence d'une pression anisotrope. Je ne peux pas confirmer la thermalisation rapide d'axions causé par leurs interactions gravitationnelles. Des nouveaux bosons de jauges comme le Z0 apparaissent dans un grand nombre d'extensions du modèle standard. On les recherche le plus souvent comme une résonance dans le spectre de masse invariante de leurs produits de désintégration. Le Z0 doit être produit sur couche de masse dans ces recherches résonantes. Mais la présence d'un Z0 peut aussi influencer d'autres observables cinématiques sans être produit directement, ce qu'on peut utiliser dans des recherches non-résonantes. Je compare ces deux types de recherches au LHC et trouve que pour des petits couplages les recherches résonantes sont plus adaptées mais pour de plus grandes masses et couplages les recherches non-résonantes sont plus performantes.

In this thesis we study the viability for ultra-light axions coming from moduli stabilisation in the Large volume scenario to fit recent observations about the dark matter presence in several galaxies and clusters. After a historical introduction on the dark matter problem and a review of the candidates proposed as dark matter constituents through the years, in Chapter 1 we present WIMPs and ALPs, which are nowadays believed to be the most likely constituents. In Chapter 2 we give the theoretical basis to analyse these particles, that is Supersymmetry and String theory, focusing especially on dimensional reduction and string compactifications in order to derive an effective theory. Then, in Chapter 3 we discuss the issue of moduli stabilisation and we deal with it in the Large volume scenario, examining the examples of Swiss cheese and Fibred Calabi-Yau manifolds. Finally, in Chapter 4 we present the original results of this thesis. Starting from recent observations claiming the existence of a preferred range of masses for the ultra-light axions constituting dark matter, we provide a theoretical explanation in the Large volume scenario. We demonstrate that a preferable mass exists for these axions and that axions having this mass could account for the total observed dark matter abundance in a natural way. In addition to this, we examine how to provide several axions with different masses by imposing reasonable cosmological hierarchies. Finally, we show how to generalise these results to Calabi-Yau manifolds with any number of axions, while in Chapter 5 some interesting outlooks of the present work are discussed.

En dépit des récents succès de la cosmologie observationnelle, la majeure partie de l'univers demeure méconnue: la matière usuelle, dite baryonique, ne représente que 5% du contenu total de l'univers. Dans le modèle cosmologique standard, deux autres composantes complètent notre description: l'énergie noire et la matière noire (respectivement 70% et 25% du contenu total). Dans cette thèse, nous nous intéressons à la matière noire, une nouvelle forme de matière qui doit être non-relativiste, non-baryonique et neutre de charge. Nous avons étudié deux candidats : les WIMPs et les axions. Toutes nos analyses ont été menées au sein de la collaboration EDELWEISS, qui opère des détecteurs sensibles à un éventuel signal de WIMP ou d'axion. Les axions ont d'abord été introduits pour résoudre le problème de la symétrie CP en chromodynamique quantique. Ils peuvent être produits dans le soleil par des processus divers et, dans certains modèles, peuvent contribuer à la densité de matière noire. Nous avons utilisé les données d'EDELWEISS pour la recherche d'axions suivant quatre modes de production-détection distincts. Ces mécanismes font intervenir le couplage des axions aux nucléons, aux photons et aux électrons. Nous n'avons observé aucun excès de signal par rapport au bruit de fond. Ces constatations nous ont permis d'obtenir des contraintes fortes sur la valeur de chaque couplage d'axion et d'exclure plusieurs ordres de grandeur de la masse de l'axion dans le cadre de modèles spécifiques de QCD. Les WIMPs font partie des candidats à la matière noire les plus étudiés. Ce sont des particules interagissant faiblement avec une masse pouvant aller du GeV au TeV. Des modèles théoriques et des résultats expérimentaux récents semblent converger vers des masses faibles (de l'ordre de quelques GeV). à la lumière de ces développements, nous avons donc choisi de privilégier l'étude des WIMPs de basse masse (de 3 à 25 GeV). Nous avons mis en place une analyse multivariée particulièrement adaptée à la recherche de WIMPs de basse masse. Cette analyse a été optimisée sur une fraction de 35 kg.jour du jeu de données EDELWEISS complet. Nous n'avons pas observé d'excès de signal par rapport au bruit de fond attendu. Par conséquent, nous avons calculé une limite supérieure sur la section efficace WIMP-nucléon spin-indépendante de 1.48 × 10⁻⁶ pb à 10 GeV
In spite of the recent successes of observational cosmology, most of the universe remains poorly known. Known particles (which we call baryons) only make up 5% of the total content of the universe. The standard cosmological model contains two other components: Dark Energy and Dark Matter (respectively 70% and 25% of the total content). Dark Matter, which is generally believed to be a non-relativistic, charge neutral and non-baryonic new form of matter, is the central focus of this work. We studied two likely candidates, namely WIMPs and axions. Our analyses were carried out within the EDELWEISS collaboration which operates detectors sensitive to both WIMP and axion signals. Axions were first introduced to solve the strong CP problem. They can be produced in the Sun through a variety of processes and in some models, they may also contribute to the Dark Matter density. In this work, we used EDELWEISS data to search for axions through four distinct production-detection mechanisms. These mechanisms involve the coupling of axions to nucleons, photons and electrons. No excess over background was found. These null observations allowed us to set stringent constraints on the axion couplings and exclude several orders of magnitude of the axion mass within specific QCD axion models. On the other hand, WIMPs are the canonical dark matter candidate whose mass lies in the GeV-TeV range. With the motivation of recent theoretical developments and possible signal hints, we focused our effort on so-called low mass WIMPs (3 to 25 GeV). This thesis describes a new multivariate analysis specifically designed for this mass range, which we tuned using an unblinded fraction of the data set (35 kg.d) from a single EDELWEISS detector. No significant signal over background excess was found and we set an upper limit on the spin-independent WIMP-nucleon cross section of 1.48 × 10⁻⁶ pb at 10 GeV

