The problem of the cosmological constant together with the tension in the observations of the present value of the Hubble parameter has brought about the search of alternative theories to the...Show moreThe problem of the cosmological constant together with the tension in the observations of the present value of the Hubble parameter has brought about the search of alternative theories to the Standard Model of Cosmology. One of the most promising ones is Modified Gravity. In this thesis, we explore scalar-tensor theories that are invariant under weakly broken galileon (WBG) transformations. We have derived the background cosmology and found attractor solutions that track a De Sitter Universe at late times, solving the coincidence problem and preventing from fine tuning issues. We have implemented the model into EFTCAMB, an Einstein- Boltzmann solver that employs the effective field theory of dark energy, and computed the power spectrum of temperature-temperature CMB anisotropies and the matter power spectrum. Our results, based on the theoretical predictions, are promising. While being compatible with LCDM at small scales, WBG can lower the ISW tail of the TT models on the CMB power spectrum, making it potentially favoured by observations.Show less
We present a method to link phenomenological parameterizations of dark energy on cosmological scales with theory. We will use the phenomenological parameterization adopted by the Planck...Show moreWe present a method to link phenomenological parameterizations of dark energy on cosmological scales with theory. We will use the phenomenological parameterization adopted by the Planck collaboration [], parameterizing the phenomenological functions $\mu$ and $\eta$ as functions of scale $k$, and time using the scale factor $a$. When linking the phenomenological functions with theory, we restrict ourselves to the Horndeski class of theories [] and neglect time derivatives, i.e. we work with the general and model independent quasi static approximation (QSA), to be able to find analytical expressions for our phenomenological parameterizations. Despite its generality, the QSA forces the phenomenological parameterization adopted by the Planck collaboration into a scale-independent one; which motivated us to propose an alternative phenomenological parameterization using a parameterization of the phenomenological functions $\mu(a,k)$ and $\Sigma(a,k)$. This phenomenological parameterization does allow for scale dependence in $\mu$ under our conditions. The effect of imposing extra conditions on our models, e.g. imposing the speed of gravitational waves to equal the speed of sound, is investigated by searching the parameter space of the phenomenological parameterizations for points that yield physically viable models. The viability here is evaluated by means of ghost and gradient stability conditions.Show less
We will discuss non-local solutions to some of the problems of the standard model of cosmology, L cold dark matter (LCDM), focusing on two models of gravity and their applications to cosmology. The...Show moreWe will discuss non-local solutions to some of the problems of the standard model of cosmology, L cold dark matter (LCDM), focusing on two models of gravity and their applications to cosmology. The first comes from modifying the Einstien-Hilbert action by including an m2R 1 2 R term and the second by including an m2 1 R term. Both models posses self-accelerating solutions. I will demonstrate that their background cosmology is consistent with data, and testable primarily through the equation of state of our universe’s effective stress-energy tensor. At the perturbative level, these models have more galaxy clustering and weak lensing, so they are be highly testable using up coming cosmological surveys. My contribution to this work is the perturbation theory of the m2 1 R model and the recovery of these results for the m2R 1 2 R model.Show less
