Probing of dark energy properties in the Universe using astrophysical observations

Abstract

The astrophysical data of the last two decades have allowed cosmologists to conclude that the present Universe is accelerating. The research carried out to find the origin of this phenomenon has led to the creation of a vast number of dark energy and modified gravity theories, of which the simplest is the ˄CDM model. The latter is, however, plagued with very difficult problems awaiting a solution. The work here presented seeks to contribute to the discussion of the possible explanation for the Cosmos' acceleration and other important questions in modern cosmology using the newest astrophysical observations available. This thesis starts by exploring a dark energy model dubbed thawing quintessence which is characterised by allowing a non constant ratio of pressure to density for dark energy that is however still close to -1 for most of the cosmological evolution, shifting away from this value when the domination of the radiation and matter components fades away. The findings are the most up-to-date constraints for which this model gives a viable theory for dark energy, including a bound on the equation of state at present of w < -0:88. This exact approach was contrasted with the use of an approximate equation-of-state parametrisation for thawing theories. The analysis also includes different parametrisation choices, and comments on the accuracy of the constraints imposed by CMB anisotropies alone. Next, the cosmology of hybrid metric-Palatini gravity is presented. This is a type of Modified Gravity theory in which the Lagrangian density for the gravitational action is a function of the Ricci scalars of both the connection and the metric. The background evolution of two models of this kind is examined explicitly showing the recovery of standard General Relativity at late times. The maximum deviation from the gravitational constant G at early times is constrained using a combination of geometrical data, finding it to be around 1%. A designer scenario, also introduced under the hybrid metric-Palatini formulation, is then used to explore to what extent early modifications of gravity, which become significant after recombination but then decay towards the present, can be constrained by current and future cosmological observations. This model is embedded in the effective field theory description of Horndeski scalar-tensor gravity with an early-time decoupling of the gravitational modification. Applying cosmological data, the constraints on the early-time deviations from General Relativity are obtained. These are dependent on the redshift at which the oscillations in the slip between the gravitational potentials are turned on. For zon = 1000, the deviation from Einstein's theory is ≤ 10-2 with 95% confidence. An explanation of the effect that these divergences have on the CMB power spectrum are discussed, as well as the effect that future 21 cm survey data will have on this study. The last part of this work is a move towards inflation, the early epoch of accelerated expansion undergone by the Universe. Here a parametrisation of the acceleration trajectory is investigated with the aim of measuring the rolling of the inflaton corresponding to the value of the tensor-to-scalar ratio r to be compared with future observations. Considering five ln ε amplitudes and 14 e-foldings, it was found that the posterior distribution of (r,∆Φ) is in very good agreement with Lyth's bound. The analysis included a histogram depiction of the latter result, from which later a minimum constraint on ∆ϕ for each of the bins was found. These outcomes constitute the intermediate step of this project which will be made more accurate by extending it to ~ 50 e-folds, a larger set of cosmological parameters and observational bounds that are restrictive on small scales.