Do primordial quark pellets solve the dark matter puzzle?

Abstract

We show that primordial quark pellets (PQPs) - ultra-dense quark-matter mini-stars formed at $T\sim 1$ GeV - could naturally arise in a radiation dominated early universe if rare baryon overdensities are produced by contracting Peccei-Quinn domain walls or similar super-horizon structures. Solving the Tolman-Oppenheimer-Volkoff equation with a hot quark equation of state, we find stable solutions with a maximum theoretical mass of $10^{-2}$ solar masses and radii of order 100 m, although the formation mechanism favours much lighter objects that would indeed be dominant. Once formed, PQPs cool and evolve into either mini neutron stars or stable strange-matter nuggets, depending on the QCD ground state. The mechanism we present could suffice to reproduce the present dark matter density without altering Big Bang Nucleosynthesis or requiring a posterior entropy injection. PQPs may evade current microlensing constraints if their masses are below $10^{-8}$ solar masses, a range that in view of their mass function could easily accommodate a large fraction of dark matter. PQPs thus potentially constitute a conservative Standars-Model based, and observationally testable candidate.