Challenges and Future Directions in Neutron Star Research

Dear Colleagues,

In recent years, scientific knowledge originating from the theoretical study of neutron stars and other related compact objects has been extensively developed. Concurrently, elaborated observation and detection methods of compact stars, emphasizing the signals from gravitational waves, have provided new aspects and have shed new light on the outstanding problems of nuclear physics and nuclear astrophysics. Nonetheless, there are still questions and relevant issues that require an answer, on which research is now focused.

In summary, these issues concern the following: (a) the hadron–quark phase transition in dense nuclear matter, which has implications on the structure of neutron stars, determining the possible existence of other phases in the phase diagram of dense matter at low temperatures and how we can use neutron star observations to learn about these phases; (b) hybrid stars as confirmations of phase transitions in dense nuclear matter, including twin stars and back-bending phenomena; (c) the possible existence of a mass gap between neutron stars and black holes; (d) the determination of what limits the spin frequencies of millisecond pulsars and why (r-modes, f-modes, etc.); (e) the investigation of the mystery related to the appearance of glitches and starquakes; (f) the roles of superfluidity and the crust–core interface; (g) the study of neutron star–dark matter admixture and its application to the existence and possible determination of dark matter in the Universe; (h) the role of pions and kaons’ condensation in dense nuclear matter; (i) the origin of the strong magnetic field in neutron stars, elucidating the physics of magnetars; (j) neutron star mergers as major sources of gravitational wave radiation and the roles of the stars’ structures and deformabilities; (k) the investigation of neutron star binary mergers, including the creation (nucleosynthesis) and existence of heavy elements in the Universe; (l) the investigation of collisions among neutron stars as sources of short gamma-ray bursts, which are among the most powerful and violent explosions in the known Universe; and (m) what we can learn from the interiors of neutron stars by investigating exotic stars (quark stars, strange stars, pion stars, preon stars, Thorne–Zytkow objects, and gravastars), focusing on their origin, structure, observation, and verification.

The aim of this Special Issue is to collect works that will address these problems by proposing innovative potential solutions, possibly going beyond the scope of prevailing theories. We hope that the current Special Issue will be the springboard for further studies that will provide new perspectives and shed new light on the underlying physics of these objects.

Dr. Polychronis Koliogiannis
Prof. Dr. Charalampos Moustakidis
Guest Editors

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• compact stars
• neutron stars
• quark stars
• hybrid stars
• neutron star mergers
• gravitational waves