QCD Axion Searches in Non-Standard Cosmology

The entropy production is an effective way to avoid the overclosure of the Universe from a QCD axion with a decay constant F_a ∈ [10¹² GeV,10¹⁶ GeV] (misalignment angle θ_i = Ο(1)). Indeed, if an axion model is considered with supersymmetry or in light of string theory, we have a heavy saxion and moduli fields as generic predictions. The late time decay of such heavy particles would inject entropy to the Universe leading to a dilution of the axion abundance. At the end of the day, under right conditions, we end up with a non-standard cosmology featured by an Early Matter Dominated (EMD) era with a reheating prior to Big-Bang Nucleosynthesis. In this talk, we discuss the possibility of formation of primordial black holes (PBHs) during the EMD era assuming enhanced primordial scalar perturbation on small scales. In the mixed DM scenario constituted by a dominant QCD axion plus a small fraction of PBHs (less than 1%), these PBHs will capture axions from the background leading to the formation of ultra compact minihalos (UCMHs). We study how these UCMHs would be of great use in the experimental search for QCD axion DM with F_a ∈ [10¹² GeV,10¹⁶ GeV] . In particular, for PBHs with masses ≤ 10^-6 M_⊙, we predict the formation of axion tidal streams in the Milky Way galactic halo due to disruptive collisions between UCMHs and stars. For PBHs with masses ≤10^-14 M_⊙, the Earth would encounter the high dense core of an axion tidal stream once every (1-10) years. On the other hand, for solar mass PBHs, UCMH-neutron star encounters in the galaxy would produce potentially detectable transient radio signatures.