In this Thesis I explore aspects of dark radiation and its role in String Phenomenology. Dark radiation is any additional hidden type of relativistic matter present in the Universe today, conventionally labelled as an "excess effective number of neutrino species", Δ N_eff. It provides a powerful test of hitherto untested theoretical models based on fundamental theories such as String Theory. I begin by considering dark radiation in the LARGE Volume Scenario, a phenomenologically viable class of string compactifications. First I review how the minimal setup slightly overproduces axionic dark radiation via modulus decay. I then demonstrate that loop corrections to the main competing visible-sector decay process have a negligible effect and are unable to alleviate the tension with observations. In the following chapter I explore fibred extensions of the LARGE Volume Scenario. The predictions for Δ N_eff are qualitatively different: in particular, models with a sequestered visible sector on D3 branes at a singularity are swamped by massless axions and decisively ruled out. I then consider TeV-scale supersymmetry in a model with anisotropic modulus stabilisation. If the Standard Model is realised on D7 branes wrapping the small volume cycle a hierarchy of soft terms is generated, which may have applications to natural supersymmetry. The final chapter takes a different approach and investigates the proposition that dark radiation, in the form of a Cosmic Axion Background, could explain the long-standing soft X-ray excess from galaxy clusters. I show for the Coma cluster that the morphology of the excess can be reproduced by axion-photon conversion in the intracluster magnetic field, provided the field is allowed to have more structure on smaller scales than typically assumed based on Faraday rotation data. This explanation requires an inverse axion-photon coupling M ∼ 10¹¹ - 10¹² GeV and a mean axion energy (E_CAB) ∼ 50 - 250 eV.

