The impact of a modified CMB photon density on UHECR propagation

Abstract

The existing discrepancies between the observation of local and extraction of global cosmological parameters motivate an extension of the ΛCDM cosmological model. A proposed extension called SU(2) CMB describes cosmic microwave background (CMB) photons with an SU(2) instead of a U(1) gauge group. This mitigates some of these tensions, for example 𝐻 0 , Ω 𝑚 , 𝜎 8 , pushes the recombination epoch to higher redshifts, and thereby effectively reduces CMB photon densities. In this work, we study the impact of the SU(2) modified CMB evolution on the propagation of ultra-high energy cosmic rays (UHECRs) and their related fluxes of cosmogenic photons and neutrinos. The measured and predicted fluxes are the basis used to constrain source properties and rely on the ΛCDM CMB evolution. Thus, a modification of the past CMB densities impacts these flux predictions and possibly the constraints on the sources. In particular, we show an increased proton flux below the ankle (10 18.5 eV), and slightly increased cosmogenic neutrino fluxes in comparison to ΛCDM.