arXiv subjects

Kazuharu Bamba

Publications and source records attributed to Kazuharu Bamba.

At least 55 records · Page 3Linked to original sources

arXiv2025

Fuzzy Dark Matter Less-complex Wormhole Structures in Higher-Order Curvature Gravity

Fuzzy dark matter wormhole solutions coupled with anisotropic matter distribution are explored in higher-order curvature gravity. We derive the shape function for fuzzy wormholes and explore their possible stability. We study the embedding diagrams of the active gravitational mass associated with fuzzy dark matter wormholes by taking a certain shape function. Aiming to highlight the role of higher-order curvature gravity in the modeling of less complex fuzzy wormhole structures, we evaluate the complexity factor, the conservation equation, and null energy conditions. Our study reinforces more importance of uniformly distributed pressure effects throughout the less complex region than the emergence of energy density homogeneity in the stability of fuzzy wormholes. It is shown that the active gravitational mass of the fuzzy wormhole structures varies inversely with the radial distance, thereby suggesting the breaching of energy conditions at some arena of the Einasto index. Furthermore, it is revealed that stable fuzzy dark matter wormhole structures exist in nature in the surroundings of cold dark matter halos and galactic bulges. The important physics understood from our analysis is that in higher-order curvature gravity, feasible geometries of fuzzy dark matter wormholes exist naturally in the environments of different galactic haloes.

gr-qc↗

arXiv2025

A Double-Sine-Gordon Early Universe

A solitonic model of the early universe is introduced by employing the Double-Sine-Gordon (DSG) potential. The model predicts the appropriate number of e-foldings ($N_e$) required for favored inflation and is an advantage for the model in addressing the flatness, horizon, and magnetic monopole problems. Compatibility of the model with observations, including the Planck $2018$ data \cite{Akrami et al. (2020)} and the Planck $2018$ data+BK$18$+BAO \cite{Ade et al. (2021)} paves the way to estimate the model's free parameters. The results generate acceptable and proper values for the spectral index ($n_s$) and the tensor-to-scalar ratio ($r$) in agreement with the Planck $2018$ data \cite{Akrami et al. (2020)} and the Planck $2018$ data+BK$18$+BAO \cite{Ade et al. (2021)}. Correspondingly, a consistent description of the reheating era is obtained, yielding positive reheating number of e-foldings ($N_{\mathrm{reh}}$) and reheating final temperature ($T_{\mathrm{reh}}$) from $10^{-2}$ GeV to $10^{16}$ GeV. Overall, the model seems viable at the inflationary and reheating eras.

astro-ph.CO↗

arXiv2025

Exploring the Effects of Generalized Entropy onto Bardeen Black Hole Surrounded by Cloud of Strings

This work explores the thermodynamic characteristics and geothermodynamics of a Bardeen black hole (BH) that interacts with a string cloud and is minimally connected to nonlinear electrodynamics. To avoid the singularities throughout the cosmic evolution, we consider an entropy function which comprises five parameters. In addition, by employing this entropy function for the specific range of parameters, we obtain the representations of BH entropy based on the holographic principle. Moreover, we employ this entropy function to investigate its impact on the thermodynamics of the BH by studying various thermodynamic properties like mass, temperature, heat capacity, and Gibbs free energy for numerous scalar charge and string cloud values. To support our investigation, we use various geothermodynamics formalisms to evaluate the stable behavior and identify different physical scenarios. Furthermore, in this analysis, we observe that only one entropy formalism provides us with better results regarding the thermodynamic behavior of the BH. Moreover, it is shown that one of the entropy models provides a thermodynamic geometric behavior compared to the other entropy models.

gr-qc↗

arXiv2025

Thermodynamic analysis of black holes with cloud of strings and quintessence via Barrow entropy

We explore a Reissner-Nordström Anti-de Sitter (RN-AdS) black hole with a cloud of string and quintessence to study thermodynamics and thermodynamic topology in the presence of Barrow entropy, which is currently being used widely as the horizon of a black hole may not be a smooth surface as described in classical general relativity but instead could have a more intricate fractal-like structure. Here, we study the impact of the fractal correction parameter of Barrow entropy on the thermodynamics of such BHs. We compute the first law of black hole thermodynamics and the Smarr relation for RN-AdS black hole with a cloud of string and quintessence in terms of Barrow entropy by employing the generalized formula for spherically symmetric spacetime which is directly derived from the Einstein field equation. The significance of Barrow entropy has been verified from thermodynamic topology as well. We also found that the non-zero topological charge indicates the presence of the critical point.