One of the primary targets of current and especially future cosmological observations are light thermal relics of the hot big bang. Within the Standard Model of particle physics, an important thermal relic are cosmic neutrinos, while many interesting extensions of the Standard Model predict new light particles which are even more weakly coupled to ordinary matter and therefore hard to detect in terrestrial experiments. On the other hand, these elusive particles may be produced efficiently in the early universe and their gravitational influence could be detectable in cosmological observables. In this thesis, we describe how measurements of the cosmic microwave background (CMB) and the large-scale structure (LSS) of the universe can shed new light on the properties of neutrinos and on the possible existence of other light relics. These cosmological observations are remarkably sensitive to the amount of radiation in the early universe, partly because free-streaming species such as neutrinos imprint a small phase shift in the baryon acoustic oscillations (BAO) which we study in detail in the CMB and LSS power spectra. Building on this analytic understanding, we provide further evidence for the cosmic neutrino background by independently confirming its free-streaming nature in different, currently available datasets. In particular, we propose and establish a new analysis of the BAO spectrum beyond its use as a standard ruler, resulting in the first measurement of this imprint of neutrinos in the clustering of galaxies. Future cosmological surveys, such as the next generation of CMB experiments (CMB-S4), have the potential to measure the energy density of relativistic species at the sub-percent level and will therefore be capable of probing physics beyond the Standard Model. We demonstrate how this improvement in sensitivity can indeed be achieved and present an observational target which would allow the detection of any extra light particle that has ever been in thermal equilibrium. Interestingly, even the absence of a detection would result in new insights by providing constraints on the couplings to the Standard Model. As an example, we show that existing bounds on additional scalar particles, such as axions, may be surpassed by orders of magnitude.

It is shown in three examples that future cosmological data may allow us to constrain fundamental physics in interesting ways. The first example illustrates that correlations in the polarization of the cosmic microwave background may allow us to put the strongest limit yet on the mass of a particle, the graviton, at a level of m . 10−30 eV. In the second example, it is shown that observations of the correlations of temperature anisotropies and polarization of the cosmic microwave background may reveal hints for the realization of a class of string theoretic inflationary models that go by the name of axion monodromy inflation, or, rule them out. If the evidence for inflation strengthens substantially, just the requirement that inflation occurred may be used to constrain models of fundamental physics. The third example shows that a class of string compactifications that are commonly used in the context of string phenomenology cannot support inflation and might thus be ruled out by cosmology. For completeness, a review of the physics underlying the cosmic microwave background radiation is included and some analytical results for the signatures of primordial gravitational waves in the cosmic microwave background are given.
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Over the last decades our knowledge of the Universe has reached an unprecedented level of accuracy. The observations of the Cosmic Microwave Background and the large scale structure of the Universe opened the so-called epoch of precise cosmology, enabling us to test with increasing precision several aspects of fundamental physics, from the first principles of the cosmological model to global theoretical scenarios beyond General Relativity and the Standard Model of elementary particles. The research subject of this PhD thesis goes exactly in this direction: working at the interface of cosmology, gravitation and (astro)particle physics, I analyze cosmological and astrophysical observations to identify, characterize and constrain possible hints for new physics in light of their implications for the Early Universe.

The existence of dark matter is one of the unsolved fundamental problems of physics and cannot be explained by established theories such as general relativity or the standard model of particle physics. However, an extension of Quantum Chromodynamics (QCD), a part of the Standard Model, can explain dark matter. It is known that QCD is invariant under charge and parity symmetry (CP symmetry), the violation of symmetry is quantified by the parameter theta. The invariance of the QCD under CP transformations is not explicitly required, but only implicitly fulfilled by a very small theta parameter. This seemingly arbitrary conservation of CP symmetry is known as the strong CP problem. A promising solution to this problem was presented by Peccei and Quinn, who consider the $\theta$ parameter not as a constant, but as a dynamical field. This requires a new particle that is extremely light and hardly interacts with the matter we know, as it is expected for dark matter. Therefore, this new particle, called axion, potentially solves two problems; the strong CP problem and the origin of dark matter. To date, however, it was not possible to detect this hypothetical particle experimentally, which is, if the particle exist, because of its very weak interaction and therefore very complex experimental set-ups are required for a successful measurement. In addition, there are only a few precise theoretical predictions for the mass of the axion. In this thesis we present a new method to determine the axion mass. This method is able to correctly describe the responsible production mechanisms for the first time and thus predict a precise value of the axion mass by lattice simulations. The challenge lies in the correct description of the production mechanisms, more precisely in the simulation of the correct string tension. Moreover we also present new methods for the microscopic investigation of the string dynamics themselves, with which theoretical equations of motion can be simulated and compared for the first time. This also provides information about the reliability of string simulations in general.