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Universe
Special Issues
Exotic Scenarios for Compact Astrophysical Objects
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Exotic Scenarios for Compact Astrophysical Objects

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Compact Objects".

Deadline for manuscript submissions: closed (31 December 2024) | Viewed by 3471

Special Issue Editors

Dr. Vlasios Petousis

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Guest Editor
Institute of Experimental and Applied Physics, Czech Technical University, 11000 Prague, Czechia
Interests: the nature of dark matter; physics of neutron stars
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Charalampos Moustakidis

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Guest Editor
Department of Nuclear and Elementaty Particle Physics, School of Physics Faculty of Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Interests: theoretical nuclear physics; nuclear astrophysics
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Martin Veselsky

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Guest Editor
Institute of Experimental and Applied Physics, Czech Technical University, 11000 Prague, Czechia
Interests: theoretical and experimental nuclear physics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

One of the most important problems of physics and astrophysics is the internal structure of compact objects, including neutron stars, boson stars, white dwarfs and compact dark objects as well. In particular, the internal structure of all the above became subject of a thorough study by researchers from different branches of physics including nuclear physics, astrophysics, particle physics, general relativity and thermodynamics. Problems such as a) the composition and properties of dense nuclear matter; b) the existence of exotic phases involving strangeness, such as hyperons, kaon or pion condensates, and deconfined quark matter; c) the possibility of dark matter being absorbed inside them; or even d) the existence of compact dark objects, all of them today are included in the researcher’s agenda. On the other hand, the accelerated improvement of the research, concerning the study of gravitational waves, has opened important perspectives to study the above theoretical predictions. We expect that in the near future, with the help of the detection of gravitational waves from events associated with compact objects (binary neutron star merger, various instability modes, etc.) and other observations including mainly the X-ray radiation and gamma-rays burst, the theoretical predictions and speculations will be able to be tested, providing us answers to some of the most fundamental problems of physics. The scope of the present Special Issue is to expose the above important problems and to highlight how they can be addressed both from the theoretical and observational point of view.

Dr. Vlasios Petousis
Dr. Charalampos Moustakidis
Dr. Martin Veselsky
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • compacts objects
  • neutron stars
  • hybrid stars
  • strange quark stars
  • boson stars
  • white dwarfs
  • dark objects

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Published Papers (4 papers)

