arXiv subjects

Taishi Katsuragawa

Publications and source records attributed to Taishi Katsuragawa.

At least 19 recordsLinked to original sources

arXiv2026

Technically Natural Suppression of Fifth Force

Light scalars generically mediate a fifth force incompatible with local tests of gravity unless their couplings are parametrically suppressed or screening mechanisms are introduced. We demonstrate that such suppression can arise from symmetry. We propose a $Z_2$-symmetric mirror extension of the Standard Model within a bi-conformal gravity construction, where spontaneous breaking of scale invariance produces a light scalaron as a pseudo-Nambu-Goldstone boson. This scalaron couples to the difference of trace anomalies between the Standard Model and mirror sectors. We find a parameter-independent correlation between the fifth-force strength $α$ and the scalaron mass $m_σ$, with the proportionality set by QCD observables and the electroweak scale. The Standard Model predicts $α\lesssim 10^{-3}$ above 10~cm scales for $m_σ\lesssim 10^{-6}$ eV, which is directly in the target window of next-generation experiments. In contrast to environmental screening mechanisms, this suppression mechanism follows directly from symmetry rather than nonlinear scalar dynamics.

hep-ph↗

arXiv2026

Revisiting Coincident GR in Internal STEGR Formulation

We revisit Coincident General Relativity (CGR) in the gauge approach to gravity based on Symmetric Teleparallel Equivalent to General Relativity (STEGR) in the \textit{internal-space formulation}, which one of the authors recently proposed in Ref.~[J. Math. Phys. 66 (2025) 5, 052505]. First, we review the standard formulation of STEGR theories in the Palatini approach to gravity, in which formulation we impose the teleparallel and torsion-free conditions by using Lagrange multipliers. Second, we introduce the STEGR theories in the gauge approach to gravity, which is formulated in the internal space, and derive its field equations. We briefly discuss whether the Ostrogradski ghost instability exists and find that the theory may require a degenerate condition to be imposed. Finally, assuming the coincident gauge, we derive CGR in both terms of the action integral and the field equation. Discussing the possible kinematics of the STEGR theory, we formulate a motion of a test scalar particle in the spacetime with non-metricity.

gr-qc↗

arXiv2026

Theoretical and observational constraints on early dark energy in $F(R)$ gravity

This work examines an early dark energy (EDE) scenario in the context of $F(R)$ gravity. EDE is introduced to alleviate the Hubble tension by temporarily injecting approximately $10\%$ of the energy fraction around the matter-radiation equality epoch ($z \approx 10^{3}$--$10^{4}$). Building on several benchmark models, we focus on the potential-driven EDE scenario and investigate the conditions required within $F(R)$ gravity. We first introduce a dimensionless quantity to analytically visualize the evolution of the density ratio between EDE and other matter components. Considering several examples, we demonstrate that the desired EDE can indeed be realized in $F(R)$ gravity. However, stringent constraints arising from violations of the equivalence principle could exclude the allowed parameter space. Our result provides a generic constraint on the potential-driven EDE in $F(R)$ gravity at the background level. This work also concludes that nonperturbative effects or nontrivial mechanisms are indispensable for studying EDE in $F(R)$ gravity while maintaining compatibility with local tests of gravity.

gr-qc↗

arXiv2026

Scalar-Induced Electromagnetic Radiation: Comparison with Axion-Like Particles and Implications for Modified Gravity

The scalar-tensor theory of gravity, a modified gravity theory, introduces a fundamental scalar field that can serve as dynamical dark energy, driving the late-time accelerated expansion of the Universe. In this work, we analyze electromagnetic (EM) radiations arising from scalar fields and compare these features with those induced by axion-like particles (ALPs). Scalar and ALP fields couple differently to the EM field due to their distinct parity properties, $ϕF_{μν} F^{μν}$ for scalar fields and $ϕF_{μν} \tilde{F}^{μν}$ for ALPs. Building on analytical methods developed for ALPs, this work presents a theoretical feasibility analysis that demonstrates how the scalar field could produce observable EM signatures from oscillating field configurations. We also show that resonance effects can amplify the EM radiation for the scalar field under specific conditions, and that the enhancement mechanisms depend on the coupling structure and the configuration of the background magnetic field. Resonance phenomena can accentuate the differences in signal strength and spectral features, potentially aiding future observations in distinguishing scalar fields from ALPs. Although our studies apply to general scalar fields, we embed them within the framework of scalar-tensor theory and discuss the mass and coupling parameter in the context of testing modified gravity. This work provides a theoretical framework for studying generic pure and pseudo-scalar fields on an equal footing and suggests new avenues for observational tests of modified gravity scenarios alongside ALP models.

hep-ph↗

arXiv2026

Cosmological Perturbation in New General Relativity: Propagating mode from the violation of local Lorentz invariance

