arXiv subjects

Kazuharu Bamba

Publications and source records attributed to Kazuharu Bamba.

At least 19 recordsLinked to original sources

arXiv2026

Noether and Mei symmetries in static spherically symmetric quadratic gravity: variational consistency, constraints, and conserved curvature flux

The symmetry content of static spherical $f(R)$ gravity is reconsidered for the pure quadratic model. We first derive the radial action without prematurely eliminating the equation carried by the radial metric variable. The Schwarzschild-type gauge may then be imposed while retaining its associated gravitational constraint. For the resulting system, radial translations and one combined scaling are exact off-shell Noether symmetries. The scaling charge reduces, on the constraint surface, to a conserved radial flux of the scalar curvature. The same flux follows independently from the trace of the four-dimensional field equations. Noether invariance, the strong point-Mei criterion, and Lie invariance of the Euler--Lagrange system are examined separately. Within the full polynomial point ansatz of total degree at most two, the Lie algebra contains precisely three independent generators. The flux also separates the solution space into a nonzero constant-curvature Einstein sector, a degenerate scalar-flat sector, and a dynamical-curvature sector. In particular, the scalar-flat sector permits a Reissner--Nordström-form metric, although its inverse-square coefficient has no electromagnetic meaning in the absence of a Maxwell field.

gr-qc↗

arXiv2026

Evolving wormhole cosmology: modified Friedmann dynamics and observational constraints

We construct an evolving wormhole cosmological model from a Morris--Thorne metric with a separable, time-dependent shape function in a spatially flat Friedmann--Lemaitre--Robertson--Walker background, deriving the corrected wormhole energy-momentum tensor and a volume-averaging procedure that yields a modified Friedmann equation with a redshift-dependent wormhole correction term. We show that the shape-function index and throat-evolution exponent combine into a single macroscopic exponent $β$, and we demonstrate that the resulting wormhole equation of state is consistent with this geometric averaging. All traversability conditions are verified. Confronting the model with cosmic chronometer, DESI Data Release~2 BAO, and standard BAO data across seven dataset combinations, our findings show that all combinations are mutually consistent, with the joint fit giving a present-day expansion rate intermediate between local and CMB-inferred values, a deceleration-to-acceleration transition consistent with independent determinations, and an effective dark-energy equation of state close to, but distinguishable from, a pure cosmological constant. The evolving throat radius offers a novel, geometry-specific observable for discriminating wormhole spacetimes from phenomenological dark-energy parametrisations.

gr-qc↗

arXiv2026

On the Maldacena-Shenker-Stanford chaos bound in black hole spacetimes: Saturation, apparent violations, and their geometric origin

Reported violations of the Maldacena-Shenker-Stanford (MSS) chaos bound in black hole spacetimes raise the question of whether they reflect a genuine breakdown, a feature of particular geometries, or a limitation of the probe used to test the bound. We show that the violations are apparent and arise predominantly in the near-extremal regime, where the black hole develops an $AdS_2\times S^2$ throat, the surface gravity becomes parametrically small, while the Lyapunov exponent of an orbit separated from the throat remains nonzero. We establish a geometric criterion governing this behavior and demonstrate its resolution in two complementary ways: a charged-particle construction that localizes an otherwise excluded orbit within the throat and restores exact saturation, and an out-of-time-order correlator (OTOC) calculation in the Jackiw-Teitelboim (JT) throat, which identifies the near-horizon Lyapunov scale and shows why the photon-sphere exponent does not reproduce it when the orbit remains outside the throat. We illustrate the criterion for static and rotating black holes in Einstein, scalar-tensor, and higher-curvature gravity, representative of a broader class of geometries to which it applies. In the absence of external effects that displace the relevant trajectories from the throat, the MSS bound remains respected; the apparent violations instead reflect a geometric decoupling between the probe and the near-horizon region.

hep-th↗

arXiv2026

Reconstruction of a dark energy model for the Dirac-Born-Infeld scalar field with the Hubble and DESI data via Gaussian process

