Addressing the overabundance of early bright galaxies in JWST observations by exploring ΛCDM modifications

The James Webb Space Telescope (JWST) is detecting farther and older galaxies than any other telescope has ever done. These observations are undermining our current theories on the early Universe, as JWST is revealing an overabundance of bright early galaxies compared to theoretical predictions based on previous observations. The aim of this thesis is to investigate the effects of modifications to the most successful cosmological model (ΛCDM) on the abundance of early bright galaxies. First, we develop an effective model to compute the theoretical abundance of galaxies within the ΛCDM, taking into account their luminosity through a novel fitting model. Without ΛCDM modifications, we check that our results retain the overabundance problem. Then, we test several cosmological modifications, such as non-Gaussianities in the primordial fluctuations and Primordial Black Holes (PBH). We try to alleviate the tension between theory and observations in the early Universe, without affecting the agreement in the late Universe. Among the different modifications tested, PBHs turn out to be the most promising. We consider two different contributions of PBHs to the abundance of bright galaxies: an effective increase in the number of galaxies and a boost in their luminosity (AGN emission). We find that either very massive non-emitting PBHs (billions of solar masses) or smaller emitting ones (hundreds of thousands of solar masses) could alleviate the aforementioned tension.