We investigate the propagating modes of New General Relativity (NGR) in second-order linear perturbations in the Lagrangian density (first-order in field equations). The Dirac-Bergmann analysis has revealed a violation of local Lorentz invariance in NGR. We review the recent status of NGR, considering the results of its Dirac-Bergmann analysis. We then reconsider the vierbein perturbation framework and identify the origin of each perturbation field in the vierbein field components. This identification is mandatory for adequately fixing gauges while guaranteeing consistency with the invariance ensured by the Dirac-Bergmann analysis. We find that the spatially flat gauge is adequate for analyzing a theory with the violation of local Lorentz invariance. Based on the established vierbein perturbative framework, introducing a real scalar field as matter, we perform a second-order perturbative analysis of NGR with respect to tensor, scalar, pseudo-scalar, and vector and pseudo-vector modes. We reveal the possible propagating modes of each type of NGR. In particular, we find that Type 3 has stable five propagating modes, \textit{i.e.}, tensor, scalar, and vector modes, compared to five non-linear degrees of freedom, which results in its Dirac-Bergmann analysis; the linear perturbation theory of Type 3 is preferable for applications to cosmology. Finally, we discuss our results in comparison to previous related work and conclude this study.

gr-qc↗

arXiv2026

Stimulated radiation from superradiant scalar cloud in scalar-tensor theory

Scalar-tensor theories predict fundamental scalar fields of considerable interest in astrophysics and cosmology. We investigate the superradiant instability of scalar clouds around Kerr black holes, showing that stimulated decay generates detectable electromagnetic signals. The growth of the superradiant scalar cloud differs from that of other bosonic fields and depends sensitively on the matter distribution surrounding the black hole, which originates from the scalar-matter coupling realized by the chameleon mechanism in modified gravity theories. In non-uniform matter distributions, stimulated emission from scalar clouds offers an observational signature that distinguishes fundamental scalars from other light bosonic fields.

gr-qc↗

arXiv2026

Wormhole spacetimes in an expanding universe: Energy conditions and future singularities

We study wormhole geometries embedded in an expanding universe within a four-scalar non-linear $σ$ model, where the target-space metric is identified with the spacetime Ricci tensor. In this framework, wormholes can remain stable even when conventional energy conditions are violated. However, once cosmological expansion is included, the effective energy density and pressure are modified by the cosmological fluid, enabling the energy conditions to be satisfied. We further present intriguing geometries in which a finite future singularity appears in our universe but not in another universe connected by the wormhole. Near the throat, the hypersurface becomes timelike, allowing trajectories to traverse to the other universe before the singularity and return afterwards. We also construct wormhole solutions motivated by galactic dark-matter halo profiles, where the required non-vanishing pressure arises naturally from the four-scalar non-linear $σ$ model.

gr-qc↗

arXiv2024

Solar system tests in covariant f(Q) gravity

We study the Solar System constraints on covariant $f(Q)$ gravity. The covariant $f(Q)$ theory is described by the metric and affine connection, where both the torsion and curvature vanish. Considering a model including a higher nonmetricity-scalar correction, $f(Q)= Q +αQ^{n} - 2Λ$, we derive static and spherically symmetric solutions, which represent the Schwarzschild-de Sitter solution with higher-order corrections, for two different ansatz of the affine connection. On the obtained spacetime solutions, we investigate the perihelion precession, light deflection, Shapiro delay, Cassini constraint, and gravitational redshift in the $f(Q)$ gravity. We place bounds on the parameter $α$ with $n=2, 3$ in our model of $f(Q)$ gravity, using various observational data in the Solar System.

gr-qc↗

arXiv2024

Compact star in noninteger power model of $f(R)$ gravity

We investigate compact stars in the noninteger power (NIP) model of $f(R)$ gravity theory, which includes the higher-curvature correction to the Einstein-Hilbert action. The mass-radius relation of the compact stars in the NIP model predicts large deviations from those in the general relativity in the low-mass region, potentially allowing us to test the NIP model by future astrophysical observations. We also study the nonvanishing scalar hair surrounding the compact star and demonstrate that the chameleon mechanism works efficiently. They result in distinct scalar profiles inside and outside the star, which implies screening the fifth force mediated by the scalar field.

gr-qc↗

arXiv2024

Future singularity in an anisotropic universe

We investigate future singularities originating from the anisotropy in the Universe. We formulate a new class of singularities in the homogeneous and anisotropic universe, comparing them with the known singularities in the homogeneous and isotropic universe. We also discuss the physical consequences of the new singularities. Moreover, we develop a novel reconstruction method for the anisotropic universe by introducing four scalar fields to reconstruct cosmological models in which future singularities appear. We present an explicit example where the anisotropy may grow in the future up to singularity.

gr-qc↗

arXiv2023

Nonpropagating ghost in covariant $f(Q)$ gravity

$f(Q)$ gravity is an extension of the symmetric teleparallel equivalent to general relativity (STEGR). This work shows that based on the scalar-nonmetricity formulation, a scalar mode in $f(Q)$ gravity has a negative kinetic energy. This conclusion holds regardless of the coincident gauge frequently used in STEGR and $f(Q)$ gravity. To study the scalar mode, we further consider the covariant $f(Q)$ gravity as a special class in higher-order scalar tensor (HOST) theory and rewrite the four scalar fields, which play a role of the Stüeckelberg fields associated with the diffeomorphism, by vector fields. Applying the standard Arnowitt-Deser-Misner (ADM) formulation to the new formulation of the $f(Q)$ gravity, we demonstrate that the ghost scalar mode can be eliminated by the second-class constraints, thus ensuring that $f(Q)$ gravity is a healthy theory.

