Existence of exotic matter in neutron star inner cores

Ascertain whether exotic degrees of freedom—hyperons (Λ, Σ, Ξ) or deconfined quarks (u, d, s)—are present in the inner core of neutron stars at high densities, beyond the conventional npeμ composition assumed in the outer core.

Background

The authors construct neutron star models using a conventional npeμ composition for the core and classical crust EoSs, explicitly excluding exotic phases in the inner core.

They state that the presence of hyperonic or quark matter in the deepest region of neutron stars remains uncertain and identify this as an open question in nuclear astrophysics, motivating their choice to omit such phases in the present work.

References

For the core region, we assume a conventional $npe\mu$ composition of nuclear matter, disregarding the presence of exotic phases (the inner core), considering the fact that the presence of such exotic matter remains uncertain, offering an open question in nuclear astrophysics.

— Fully general relativistic description of rapidly-rotating axially-symmetric neutron stars for constraining nuclear matter equations of state  (2505.20990 - Kwon et al., 27 May 2025) in Section FORMULATION, Modeling of neutron star matter

What this matter is made of --- nucleons, hyperons, meson condensates or deconfined quarks --- remains a central open problem of the strong interaction.

— Generative artificial intelligence for reconstructing neutron-star matter  (2608.17457 - Panteleeva et al., 18 Aug 2026) in Section 1, Introduction

Although such exotic degrees of freedom have not been observationally established, their possible presence remains an open question, motivating theoretical studies constrained by multimessenger observations.

— Trace-anomaly decomposition and universal dark matter scaling in compact stars  (2610.01991 - Issifu et al., 1 Oct 2026) in Section 1, Introduction