Astronomy doi: 10.3390/astronomy2040019

Authors: Ralf Hofmann Janning Meinert

We point out that a modified temperature&ndash;redshift relation (T-z relation) of the cosmic microwave background (CMB) cannot be deduced by any observational method that appeals to an a priori thermalisation to the CMB temperature T of the excited states in a probe environment of independently determined redshift z. For example, this applies to quasar-light absorption by a damped Lyman-alpha system due to atomic as well as ionic fine-splitting transitions or molecular rotational bands. Similarly, the thermal Sunyaev-Zel&rsquo;dovich (thSZ) effect cannot be used to extract the CMB&rsquo;s T-z relation. This is because the relative line strengths between ground and excited states in the former and the CMB spectral distortion in the latter case both depend, apart from environment-specific normalisations, solely on the dimensionless spectral variable x=h&nu;kBT. Since the literature on extractions of the CMB&rsquo;s T-z relation always assumes (i) &nu;(z)=(1+z)&nu;(z=0), where &nu;(z=0) is the observed frequency in the heliocentric rest frame, the finding (ii) T(z)=(1+z)T(z=0) just confirms the expected blackbody nature of the interacting CMB at z&gt;0. In contrast to the emission of isolated, directed radiation, whose frequency&ndash;redshift relation (&nu;-z relation) is subject to (i), a non-conventional &nu;-z relation &nu;(z)=f(z)&nu;(z=0) of pure, isotropic blackbody radiation, subject to adiabatically slow cosmic expansion, necessarily has to follow that of the T-z relation T(z)=f(z)T(z=0) and vice versa. In general, the function f(z) is determined by the energy conservation of the CMB fluid in a Friedmann&ndash;Lemaitre&ndash;Robertson&ndash;Walker universe. If the pure CMB is subject to an SU(2) rather than a U(1) gauge principle, then f(z)=1/41/3(1+z) for z&#8811;1, and f(z) is non-linear for z&sim;1.

]]>Astronomy doi: 10.3390/astronomy2040018

Authors: Mechid Paiman Horia Cornean Christoph Köhn

Black holes are one of the most extreme phenomena in the Universe, bridging the gap between the realms of general relativity and quantum physics. Any matter that crosses the event horizon moves towards the core of the black hole, creating a singularity with infinite mass density&mdash;a phenomenon that cannot be comprehended within present theories of relativity and quantum physics. In this study, we undertake an investigation of non-rotating, non-charged Schwarzschild black holes in an extended spacetime framework with two time dimensions. To accomplish this, we extend Einstein&rsquo;s field equations by one more temporal dimension. We solve the corresponding equations for a spherical central mass, which leads to an Abel-type equation for the 5D Schwarzschild metric. By exploring distinct solution classes, we present an approximate solution for the 5D metric. Our proposed solution maintains consistency with Schwarzschild&rsquo;s 4D solution. Finally, we address the central black hole singularity and demonstrate a potential breakthrough, as our solution effectively avoids the singularity quandary, providing valuable insight into the fundamental properties of black holes in this augmented framework.

]]>Astronomy doi: 10.3390/astronomy2040017

Authors: Dimitris M. Christodoulou Demosthenes Kazanas

Natural systems of units {Ui} need to be overhauled to include the dimensionless coupling constants {&alpha;Ui} of the natural forces. Otherwise, they cannot quantify all the forces of nature in a unified manner. Thus, each force must furnish a system of units with at least one dimensional and one dimensionless constant. We revisit three natural systems of units (atomic, cosmological, and Planck). The Planck system is easier to rectify, and we do so in this work. The atomic system discounts {G,&alpha;G}, thus it cannot account for gravitation. The cosmological system discounts {h,&alpha;h}, thus it cannot account for quantum physics. Here, the symbols have their usual meanings; in particular, &alpha;G is the gravitational coupling constant and &alpha;h is Dirac&rsquo;s fine-structure constant. The speed of light c and the impedance of free space Z0 are resistive properties imposed by the vacuum itself; thus, they must be present in all systems of units. The upgraded Planck system with fundamental units UPS:={c,Z0,G,&alpha;G,h,&alpha;h,&hellip;} describes all physical scales in the universe&mdash;it is nature&rsquo;s system of units. As such, it reveals a number of properties, most of which have been encountered previously in seemingly disjoint parts of physics and some of which have been designated as mere coincidences. Based on the UPS results, which relate (sub)atomic scales to the Planck scale and the fine-structure constant to the Higgs field, we can state with confidence that no observed or measured physical properties are coincidental in this universe. Furthermore, we derive from first principles Koide&rsquo;s K=2/3 enigmatic constant and additional analogous quark and vector boson constants. These are formal mathematical proofs that justify a posteriori the use of geometric means in deriving the quark/boson mass ladder. This ladder allows us to also calculate the Higgs couplings to the vector bosons and the Weinberg angle in terms of K only, and many of the &ldquo;free&rdquo; parameters of the Standard Model of particle physics were previously expected to be determined only from experiments.

