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We propose a novel cosmological scenario in which baryonic neutron stars could plausibly form in the early universe. If baryogenesis initially produces an excessively large baryon asymmetry, $Y_B \gg 10^{-10}$, the baryonic mass inside the horizon can exceed the minimum neutron star mass before big bang nucleosynthesis (BBN). While this large asymmetry is present, non-relativistic baryons can dominate the universe, and enhanced density perturbations on small scales can gravitationally collapse Hubble patches shortly after horizon re-entry. For some initial perturbations, just below the threshold for black hole formation, this collapse is arrested only by nuclear pressure, possibly resulting in neutron star formation. Afterwards, a large entropy injection must restore the observed baryon asymmetry, $Y_B \sim 10^{-10}$, and preserve the successful predictions of standard BBN. Unlike neutron stars formed through stellar collapse, primordial neutron stars can, in principle, be as light as $\sim 0.1,M_\odot$, limited only by the nuclear equation of state.

Bio: Gordan Krnjaic’s research focuses on theoretical particle physics and cosmology beyond the Standard Model. He has developed new experiments and search strategies to discover dark matter and hidden forces in non-traditional settings, including electron beam-dump fixed-target experiments and neutrino factories. He also works on models of new physics at the electroweak scale, motivated both by naturalness and by upcoming searches at the Large Hadron Collider.