Within the inflationary paradigm, our patch of the universe near the end of inflation is highly homogeneous and isotropic as necessitated by cosmic microwave background observations. This patch, however, is also in a cold and non-thermal state. A successful model of an inflationary primordial universe should account for how the universe transitioned from an inflationary to a radiation-dominated, hot, thermal phase required for the production of light elements via big-bang nucleosynthesis. It is desirable for such a model also to include a mechanism for the generation of the observed matter-antimatter asymmetry and perhaps a primordial mechanism for the generation of cosmic magnetic fields. The transition from an inflationary to a radiation-dominated, thermal phase (reheating) is likely to be phenomenologically rich. Reheating could include explosive particle production and various other non-perturbative, non-linear and non-equilibrium phenomena. Reheating can leave its own observational signatures in the form of gravitational waves and non-Gaussianities. Importantly, reheating can also affect the observational predictions of the preceding phase of inflation. Reheating remains an active field of research, with significant gaps in our understanding of the process. This thesis is an attempt to improve our understanding of the period following inflation, including reheating, through an exploration and analysis of realistic post-inflationary models with the aid of detailed numerical simulations. The focus of the studies is on aspects of the models with potential observational implications. In Part I of this thesis, we provide an overview of inflation and its end, concentrating on our current understanding of reheating and the challenges we face in trying to constrain reheating observationally. In Part II, we consider the post-inflationary expansion history in a broad class of observationally-favoured single-field models of inflation. Generally, the ambiguity in the expansion history of reheating can cause significant uncertainty in predictions for inflationary observables such as the spectral index, n_s, and the tensor-to-scalar ratio, r. The work in this part considers the full non-linear evolution of the inflaton during the initial stages of reheating and places bounds on the post-inflationary expansion history when perturbative couplings of the inflaton to other relativistic fields are included. In Part III, we investigate non-perturbative particle production and non-linear dynamics after inflation in models where the inflaton is charged under global/local symmetries. We first explore the effects of the non-linear inflaton dynamics for the generation of matter-antimatter asymmetry in the case where a global U(1) symmetry of the inflaton is weakly broken. We find a parameter range in which the model successfully predicts the observed baryon-to-photon ratio. We then consider the particle production during and after inflation in models with a charged inflaton under Abelian, U(1), and non-Abelian, SU(2) and U(1) x SU(2), gauge symmetries. Finally, we present a novel algorithm for evolving the full set of coupled, non-linear equations describing the U(1) charged inflaton and accompanying gauge fields on a lattice in an expanding universe. The novel feature here is that the gauge constraints are satisfied to machine precision when the gravitational dynamics are self-consistently included at the background level, and there are no restrictions on the order of the time-integrators.

The inflationary ΛCDM cosmology currently provides an accurate description of the Universe. It has been tested using several observational techniques over a wide redshift range, and it provides a good fit to most of them. In addition, it is a surprisingly economical model, requiring only six parameters to characterize the background cosmology and its fluctuations. In this model, the Universe is dominated by a cosmological constant Λ driving an accelerated expansion, and by cold dark matter. The strongest constraints on parameters to date come from observations of the temperature and polarization anisotropies of the cosmic microwave background measured by the Planck satellite. There are, however, indications of features in the Planck power spectra, possible differences with high redshift ground-based CMB experiments, and 'tensions' between Planck and low redshift measurements of the Hubble constant and weak gravitational lensing. In this thesis, we review possible tensions and extensions to the Planck cosmology, at both high and low redshifts. We begin with the high redshift analysis, using the Planck data to test models which introduce oscillatory features in the primordial power spectrum. We also study possible departures from slow roll inflation using the generalized slow-roll formalism, which allows for order unity deviations. Although we find models which give marginal improvements on the temperature or polarization power spectra, the combination of temperature and polarization is found to be consistent with a featureless power-law primordial spectrum. We then focus on measurements of the polarized CMB sky by the South Pole Telescope collaboration, who report tension between their measurements and the ΛCDM cosmology and with the cosmological parameters determined by Planck. We find evidence of a high χ2 in the SPTpol spectra which is unlikely to be cosmological. We report consistency between the Planck and SPTpol polarization spectra over the multipoles accessible to Planck (l ∼< 1500). We then investigate tension at low redshifts. We begin with weak gravitational lensing in which a number of surveys have suggested that the amplitude of the fluctuation spectra is lower than the Planck value. We review the small-angle approximations commonly used in galaxy weak lensing analyses and their effect on cosmological parameters. We find that these approximations are perfectly adequate for present and near future experiments. We find internal inconsistencies in the recent KiDS-450 analysis involving photometric redshifts and the KiDS covariance matrix at large scales. Finally, we investigate the difference between measurements of the present day expansion rate of the Universe. We apply a novel parameterization of the inverse distance ladder to determine the present date value of the Hubble parameter H0, which assumes General Relativity but makes no further assumptions about systematic errors or the nature of dark energy. Our analysis uses baryon acoustic oscillation data and Type Ia Supernovae to constrain the expansion history assuming a value of the sound horizon determined from the CMB. Our results are in tension with recent direct determinations of H0. We conclude that this tension, if real, cannot be solved by modifications of the ΛCDM model at late times. Instead, we would require a modification of the theory at early times which reduces the sound horizon. We conclude that at this time there is no compelling evidence that conflicts with the ΛCDM cosmology either at low or at high redshifts.

