Primordial Neutron Stars

TL;DR

This paper proposes a new cosmological scenario where primordial neutron stars could form in the early universe due to large baryon asymmetries and density perturbations, differing from stellar-collapse neutron stars.

Contribution

It introduces a mechanism for primordial neutron star formation from early universe density perturbations caused by large baryon asymmetries.

Findings

Primordial neutron stars could be as light as 0.1 solar masses.

Density perturbations can lead to neutron star formation before BBN.

Large entropy injection is needed to match observed baryon asymmetry.

Abstract

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 will be arrested only by nuclear pressure, possibly resulting in neutron star formation. Afterwards, there must be a large entropy injection to restore the observed baryon asymmetry, $Y_B \sim 10^{-10}$, and preserve the successful predictions of standard BBN. Unlike neutron stars that form from 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.