Astronomy doi: 10.3390/astronomy2020007

Authors: Fabrizio Nesti Paolo Salucci Nicola Turini

The phenomenon of the Dark matter baffles the researchers: the underlying dark particle has escaped so far the detection and its astrophysical role appears complex and entangled with that of the standard luminous particles. We propose that, in order to act efficiently, alongside with abandoning the current &Lambda;CDM scenario, we need also to shift the Paradigm from which it emerged.

]]>Astronomy doi: 10.3390/astronomy2020006

Authors: G. G. Adamian N. V. Antonenko H. Lenske V. V. Sargsyan

Applying the ideas from microscopic objects to macroscopic stellar and galactic systems, the evolution of compact di-stars and di-galaxies is studied in the mass asymmetry coordinate. The formation of stable binary systems is analyzed. The role of symmetrization of an initially asymmetric binary system is revealed in the transformation of gravitational energy into internal energy of stars or galaxies accompanied by the release of a huge amount of energy. For the contact binary stars, the change of the orbital period is explained by evolution to symmetry in mass asymmetry coordinates. The matter transfer in binary black holes is studied. The conditions for the merger of black holes in a binary system are analyzed regarding the radiation of gravitational waves. Using the model based on the Regge-like laws, the Darwin instability effect in binary systems is discussed. New analytical formulas are derived for the period of orbital rotation and the relative distance between the components of a binary system. The impossibility of the appearance of a binary cosmic object from a single cosmic object is revealed.

]]>Astronomy doi: 10.3390/astronomy2010005

Authors: Daniel Frolovsky Sergei V. Ketov

A simple phenomenological fit for the power spectrum of scalar (curvature) perturbations during inflation is proposed to analytically describe slow roll of inflaton and formation of primordial black holes (PBH) in the early universe, in the framework of single-field models. The fit is given by a sum of the power spectrum of slow-roll inflation, needed for a viable description of the cosmic microwave background (CMB) radiation in agreement with Planck/BICEP/Keck measurements, and the log-normal (Gaussian) fit for the power spectrum enhancement (peak) needed for efficient PBH production, in the leading (model-independent) approximation. The T-type &alpha;-attractor models are used to get the simple CMB power spectrum depending upon the e-folds as the running variable. The location and height of the peak are chosen to yield the PBH masses in the asteroid-size window allowed for the whole (current) dark matter. We find the restrictions on the peak width.

]]>Astronomy doi: 10.3390/astronomy2010004

Authors: Wei-Xiang Feng

In this note, I derive the Chandrasekhar instability of a fluid sphere in (N + 1)-dimensional Schwarzschild&ndash;Tangherlini spacetime and take the homogeneous (uniform energy density) solution for illustration. Qualitatively, the effect of a positive (negative) cosmological constant tends to destabilize (stabilize) the sphere. In the absence of a cosmological constant, the privileged position of (3 + 1)-dimensional spacetime is manifest in its own right. As it is, the marginal dimensionality in which a monatomic ideal fluid sphere is stable but not too stable to trigger the onset of gravitational collapse. Furthermore, it is the unique dimensionality that can accommodate stable hydrostatic equilibrium with a positive cosmological constant. However, given the current cosmological constant observed, no stable configuration can be larger than 1021M&#8857;. On the other hand, in (2 + 1) dimensions, it is too stable either in the context of Newtonian Gravity (NG) or Einstein&rsquo;s General Relativity (GR). In GR, the role of negative cosmological constant is crucial not only to guarantee fluid equilibrium (decreasing monotonicity of pressure) but also to have the Ba&ntilde;ados&ndash;Teitelboim&ndash;Zanelli (BTZ) black hole solution. Owing to the negativeness of the cosmological constant, there is no unstable configuration for a homogeneous fluid disk with mass 0&lt;M&le;0.5 to collapse into a naked singularity, which supports the Cosmic Censorship Conjecture. However, the relativistic instability can be triggered for a homogeneous disk with mass 0.5&lt;M&#8818;0.518 under causal limit, which implies that BTZ holes of mass MBTZ&gt;0 could emerge from collapsing fluid disks under proper conditions. The implicit assumptions and implications are also discussed.

]]>Astronomy doi: 10.3390/astronomy2010003

Authors: Jan Chojnacki Jan Henryk Kwapisz

The swampland criteria in string theory assert the no eternal inflation scenario. This work studied the impact of generic gravitational quantum corrections on eternal inflation. In particular, we find that the Starobinsky model of inflation should receive higher-order corrections stemming from quantum gravity. In this work, we studied the effect of the R3/2 and R4 corrections on the eternal inflation conditions for the Starobinsky model.