hep-th↗

arXiv2025

Autonomous systems and attractor behaviors in non-metricity gravity: stability analysis and cosmic acceleration

The cosmological dynamics are rigorously investigated through the systematic application of autonomous system analysis to the gravitational field equations in non-metricity gravity. The systematic procedure to analyze the late-time cosmic acceleration in higher-order non-metricity gravity is demonstrated by exploring non-hyperbolic critical points with the center manifold theory. The stability properties of these critical points are also evaluated based on the analysis of eigenvalues and phase portraits. It is explicitly shown that the stable node can be realized. The critical points of each model are individually analyzed, and their corresponding cosmological implications are derived. The stability properties of these critical points are evaluated based on the analysis of eigenvalues and phase portraits, revealing that each model includes at least one stable node. Furthermore, the evolution plots of the cosmological parameters confirm the models capacity to exhibit accelerated expansion.

gr-qc↗

arXiv2025

The Stability of Anisotropic Compact Stars Influenced by Dark Matter under Teleparallel Gravity: An Extended Gravitational Deformation Approach

In our investigation, we pioneer the development of geometrically deformed strange stars within the framework of teleparallel gravity theory through gravitational decoupling via the complete geometric deformation (CGD) technique. The significant finding is the precise solution for deformed strange star (SS) models achieved through the vanishing complexity factor scenario. Further, we introduce the concept of space-time deformation caused by dark matter (DM) content in DM haloes, leading to perturbations in the metric potentials $g_{tt}$ and $g_{rr}$ components. Mathematically, this DM-induced deformation is achieved through the CGD method, where the decoupling parameter $α$ governs the extent of DM influence. To validate our findings, we compare our model predictions with observational constraints, including GW190814 (with a mass range of $2.5-2.67 M_{\odot}$) and neutron stars (NSTRs) such as EXO 1785-248 [mass=$1.3_{-0.2}^{+0.2}~M_{\odot}$], 4U 1608-52 [mass=$1.74_{-0.14}^{+0.14}~M_{\odot}$], and PSR J0952-0607 [mass=$2.35_{-0.17}^{+0.17}~M_{\odot}$]. Our investigation delves into the stability of the model by considering causality conditions, Herrera's Cracking Method, the adiabatic index, and the Harrison-Zeldovich-Novikov criterion. We demonstrate that the developed model mimics a wide range of recently observed pulsars. To emphasize its compatibility, we highlight the predicted mass and radius in tabular form by varying both the parameters $α$ and $ζ_1$. Notably, our findings are consistent with the observation of gravitational waves from the first binary merger event. Furthermore, we compare our results with those obtained for a slow-rotating configuration. In addition to this, we discuss the moment of inertia using the Bejger-Haensel approach in this formulation.

gr-qc↗

arXiv2025

Perturbed $f(R)$ gravity coupled with neutrinos: exploring cosmological implications

We conduct a thorough examination of cosmological parameters within the context of $f(R)$ gravity coupled with neutrinos, leveraging a diverse array of observational datasets, including Cosmic Microwave Background (CMB), Cosmic Chronometers (CC), Baryon Acoustic Oscillations (BAO), and Pantheon supernova data. Our analysis unveils compelling constraints on pivotal parameters such as the sum of neutrino masses ($\sum m_ν$), the interaction strength parameter ($Γ$), sound speed ($c_s$), Jean's wavenumbers ($k_J$), redshift of non-relativistic matter ($z_{\rm nr}$), and the redshift of the Deceleration-Acceleration phase transition ($z_{\rm DA}$). The incorporation of neutrinos within the $f(R)$ gravity framework emerges as a key factor significantly influencing cosmic evolution, intricately shaping the formation of large-scale structures and the dynamics of cosmic expansion. Additionally, a detailed analysis of bulk flow direction and amplitude across various redshifts provides valuable insights into the nature of large-scale structures. A notable aspect of our model is the nuanced integration of $f(R)$ gravity theory with neutrinos, representing a distinctive approach to unraveling cosmological phenomena. This framework, unlike previous models, explicitly considers the impact of neutrinos on gravitational interactions, the formation of large-scale structures, and the overarching dynamics of cosmic expansion within the $f(R)$ gravity paradigm. Furthermore, our study addresses the Hubble tension problem by comparing $H_0$ measurements within our model, offering a potential avenue for reconciling discrepancies. Our findings not only align with existing research but also contribute novel perspectives to our understanding of dark energy, gravitational interactions, and the intricate challenges posed by the Hubble tension.