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Research

23 pages, 655 KiB  
Article
Screening Mechanisms on White Dwarfs: Symmetron and Dilaton
by Joan Bachs-Esteban, Ilídio Lopes and Javier Rubio
Universe 2025, 11(5), 158; https://doi.org/10.3390/universe11050158 - 12 May 2025
Viewed by 192
Abstract
This work provides the first comparison of the symmetron and dilaton fields in white dwarfs. We show how these screening mechanisms behave inside such stars and their impact on stellar properties. Employing a custom-developed shooting method, we solve the scalar–tensor equilibrium equations in [...] Read more.
This work provides the first comparison of the symmetron and dilaton fields in white dwarfs. We show how these screening mechanisms behave inside such stars and their impact on stellar properties. Employing a custom-developed shooting method, we solve the scalar–tensor equilibrium equations in the Newtonian approximation. We consider a Chandrasekhar equation of state and examine a range of potential mass scales and coupling strengths for both fields. Both fields enhance the pressure drop in low-density white dwarfs, leading to smaller stellar masses, radii, and luminosities. Unlike chameleon models, their effects are suppressed in more massive stars, with symmetron fields fully decoupling and dilaton fields weakening but not vanishing. Consequently, no mass–radius curve for screened white dwarfs exceeds the Newtonian prediction in any of these three mechanisms. The mass–radius deviations are generally more pronounced at lower densities, depending on model parameters. Due to their common runaway potential, we confirm that dilaton and chameleon fields display similar field and gradient profiles. In contrast, due to their environment-dependent coupling, the dilaton and symmetron mechanisms exhibit stronger density-dependent screening effects. These findings highlight both phenomenological differences and theoretical similarities among these mechanisms, motivating asteroseismology studies to constrain the symmetron and dilaton parameter spaces. Full article
(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)
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23 pages, 1793 KiB  
Article
Restrictions on Regularized Fisher and Dilatonic Spacetimes Implied by High-Frequency Quasiperiodic Oscillations Observed in Microquasars and Active Galactic Nuclei
by Jaroslav Vrba and Zdeněk Stuchlík
Universe 2025, 11(3), 99; https://doi.org/10.3390/universe11030099 - 17 Mar 2025
Viewed by 282
Abstract
The Bronnikov generalization of the Fisher naked singularity and Dilatonic black hole spacetimes attracts high interest, as it combines two fundamental transitions of the solutions of Einstein equations. These are the black hole/wormhole “black bounce” transition of geometry, and the phantom/canonical transition of [...] Read more.
The Bronnikov generalization of the Fisher naked singularity and Dilatonic black hole spacetimes attracts high interest, as it combines two fundamental transitions of the solutions of Einstein equations. These are the black hole/wormhole “black bounce” transition of geometry, and the phantom/canonical transition of the scalar field, called trapped ghost scalar, combined with an electromagnetic field described by a non-linear electrodynamics. In the present paper, we put restrictions on the parameters of the Fisher (wormhole) and Dilatonic (black hole or wormhole) regularized spacetimes by using frequencies of the epicyclic orbital motion in the geodesic model for explanation of the high-frequency oscillations observed in microquasars or active galactic nuclei, where stellar mass or supermassive black holes are usually assumed. Full article
(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)
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13 pages, 462 KiB  
Communication
Stellar Modeling via the Tolman IV Solution: The Cases of the Massive Pulsar J0740+6620 and the HESS J1731-347 Compact Object
by Grigoris Panotopoulos
Universe 2024, 10(9), 342; https://doi.org/10.3390/universe10090342 - 27 Aug 2024
Cited by 1 | Viewed by 726
Abstract
We model compact objects of known stellar mass and radius made of isotropic matter within Einstein’s gravity. The interior solution describing hydrostatic equilibrium we are using throughout the manuscript corresponds to the Tolman IV exact analytic solution obtained a long time ago. The [...] Read more.
We model compact objects of known stellar mass and radius made of isotropic matter within Einstein’s gravity. The interior solution describing hydrostatic equilibrium we are using throughout the manuscript corresponds to the Tolman IV exact analytic solution obtained a long time ago. The three free parameters of the solutions are determined by imposing the matching conditions for objects of known stellar mass and radius. Finally, using well established criteria, it is shown that, contrary to the Kohler Chao solution, the Tolman IV solution is compatible with all requirements for well-behaved and realistic solutions, except for the relativistic adiabatic index that diverges at the surface of stars. The divergence of the index Γ may be resolved, including a thin crust assuming a polytropic equation of state, which is precisely the case seen in studies of neutron stars. To the best of our knowledge, we model here for the first time the recently discovered massive pulsar PSR J0740+6620 and the strangely light HESS compact object via the Tolman IV solution. The present work may be of interest to model builders as well as a useful reference for future research. Full article
(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)
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12 pages, 563 KiB  
Article
Role of Quark Matter and Color Superconductivity in the Structure and Tidal Deformability of Strange Dwarfs
by Loïc Perot and Nicolas Chamel
Universe 2023, 9(9), 382; https://doi.org/10.3390/universe9090382 - 25 Aug 2023
Cited by 2 | Viewed by 1296
Abstract
In 1995, Glendenning, Kettner and Weber postulated the existence of a new class of compact stars resembling white dwarfs but containing a small strange quark-matter core surrounded by hadronic layers attaining much higher densities than those found in white dwarfs. In our previous [...] Read more.
In 1995, Glendenning, Kettner and Weber postulated the existence of a new class of compact stars resembling white dwarfs but containing a small strange quark-matter core surrounded by hadronic layers attaining much higher densities than those found in white dwarfs. In our previous study, we have shown that it could be possible to unmask these so-called strange dwarfs through gravitational-wave observations with future space-based detectors such as the Laser Interferometer Space Antenna. We calculated more realistic equations of state for the hadronic envelope, but the quark core was treated using the simplest MIT bag model. In this paper, we investigate more closely the role of the possibly solid core in the structure and the tidal deformability of strange dwarfs in the full general relativistic framework by considering different models of strange quark matter in the crystalline color -superconducting phase. We find that the effect of the extreme rigidity of the elastic core on the tidal deformability is almost completely canceled by the surrounding hadronic layers. However, in all cases, the tidal deformability of strange dwarfs remains sufficiently lower than that of white dwarfs, to be potentially observable with gravitational waves despite the uncertainties in the strange quark-matter equation of state. Full article
(This article belongs to the Special Issue Exotic Scenarios for Compact Astrophysical Objects)
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