arXiv subjects

Kazuharu Bamba

Publications and source records attributed to Kazuharu Bamba.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Probe Cosmological Constraints on Clustering DBI Dark Energy Scenario

We present a multi-probe observational analysis of a clustering tachyonic Dirac-Born-Infeld~(DBI) dark energy model. Its noncanonical kinetic term gives a subluminal sound speed, so dark energy perturbations can grow on subhorizon scales. Working in a multifluid framework that tracks both the background expansion and linear perturbations, we constrain the model with nested sampling, using Planck 2018 compressed CMB distance priors, DESI DR2 BAO, cosmic chronometers, $fσ_8$ growth data, and three Type Ia supernova compilations: Pantheon$+$, DES-SN5YR, and Union3. The standard cosmological parameters agree with $Λ$CDM within $2σ$, with DBI preferring a slightly lower $H_0$ and higher $Ω_{m0}$, with the largest shift being $\simeq1.8σ$ in $H_0$ for Pantheon$+$. The Bayesian evidence favours DBI in every compilation, moderately for Pantheon$+$ ($Δ\ln\mathcal{Z}\simeq3.4$) and inconclusively for DES-SN5YR and Union3 ($Δ\ln\mathcal{Z}\simeq0.6$-$0.7$). The field thaws naturally. Dark energy perturbations stay several orders of magnitude below matter perturbations, yet the noncanonical sound speed still alters the Hubble friction term, raising $S_8$ by only $\approx0.5σ$ relative to $Λ$CDM. Four independent diagnostics, namely the CPL parametrization, $Om(z)$, the statefinder hierarchy, and the growth index, all point to genuinely evolving dark energy rather than a cosmological constant. The thermal history is left intact, with negligible early dark energy. However, one limitation remains. Because $w_ϕ=\dotϕ^2-1\geq-1$, the phantom regime is out of reach, which is in line with the no-go theorem for a single minimally coupled scalar field, so the model cannot follow the DESI DR2 preference for an effective equation of state crossing $w=-1$.

astro-ph.CO↗

arXiv2026

Transitioning late-time cosmology with the Hubble parameterization

We investigate a late-time cosmological model for a homogeneous and isotropic space-time in the Rastall theory. We explore the observational constraints on the Hubble parameter by using the latest cosmological datasets such as cosmic microwave background radiation (Planck), baryon acoustic oscillations (DESI) and Type Ia Supernovae (Union 3.0). As a result, we explicitly demonstrate that the specific redshift transition occurs, namely, there happens a phase shift in the evolution of the universe from the initial deceleration era to the current accelerating phase of the cosmological scenario. Furthermore, we show that with the latest dataset of DESI-BAO clubbed with CC, CMB, and Union 3.0, the current value of the Hubble parameter is estimated as $H_0 = 66.945 \pm 1.094$, which can be compatible with the available observations.

gr-qc↗

arXiv2026

Regular black holes with Minkowskian cores: energy-condition obstruction and optical screening

We construct an asymptotically flat black-hole geometry with a Minkowskian core using gravitational decoupling and analyze its static and rotating extensions. Rather than imposing global energy-condition viability, we show that the vanishing central density required by the Minkowskian core produces a local null-energy-condition obstruction for the present static closure class. The exact mass profile can be written in terms of the regularized incomplete gamma function, which makes transparent both the horizon structure and the exponential localization of the deformation. This localization yields an analytic optical-screening mechanism: on the black-hole branch, deviations of the critical impact parameter from Schwarzschild are controlled by the exponentially suppressed mass deficit at the photon sphere. We also clarify the status of the rotating extension: curvature invariants remain finite on the physical domain $r\geq0$, but this local statement does not by itself establish a globally regular analytic extension through the Kerr disk. These results separate local core regularization from global completeness and explain why strong modifications of the interior can coexist with very small changes in exterior optical observables.

gr-qc↗

arXiv2026

Decoupled asymmetric thin-shell wormholes

We construct reflection-asymmetric thin-shell wormholes by joining a Schwarzschild exterior to its minimal geometric deformation, which shares the same mass and redshift function, so that the asymmetry is carried entirely by the gravitational-decoupling sector. The deformed side is a black hole sourced by a scalar field coupled to nonlinear electrodynamics, in its branch with a real scalar. We show that the total radial null energy of shell and bulk obeys an exact identity and becomes positive for a sufficiently strong deformation. The momentum flux across the shell equals the radial null-energy density of the bulk, and retaining it yields the linear stability condition in closed form. We prove that stability with a conventional shell response requires the bulk to violate the radial null energy condition at the throat; the deformation instead enlarges the stable domain and moves the transition radius inside the photon sphere. For a stationary extension built on Kerr, the junction is solved exactly in the spin, which fixes how the angular momentum is shared between shell and bulk and where the shell behaves as a rotating fluid. A free-precession model provides an estimate of the gravitational-wave signal of the throat.