In this study, we reconstruct the dark energy (DE) as a Dirac-Born-Infeld (DBI) scalar field from the Hubble dataset (32 CC + 26 BAO) and the DESI dataset using the Gaussian process (GP). As the GP is a non-parametric and model-independent way to reconstruct a function and its derivative using the data, our reconstruction of the DE equation of state, the DE density parameter, and the potential does not assume any particular model of cosmology. Using Monte Carlo realizations of the GP-reconstructed expansion history, we derive a posterior estimate of the Hubble constant, obtaining $H_0 = 69.53 \pm 2.68$ km s$^{-1}$ Mpc$^{-1}$. This method offers a fully model-independent estimate of $H_0$, relying only on data and GP priors, and provides an unbiased intermediate value useful for reassessing the Planck-SH0ES tension. Using the reconstructed profiles of the scalar potential as a function of the field $ϕ$, along with their associated uncertainties, we perform a chi-square curve fitting procedure to assess the viability of four different scalar field potentials, such as Exponential, Power-law, Free Field (quadratic), and Higgs-like potential. This allows us to identify which potential best fits the reconstructed data. We also employ MCMC analysis to place quantitative constraints on the model parameters associated with each potential. Furthermore, we do a $χ^2$ analysis for all four potentials and comment on the goodness of the fit for each of them. Finally, we discuss possible generalizations of our model-independent framework and outline the phenomenological implications of our findings.

physics.gen-ph↗

arXiv2026

Gravitational wave echoes as probes of the maximum mass of strange stars in quadratic curvature-matter coupled gravity

Gravitational wave astronomy provides an exemplary avenue to study exotic compact stars with utmost precision. Recent analyses of GW170817 have reported possible post-merger gravitational wave echoes with a significance of $4.2σ$ and a dominant frequency near $72$ Hz. Such echoes may originate from ultracompact remnants possessing photon spheres that partially trap gravitational perturbations. In general relativity, photon-sphere formation requires the stellar compactness to lie within one-third and four-ninths, which is challenging even for a realistic equations of state. Here, we explore this possibility in quadratic curvature gravity with non-minimal matter coupling, considering strange stars described by the MIT bag model equation of state. By solving the modified Tolman-Oppenheimer-Volkoff equations, we obtain the mass-radius relations and identify configurations capable of supporting photon spheres and GW echoes. In the proposed framework, the modified Buchdahl limit allows more compact stellar solutions, while photon-sphere constraints restrict the viable parameter space. We find that increasing the bag constant, decreases the maximum mass and echo time, shifting the echo frequency toward the kHz regime. The echo constraints yield more stringent maximum mass-radius limits than hydrostatic equilibrium, suggesting a revised maximum mass bounds for strange stars. These results highlight the potential of post-merger strange stars as GW echo sources and demonstrate the role of echoes as probes of modified gravity and high-frequency gravitational waves.

gr-qc↗

arXiv2026

A sound-horizon-free measurement of the Hubble constant from DESI DR2 baryon acoustic oscillations using artificial neural networks

We present a model-independent, sound-horizon-free measurement of the Hubble constant $H_0$ using baryon acoustic oscillation tracers from the Dark Energy Spectroscopic Instrument Data Release 2. The function reconstructions are performed using the artificial neural network method, which is a completely data-driven approach that avoids the mild $Λ$CDM prior dependence. Our approach is based on the distance duality relation and combines three complementary observational probes, such as Type\,Ia supernovae, cosmic chronometer, and DESI DR2 BAO -- without requiring any knowledge of the sound horizon scale $r_d$ or any assumption about the absolute luminosity of SNe\,Ia. We obtain a joint constraint of $H_0 = 71.5\pm2.2$\,km\,s$^{-1}$\,Mpc$^{-1}$ at 68\% confidence for 1000 bootstrap realisations and 4096 neurons, which is consistent with the TRGB result and the SH0ES measurement within $0.6σ$, consistent with the Planck 2020 result within $2σ$. Our results favor a higher value of $H_0$ compared to the Planck CMB inference, adding independent support for the reality of the Hubble tension.