Information about the very early universe can be accessed from observations of the cosmic microwave background (CMB) radiation and the later formation of large-scale structure (LSS) that are produced from cosmological perturbations of the early universe. The most developed theoretical explanation for the origin of these perturbations is the theory of inflation, in which the early universe undergoes a period of accelerated expansion, amplifying quantum fluctuations to macroscopic size, which act as the seeds for the CMB anisotropies and the cosmic web of the LSS. The work in this thesis aims to connect the theory of inflation to properties of these observables in a highly detailed way, suitable for future high-precision astronomical surveys. After some introductory review chapters, we begin with new research on a study of inflation from string theory, deriving an observably-large value of the tensor-to-scalar ratio, which had been previously difficult to achieve theoretically. The next study investigates the link between the observed CMB power asymmetry and non-Gaussianity, including a novel non-zero value for the trispectrum. Next we study soft limits of non-Gaussian inflationary correlation functions, focussing first on the squeezed limit of the bispectrum and then generalizing to soft limits of higher-point correlation functions, giving results valid for multi-fi eld models of inflation.

The so-called 'cosmic web', comprising cosmic voids and galaxy clusters, has been proven to be extremely sensitive to deviations from General Relativity. This could be further investigated by future large-scale surveys, such as with the European Space Agency satellite Euclid. In this study, the parameter |fR0| from f(R) gravity is constrained by considering the Euclid survey specications to predict the observed numbers of voids and clusters in bins of redshift, mass and, only for voids, density contrast. From these values, the Fisher matrix is computed for three values of |fR0|, 10-4, 10-6 and 10-8, by assuming a flat Universe with a component that mimics the cosmological constant. The probability density functions are obtained for |fR0| and seven other parameters from the fiducial model considered (ns, h, Ωb, Ωm, σ8, w0 and wa).

Weak gravitational lensing has developed to be one of the most powerful tools for studying the (dark) matter distribution in the Universe. Most weak lensing studies thus far were con- ducted in the optical and near infrared. Measuring weak lensing in the radio though, provided it is feasible, can be very advantageous. One can exploit the well known and deterministic beam pattern of a radio telescope and the polarisation information in radio data to reduce shape biases and intrinsic alignment effects respectively. Combining the information from an optical and a radio survey can also help remove systematics from both datasets. This has motivated this study that uses archival radio and optical data to treat telescope systematics and measure an unbiased weak lensing signal using shape information derived from radio observations. Using simulations I have shown that an unbiased convergence cross power spectrum can be measured in the presence of the large scale (θ > 1◦) systematics detected in FIRST and SDSS. The method however amplifies the uncertainties by a factor ∼2.5 compared to the errors due to cosmic variance and noise due to galaxy intrinsic shape alone. Using the shape information from the two surveys I measure a Ckappakappa spectrum signal that is inconsistent with zero at the 2.7sigma. The placed constraints are consistent with the expected signal in the concordance cosmological model assuming recent estimates of the cosmological parameters from the Planck satellite and literature values for the median redshifts of SDSS and FIRST.Through simulations I also show that I can successfully remove position based small scale systematics (θ5). Using the deconvolved information for the resolved sources I calculate a FWHM median size and flux density of 0.5'' and 300μJy respectively. Comparing the source number density and RMS noise of the study with those of FIRST, I extrapolate to predict that the number density of sources at > 5sigma will be ∼5arcmin-2, assuming the target noise threshold for the survey is reached.