astro-ph.CO↗

arXiv2024

Reconstruction of the scalar field potential in nonmetricity gravity through Gaussian processes

The accelerated expansion of the universe has been widely confirmed, posing challenges to the standard $Λ$CDM model, particularly the cosmological coincidence problem. This has motivated the exploration of modified gravity theories, including non-metricity gravity, which explains cosmic acceleration without dark energy. In this work, we incorporate a quintessence scalar field into the non-metricity framework to model both inflation and late-time acceleration. Employing the Gaussian process method with a square exponential kernel, we reconstruct the scalar field potential, $V(ϕ)$, from observational Hubble data sets coming from cosmic chronometers (CC) as well as from the method of radial baryon acoustic oscillations (BAO) in a model-independent approach. This approach allows us to obtain a suitable quintessence scalar field model that aligns with the observational Hubble data under the framework of power-law non-metricity gravity. Additionally, we compare our reconstructed potential with power-law scalar field potentials, revealing that these models show better agreement with the observational data, providing new insights into the dynamics of the universe. In contrast, we find that the early dark energy has minimal effect on the present-time accelerated expansion of the universe.

gr-qc↗

arXiv2024

Charged Fuzzy Dark Matter Black Holes

We investigate the impact of fuzzy dark matter (FDM) on supermassive black holes (SMBHs) characterized by a spherical charge distribution. This work introduces a new class of spherically symmetric, self-gravitational relativistic charged models for FDM haloes, using the Einasto density model. This study enables the dark matter (DM) to appear as the matter ingredient, which constructs the black hole and extends the non-commutative mini black hole stellar solutions. By considering the charged anisotropic energy-momentum tensor with an equation of state (EoS) $p_{r}=-ρ$, we explore various black hole solutions for different values of the Einasto index and mass parameter. Our approach suggests that the central density of the resulting black hole model mimics the usual de Sitter core. Furthermore, we discuss the possibility of constructing a charged self-gravitational droplet by replacing the above-mentioned EoS with a non-local one. However, under these circumstances, the radial pressure is observed to be negative. Ultimately, we consider various possibilities of constructing DM black holes, featuring intermediate masses that could evolve into galaxies. Consequently, some of these theoretical models have the potential to replace the usual black hole solutions of the galactic core. Simultaneously, these models are physically beneficial for being comprised of the fundamental matter component of the cosmos. Due to the outcomes of this paper, we would be able to study the connection between BH and DM by formulating stable stellar structures featuring fuzzy mass distributions derived from the Einasto distribution of DM halos.

gr-qc↗

arXiv2024

Orthogonal Splitting in Degenerate Higher-order Scalar-tensor Theories

We explore a comprehensive analysis of the formalism governing the gravitational field equations in degenerate higher-order scalar-tensor theories. The propagation of these theories in the vacuum has a maximum of three degrees of freedom and is at most quadratic in the second derivative of the scalar field. We investigate the gravitational field equation for spherically symmetric anisotropic matter content along with its non-conserved equations. Our analysis focuses on the evaluation of structure scalars to assess their behavior under Einstein's modification. We present a realistic mass contribution that sheds light on both geometric mass and total energy budget evaluations for celestial objects. Ultimately, we discuss two viable models restricted as minimal complexity and conformal flatness to enhance the scientific contribution of the present manuscript.

gr-qc↗

arXiv2024

Imprints of dark matter on the structural properties of minimally deformed compact stars

In this manuscript, we investigate the possibility of constructing anisotropic dark matter compact stars motivated by the Einasto density profile. This work develops analytical solutions for an anisotropic fluid sphere within the framework of the well-known Adler-Finch-Skea metric. This toy model incorporates an anisotropic fluid distribution that includes a dark matter component. We use the minimal geometric deformation scheme within the framework of gravitational decoupling to incorporate anisotropy into the pressure profile of the stellar system. In this context, we model the temporal constituent of the $Θ$-field sector to characterize the contribution of dark matter within the gravitational matter source. We present an alternative approach to studying anisotropic self-gravitating structures. This approach incorporates additional field sources arising from gravitational decoupling, which act as the dark component. We explicitly verify whether the proposed model satisfies all the requirements for describing realistic compact structures in detail. We conclude that the modeling of the Einasto density model with the Adler-Finch-Skea metric gives rise to the formation of well-behaved and viable astrophysical results that can be employed to model the dark matter stellar configurations.

gr-qc↗

arXiv2024

Reconstruction of the singularity-free $f(\mathcal{R})$ gravity via Raychaudhuri equations

