Radiation fields in the early Universe and their feedback effects

This Thesis investigates how radiation fields shaped the early Universe by regulating star formation through feedback and by providing observational probes of the first luminous sources. Using the transformative capabilities of JWST as motivation, we refine theoretical models of cosmic evolution across both galactic and cosmological scales. At small scales, we studied how Ly$alpha$ radiation pressure limits star formation in dense, metal-poor environments typical of primordial galaxies. We developed an analytical shell model incorporating Ly$alpha$ feedback via a force multiplier formulation that includes photon destruction by dust, validated through one-dimensional hydrodynamical simulations. By modeling feedback from a central stellar cluster, we derived upper limits to the star formation efficiency (SFE) before Ly$alpha$ feedback disperses the parent cloud. We find that, for $Sigma_g gtrsim 10^3 mathrm{M_{odot},pc^{-2}}$, Ly$alpha$ radiation disrupts molecular clouds within less than a free-fall time—well before supernova onset. At high-redshift metallicities ($Z/Z_{odot} sim 10^{-2}$), the SFE is severely constrained ($epsilon_* sim 0.01$–{jumi [sns/pths/master.php]}.6$), with order-unity efficiencies achievable only for near-solar metallicity and extreme surface densities. We further modeled Ly$alpha$-driven bubbles around massive stars, linking their evolution to the global SFE through the filling factor of feedback-inflated regions. For typical cloud surface densities $(37 - 1.7times10^5) mathrm{M_{odot},pc^{-2}}$, we find $epsilon_* < 0.08$, increasing only modestly with $Sigma_g$ (from 0.023 to 0.27). Even under optimistic conditions, Ly$alpha$ radiation pressure alone imposes strong limits on SFE, suggesting that a feedback-free, near-unity efficiency phase of star formation was physically implausible in the early Universe. At cosmological scales, we explored whether primordial black holes (PBHs) could account for the observed excess in near-infrared background (NIRB) fluctuations. Treating PBHs as a dark matter component, we modeled gas accretion, radiative emission, and feedback, including both monochromatic and lognormal mass functions (jumi$–^3 mathrm{M_{odot}}$). We find that even if PBHs constitute all dark matter, their contribution to NIRB anisotropies remains below 1%, and under current constraints below 0.1%. PBHs are thus excluded as a viable explanation for the NIRB excess. Overall, this Thesis provides predictive frameworks for Ly$alpha$ feedback-regulated star formation and for PBH-induced background fluctuations, establishing new theoretical benchmarks for interpreting early-Universe observations.