astro-ph.CO↗

arXiv2026

Tidal deformation of an accreting compact object

Tidal deformation of a compact object serves as a sensitive probe of the strong-gravity regime and nature of the compact object. It captures how a compact object responds to the external perturbing field of a companion. In realistic astrophysical settings, compact objects are typically immersed in matter-rich environments, which can significantly alter the response. In this work, we investigate the static deformability of Schwarzschild-like exotic compact objects (ECOs) embedded in a quasi-stationary, self-gravitating thin accretion disk. By modelling the external spacetime with a relativistic thin-disk solution, we isolate environmental contributions to the scalar and spin-1 response while maintaining analytical control. We show that, for perfectly reflecting ECOs, the characteristic logarithmic dependence of the scalar and spin-1 response on compactness, set by near-horizon physics, remains intact even in the presence of accretion. The disk primarily amplifies the overall magnitude of the response significantly. These findings highlight that environmental effects can seriously impact tidal signatures, while still permitting, under suitable conditions, the distinguishability of horizonless compact objects from black holes in gravitational-wave observations.

gr-qc↗

arXiv2026

Higher-order generalized uncertainty principle corrections to Casimir-supported traversable wormholes

We investigate traversable wormholes supported by Casimir vacuum energy with second-order generalized uncertainty principle (GUP) corrections. For two representative GUP models, we derive higher-order corrections to the Casimir energy and construct exact wormhole solutions in general relativity. The resulting geometries satisfy the throat, flare-out, and asymptotic-flatness conditions. Higher-order corrections modify the wormhole mass and reduce the exotic matter required, although the null and weak energy conditions remain violated near the throat. We further analyze weak gravitational lensing and gravitational-wave echoes, finding model-dependent signatures that may distinguish different GUP realizations. By relating the dimensionless parameter used in the solutions to the conventional phenomenological GUP parameter, we show that its physical interpretation depends strongly on the throat radius. Most current experimental bounds favor near-Planckian throats for appreciable GUP corrections, while macroscopic throats generally require much weaker corrections. These results provide a framework for confronting GUP-corrected Casimir wormholes with laboratory constraints and astrophysical observations.

gr-qc↗

arXiv2026

Regular black holes with Minkowskian cores: causal structure and observational degeneracy

We construct a new family of asymptotically flat static and rotating regular black holes characterized by a Minkowskian core and a finite two-horizon structure. The solutions are obtained within the framework of gravitational decoupling and provide an analytically tractable realization of non-singular black hole geometries with a regular interior and well-defined asymptotic properties. We investigate the optical appearance of the rotating spacetime through shadow observables and ray-traced images of geometrically thin, optically thin accretion disks. Despite the substantial differences between the interior geometry of these solutions and that of singular black holes, their optical signatures are found to remain remarkably close to those of Schwarzschild and Kerr spacetimes with the same asymptotic parameters. Our results show that markedly different black hole interiors may lead to nearly indistinguishable optical appearances, highlighting the challenges of probing the internal structure of compact objects using current shadow and imaging observations.

gr-qc↗

arXiv2026

Geometric effects of torsion on black hole ringdown and shadows in Poincaré gauge gravity

Spacetime torsion provides a natural extension of general relativity and may lead to black hole solutions that differ significantly from their Einsteinian counterparts. We investigate a class of Reissner-Nordström-like black holes in Poincaré gauge gravity, where the effective charge is generated entirely by spacetime torsion instead of an electromagnetic field. Within the physically relevant torsion sector, the spacetime exhibits a single-horizon structure, free from the inner horizons and extremal states characteristic of charged black holes. Using the sixth-order Wentzel-Kramers-Brillouin (WKB) approximation, Leaver's continued-fraction method, and the eikonal correspondence between quasinormal modes and unstable null geodesics, we study scalar perturbations and spin-2 test fields on the torsion-modified background. We find that increasing torsion decreases both the oscillation frequencies and damping rates, leading to longer-lived ringdown signals. We further compare the model predictions with Event Horizon Telescope observations of Sgr A* and M87*, along with representative LIGO-Virgo-KAGRA (LVK) ringdown scales, to derive constraints on the torsion parameter via a profile-$χ^{2}$ analysis supplemented by Monte Carlo sampling. Although the resulting bounds remain consistent with the Schwarzschild limit within current observational uncertainties, our results show that spacetime torsion leaves correlated imprints on both black hole shadows and ringdown observables.

