The impact of primordial black holes on the high redshift Universe

Recent measurements of the cosmic X-ray and radio backgrounds (CXB/CRB, respectively), obtained with Chandra and ARCADE2, report signals in excess of those expected from known sources. Similarly, measurements of the near infrared background (NIRB) angular power spectrum on angular scales θ ≥ 1 arcmin exceeds by roughly two order of magnitudes predictions from known galaxy populations. The nature of the sources producing the CXB/CRB excesses, and the NIRB fluctuations remains unknown, suggesting the presence of a yet undiscovered population of emitters. Interestingly, the NIRB has been found to cross-correlate with the soft-X-ray background (SXB), possibly justified by X-ray emission from elusive, high redshift accreting black holes (BHs). In this context, the most recent James Webb Space Telescope (JWST) observations have revealed the presence of z ∼ 6-11 active galactic nuclei (AGN) powered by accreting MBHs (M_{BH} ∼ 10^{6-8} M_{sun}). The existence of these sources, along with supermassive black holes (SMBHs, M_{BH} ∼ 10^{8-10} M_{sun}) powering z ∼ 6-7.5 quasars, poses a puzzle for current theoretical models of BH formation and evolution. It is indeed still unclear both the nature of the seeds from which these SMBHs are formed and their ability to grow fast enough to assemble an SMBH in less than 1 Gyr (the age of the Universe at z ∼ 6). These results can be interpreted as requiring either massive (M_{BH} ∼ 10^{4-6} M_{sun}) seeds  and/or less extreme BHs experiencing bursts of super-Eddington accretion. In this Thesis, we tackle the aforementioned puzzles by ascribing their solution to a population of accreting primordial black holes (PBHs). PBHs are black holes that are expected to have formed during the radiation dominated era from the collapse of overdense regions, and have been considered as potential dark matter (DM) candidates. In particular, the questions we would like to answer are:Can PBHs be the sources of the observed backgrounds excess?Can PBHs lead to the formation of SMBHs seeds?This thesis describes the PBH theoretical model we developed and presents the analysis we carried out to answer the aforementioned two open questions.