Galaxy rotation curves and the tight scaling relations that accompany them are not easily reproduced by collisionless cold dark matter alone, while modified-gravity descriptions that succeed at galactic scales encounter difficulties elsewhere. We examine a minimal alternative that leaves general relativity untouched and instead gives the dark sector a small internal pressure. The dark component is treated as a barotropic fluid in hydrostatic equilibrium with an isothermal equation of state, so that pressure resists gravitational collapse and halos settle into finite-density cores rather than cusps. The resulting rotation law is analytic and carries two free parameters per galaxy, the same number as a standard halo. Fitting 165 galaxies from the SPARC database under a fixed-baryon protocol applied identically to every model, we obtain a median coefficient of determination R^2 = 0.961 , against 0.924 for the standard halo profile at the same two-parameter count, the comparison that isolates the dark sector. The model is favoured by the Akaike criterion, which penalises parameters, in 99 of the 165 galaxies. Calibrating two population scaling relations on photometry alone and predicting each rotation curve with no per-galaxy fitting reproduces the radial acceleration relation with a scatter of 0.192 dex, matching the zero-parameter modified-gravity value of 0.202 dex on the same points. The model also generates an acceleration scale of order the observed one, but a structured rather than universal one: it correlates with baryonic surface density ( p ∼ 10^-14 ), a correlation forbidden for a universal scale and thus a direct point of separation from modified gravity. It recovers the baryonic Tully-Fisher relation with a slope near four. Pressure support thus accounts for the principal galactic regularities without modifying gravity, as a modest extension of the cold dark matter picture.
In this work, we analyze a spatially homogenous but anisotropic locally rotationally symmetric (LRS) Bianchi type‑I spacetime in order to investigate a cosmological scenario described in the framework of f(R,T) gravity. The modified Einstein field equations, which show a smooth development of the cosmos from an early decelerated expansion to the currently observable accelerated phase, can be solved exactly when the deceleration value is chosen appropriately. The discovered solutions also demonstrate that, in later eras, the cosmos progressively approaches isotropy. We analyze the state-finder diagnostics and discover that the cosmic development moves from an Einstein static phase to an era dominated by ΛCDM. By examining basic cosmological characteristics including the energy density, pressure, equation of state parameter, and different energy conditions, the model’s physical plausibility is evaluated. Additionally, the evolution of temperature and entropy density is used to examine the model’s thermodynamic behavior. In accordance with the second law of thermodynamics, it is found that although the entropy density drops with cosmic time, the overall entropy stays positive. It is discovered that the temperature diverges in the first epoch, then falls and eventually approaches a stable value. These results validate the suggested cosmological framework’s consistency and physical robustness.
We analyze the dependence between magnetic field evolution (HMI magnetograms) and the AIA 94Å, 131Å, 171Å, 193Å, 211Å, 304Å, 335Å, and Fe xviii emissions associated to flares through copula theory, which captures both linear and nonlinear correlations of variables. We utilize the Archimedean copulas to characterize the correlations (dependence structures) between the magnetic field and EUV observations for 66 active regions. The copula parameters are determined using the canonical maximum likelihood estimation method to capture the dependency structure effectively, that verified by goodness of fit which shows the validity of modeling based on Frank copula. We demonstrate that the magnetic field configurations are nonlinearly correlated with the onset of flare emissions, which typically occur 2–5.5h later. Therefore, copula approach provides an effective tool to capture the relationships between solar magnetic fields and emissions, offering new insights into solar flare initiation.
A two-dimensional nonlinear integral equation with vacuum boundary condition, arising in radiative transfer theory is investigated. In addition to the trivial solution, the existence of a nontrivial solution is established, together with its uniqueness within an appropriate class of functions. A uniform estimate between the n-th iteration and the exact solution is obtained, allowing the approximation error to be evaluated with any prescribed level of accuracy. Several examples of nonlinear functions satisfying the assumptions of the main theorem are presented. Numerical computations are carried out for a square-root type nonlinearity. The results reveal a qualitative distinction between linear and nonlinear models: while the linear equation admits only the trivial solution (or solution with unbounded growth in limiting case), the nonlinear problem exhibits bounded, physically meaningful solutions with fundamentally different behavior.
