Is there concordance within the concordance Λ\LambdaCDM model?

We use a complete and rigorous statistical indicator to measure the level of concordance between cosmological data sets, without relying on the inspection of the marginal posterior distribution of some selected parameters. We apply this test to state of the art cosmological data sets, to assess their agreement within the $\Lambda$CDM model. We find that there is a good level of concordance between all the experiments with one noticeable exception. There is substantial evidence of tension between the CMB, temperature and polarization, measurements of the Planck satellite and the data from the CFHTLenS weak lensing survey even when applying ultra conservative cuts. These results robustly point toward the possibility of having unaccounted systematic effects in the data, an incomplete modelling of the cosmological predictions or hints toward new physical phenomena.Comment: 5 pages, 2 figure

Can modified gravity models reconcile the tension between CMB anisotropy and lensing maps in Planck-like observations?

Planck-2015 data seem to favour a large value of the lensing amplitude parameter, $A_{\rm L}=1.22\pm0.10$, in CMB spectra. This result is in $2\sigma$ tension with the lensing reconstruction result, $A_{\rm L}^{\phi\phi}=0.95\pm0.04$. In this paper, we simulate several CMB anisotropy and CMB lensing spectra based on Planck-2015 best-fit cosmological parameter values and Planck blue book beam and noise specifications. We analyse several modified gravity models within the effective field theory framework against these simulations and find that models whose effective Newton constant is enhanced can modulate the CMB anisotropy spectra in a way similar to that of the $A_{\rm L}$ parameter. However, in order to lens the CMB anisotropies sufficiently, like in the Planck-2015 results, the growth of matter perturbations is substantially enhanced and gives a high $\sigma_8$ value. This in turn proves to be problematic when combining these data to other probes, like weak lensing from CFHTLenS, that favour a smaller amplitude of matter fluctuations.Comment: 7 pages, 3 figure

The Threefold Way to Cosmological Tests of Gravity

Explaining the physical origin of cosmic acceleration still poses a challenge to modern cosmology. On one hand, observational evidence corroborating this phenomenon is compelling and continuously becoming stronger and stronger. On the other hand a physical explanation for it is still missing. Cosmic acceleration might be explained by a cosmological constant having the same effect of vacuum energy. Indeed cosmological observations point in this direction and the cosmological constant is a cornerstone of the standard cosmological model. This explanation, however, suffers from several naturalness problems that reflect the fact that, while a cosmological constant is allowed in the gravitational sector by symmetry arguments, there is no theory for the gravitational effect of quantum vacuum. To address this issue, or at least to have an intuition of the phenomenology related to the solution of this problem, one might want to add other dark fluids to the cosmic budget or modify the laws of gravity on large scales to drive the accelerated expansion of the universe. In this thesis we develop and exploit a threefold approach to the study of the phenomenological aspects of cosmic acceleration with the aim of systematizing the investigation of models beyond the standard one. The first path that we shall follow is that of quantifying the level of agreement of cosmological observations, within the standard cosmological model; this will allow us to determine whether there is already some indication that this model might be inappropriate in describing present day observations. Then we shall move along the second path to study parametrized approaches to the phenomenology of Dark Energy and Modified Gravity theories. We de- velop the relevant tools to exploit an Effective Field Theory description for this phenomenon and we investigate some of its observational consequences. At last we shall move along the third path that consists in testing specific non-standard models. Exploiting the unifying power of the Effective Field Theory approach, we study the cosmological implications and corresponding data constraints on two f(R) models and on Horava gravity. Overall we find that, already at present, cosmological observations are precise enough to substantially improve our knowledge about the space of Dark Energy and Modified Gravity models. While doing so we developed the relevant tools to perform massive and systematic studies of non-standard cosmologies aiming at explaining the physical origin of Cosmic Acceleration with present data and the next generation of cosmological surveys

Resolving the Hubble tension at late times with Dark Energy

Within the standard cosmological model, the presence of Dark Energy (DE) is the only structural difference between the early and late times Universe. While its presence is in full display at late times, it is irrelevant at early times, especially when all other physical ingredients of the standard model are contributing to the formation of CMB anisotropies. This makes DE a natural candidate to try to solve cosmological tensions between measurements from these two different epochs. We will analyze how DE presence affects relevant cosmological observables and how it could change them. This will allow us to discuss how late-time measurements constrain DE models that aim to resolve the Hubble constant tension and the achievable performances of these models.Comment: Invited chapter for the edited book "The Hubble Constant Tension" (Eds. E. Di Valentino and D. Brout, Springer Singapore, expected in 2024); 13 pages, 2 figure

Testing Hu-Sawicki f(R) gravity with the Effective Field Theory approach

We show how to fully map a specific model of modified gravity into the Einstein-Boltzmann solver EFTCAMB. This approach consists in few steps and allows to obtain the cosmological phenomenology of a model with minimal effort. We discuss all these steps, from the solution of the dynamical equations for the cosmological background of the model to the use of the mapping relations to cast the model into the effective field theory language and use the latter to solve for perturbations. We choose the Hu-Sawicki f(R) model of gravity as our working example. After solving the background and performing the mapping, we interface the algorithm with EFTCAMB and take advantage of the effective field theory framework to integrate the full dynamics of linear perturbations, returning all quantities needed to accurately compare the model with observations. We discuss some observational signatures of this model, focusing on the linear growth of cosmic structures. In particular we present the behavior of $f\sigma_8$ and $E_G$ that, unlike the $\Lambda$CDM scenario, are generally scale dependent in addition to redshift dependent. Finally, we study the observational implications of the model by comparing its cosmological predictions to the Planck 2015 data, including CMB lensing, the WiggleZ galaxy survey and the CFHTLenS weak lensing survey measurements. We find that while WiggleZ data favor a non-vanishing value of the Hu-Sawicki model parameter, $\log_{10}(-f^0_{R})$, and consequently a large value of $\sigma_8$, CFHTLenS drags the estimate of $\log_{10}(-f^0_{R})$ back to the $\Lambda$CDM limit.Comment: 13 pages, 8 figure