With new methods and technology, the accuracy in cosmological measurements has vastly improved in the past years. The recent observations of the Cosmic Microwave Background Anisotropies from the Planck satellite experiment, for example, have spectacularly confirmed the expectations of the standard cosmological model. This is of crucial importance since this model is based on three assumptions that require new physics beyond the standard model of particles: dark matter, a cosmological constant and inflation. However few but interesting tensions between cosmological datasets are emerging that clearly deserve further investigations since could reveal new properties of this new physics sector. For example, the cosmic microwave background data seems to predict a value of the cold matter density background and perturbations that is larger than the one directly measured by cosmic shear surveys and galaxy clusters number counts. This tension is currently above the two standard deviations and future datasets can play a key role in confirming or falsifying it.
Moreover the current determination of the Hubble constant based on luminosity distances from Riess et al. 2018 is about 3.8 sigma away from the determination based on Planck data under the assumption of the standard cosmological model.
Again, a combined analysis of GC and CS data could provide new constraints on the Hubble constant and confirm or rule out the present tension. In this proposal we aim to study in detail those tensions and anomalies by identifying possible systematics and investigating at the same time possible extensions to the standard model that could explain them. We will focus on three main cosmological observables: Cosmic Microwave Background (CMB) anisotropies, Cosmic Shear (CS) and Galaxy Clusters (GC).
The current tensions present between cosmological datasets are clearly one of the most hot topic in modern cosmology. Experiments are reaching only now the accuracy needed for testing second order effects on CMB anisotropies as lensing. Since CMB lensing depends not only from the physics at recombination but also on the late time evolution of the Universe, when the cosmological constant starts to dominate, anomalies in the CMB lensing could possibly hint for new physics in the dark sector. Indeed, a dark energy component different from a cosmological constant could well be at work and future CMB lensing measurements could discriminate between a cosmological constant and a time evolving component. Moreover there is the possibility that dark energy interacts with other components as cold dark matter and, again, CMB lensing can shed light on this. Even more extreme hypothesis of modifications to General Relativity could be considered. Finally, according to current bounds on neutrino masses, neutrino also change from a relativistic to a non relativistic regime in an epoch after CMB recombination, also leaving characteristic imprints on CMB lensing. Understanding and solving current anomalies in the CMB lensing could therefore help in placing new strong bounds on neutrino masses, testing the neutrino mass hierarchy and possibly providing the first measurement for the neutrino absolute mass scale.
Part of this project is also focused on the study of clusters of galaxies to strengthen the effectiveness of those objects as cosmological probes by employing hydrodynamic simulations and observations. The participants of this project have full access to accurate cosmological hydrodynamic simulations such as MUSIC and, the more recent one, The Three Hundred Project. These datasets will efficiently help the comparison with incoming SZ observations at mm-wavelengths. High angular resolution SZ observations are planned in the on-going Sunyaev-Zeldovich Large Program, 50 clusters to be observed with the new camera NIKA2 at IRAM 30m telescope, and medium spectral resolution observations with the new spectrometer, KISS, scheduled at the focal plane of QUIJOTE telescope in Tenerife before the end of 2018. We have priority access to all these observational data. All this information will allow us to better constraint the thermodynamics of the IntraCluster Medium and, with the merging of lensing observations, to infer cluster total mass radial profiles and their dependence with the dynamical state and hydrostatic equilibrium. Thanks to an ongoing collaboration with Stefano Ettori (INAF-OABO) we have access to the XMM Cluster Outskirts Project (X-COP) data. This will allow us to approach the regime where systematics in the determination of H0 with the SZ/X-ray method approach the statistical error.
Concerning Gravitational Lensing, the high image quality and large statistics of Euclid data push the accuracy of cosmic shear measurements to such an exquisite level that the experiment error budget will be dominated by systematics. We will focus on characterizing the bias on the shear
measurement due to the colour gradient of galaxies. The large waveband of the visible filter of Euclid makes the wavelength dependence of the PSF no more negligible thus
asking for a correction due to the non flat spectral energy distribution of source galaxies. We will find out correlations between the colour gradient bias and the observed galaxy properties in order to remove problematic source or correct the shear estimate for this effect. To maximize the outcome of the Euclid lensing data, one can add further statistical tools to boost the Figure of Merit.
Going beyond the standard two points statistics, one can wonder whether using higher order moments can narrow down the confidence ranges on cosmological parameters by partially breaking some degeneracies among them. Rather than using shear, it is more convenient to Fourier transform it to reconstruct the convergence field and then quantify its up to fourth order moments.