The knowledge of the dynamics and characteristics of space plasma during solar-terrestrial coupling has been greatly enriched by process that aids the determination of the instantaneous frequencies which support the non-stationary and non-linear nature of signals. Such plasmas are observed in the magnetosheath in the downstream of bow shock. In this thesis a technique was applied which extracts the various contributing oscillatory modes reflecting the waveforms observed in the space by Cluster spacecraft instruments such as FGM, CIS and EFW, and decompose the frequency of each extracted mode using Instantaneous Frequency method that is based on Simple Hilbert Transform (SHT). This is achieved through the use of Empirical Mode Decomposition (EMD). To eliminate the negative frequency of the various extracted modes referred to as intrinsic mode function which appears with Fourier transform, we apply Hilbert transform leading to analytic representation of the signals. This process aids the determination of the instantaneous frequencies of the extracted modes. The combined process of EMD and Hilbert transform is called the Hilbert-Huang transform. The results in this thesis have been based on the improved EMD. To contribute to the understanding of plasma dynamics, the computed instantaneous frequencies are compared with the results obtained from the application of Simple Hilbert Transform. Instantaneous frequencies of overriding waves are easily separated as opposed to the application of just SHT. They offer the advantage of 3-dimensional study of the spatial characteristics of waves. The understanding of the instantaneous wave number has been achieved through the EMD and SHT combination. This provides the results which give the wave vector for a known frequency at a given instant of time. The instantaneous dispersion relation is determined using the knowledge of the instantaneous frequency and wave vector in the satellite frame, the plasma bulk velocity and the spacecraft velocity (found to be negligible compared with the plasma bulk velocity). This is accomplished using a Doppler shift relation. Wave modes identifications have been carried out by considering the proton temperature anisotropies, plasma beta and plasma bulk velocity and instantaneous phase velocity in the satellite frame. We report Alfvén mode close to the bow shock, spreading out to mirror mode which dominates the middle of magnetosheath. The mirror mode then diminishes towards the magnetopause.

Após uma revisão das bases da cosmologia moderna e dos mecanismos de produção de anisotropias na radiação cósmica de fundo, calculamos a relação entre a polarização da radiação cósmica de fundo causada por espalhamento Thomson no gás ionizado presente em aglomerados de galáxias e o potencial gravitacional da época do desacoplamento dos fótons com a matéria, em z \' 1100. Mostramos como é possível realizar, em teoria, uma tomografia desse potencial gravitacional em todo o universo observável e como a correlação desse sinal de polarização com o contraste de densidade de matéria poderia nos ajudar a restringir parâmetros cosmológicos. Entretanto, o fraco sinal esperado para essa polarização nos leva à conclusão de que uma tomografia do potencial gravitacional, através desse método, é impraticável no futuro próximo. / After a review of the foundations of modern cosmology and the cosmic microwave background anisotropies production mechanisms, we calculated the relation between the cosmic microwave background polarization caused by Thomson scattering in the ionized gas found in galaxy clusters and the gravitational potential from the photon decoupling epoch, on z \' 1100. We have shown how it is possible to make, in theory, a tomography of this potential over all the observable universe and how the correlation of this polarization signal with the matter density contrast could help us constrain cosmological parameters. However, the weak signal expected for this polarization shows that a gravitational potential tomography using this method is unfeasible in the near future.