We apply the holographic principle based on the generalized infrared cut-off introduced by Nojiri-Odintsov [1, 2] for describing cosmological models of the universe induced by a viscous fluid in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) metric. We study the evolution of the universe in terms of the generalized equation of state (EoS) parameters. We consider the early universe, including the bounce cosmology, the inflation, and the model of the unified early and late universe. Finally, we derive the energy conservation equation from the holographic point of view in terms of generalized infrared cut-offs. As a result, we obtain an equivalent description of the bounce, inflation, unified early and late-time universe, in terms of a viscous fluid and holographic fluid.
M dwarf stars, the most common stellar type in our solar neighborhood, play a crucial role in galactic composition and stellar evolution studies. Among the mechanisms influencing pulsations in M dwarfs, the role of magnetic fields is pivotal, particularly through the magnetic inhibition parameter. This study develops a comprehensive theoretical model for the magnetic inhibition parameter in M dwarfs, incorporating its dependence on both radial distance and latitude. Extending previous model which advocates for the sole dependence of radial distance on magnetic inhibition parameter, we introduce a latitude dependent factor characterized by a monotonic increasing profile to account for the observed growing trend of magnetic inhibition parameter from the equator to the pole. Profiles are presented demonstrating the variation in magnetic inhibition parameter at interior points as well as on the surface. This framework offers a path way to refine our understanding of M dwarf physics and their role in stellar and planetary systems.
In this paper, we investigate a non-minimally coupled cosmological model in f(Q,Lm) gravity within the framework of the FLRW metric in the presence of a perfect fluid. We consider the functional form f(Q,L_m)=-Q+α L_m+β QL_m, where Q is the non-metricity scalar, Lm is the matter Lagrangian and α , β are the model parameters. The coupling between geometry and matter leads to a density-dependent gravitational interaction and modifies the cosmological dynamics in the presence of a perfect fluid. We derive the modified Friedmann equations, constrain the model parameters H0, ω and λ =6β H_0^2/α using the observational datasets H(z), Pantheon+SH0ES and DESI BAO, and discuss their physical implications for the cosmological evolution. The values of equation of state (EoS) parameter ω is obtained in the range -1< ω < -1/3 , represent the quintessence type dark energy behaviour of a perfect fluid. From all the datasets, the negative values of deceleration parameter q(z) at z=0, confirms the late-time accelerated expansion of the universe, whereas the transition redshift zt indicates the transition of the universe from an earlier decelerating phase to the present accelerating phase. This shows that our f(Q,Lm) gravity model is capable of reproducing the late-time accelerated expansion through the non-minimal coupling between geometry and matter, without introducing an explicit cosmological constant term in the model under consideration.
The plasma frequency, the transverse permittivity, the spectrum, and the energy density of the thermal electromagnetic radiation of stellar matter were determined. For the non-degenerate matter, the calculations were performed using Vlasov’s equations. The degree of ionization was determined by the Saha’s equations. The chemical composition of the matter was chosen to be close to that of the Sun. The unperturbed particle energy distribution function was obtained by expanding the Fermi distribution function. This allows calculations to be made for density and temperature values close to degeneracy. The decrease in the thermal radiation density due to the spectrum distortion in stellar matter was determined. For hot ( T∈ (20÷ 100)MeV ) neutrino-opaque quark matter the calculations were performed in the hydrodynamic approximation. For the ratio of the electrons leptonic and baryonic charge densities was set 0.4. It was shown that the contribution of u quarks and electrons to the numerical value of the plasma frequency is several times greater than the contributions of other charged particles. For certain temperatures 20÷ 100MeV the temperature dependence of the plasma frequency of quark matter is weak. At lower temperatures ∼ 1.7× 10^11𝐾 the thermal radiation density in such matter is only 46
The present study explores the propagation characteristics of small-amplitude dust ion-acoustic (DIA) solitary waves in an unmagnetized dusty plasma with kappa-distributed electrons, inertial ions, and negatively charged dust grains. Using the reductive perturbation method, the governing plasma equations are reduced to the Korteweg-de Vries (KdV) and modified KdV (mKdV) equations, and their corresponding soliton solutions are analyzed numerically. The findings indicate that both polarities coexist for the KdV equation, whereas only compressive DIA solitons exist for the mKdV equation. It is further observed that compressive and rarefactive fast DIA solitary waves occur for μ < μ_c and μ > μ_c , respectively. Moreover, a comprehensive analysis is performed to explore the effects of key plasma parameters, viz., the spectral index (κ) , dust concentration ratio (μ) , and temperature ratio (α) on the characteristics of DIA solitary waves. Our present investigation may provide valuable insight into nonlinear wave phenomena in the solar wind.