]]>Astronomy doi: 10.3390/astronomy2010002

Authors: Astronomy Editorial Office Astronomy Editorial Office

High-quality academic publishing is built on rigorous peer review [...]

]]>Astronomy doi: 10.3390/astronomy2010001

Authors: Mitko K. Gaidarov Martin V. Ivanov Yordan I. Katsarov Anton N. Antonov

The incompressibility of both nuclear matter and finite nuclei is estimated by the monopole compression modes in nuclei in the framework of a nonrelativistic Hartree&ndash;Fock&ndash;Bogoliyubov method and the coherent density fluctuation model. The monopole states originate from vibrations of the nuclear density. The calculations in the model for the incompressibility in finite nuclei are based on the Brueckner energy&ndash;density functional for nuclear matter. Results for the energies of the breathing vibrational states and finite nuclei incompressibilities are obtained for various nuclei and their values are compared with recent experimental data. The evolution of the isoscalar giant monopole resonance (ISGMR) along Ni, Sn, and Pb isotopic chains is discussed. This approach can be applied to analyses of neutron stars properties, such as incompressibility, symmetry energy, slope parameter, and other astrophysical quantities, as well as for modelling dynamical behaviors within stellar environments.

]]>Astronomy doi: 10.3390/astronomy1030016

Authors: Pravin Kumar Dahal

We present the geometric optics expansion for circularly polarized gravitational waves on a curved spacetime background, to subleading order. We call spin optics to the subleading order geometric optics expansion, which involves modifying the standard eikonal function by including a specially chosen helicity-dependent correction. We show that the techniques developed for the propagation of electromagnetic waves can also be applied to gravitational waves in the limit of spin optics. However, one needs to account for the difference in the photon and graviton helicity, which we do here.

]]>Astronomy doi: 10.3390/astronomy1030015

Authors: Chamani M. Gunasekera Marios Chatzikos Gary J. Ferland

Atomic and molecular data are required to conduct the detailed calculations of microphysical processes performed by cloudy to predict the spectra of a theoretical model. cloudy now utilizes three atomic and molecular databases, one of which is CHIANTI version 7.1. CHIANTI version 10.0.1 is available, but its format has changed. cloudy is incompatible with the newer version. We have developed a script to convert the version 10.0.1 database into its version 7.1 format so that cloudy does not have to change every time there is a new CHIANTI version with an evolved format. This study outlines the steps taken by the script for this version format change. We have also found a modest number of significant changes to spectral line intensities/luminosities calculated by cloudy with the adoption of CHIANTI version 10.0.1. These changes are a result of improvements to collision strength data.

]]>Astronomy doi: 10.3390/astronomy1030014

Authors: Alexander P. Yefremov

A comparison of gravitational forces and a space probe&rsquo;s trajectory parameters is made for two different models of the sun&rsquo;s field, expressed in Schwarzschild and isotropic coordinates. It is shown that these two representations of a single Schwarzschild solution give, in the tangent space format, different deflections from classical finite trajectories and, hence, from one other; greatly amplified by a planet&rsquo;s (Venus&rsquo;) gravity assist, this effect renders it possible to experimentally specify the format of the gravity law that dominates the solar system.

]]>Astronomy doi: 10.3390/astronomy1030013

Authors: Li Lu Qinglong Yu Shuai Jia Yuan Chang

We doubt whether the &ldquo;Energetic Neutral Atom (ENA) ribbon&rdquo; signals, especially the peak ones, scanned remotely by IBEX-Hi at the lunar resonance orbit, are really from the heliopause, which involves assessing the scale of solar wind particle energy loss throughout the solar system. The ENA imaging simulation results at the Earth&rsquo;s orbit show that the scale of the planetary magnetosphere with a telemetry distance of AU magnitude is too small to contribute to the IBEX-Hi ribbon. However, the simulated effective ENA differential fluxes provide a reference for the physical scale evaluation of the huge magnetic structure in the heliopause. The ENA differential flux of the &ldquo;ENA emission cone&rdquo; generated by the charge exchange between the solar wind ion flow and local neutral gas near the Earth&rsquo;s orbit is also comparable to the measured peak of the IBEX-Hi ribbon, which may be the main ENA emission source of the ribbon&rsquo;s measured peak. The 2D ENA imaging measurements at the Lagrange points proposed here can be used to investigate the ENA ribbon origination by using the energy spectral lag vs the disparity of the ENA images.

