The old and new universe in the era of precision cosmology

These are privileged times to be a cosmologist. Recent years have witnessed unprecedented progress in observational and computational techniques and we now are able to quantify cosmological properties with unprecedented accuracy. My work builds upon this observational accuracy by establishing a connection with viable theoretical models. I focus on two specifics eras of the universe’s evolution, namely inflation and today’s cosmic acceleration. In the context of single field inflationary models I illustrate the relation between the spectra of curvature and gravitational wave perturbations. I conclude that their mutual interdependence extends beyond the usual amplitude consistency relation and can be traced all the way to infinite order of accuracy. This yields an infinite hierarchy of consistency relations between these spectra and their derivatives. On a observational perspective, using WMAP’s data, I explore the dependence of CMB constraints on inflation with the cosmological scale at which these are chosen to be presented. I develop a technique that allows for an appropriate choice of this scale and show that this way constraints may be improved by as much as 5 times. In the context of the particle physics motivated quintessence models I have looked at the ability of early universe probes - namely Big Bang Nucleosynthesis - for distinguishing between different dark energy proposals when combined with standard distance modulus or the Hubble rate techniques. I conclude that more yet more accurate measurements are required if observations are to successfully confirm or rule out these models as potential candidates against a cosmological constant. I also analyze possible effects that may mimic or underlie cosmic acceleration effects. I focus on a potential lack of knowledge of the precise values of particular cosmological parameters such as the curvature and matter content of the universe. I find that even a small uncertainty in any of this two quantities leads to significant bias on the reconstruction of dark energy properties, when typical probes like the distance luminosity and the Hubble rate are considered. I conclude that in order to disentangle between these effects a combination of distance and expansion history measurements is required

On what scale should inflationary observables be constrained?

We examine the choice of scale at which constraints on inflationary observables are presented. We describe an implementation of the hierarchy of inflationary consistency equations which ensures that they remain enforced on different scales, and then seek to optimize the scale for presentation of constraints on marginalized inflationary parameters from WMAP3 data. For models with spectral index running, we find a strong variation of the constraints through the range of observational scales available, and optimize by finding the scale which decorrelates constraints on the spectral index n_S and the running. This scale is k=0.017 Mpc^{-1}, and gives a reduction by a factor of more than four in the allowed parameter area in the n_S-r plane (r being the tensor-to-scalar ratio) relative to k=0.002 Mpc^{-1}. These optimized constraints are similar to those obtained in the no-running case. We also extend the analysis to a larger compilation of data, finding essentially the same conclusions.Comment: 7 pages RevTeX4 with 9 figures included. v2: References added, new section added analyzing additional datasets alongside WMAP3. v3: Minor corrections to match version accepted by PR

Comprehensive analysis of the simplest curvaton model

We carry out a comprehensive analysis of the simplest curvaton model, which is based on two non-interacting massive fields. Our analysis encompasses cases where the inflaton and curvaton both contribute to observable perturbations, and where the curvaton itself drives a second period of in inflation. We consider both power spectrum and non-Gaussianity observables, and focus on presenting constraints in model parameter space. The fully curvaton-dominated regime is in some tension with observational data, while an admixture of inflaton-generated perturbations improves the fit. The inflating curvaton regime mimics the predictions of Nflation. Some parts of parameter space permitted by power spectrum data are excluded by non-Gaussianity constraints. The recent BICEP2 results [1] require that the in inflaton perturbations provide a significant fraction of the total perturbation, ruling out the usual curvaton scenario in which the inflaton perturbations are negligible, though not the admixture regime where both inflaton and curvaton contribute to the spectrum

Viable inflationary models ending with a first-order phase transition

We investigate the parameter space of hybrid inflation models where inflation terminates via a first-order phase transition causing nucleation of bubbles. Such models experience a tension from the need to ensure nearly scale invariant density perturbations, while avoiding a near scale-invariant bubble size distribution which would conflict observations. We perform an exact analysis of the different regimes of the models, where the energy density of the inflaton field ranges from being negligible as compared to the vacuum energy to providing most of the energy for inflation. Despite recent microwave anisotropy results favouring a spectral index less than one, we find that there are still viable models that end with bubble production and can match all available observations. As a by-product of our analysis, we also provide an up-to-date assessment of the viable parameter space of Linde's original second-order hybrid model across its full parameter range.Comment: 9 pages, 7 figures. Revised version: corrections to description of the historical development of the models. v3: Minor corrections to match version accepted by PR

On dataset tensions and signatures of new cosmological physics

Can new cosmic physics be uncovered through tensions amongst datasets? Tensions in parameter determinations amongst different types of cosmological observation, especially the `Hubble tension' between probes of the expansion rate, have been invoked as possible indicators of new physics, requiring extension of the $\Lambda$CDM paradigm to resolve. Within a fully Bayesian framework, we show that the standard tension metric gives only part of the updating of model probabilities, supplying a data co-dependence term that must be combined with the Bayes factors of individual datasets. This shows that, on its own, a reduction of dataset tension under an extension to $\Lambda$CDM is insufficient to demonstrate that the extended model is favoured. Any analysis that claims evidence for new physics {\it solely} on the basis of alleviating dataset tensions should be considered incomplete and suspect. We describe the implications of our results for the interpretation of the Hubble tension.Comment: 5 page

The consistency equation hierarchy in single-field inflation models

Inflationary consistency equations relate the scalar and tensor perturbations. We elucidate the infinite hierarchy of consistency equations of single-field inflation, the first of which is the well-known relation A_T^2/A_S^2 = -n_T/2 between the amplitudes and the tensor spectral index. We write a general expression for all consistency equations both to first and second-order in the slow-roll expansion. We discuss the relation to other consistency equations that have appeared in the literature, in particular demonstrating that the approximate consistency equation recently introduced by Chung and collaborators is equivalent to the second consistency equation of Lidsey et al. (1997).Comment: 5 pages RevTe