DARK DEGENERACY: THEORETICAL AND OBSERVATIONAL ASPECTS

Abstract

The fact that Einstein’s equations connect the space-time geometry to the total dark content of the cosmic substratum, but not to individual contributions of its constituents, can be translated into a degeneracy in the cosmological dark sector. Consequently, cosmological models featuring distinct decompositions within the dark sector, while sharing identical values for the sum of dark components' energy-momentum tensor, remain indistinguishable when assessed through observables based on distance measurements. In this thesis, we investigate cosmological aspects of this degeneracy in the dark sector in the context of two approaches: (i) an interacting vacuum; (ii) a dynamical dark energy model. In the first work, we derive and study some physical aspects of a varying one-parameter dynamical DE parametrization, obtained from an interacting non-adiabatic generalized Chaplygin gas (gCg) model. We find that this dynamical model does not allow phantom crossing. We perform a parameter selection using the most recent publicly available data, such as the data from Planck 2018, eBOSS DR16, Pantheon and KiDS-1000. In light of this analysis, we assess how our model responds to the S_8 ≡ σ_8(Ω_m/0.3)^{0.5} tension, a quantity associated to the growth of cosmological perturbations}. We find that the {data for the cosmic radiation background (CMB) impose strong constraints on the model and conclude that the S_8 tension can be alleviated only for values of the model parameters very close to the ΛCDM cosmology. In the second work, we explore the possibility of breaking this degeneracy by using measurements of the gas mass fraction observed in massive and relaxed galaxy clusters. This data is particularly interesting for this purpose because it isolates the matter contribution, possibly allowing the degeneracy breaking. We study the particular case of the wCDM model with its interactive counterpart. We compare the results obtained from both descriptions with a non-parametric analysis obtained through Gaussian Process. Even though the degeneracy may be broken at background level from the theoretical point of view, we find that current gas mass fraction data seems to be insufficient for a final conclusion about which approach is favored, even when combined with Supernovae type Ia (SN Ia), Baryon Acoustic Oscillations (BAO) and CMB measurements.