]]>Astronomy doi: 10.3390/astronomy1030012

Authors: John Herbert Marr

After their initial formation, disk galaxies are observed to be rotationally stable over periods of &gt;6 Gyr, implying that any large velocity disturbances of stars and gas clouds are damped rapidly on the timescale of their rotation. However, it is also known that despite this damping, there must be a degree of random local motion to stabilize the orbits against degenerate collapse. A mechanism for such damping is proposed by a combination of inter-stellar gravitational interactions, and interactions with the Oort clouds and exo-Oort objects associated with each star. These mechanisms may produce rapid damping of large perturbations within a time period that is short on the scale of observational look-back time, but long on the scale of the disk rotational period for stars with small perturbations. This mechanism may also account for the locally observed mean perturbations in the Milky Way of 8&ndash;15 km/s for younger stars and 20&ndash;30 km/s for older stars.

]]>Astronomy doi: 10.3390/astronomy1030011

Authors: Pierre-Henri Chavanis

We determine the k-essence Lagrangian of a relativistic barotropic fluid. The equation of state of the fluid can be specified in different manners depending on whether the pressure is expressed in terms of the energy density (model I), the rest-mass density (model II), or the pseudo rest-mass density for a complex scalar field in the Thomas-Fermi approximation (model III). In the nonrelativistic limit, these three formulations coincide. In the relativistic regime, they lead to different models that we study exhaustively. We provide general results valid for an arbitrary equation of state and show how the different models are connected to each other. For illustration, we specifically consider polytropic and logotropic dark fluids that have been proposed as unified dark matter and dark energy models. We recover the Born-Infeld action of the Chaplygin gas in models I and III and obtain the explicit expression of the reduced action of the logotropic dark fluid in models II and III. We also derive the two-fluid representation of the Chaplygin and logotropic models. Our general formalism can be applied to many other situations such as Bose-Einstein condensates with a |&phi;|4 (or more general) self-interaction, dark matter superfluids, and mixed models.

]]>Astronomy doi: 10.3390/astronomy1020010

Authors: Lorenzo Sebastiani Sergio Zerbini

A short review of spherically symmetric static regular black holes and spherically symmetric non-singular cosmological space-time is presented. Several models, including new ones, of regular black holes are considered. First, a large class of regular black holes having an inner de Sitter core with the related issue of a Cauchy horizon is investigated. Then, Black Bounce space-times, where the Cauchy horizon and therefore the related instabilities are absent, are discussed as valid alternatives to regular black holes with inner de Sitter cores. Friedman&ndash;Lema&icirc;tre&ndash;Robertson&ndash;Walker space-times admitting regular bounce solutions are also discussed. In the general analysis concerning the presence or absence of singularities in the equations of motion, the role of a theorem credited to Osgood is stressed.

]]>Astronomy doi: 10.3390/astronomy1020009

Authors: Vyacheslav I. Dokuchaev

We elucidate the physical origin of the dark spot in the image of supermassive black hole SgrA* presented very recently by the EHT collaboration. It is argued that this dark spot, which is noticeably smaller than the classical black hole shadow, is the northern hemisphere of the event horizon globe. The classical black hole shadow is unseen in the image of SgrA*. The dark spot in the image of SgrA* is projected within the position of the classical black hole shadow on the celestial sphere. The outer boundary of this dark spot is an equator on the event horizon globe.

]]>Astronomy doi: 10.3390/astronomy1020008

Authors: Iván Gallardo Cava Valentín Bujarrabal Javier Alcolea Miguel Gómez-Garrido Arancha Castro-Carrizo Hans Van Winckel Miguel Santander-García

There is a class of binary post-AGB stars (binary system including a post-AGB star) that are surrounded by Keplerian disks and outflows resulting from gas escaping from the disk. To date, there are seven sources that have been studied in detail through interferometric millimeter-wave maps of CO lines (ALMA/NOEMA). For the cases of the Red Rectangle, IW Carinae, IRAS 08544-4431, and AC Herculis, it is found that around &ge;85% of the total nebular mass is located in the disk with Keplerian dynamics. The remainder of the nebular mass is located in an expanding component. This outflow is probably a disk wind consisting of material escaping from the rotating disk. These sources are the disk-dominated nebulae. On the contrary, our maps and modeling of 89 Herculis, IRAS 19125+0343, and R Scuti, which allowed us to study their morphology, kinematics, and mass distribution, suggest that, in these sources, the outflow clearly is the dominant component of the nebula (&sim;75% of the total nebular mass), resulting in a new subclass of nebulae around binary post-AGB stars: the outflow-dominated sources.Besides CO, the chemistry of this type of source has been practically unknown thus far. We also present a very deep single-dish radio molecular survey in the 1.3, 2, 3, 7, and 13 mm bands (&sim;600 h of telescope time). Our results and detections allow us to classify our sources as O- or /C-rich. We also conclude that the calculated abundances of the detected molecular species other than CO are particularly low, compared with AGB stars. This fact is very significant in those sources where the rotating disk is the dominant component of the nebula.

