“…Two types of models are promptly identifiable in the literature concerning UHECRs origin: top-down models (41,42), which propose that cosmic rays are generated by decays of very heavy particles born in the early primordial universe , and bottom-up models (43), which try to explore processes capable of accelerating particles up to the very-high energy regime. Since top-down models predict a large number of high-energy photons and current experimental data set a low upper limit for high-energy photon flux (44,45), they are strongly disfavored.…”