New massive bigravity cosmologies with double matter coupling

We study a previously largely unexplored branch of homogeneous and isotropic background solutions in ghost-free massive bigravity with consistent double matter coupling. For a certain family of parameters we find `self-inflated' FLRW cosmologies, i.e. solutions with an accelerated early-time period during the radiation-dominated era. In addition, these solutions also display an accelerated late-time period closely mimicking GR with a cosmological constant. Interestingly, within this family, the particular case of $\beta_1=\beta_3=0$ gives bouncing cosmologies, where there is an infinite contracting past, a non-zero minimum value of the scale factor at the bounce, and an infinite expanding future.Comment: Last version includes minor changes to text and reference

A general theory of linear cosmological perturbations: bimetric theories

We implement the method developed in [1] to construct the most general parametrised action for linear cosmological perturbations of bimetric theories of gravity. Specifically, we consider perturbations around a homogeneous and isotropic background, and identify the complete form of the action invariant under diffeomorphism transformations, as well as the number of free parameters characterising this cosmological class of theories. We discuss, in detail, the case without derivative interactions, and compare our results with those found in massive bigravity.Comment: Published version with extra comments in conclusio

A covariant approach to parameterised cosmological perturbations

We present a covariant formulation for constructing general quadratic actions for cosmological perturbations, invariant under a given set of gauge symmetries for a given field content. This approach allows us to analyse scalar, vector and tensor perturbations at the same time in a straightforward manner. We apply the procedure to diffeomorphism invariant single-tensor, scalar-tensor and vector-tensor theories and show explicitly the full covariant form of the quadratic actions in such cases, in addition to the actions determining the evolution of vector and tensor perturbations. We also discuss the role of the symmetry of the background in identifying the set of cosmologically relevant free parameters describing these classes of theories, including calculating the relevant free parameters for an axisymmetric Bianchi-I vacuum universe.Comment: Updated to match published versio

Bayesian evidence and predictivity of the inflationary paradigm

In this paper we consider the issue of paradigm evaluation by applying Bayes' theorem along the following nested hierarchy of progressively more complex structures: i) parameter estimation (within a model), ii) model selection and comparison (within a paradigm), iii) paradigm evaluation. In such a hierarchy the Bayesian evidence works both as the posterior's normalization at a given level and as the likelihood function at the next level up. Whilst raising no objections to the standard application of the procedure at the two lowest levels, we argue that it should receive a considerable modification when evaluating paradigms, when testability and fitting data are equally important. By considering toy models we illustrate how models and paradigms that are difficult to falsify are always favoured by the Bayes factor. We argue that the evidence for a paradigm should not only be high for a given dataset, but exceptional with respect to what it would have been, had the data been different. With this motivation we propose a measure which we term predictivity, as well as a prior to be incorporated into the Bayesian framework, penalising unpredictivity as much as not fitting data. We apply this measure to inflation seen as a whole, and to a scenario where a specific inflationary model is hypothetically deemed as the only one viable as a result of information alien to cosmology (e.g. Solar System gravity experiments, or particle physics input). We conclude that cosmic inflation is currently hard to falsify, but that this could change were external/additional information to cosmology to select one of its many models. We also compare this state of affairs to bimetric varying speed of light cosmology.Comment: Final version with corrections adde

Testing gravity at cosmological scales

Our understanding of the Universe is based on the ΛCDM model which, although the best cosmological model so far, relies on the presence of major unknown components – dark matter, dark energy, and an inflationary field – which in turn play a crucial role in the evolution of the Universe. These limitations of the model suggest that we may need to introduce modifications at cosmological scales. Indeed, a large variety of modified gravity theories have been proposed (see [1] for a review) and in order to better understand the behaviour of gravity in this regime, we must begin by constructing theoretical and observational tests of the ΛCDM model and the various alternative proposals. This thesis is concerned with testing gravity on cosmological scales, by analysing the viability of alternative gravitational theories, and scrutinising their theoretical consistency. In order to do this, we take two approaches. On the one hand, we explore the viability of a specific modified gravity theory, namely massive bigravity. The evolution of a perfectly homogeneous and isotropic Universe has been previously studied in detail in this model, and has been found to fit observational data. Hence, in this thesis we analyse the evolution of linear cosmological perturbations, where we find a number of interesting instabilities. On the other hand, we take a broader view and develop a method for parametrising linear cosmological perturbations that stays agnostic about the underlying theory of gravity. We apply this method to three classes of models: scalar-tensor, vector-tensor and bimetric theories, and as a result, in this case, we identify the complete forms of the quadratic actions for linear perturbations, and the number of free parameters that need to be defined, to cosmologically characterise these broad classes of gravity theories.Open Acces