]]>Astronomy doi: 10.3390/astronomy1020007

Authors: Anne M. Hofmeister Robert E. Criss Everett M. Criss

Extremely slow recession of the Moon from the Earth has been recently proposed and attributed to conversion of Earth&rsquo;s axial spin to lunar orbital momentum. This hypothesis is inconsistent with long-standing recognition that the Moon&rsquo;s orbit involves three-body interactions. This and other short-comings, such as Earth&rsquo;s spin loss being internal, are summarized here. Considering point-masses is justified by theory and observational data on other moons. We deduce that torque in the Earth-Moon-Sun system increases eccentricity of the lunar orbit but decreases its inclination over time. Consequently, the average lunar orbital radius is decreasing. We also show that lunar drift is too small to be constrained through lunar laser ranging measurements, mainly because atmospheric refraction corrections are comparatively large and variations in lunar cycles are under-sampled. Our findings support co-accretion and explain how orbits evolve in many-body point-mass systems.

]]>Astronomy doi: 10.3390/astronomy1010006

Authors: Igor I. Smolyaninov

Localization phenomena in light, scattering from random fluctuations of matter fields and space&ndash;time metrics near a black hole horizon, were predicted to produce a pronounced peak in the angular distribution of second-harmonic light in the direction normal to the horizon. Therefore, the detection of second-harmonic generation may become a viable observational tool to study spacetime physics near event horizons of astronomical black holes. The light localization phenomena near the horizon may be facilitated by the existence of surface electromagnetic wave solutions. In this communication, we study such surface electromagnetic wave solutions near the horizon of a Schwarzschild metric, describing a black hole in vacuum. We demonstrate that such surface wave solutions must appear when quantum gravity effects are taken into account. Potential observational evidence of this effect is also discussed.

]]>Astronomy doi: 10.3390/astronomy1010005

Authors: Rico Zöllner Burkhard Kämpfer

We parameterize the core of compact spherical star configurations by a mass (mx) and a radius (rx) and study the resulting admissible areas in the total-mass&ndash;total-radius plane. The employed fiducial equation-of-state models of the corona at radii r&gt;rx and pressures p&le;px with p(r=rx)=px are that of constant sound velocity and a proxy of DY&Delta; DD-ME2 provided by Buchdahl&rsquo;s exactly solvable ansatz. The core (r&lt;rx) may contain any type of material, e.g., Standard-Model matter with unspecified equation of state or/and an unspecified Dark-Matter admixture. Employing a toy model for the cool equation of state with first-order phase transition, we also discuss the mass-radius relation of compact stars with an admixture of Dark Matter in a Mirror-World scenario.

]]>Astronomy doi: 10.3390/astronomy1010004

Authors: Spiros Cotsakis Ifigeneia Klaoudatou Georgios Kolionis John Miritzis Dimitrios Trachilis

We discuss qualitative features of the conformal relation between certain classes of gravity theories and general relativity, common to different themes such as f(R), Brans-Dicke-type, and string theories. We focus primarily on the frame relations of the fields involved, slice energy, traceless and Palatini extensions, and selected cosmological applications.

]]>Astronomy doi: 10.3390/astronomy1010003

Authors: Ignatios Antoniadis

Astronomy has a long history over thousands of years, since the creation of ancient civilization [...]

]]>Astronomy doi: 10.3390/astronomy1010002

Authors: Pio J. Arias Pedro Bargueño Ernesto Contreras Ernesto Fuenmayor

In this work we study the 2+1-Einstein&ndash;Klein&ndash;Gordon system in the framework of Gravitational Decoupling. We associate the generic matter decoupling sector with a real scalar field so we can obtain a constraint which allows us to close the system of differential equations. The constraint corresponds to a differential equation involving the decoupling functions and the metric of the seed sector and will be independent of the scalar field itself. We show that when the equation admits analytical solutions, the scalar field and the self-interacting potential can be obtained straightforwardly. We found that, in the cases under consideration, it is possible to express the potential as an explicit function of the scalar field only for certain particular cases corresponding to limiting values of the parameters involved.

]]>Astronomy doi: 10.3390/astronomy1010001

Authors: Yu Lin Ioana Craciun

Have you ever looked at the stars [...]

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