]]>Astronomy doi: 10.3390/astronomy2040016

Authors: Zaza N. Osmanov Swadesh M. Mahajan

Based on the recently demonstrated resonant wave&ndash;wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein&ndash;Gordon (KG) equation) moving in unison with an intense electromagnetic (EM) wave; the KG wave/particle continuously draws energy from EM. In a brief recapitulation of the general theory, the high-energy (resonantly enhanced) electron states are investigated by solving the KG equation, minimally coupled to the EM field. The restricted class of solutions that propagate in phase with EM radiation (functions only of &zeta;=&omega;t&minus;kz) are explored to serve as a possible basis for the proposed electron energization in the radio pulsars. We show that the wave&ndash;wave resonant energization mechanism could be operative in a broad class of radio pulsars with periods ranging from milliseconds to normal values (&sim;1 s); this could drive the magnetospheric electrons to acquire energies from 100 s of TeVs (millisecond pulsars) to 10 ZeVs (normal pulsars).

]]>Astronomy doi: 10.3390/astronomy2030015

Authors: Ivano Basile

We investigate the infinite-distance properties of families of unstable flux vacua in string theory with broken supersymmetry. To this end, we employ a generalized notion of distance in the moduli space and we build a holographic description for the non-perturbative regime of the tunneling cascade in terms of a renormalization group flow. In one limit, we recover an exponentially-light tower of Kaluza-Klein states, while in the opposite limit, we find a tower of higher-spin excitations of D1-branes, realizing the emergent string proposal. In particular, the holographic description includes a free sector, whose emergent superconformal symmetry resonates with supersymmetric stability, the CFT distance conjecture and S-duality. We compute the anomalous dimensions of scalar vertex operators and single-trace higher-spin currents, finding an exponential suppression with the distance which is not generic from the renormalization group perspective, but appears specific to our settings.

]]>Astronomy doi: 10.3390/astronomy2030014

Authors: Marco Roncadelli Giorgio Galanti

First of all, we show that any spherically symmetric galactic model with integrated mass profile M(r)&rarr;0 as r&rarr;0 is physically correct close to the centre only provided that the circular velocity vc(r)&rarr;0 and the gravitational field g(r)&rarr;0 as r&rarr;0. Next, we apply this statement to a broad class of five-parameter spherical galactic models, including most of those used in astrophysics and cosmology. Specifically, we show that the Jaffe and Hernquist models can be trusted only for r&#8819;0.2Re (Re being the effective radius), while the Navarro&ndash;Frank&ndash;White (NFW) model cannot describe galaxies in the central region of regular clusters. We also briefly discuss the relevance of our result for the NFW profile of pure dark matter halos. However, we are unable to tell at which central distance the NFW model breaks down in either case, and this is a challenge for future investigations.

]]>Astronomy doi: 10.3390/astronomy2030013

Authors: Cesare Barbieri Giampiero Naletto Luca Zampieri

Twenty years ago, we started to apply quantum optics to the astronomical research carried out inside the Department of Physics and Astronomy and the INAF Astronomical Observatory in Padova, Italy. The initial activities were stimulated by the project of the European Southern Observatory (ESO) to build a 100 m diameter telescope, the Overwhelmingly Large (OWL) telescope. The enormous photon flux expected from such an aperture suggested that quantum optics concepts be utilized in order to obtain novel astrophysical results. Following initial successful attempts to utilize the orbital angular momentum of the light beam to enhance the visibility of faint companions to bright stars, the Padova team concentrated its efforts on very high time resolution, in order to measure and store the arrival time of celestial photons to better than one nanosecond. To obtain observational results, we built two photon counting photometers (AquEye and IquEye) to be used with our telescopes of the Asiago Observatory and with 4 m class telescopes such as the ESO New Technology Telescope (NTT) in Chile. This paper firstly describes these two instruments and then expounds the results obtained on pulsar light curves, lunar occultations and the first photon counting intensity interferometry measurements of the bright star Vega. Indeed, the correlation of photon arrival times on two or more apertures can lead to extremely high angular resolutions, as shown around 1970 by Hanbury Brown and Twiss. Prospects for quantum intensity interferometry with arrays of Cherenkov light telescopes will also be described.