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

Swampland diagnostics and observational tests of the inflationary universe in the Palatini formalism

We consider quintessential inflation in a quadratic Palatini model with a generalized Peebles-Vilenkin potential, where the same scalar field describes both the inflationary plateau and the later inverse-power-law regime. Passing to the Einstein frame gives a canonically normalized field and an explicit effective potential. The inflationary predictions are then reassessed, together with the further-refined de Sitter, scalar weak-gravity, strong scalar weak-gravity, distance, and trans-Planckian censorship conjectures. The value of the tensor-to-scalar ratio stays below the observational bounds used in this work, whereas the spectral index of scalar fluctuations changes with the potential exponent and with the duration of inflation. The further-refined de Sitter bound is satisfied only over part of the trajectory. At the pivot, both weak-gravity diagnostics are negative in the reduced-Planck-mass convention, although they become positive at finite field values. The field excursion is super-Planckian, but this does not by itself imply a failure of the distance conjecture; the trans-Planckian censorship criterion is not met in the assumed slow-roll regime.

physics.gen-ph↗

arXiv2026

Generalized First Law and Smarr-type Relation: Beyond Additivity and Extensivity

In this work, we develop a general framework for deriving the first law of black hole thermodynamics and the associated Smarr-type relation for 4-dimensional static spherically symmetric spacetimes within a broad class of generalized entropy models. In addition, by employing the entropy representation for the Reissner--Nordström (RN) black hole, we determine a concise curvature functional that depends only on this representation and its derivatives. A general flatness condition is derived, demonstrating that when the entropy Hessian metric is regular and nondegenerate, both homogeneous power-law transformations and a separate logarithmic branch leave the RN black hole's Ruppeiner geometry flat. On the other hand, Rényi, Sharma-Mittal, Kaniadakis, and LQG-inspired entropy deformations typically give rise to nonvanishing curvature in the regular thermodynamic regime, demonstrating that the extremality of the RN black hole does not by itself require a curvature divergence.

gr-qc↗

arXiv2026

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

Gravitational wave astronomy provides a powerful avenue for probing exotic compact stars. Recent analyses of GW170817 have reported possible post-merger gravitational wave echoes with a significance of $4.2σ$ and a dominant frequency near $72,\text{Hz}$. Such echoes may arise from ultracompact remnants possessing photon spheres that partially trap gravitational perturbations. In general relativity, photon-sphere formation requires the stellar compactness to lie between one-third and four-ninths, a condition difficult to achieve with realistic equations of state. Here, we investigate 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 mass-radius relations and identify configurations capable of supporting photon spheres and gravitational wave echoes. The modified Buchdahl limit permits more compact stellar solutions, while photon-sphere constraints restrict the viable parameter space. We find that increasing the bag constant reduces the maximum mass and echo time, shifting the echo frequency toward the kHz regime. The echo constraints impose more stringent maximum mass-radius limits than hydrostatic equilibrium, suggesting revised maximum mass bounds for strange stars. We further compute the critical impact parameter and eikonal quasi-normal modes, demonstrating how modified gravity parameters systematically alter the photon-sphere radius and associated observational signatures. These results establish post-merger strange stars as potential GW echo sources and highlight echoes, optical shadows, and QNMs as complementary probes of modified gravity for next-generation detectors such as the Einstein Telescope and Cosmic Explorer.

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↗

arXiv2026

Finslerian wormholes in squared trace gravity

We investigate traversable wormholes in squared-trace extended gravity within the framework of Finsler-Randers geometry equipped with the Barthel connection. The Einstein-Hilbert action is modified by terms involving the trace of the energy-momentum tensor and its square, generating effective anisotropies through matter-curvature coupling. The resulting field equations are studied under barotropic equations of state with exponential and power-law shape functions. Finslerian anisotropy introduces novel pressure dynamics that enable the classical energy conditions to be satisfied in specific parameter domains. Our analysis quantitatively maps how the Barthel connection scales the parameter space for non-exotic, geometrically viable wormholes compared to purely Riemannian models. These findings suggest that Finslerian modifications provide a powerful mechanism for realizing consistent wormhole structures, offering new perspectives on anisotropic and geometrically enriched space-time configurations in extended gravity.

gr-qc↗

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

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

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↗