General relativity, one of the pillars of our understanding of the universe, has been a remarkably successful theory. It has stood the test of time for more than 100 years and has passed all experimental tests so far. Most recently, the LIGO collaboration made the first-ever direct detection of gravitational waves, confirming a long-standing prediction of general relativity. Despite this, several fundamental mathematical questions remain unanswered, many of which relate to the global existence and the stability of solutions to Einstein's equations. This thesis presents our efforts to use numerical relativity to investigate some of these questions. We present a complete picture of the end points of black ring instabilities in five dimensions. Fat rings collapse to Myers-Perry black holes. For intermediate rings, we discover a previously unknown instability that stretches the ring without changing its thickness and causes it to collapse to a Myers-Perry black hole. Most importantly, however, we find that for very thin rings, the Gregory-Laflamme instability dominates and causes the ring to break. This provides the first concrete evidence that in higher dimensions, the weak cosmic censorship conjecture may be violated even in asymptotically flat spacetimes. For Myers-Perry black holes, we investigate instabilities in five and six dimensions. In six dimensions, we demonstrate that both axisymmetric and non-axisymmetric instabilities can cause the black hole to pinch off, and we study the approach to the naked singularity in detail. Another question that has attracted intense interest recently is the instability of anti-de Sitter space. In this thesis, we explore how breaking spherical symmetry in gravitational collapse in anti-de Sitter space affects black hole formation. These findings were made possible by our new open source general relativity code, GRChombo, whose adaptive mesh capabilities allow accurate simulations of phenomena in which new length scales are produced dynamically. In this thesis, we describe GRChombo in detail, and analyse its performance on the latest supercomputers. Furthermore, we outline numerical advances that were necessary for simulating higher dimensional black holes stably and efficiently.

As pesquisas em Raios Cósmicos de Ultra Alta Energia conectam várias áreas da ciência, desde a física de particulas elementares até a estrutura do universo. Compreender a sua composição pode elucidar muitos enigmas interdisciplinares. O foco de nosso trabalho será a elaboração de métodos para distinguir a composição dos raios cósmicos com energia E > 5 1019 eV usando dois parâmetros, o Xmax e a densidade de muons. Em particular, temos como objetivo discriminar chuveiros iniciados por fotons daqueles por núcleos ou nucleons. Iremos basear nosso método em simulações de chuveiros atmosféricos extensos que geraremos através do código AIRES. Iremos analisar as principais caractersticas dos chuveiros iniciados por nucleos, nucleons e fotons procurando por sinais que possibilitem a discriminação com alta estatstica. / The subject of Ultra High Energy Cosmic Rays connects scientic elds, from elementary particle physics to the structure of the universe. Understanding the composition of UHECR can elucidate many interdisciplinary enigmas. Our work is focused on the elaboration of methods to distinguish the composition of cosmic rays at energies 51019 eV using two parameters, Xmax and muon density. In particular, we want to discriminate extensive air showers initiated by photons from those initiated by nucleus and nucleons. We base our work on simulations of extensive air showers using the AIRES package. We analyze proton, nucleus and gamma showers looking for signals to discriminate them using large statistics.

This essay explores how the reader cognitively reacts to reading H. P. Lovecraft’s horror story “The Shadow Out of Time” (1936). I analyse the text to see how it invites readers to beaffected by it. I specifically look at invitations to envisionment, subjective experience andintersubjectivity. The literary horror terms uncertainty and uncanniness are used inconjunction with cognitive theory to explain how the readers may react to readingLovecraft’s stories. These are in turn reinforced by the reader's ongoing envisionment of thestory, and their sharing of the character’s experiences via subjective experience. For example,the separation between the mind and body of the protagonist creates an uncanny dissonanceto his own identity, which disrupts the reader’s understanding of the character. This maycause distress in the reader due to the subjective experience that connects the reader to thecharacter. Additionally, the disrupted sense of reality in the book affects the narrator'sperceptions of both his surroundings and his own mental faculties. Subsequently, the reader’sunderstanding of the text is also in a state of flux which affects their envisionment of thestory. This disrupts understanding and may enhance feelings of unease. My theory istherefore that unease is invited by certain horror techniques, such as uncertainty anduncanniness, which in turn influence the reader specifically through subjective experienceand envisionment.