gr-qc↗

arXiv2026

Gravastars on the brane with a timelike extra dimension

We construct a gravastar configuration within the Shtanov-Sahni (SS) braneworld scenario, characterized by a timelike extra dimension and negative brane tension. Unlike classical black holes, which inevitably culminate in central curvature singularities, our model demonstrates that the SS braneworld dynamics naturally regularize the interior geometry and prevent singularity formation. By solving the modified Einstein field equations induced on the brane, we obtain explicit interior, shell, and exterior solutions without invoking the idealized thin-shell approximation. The gravastar core is modeled as a Bose--Einstein condensate, while the intermediate shell consists of ultra-dense stiff matter. Bulk Weyl corrections induce anisotropic effective pressures on the brane, a feature that emerges intrinsically in this scenario and supports stability. We analyze the active gravitational mass, energy, entropy, and proper thickness of the shell, and establish the junction conditions at the interfaces. Our analysis reveals that the SS gravastar exhibits suppressed or even negative effective mass, reflecting the repulsive nature of the interior condensate, and admits stable equilibrium solutions consistent with energy conditions. This highlights the SS braneworld gravastar as a physically viable compact object and a compelling alternative to black holes. A key novelty of our construction is that the stabilizing pressure anisotropy and suppressed effective gravitational mass arise dynamically from higher-dimensional Weyl corrections, rather than being imposed through ad hoc matter sources or thin-shell idealizations. This provides the first fully analytic realization of a finite-thickness, stable gravastar in the Shtanov-Sahni braneworld, highlighting a genuinely geometric mechanism for singularity avoidance in compact objects.

gr-qc↗

arXiv2026

Self-consistent Hubble expansion in exponential teleparallel gravity: confrontation with recent observations

We reconstruct the Hubble expansion history in exponential teleparallel gravity using recent cosmological observations, including Observational Hubble Data (OHD), the Dark Energy Spectroscopic Instrument Data Release 2 (DESI DR2) baryon acoustic oscillation (BAO) measurements, gravitational-wave (GW) standard-siren data, and the Pantheon Plus and SH0ES Type Ia supernova compilations. We explicitly demonstrate that exponential teleparallel gravity can be consistent with current cosmological observations through statistical analyses based on the reduced $χ^2$, Akaike Information Criterion (AIC), and Bayesian Information Criterion (BIC). In addition, we show that the present model of exponential teleparallel gravity can be viable cosmologically by analysing the dynamical evolution, stability conditions, and linear matter perturbations.

gr-qc↗

arXiv2026

A model-independent measurement of the Hubble constant from gravitational-wave standard sirens and electromagnetic observations

The Hubble tension is one of the most significant challenges in modern cosmology. Developing new approaches to estimate the Hubble constant is therefore crucial, and in this work, we employ a Gaussian process, a fully model-independent method that relies solely on observational data. To determine the Hubble constant, we use not only electromagnetic observations but also include gravitational-wave standard siren data from GWTC3. Our measurements of the Hubble constant are strongly consistent with the SH0ES result, with tensions less than $2σ$, indicating no statistically significant discrepancy. This approach quantifies the impact of gravitational-wave data on the determination of the Hubble constant, examines its consistency with electromagnetic measurements, and explores its potential role in addressing the Hubble tension.

astro-ph.CO↗

arXiv2026

Rotating traversable wormholes and particle dynamics in $f(R,T)$ gravity

Traversable wormholes are among the most interesting solutions of gravitational theories, but within General Relativity they generally require exotic matter violating the null energy condition. Modified gravity theories with matter-geometry coupling provide a promising framework in which wormhole geometries may instead be supported by effective gravitational contributions. Motivated by this possibility, we investigate rotating traversable wormholes in $f(R,T)$ gravity, where $R$ is the scalar curvature and $T$ is the trace of the energy-momentum tensor, within the slow-rotation approximation. We construct stationary and axisymmetric wormhole solutions supported by an anisotropic fluid and show that the obtained geometries are regular, asymptotically flat, horizonless, and satisfy the flare-out condition at the throat. A central result is that the matter sector satisfies both the null and strong energy conditions, indicating that traversable rotating wormholes can be supported without exotic matter. We further analyze particle motion, frame dragging, and non-geodesic effects arising from matter-geometry coupling, together with shadow deformation and gravitational lensing signatures induced by rotation. A preliminary stability analysis based on sound-speed conditions indicates the physical viability of the solutions. These results demonstrate that rotating wormholes in $f(R,T)$ gravity constitute physically consistent compact configurations with potentially observable astrophysical signatures.