We propose a new analytic description of cosmic expansion-the W-Λ parametrisation-based on a time-dependent deceleration parameter that naturally captures the transition from early deceleration to late-time acceleration. The model yields exact, closed-form expressions for both the scale factor and the Hubble expansion rate, with the latter expressible using a well-known special function for efficient comparison with observations. Testing the framework against 57 cosmic-chronometer measurements of the expansion rate, we find that fixing the model’s shape parameter and the present-day Hubble constant to physically motivated values produces an excellent fit to the data without any free parameters. This fit is notably better than that of the standard flat Λ CDM model evaluated with a fixed Hubble constant and one fitted matter density parameter. Information-theoretic criteria further support the new parametrisation. A full two-parameter fit allowing both the Hubble constant and shape parameter to vary confirms that the W-Λ model performs comparably to Λ CDM when both are granted equal freedom. Although based on a purely kinematic ansatz with minimal parameter dependence, the framework provides a consistent description of the observed expansion history and yields a transition redshift compatible with current observational bounds. Given its analytic simplicity and computational efficiency, the W-Λ parametrisation may serve as a practical complementary tool for further testing the standard cosmological model, pending more comprehensive dynamical and multi-probe analyses.
We investigate the features of compact star by combining the Van der Waals equation of state (EoS) with class I spacetime embedding condition. The Van der Waals EoS, which accounts for intermolecular forces and finite particle sizes, is employed to describe the less extreme regions of compact stars, while the class one condition provides a link between four-dimensional general relativity and higher-dimensional spaces. This is done as Karmarkar condition describes the relationship between the metric funtions. The metric function z(x) is chosen on physical grounds and the other metric function y(x) is obtained through integration. The model incorporates anisotropic charged distributions governed by the Einstein–Maxwell equations, allowing exploration of electromagnetic effects and anisotropic pressures within the stellar interior. This approach reveals how additional spatial dimensions, emerging through class I embedding, influence observable quantities such as surface redshift, luminosity, and stability limits. This integrated approach links microscopic particle interactions with large-scale gravitational dynamics, offering deeper insights into the structure, stability and evolution of compact stars within higher-dimensional relativistic frameworks.
Three molecules, SiCSi, SiC2, and SiC are considered to play significant role in the formation of SiC-dust around the carbon-stars. Precise collisional rate coefficients for transitions among 78 rotational levels in SiCSi due to collisions with para‑H2 (j = 0), for kinetic temperatures up to 50 K, are now available. Using these collisional rate coefficients, we have investigated the MASER and dasar lines of SiCSi, by solving a set of 78 statistical equilibrium equations coupled with 235 radiative transfer equations (Sobolev analysis). Six, out of 235 radiative transitions have shown MASER action, whereas 13 lines are found as dasar lines. The results are compared with those obtained by using the scaled values of the collisional rate coefficients. Significant differences have been found between the two sets of results. It highlights the importance of collisional rate coefficients.
We develop a general framework for investigating the influence of topological defects on the local characteristics of a quantum scalar field in a warped geometry background. The Ricci tensor and curvature scalar are decomposed into contributions from the warp factor, the radial geometry and the angular defect structure. For an arbitrary curvature coupling parameter, the field equation is separated into independent radial, angular and warp-coordinate parts. A complete set of normalized mode functions is obtained for general values of the angular deficit parameters. The general formalism is applied to several specific cases, such as conformally flat warped spacetimes, generalized cosmic strings, global monopoles and anti-de Sitter (AdS)-type warped geometries. The Hadamard two-point function is then evaluated for a global monopole in AdS spacetime using the obtained mode functions.
