Primordial Black-Hole Mimicker in Quadratic Gravity as Dark Matter

Abstract: We discuss the astrophysical and cosmological implications of having primordial thermal 2-2-hole remnants as dark matter. Thermal 2-2-holes emanate in quadratic gravity as horizonless classical solutions for ultracompact distributions of relativistic thermal gas. In contrast to a large 2-2-hole that imitates the thermodynamic behaviour of a black hole, a small 2-2-hole at late stages of evaporation behaves as a stable remnant with the mass approaching a minimal value. These remnants as all dark matter can satisfy the corresponding observational constraints provided that both the formation and remnant masses are relatively small. The parameter space for the remnant mass is probed through possible remnant mergers that would produce strong fluxes of high-energy astrophysical particles; the high-energy photon and neutrino data appear to favor towards the Planck-mass remnants, pointing to the strong-coupling scenario for the quantum theory of quadratic gravity. The formation mass, on the other hand, is constrained by the early-universe cosmology, which turns out to require 2-2-holes to evolve into the remnant state before Big Bang Nucleosynthesis.