]]>Astronomy doi: 10.3390/astronomy2030012

Authors: Rositsa Miteva Susan W. Samwel Stela Tkatchova

The study presents a concise overview on the main effects on satellites due to space weather drivers compared to the well-known interplanetary, magnetospheric and ground-based consequences. The solar-activity-driven influences include specific physics-based effects on the spacecraft surface and on-board electronics due to electromagnetic emission and energetic particles as well as complex effects due to geomagnetic storms which may endanger the mission performance and spacecraft longevity. We select as test examples the Starlink satellites in the period 2019&ndash;2022 and present the temporal correspondence between their launches and the space weather phenomena. Based on comparative analysis, we discuss whether the occurrence vs. the intensity of solar and interplanetary drivers of space weather can be considered as a cause for orbital stability problems and satellite loss. The results suggest that a sequence of geomagnetic disturbances together with multiple weak space weather events could lead to severe levels of atmospheric drag ending in a service or satellite loss.

]]>Astronomy doi: 10.3390/astronomy2030011

Authors: Li Lu Qinglong Yu Shuai Jia Zhong Xie Jian Lan Yuan Chang

The distribution of energetic ion flux in the ring current region, such as a meteorological cumulonimbus cloud, stores up the particle energy for a geomagnetic substorm. It is helpful to study the geomagnetic substorm mechanism by using a lunar base ENA imaging simulation of the dynamic evolution of the ring current, and establishing the corresponding relationship between key node events of the substorm. Based on the previous observation experience and our simulation results of the dynamic evolution of the ring current, we propose a macroscopic model of substorms related to the dynamic evolution of ring currents and present the possibility of confirming the causal sequence of some of those critical node events of substorms with the lunar base ENA imaging measurement. IBEX, operating in the ecliptic plane, may even give examples of the telemetry of ring current ion fluxes through ENA measurements during substorms/quiets.

]]>Astronomy doi: 10.3390/astronomy2030010

Authors: Vyacheslav Ivanovich Dokuchaev

We reconstructed dark spots in the images of supermassive black holes SgrA* and M87* provided by the Event Horizon Telescope (EHT) collaboration by using the geometrically thin accretion disk model. In this model, the black hole is highlighted by the hot accretion matter up to the very vicinity of the black hole event horizon. The existence of hot accretion matter in the vicinity of black hole event horizons is predicted by the Blandford&ndash;Znajek mechanism, which is confirmed by recent general relativistic MHD simulations in supercomputers. A dark spot in the black hole image in the described model is a gravitationally lensed image of an event horizon globe. The lensed images of event horizons are always projected at the celestial sphere inside the awaited positions of the classical black hole shadows, which are invisible in both cases of M87* and SgrA*. We used the sizes of dark spots in the images of SgrA* and M87* for inferring their spins, 0.65&lt;a&lt;0.9 and a&gt;0.75, accordingly.

]]>Astronomy doi: 10.3390/astronomy2020009

Authors: Christian Käding

Generalized symmetrons are models that have qualitatively similar features to the archetypal symmetron, but have barely been studied. In this article, we investigate for what parameter values the fifth forces induced by disformally coupling generalized symmetrons can provide an explanation for the difference between baryonic and lens masses of galaxies. While it is known that the standard symmetron struggles to provide an alternative source for the lensing otherwise attributed to particle dark matter, we show that some generalized symmetron models are more suitable for complying with existing constraints on disformal couplings. This motivates future studies of these only little-explored models.

]]>Astronomy doi: 10.3390/astronomy2020008

Authors: Kostas Kleidis Apostolos Kuiroukidis Demetrios B. Papadopoulos

The general-relativistic (GR) magnetohydrodynamic (MHD) equations for a conductive plasma fluid are derived and discussed in the curved spacetime described by Thorne&rsquo;s metric tensor, i.e., a family of cosmological models with inherent anisotropy due to the existence of an ambient, large-scale magnetic field. In this framework, it is examined whether the magnetized plasma fluid that drives the evolution of such a model can be subsequently excited by a transient, plane-polarized gravitational wave (GW) or not. To do so, we consider the associated set of the perturbed equations of motion and integrate them numerically in order to study the evolution of instabilities triggered by the GW propagation. In particular, we examine to what extend perturbations of the electric and/or the magnetic field can be amplified due to a potential energy transfer from the GW to the electromagnetic (EM) degrees of freedom. The evolution of the perturbed quantities depends on four free parameters, namely, the conductivity of the fluid, &sigma;; the speed of sound square, 13&lt;Csc2&equiv;&gamma;&lt;1, which in this model may serve also as a measure of the inherent anisotropy; the GW frequency, &omega;g; and the associated angle of propagation with respect to the direction of the magnetic field, &theta;. We find that GW propagation in the anisotropic magnetized medium under consideration does excite several MHD modes; in other words, there is energy transfer from the gravitational to the EM degrees of freedom that can result in the acceleration of charged particles at the spot and in the subsequent damping of the GW.

]]>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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