The Anisotropic Dark Universe.
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Partecipanti al progetto
Descrizione del progetto
A long-standing hypothesis on the nature of dark matter [DM] is that it is composed by a new type of elementary particle. However, up to now its evidences are of purely gravitational origin and no direct and unambiguous proof that it is composed by particles has been provided. The goal of this project is to test the particle physics interpretation of DM by employing in a unified way two signals representatives of the two aspects of DM: a gravitational tracer (weak lensing, large scale structure [LSS]) combined with an electromagnetic [EM] signal emitted from DM through its self-annihilation.
This will be achieved by performing a statistical cross-correlation of the two types of signals. The idea behind this approach is that the DM distribution in the Universe is non-isotropic: even though the EM emission coming from these structures is integrated along the line of sight, and therefore largely averaged, slight anisotropies should be present. The anisotropies in the EM emission need to correlate, at some level, with any gravitational tracer of the DM distribution, since a larger emission is expected where more DM is present. A cross-correlation between the two types of signal is therefore envisaged, and this should manifest itself in the statistical two-point correlation function. The reason to use statistical correlators relies on the fact that anisotropies are quite small: the statistical approach allows to enhance the significance of the signal by performing an all-sky analysis (it cannot be totally full-sky, due to presence of foregrounds, but the unmasked fraction of the sky is still quite relevant). Another advantage is that cross-correlations bring to the particle-physics observables information that, let it alone, they do not possess: redshift information, available to the gravitational observables but not to the EM emission. Since DM emission is peaked at low redshift [1-3] while emission from astrophysical sources comes from higher redshift [1-3], cross-correlations are a powerful way to filter a DM signal from an otherwise overwhelming astrophysical background.
For the EM signal we will concentrate on the gamma-ray emission. This is relevant for DM made of WIMPs (Weakly Interacting Massive Particles), i.e. particles in the GeV-TeV mass range and with weak-type interactions. For gravitational tracers, we will study 3 observables: weak lensing, in terms of cosmic shear; distribution of galaxies; galaxy clusters. Among the 3 options, cosmic shear is the best possibility, since it provides direct, unbiased information of the DM distribution in the Universe. Moreover, a detection of the cross-correlation signal would be a first, since it has never been observed and only now large statistics surveys are becoming available with the Dark Energy Survey [DES]. Instead, galaxies and galaxy-clusters trace light, thus providing biased information on DM. However, they offer crucial additional information that allows us to test the DM clustering at different scales. Moreover, for galaxy catalogues a significant statistics is typically available.
The cross-correlation signal by its nature is clean. For example: the cosmological cosmic shear does not correlate with the galactic gamma-ray foreground. This allows us to see a cosmological gamma- ray emission in a large, overwhelming, foreground. However, the cross-correlation gets contributions from astrophysical sources present in the same DM environment that we want to study: active galactic nuclei [AGN] and star-forming galaxies [SFG] are located in the DM halos of galaxies and clusters, thus producing a cross-correlation signal which needs to be disentangled from the DM one. Redshift tomography and gamma-ray spectral features are instrumental in separating the sought- after DM signal from the astrophysical contributions . The filter offered by the gravitational tracers allows us to extract a DM signal even though the total integrated DM gamma-ray emission is much smaller than the emission from astrophysical sources . In this relies the power of the technique.
In summary, the project has 3 objectives: i) to obtain the first measurement of the cross-correlation between cosmological gamma-ray emission and cosmic shear; ii) to provide a physical interpretation of this measurement, with the main focus on implications for DM (and, as a necessary co-product, for the astrophysical sources of gamma rays); iii) to complement these results with additional cross- correlations involving the LSS of the Universe, through suitably chosen galaxies (low redshift, detected in different energy bands) and galaxy-clusters. The cross-correlations will be quantified in terms of the angular power spectrum and of the two-point correlation function of the skymaps.