We study the bounce cosmology to construct a singularity-free $f(\mathcal{R})$ model using the reconstruction technique. The formulation of the $f(\mathcal{R})$ model is based on the Raychaudhari equation, a key element employed in reconstructed models to eliminate singularities. We explore the feasibility of obtaining stable gravitational Lagrangians, adhering to the conditions $f_{\mathcal{R}}>0$ and $f_{\mathcal{R}\mathcal{R}}>0$. Consequently, both models demonstrate stability, effectively avoiding the Dolgov-Kawasaki instability. Our assessment extends to testing the reconstructed model using energy conditions and the effective equation-of-state (EoS). Our findings indicate that the reconstructed super-bounce model facilitates the examination of a singularity-free accelerating universe for both phantom and non-phantom phases. However, in the case of the reconstructed oscillatory bounce model, two scenarios are considered with $ω=-1/3$ and $ω=-2/3$. While the model proves suitable for studying a singular-free accelerating universe in the $ω=-1/3$ case, it fails to demonstrate such behavior under energy conditions for the $ω=-2/3$ scenario. The reconstructed models accommodate early-time bouncing behavior and late-

gr-qc↗

arXiv2024

Measuring 60-pc-scale Star Formation Rate of the Nearby Seyfert Galaxy NGC 1068 with ALMA, HST, VLT/MUSE, and VLA

Star formation rate (SFR) is a fundamental parameter for describing galaxies and inferring their evolutionary course. HII regions yield the best measure of instantaneous SFR in galaxies, although the derived SFR can have large uncertainties depending on tracers and assumptions. We present an SFR calibration for the entire molecular gas disk of the nearby Seyfert galaxy NGC 1068, based on our new high-sensitivity ALMA 100GHz continuum data at 55pc (=0."8) resolution in combination with the HST Paα line data. In this calibration, we account for the spatial variations of dust extinction, electron temperature of HII regions, AGN contamination, and diffuse ionized gas (DIG) based on publicly available multi-wavelength data. Especially, given the extended nature and the possible non-negligible contribution to the total SFR, a careful consideration of DIG is essential. With a cross-calibration between two corrected ionized gas tracers (free-free continuum&Paα), the total SFR of the NGC 1068 disk is estimated to be 3.2\pm0.5 Msol/yr, one-third of the SFR without accounting for DIG (9.1\pm1.4 Msol/yr). We confirmed high SFR around the southern bar-end and the corotation radius, which is consistent with the previous SFR measurements. In addition, our total SFR exceeds the total SFR based on 8μm dust emission by a factor of 1.5. We attribute this discrepancy to the differences in the young stars at different stages of evolution traced by each tracer and their respective timescales. This study provides an example to address the various uncertainties in conventional SFR measurements and their potential to lead to significant SFR miscalculations.

astro-ph.GA↗

arXiv2024

Role of Complexity on the Minimal Deformation of Black Holes

We investigate spherically symmetric classes of anisotropic solutions within the realm of a schematic gravitational decoupling scheme, primarily decoupling through minimal geometric deformation, applied to non-rotating, ultra-compact, self-gravitational fluid distributions. In this respect, we employ the minimal complexity factor scheme to generate physically realistic models for anisotropic matter distributions, using a well-behaved model. The zero-complexity factor condition enables us to determine the deformation function for solving the decoupled system. We explore all the structure-defining scalar variables, such as density inhomogeneity, strong energy condition, density homogeneity, and the complexity factor (an alloy of density inhomogeneity and pressure anisotropy) for the decoupling constant ranging between $0$ and $1$. We observe that the anisotropy vanishes when the coupling constant is set to unity. This finding holds significance as it implies that, in the context of a zero-complexity factor approach, an anisotropic matter distribution becomes perfect without requiring any isotropy requirements. This work effectively explored the impact of complexity on the composition of self-gravitational stellar distributions. This effective approach enables the development of new, physically realistic isotropic stellar models for anisotropic matter distributions. Additionally, our findings indicate that the complexity factor in static, spherically symmetric self-gravitational objects can significantly affect the nature of the matter distribution within these systems. It is concluded that the minimally deformed Durgapal-IV model features an increasing pressure profile, and the local anisotropy of pressure vanishes throughout the model under complexity-free conditions.

gr-qc↗

arXiv2024

Stability of a realistic astrophysical pulsar and its mass-radius relation in higher-order curvature gravity