Optical spectra of the well-known infrared source CPM 19, which exhibited a strong decline in brightness during the period from 1984–1987 to 2000–2005, have been obtained for the first time. A strong and broad Hα emission line has been detected, along with the possible presence of [S II] emission. No traces of an absorption spectrum are observed. It is suggested that the optical component of CPM 19 is in the pre-main-sequence stage. Various explanations of the observed properties are considered; a plausible scenario is that CPM 19 may belong to the class of UX Ori-type stars with an unusually long eclipse duration, similar to that observed in V1184 Tau. Spectra of other nebulous objects in the vicinity of CPM 19, including the HH objects HH 940 and HH 941, have also been obtained and discussed.
We study topological defects with a general structure in higher-dimensional cosmological backgrounds described by a set of angle deficit parameters. As special cases, they include higher-dimensional generalizations of cosmic strings and global monopoles. The corresponding complete set of mode functions is presented for a massive scalar field with a general curvature coupling parameter. For general scale factors and radial functions in the line element, the angular parts of the scalar modes are expressed in terms of associated Legendre functions. De Sitter and Milne universes are considered as examples of cosmological expansion. For the de Sitter bulk, we present the time-dependent parts of the mode functions in inflationary, hyperbolic, and global coordinates.
Models of neutron and strange stars are considered in the approximation of a uniform density distribution. A universal algebraic equation, valid for any equation of state, is used to find the approximate mass of a star of a given density without resorting to the integration of differential equations. Equations of state for neutron stars had been taken for degenerate neutron gas and for more realistic ones, used by Bethe, Malone, Johnson (1975). Models of homogeneous strange stars for the equation of state in the "quark bag model" have a simple analytical solution. The solutions presented in the paper for various equations of state differ from the exact solutions obtained by the numerical integration of differential equations by at most ∼ 20 %. The formation of strange stars is examined as a function of the deconfinement boundary (DB), at which quarks become deconfined. Existing experimental data indicate that matter reaches very high densities in the vicinity of the DB. This imposes strong constraints on the maximum mass of strange stars and prohibits their formation at the final stages of stellar evolution, because the limiting mass of neutron stars is substantially higher and corresponds to considerably lower matter densities.
This paper considers the problem of determining the degree of spottedness of solar-type stars with superflares. The main focus is on the application of a method previously developed by the author for estimating the spotiness of flaring stars based on the chronology of recorded flares. The method is based on determining the periodic function of flare frequency based on statistical analysis of time series of flares, interpreted as a Poisson process with a cyclic parameter related to the period of the star's axial rotation. Converting this function into a von Mises distribution density function allows us to move from the frequency of flares to estimating the angular distribution of stellar spots and determine the effective surface area covered by spots. Nine of the most active solar-type stars from the Okamoto et al. catalog were studied, for which the axial rotation periods were calculated based on the chronology of recorded flares, and the flare distribution concentration coefficients and angular distributions of spots were obtained. The estimated values of spot coverage range from 1-4% for the least spotted to 10% for the most spotted hemispheres, which significantly exceeds the values for the Sun. Comparison with the results of Okamoto et al. showed a high degree of agreement - the correlation coefficient r = 0.896. The developed approach demonstrates the possibility of determining the spotting of stars based on the time series of flare activity without the use of high-resolution photometric or spectroscopic data, which makes it a valuable tool for statistical studies of the magnetic activity of solar-type stars exhibiting super-powerful flares.
On the base of TESS telescope archive data an analysis of the spot activity of stars of five selected moving groups, associations and clusters with ages from 10 to 425 million years was performed. The values of the rotation periods P, spottedness S, and the area of spots Aspot for 704 analyzed objects were obtained. Using the data under consideration evidence of the evolution of stellar activity over time was carried out. It is shown that the average level of spottedness of older stellar groups is lower than the average level of spottedness of younger ones. Quantitative estimates of the change in S of objects of the same age depending on their effective temperature are considered and presented. The obtained estimates of the S of solar-type stars of various ages (10-790 million years) with the spottedness of the Sun are
