Bargiacchi, Giada (2024) Cosmology and cosmographic analyses with new distance indicators. [Tesi di dottorato]

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Abstract

Quasars (Quasi Stellar Objects, QSOs) are particularly luminous Active Galactic Nuclei (AGNs), that are extremely persistent powerful sources, characterised by the radiation coming from the accretion onto a supermassive black hole at the centre of a galaxy. They are observed in all cosmic epochs, from the local Universe up to when it was less than a billion years old, and they are very numerous. These properties make them very interesting objects for cosmology. Similarly, Gamma-Ray Bursts (GRBs) have recently attracted the attention of the cosmological community as incredibly powerful and luminous transient sources that reach up to redshift z ∼ 9. As high-redshift sources, QSOs and GRBs are potentially the next rung of the cosmic distance ladder beyond type Ia supernovae (SNe Ia), which have been observed up to z ∼ 2. Indeed, they allow us to investigate the evolution of the Universe in a region that is pivotal to distinguish among predictions of different cosmological models by comparison with observational data. Actually, the recently discovered unsolved cosmological discrepancies, such as the Hubble constant tension, have called into question the reliability of the standard cosmological model and triggered a search for the model that can effectively solve these tensions and reproduce all the observations at different scales. In this scenario, QSOs and GRBs stand as very promising and powerful tools. However, despite our current knowledge, there is still much to be understood about these objects. A deeper insight into them not only broadens our understanding but also enables us to harness them more effectively in cosmological studies. In this thesis, we analyse the methods and the physical correlations used to turn QSOs and GRBs into cosmological probes and we describe their impact on tests of the standard cosmological model and some of its extensions and alternatives. Additionally, we explore cosmography as a model-independent approach to investigate different cosmological models.

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