gr-qc↗

arXiv2026

Quintessence-dominated cyclic universe with negative cosmological constant

We investigate two simplified non-singular cyclic models with a negative time-varying cosmological constant to represent the non-conventional mechanism of negative cosmological constant expected to address the late-time cosmic acceleration. We show that a physically acceptable evolution with positive energy density can be realized, while negative energy density dominates in case of a positive or zero cosmological constant. In the first model, we demonstrate a sign flipping of the cosmic pressure in a quintessence-dominated universe with no violation of the null energy condition. In the second model, we propose a matter-bounce scenario with showing the crossing of the phantom divide line in the vicinity of the bounce. We find that while we get positive kinetic term and scalar potential, the sum of scalar and quantum potentials is negative.

gr-qc↗

arXiv2026

Thermodynamics and optical aspects of ModMax black holes in higher order curvature gravity with quintessence dark energy

In this work, we derive an exact black hole solution in higher-order curvature gravity by coupling an electromagnetic sector formulated within the ModMax framework to a quintessence dark energy component. Focusing on purely electrically charged configurations, we analyze the thermodynamic and geothermodynamic properties of the solution to investigate its stability and phase structure. Within this sector, the ModMax theory effectively reduces Maxwell electrodynamics up to a rescaling of the electric charge, and thus the obtained solution corresponds to a consistent subset of the broader nonlinear theory. Using thermodynamic geometry, we examine microscopic interactions and phase transitions, showing that divergences in the thermodynamic curvature coincide with the vanishing of the heat capacity, confirming the consistency of the phase structure. We further explore the optical properties of the black hole by studying null geodesics and determining the photon sphere and the corresponding shadow radius for different values of the quintessence state parameter $ω$. Exact analytical expressions for the photon-sphere radius are derived, revealing that higher-order curvature corrections and quintessence significantly enhance the shadow size, whereas the electric charge has the opposite effect. Notably, quintessence is found to have a more pronounced impact on the shadow than the charge. These results highlight that dark energy and higher-order curvature corrections can yield potentially observable signatures in black hole shadows.

gr-qc↗

arXiv2026

Generalized First Law and Smarr Formula: Beyond Additivity and Extensivity

The study of black hole thermodynamics becomes a central topic in gravitational physics, where the first law and the Smarr relation establish a deep connection between spacetime geometry and thermodynamic laws. As we know, these relations depend on the entropy; any modification to the entropy arising from quantum gravity or generalized statistical mechanics may impact the basic thermodynamic framework of black holes. In this work, we develop a general framework for deriving the first law of black hole thermodynamics and the associated Smarr relation for generic spherically symmetric spacetime under a wide class of generalized entropy models. In addition, a generalized Ruppeiner thermodynamic geometry is developed to utilize the generalized entropy model, from which the curvature scalar is determined in a general form. To demonstrate this framework, we assume the Resinser-Nordström black hole and investigate the corresponding extremal and non-extremal phase transition. Interestingly, our analysis reveals that entropy models consistent with the Abè-type composition rule result in a vanishing thermodynamic curvature, whereas violations of this rule exhibit curvature divergences, suggesting a geometric test for the consistency of generalized entropy models.

gr-qc↗

arXiv2026

Testing $Λ$CDM with ANN-Reconstructed Expansion History from Cosmic Chronometers

In modern cosmology, the rapid growth of high-precision observational data, along with significant theoretical advances, has intensified the challenge of identifying a robust, model-independent framework to probe the expansion history of the Universe. In this work, we propose a novel artificial neural network (ANN)-based framework for the non-parametric reconstruction of the late-time cosmic expansion. The framework is trained and validated through a three-stage screening pipeline prior to its application to real observational data. As a demonstration of its effectiveness, we reconstruct the Hubble parameter $H(z)$ using the latest cosmic chronometer measurements. Our results show that the reconstructed expansion history aligns with the predictions of the $Λ$CDM model within observational uncertainties, thereby supporting the robustness and reliability of the proposed approach.

astro-ph.CO↗