The objective of this research is to explore compact celestial objects while considering the framework of an extended gravitational theory known as $\mathcal{R}+f(\mathcal{G})$ gravity. The notations $\mathcal{R}$ and $\mathcal{G}$ denote the Ricci scalar and the Gauss-Bonnet invariant, respectively. Radio pulsars, which are neutron stars with masses greater than 1.8 times that of the Sun ($M_\odot$), provide exceptional opportunities for delving into fundamental physics in extraordinary environments unparalleled in the observable universe and surpassing the capabilities of experiments conducted on Earth. Through the utilization of both the linear and quadratic expressions of the function { $f(\mathcal{G}) = α_1 \mathcal{G}^2$, where $α_1$ (with dimensional units of [${\textit length}^6$]) are incorporated}, we have achieved an accurate analytical solution for anisotropic perfect-fluid spheres in a state of hydrostatic equilibrium. By integrating the dimensional parameters $α_1$ and the compactness factor, defined as ${\mathcal C=\frac{2GM}{Rc^2}}$, we showcase our capacity to encompass and depict all physical characteristics within the stellar structure. We illustrate that the model is capable of producing a stable arrangement encompassing its physical and geometric properties. We illustrate that by utilizing the quadratic form of $\mathcal{G}$ in the $\mathcal{R}+f(\mathcal{G})$ framework, the ansatz of Krori-Barua establishes connection between pressure in the radial direction ($p_r$) using semi-analytical methods, pressure in the tangential direction ($p_t$), and density ($ρ$).

gr-qc↗

arXiv2024

Observational constraints on thawing quintessence scalar field model

Thawing quintessence scalar field models with the various potential forms to explain the late-time cosmic acceleration are compared to the ΛCDM model in detail by analyzing cosmological parameters with a set of observational data including H(z), BAO, CMB, SNIa, BBN, and f(z)σ8 at the background and the perturbation levels. At low redshifts for the thawing quintessence scalar field models, the growth rate of the cosmic structure is significant. By utilizing a standard Markov Chain Monte Carlo (MCMC) procedure based on the recent expansion and the growth observational data with the statistical values of the Akaike and the Bayesian information criteria, we discuss the consistency of the thawing quintessence scalar field models with the set of different potentials with the observational data. The main consequence of this work is that despite the various considered potential forms that are very popular in the literature, we should be looking for consistent potential forms with observational data. Keywords: Thawing Dark Energy, Large Scale Structure, Markov Chain Monte Carlo Method, Observational Constraints, Matter Perturbations.

astro-ph.CO↗

arXiv2024

Topological interpretation of extremal and Davies-type phase transitions of black holes

Topological arguments are currently being used as a novel scheme to discern the properties of black holes while ignoring their detailed structure and specific field equations. Among various avenues of black hole physics, where this novel approach is being utilized, the phase transition in black hole thermodynamics lies at the forefront. There are several types of phase transition in black holes; such as the van der Waals type phase transition, Davies-type phase transition, extremal phase transition, and Hawking-Page (HP) transition. So far, the topological interpretation, where the critical point has been identified with the non-zero topological charge, has been obtained only for the van der Waals type phase transition and HP transition in different spacetimes. To complete the picture, here we provide the same interpretation for two other phase transitions: Davies-type phase transition and extremal phase transition. The entire analysis is general and is valid for any spacetime where these types of phase transitions are observed. More importantly, our analysis suggests that amid the apparent differences in these phase transitions, they share the same topological characteristics, \textit{i.e.} non-zero topological charge arising from different thermodynamic potentials in different types of phase transition.

gr-qc↗

arXiv2024

Governing accelerating Universe via newly reconstructed Hubble parameter by employing empirical data simulations

A new parametrization of the Hubble parameter is proposed to explore the issue of the cosmological landscape. The constraints on model parameters are derived through the Markov Chain Monte Carlo (MCMC) method by employing a comprehensive union of datasets such as 34 data points from cosmic chronometers (CC), 42 points from baryonic acoustic oscillations (BAO), a recently updated set of 1701 Pantheon$^+$ (P22) data points derived from Type Ia supernovae (SNeIa), and 162 data points from gamma-ray bursts (GRBs). Furthermore, the models are compared by using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC), so that a comparative assessment of model performance can be available. Additionally, we compare the Dainotti relation via Gaussian likelihood analysis versus new likelihoods and Calibration of the Dainotti relation through a model-independent method. The kinematic behavior of the models is also investigated by encompassing the transition from deceleration to acceleration and the evolution of the jerk parameter. From the analysis of the parametric models, it is strongly indicated that the Universe is currently undergoing an accelerated phase with diagnostics of the model validating the quintessence phase.

astro-ph.CO↗