gr-qc↗

arXiv2023

Revisiting compact star in $F(R)$ gravity: Roles of chameleon potential and energy conditions

We reexamine the static and spherical symmetric compact star configuration in the $R^2$ model of the $F(R)$ gravity theory. With asymptotic solutions for the additional scalar degrees of freedom, we refine analysis on the external geometry and settle the scalar-hair problem argued in previous works. Performing the numerical integration of the modified Tolman-Oppenheimer-Volkoff equations as a two-boundaries-value problem, we further discuss the scalar-field distribution inside the compact stars and its influence on the mass-radius relation. We show that the chameleon potential plays an essential role in determining the scalar-field profile inside the star. The scalar field often behaves as a quintessential field that effectively decreases the mass of compact stars with lower central energy density.

gr-qc↗

arXiv2022

Cosmological gravity probes: connecting recent theoretical developments to forthcoming observations

Since the discovery of the accelerated expansion of the present Universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality data, allowing us to explore gravity on cosmological scales. To bridge the recent theoretical developments and observations, we present an overview of a variety of modified theories of gravity and the cosmological observables in the cosmic microwave background and large-scale structure, supplemented with a summary of predictions for cosmological observables derived from cosmological perturbations and sophisticated numerical studies. We specifically consider scalar-tensor theories in the Horndeski and DHOST family, massive gravity/bigravity, vector-tensor theories, metric-affine gravity, and cuscuton/minimally-modified gravity, and discuss the current status of those theories with emphasis on their physical motivations, validity, appealing features, the level of maturity, and calculability. We conclude that the Horndeski theory is one of the most well-developed theories of modified gravity, although several remaining issues are left for future observations. The paper aims to help to develop strategies for testing gravity with ongoing and forthcoming cosmological observations.

astro-ph.CO↗

arXiv2022

Compact Star in General $F(R)$ Gravity: Inevitable Degeneracy Problem and Non-Integer Power Correction

We investigate a compact star in the general $F(R)$ gravity. Developing a novel formulation in the spherically symmetric and static space-time with the matter, we confirm that an arbitrary relation between the mass $M$ and the radius $R_s$ of the compact star can be realized by adjusting the functional form of $F(R)$. Such a degeneracy with a choice of the equation of state (EOS) suggests that only mass-radius relation is insufficient to constrain the $F(R)$ gravity. Furthermore, by solving the differential equation for $\left. \frac{dF(R)}{dR}\right|_{R=R(r)}$ near and inside the surface of the compact star with the polytropic EOS, the boundary condition demands a weak curvature correction to the Einstein gravity could be non-integer power of the scalar curvature, which gives a stringent constraint on the functional form of $F\left(R\right)$. This consequence follows that the equation of motion in $F(R)$ gravity includes the fourth-order derivative of metric, and thus, it is applicable to general $F(R)$ models.

gr-qc↗

arXiv2022

Hunting dark energy with pressure-dependent photon-photon scattering

Toward understanding of dark energy, we propose a novel method to directly produce a chameleon particle and force its decay under controlled gas pressure in a laboratory-based experiment. {\it Chameleon gravity}, characterized by its varying mass depending on its environment, could be a source of dark energy, which is predicted in modified gravity. A remarkable finding is a correspondence between the varying mass and a characteristic pressure dependence of a stimulated photon-photon scattering rate in a dilute gas surrounding a focused photon-beam spot. By observing a steep pressure dependence in the scattering rate, we can directly extract the characteristic feature of the chameleon mechanism. As a benchmark model of modified gravity consistent with the present cosmological observations, a reduced $F(R)$ gravity is introduced in the laboratory scale. We then demonstrate that the proposed method indeed enables a wide-ranging parameter scan of such a chameleon model with the varying mass around $(0.1-1)~[μ\mathrm{eV}]$ by controlling pressure values.

gr-qc↗

arXiv2022

ADM formulation and Hamiltonian analysis of $f(Q)$ gravity

$f(Q)$ gravity is an extension of the symmetric teleparallel equivalent to general relativity. We demonstrate the Hamiltonian analysis of $f(Q)$ gravity with fixing the coincident gauge condition. Using the standard Dirac-Bergmann algorithm, we show that $f(Q)$ gravity has 8 physical degrees of freedom. This result reflects that the diffeomorphism symmetry of $f(Q)$ gravity is completely broken due to the gauge fixing. Moreover, in terms of the perturbations, we discuss the possible mode decomposition of these degrees of freedom.

gr-qc↗

arXiv2022

Towards a unified interpretation of the early Universe in $R^2$-corrected dark energy model of $F(R)$ gravity

$R^2$-corrected dark energy (DE) models in $F(R)$ gravity have been widely investigated in recent years, which not only removes the weak singularity potentially present in DE models but also provide us with a unified picture of the cosmic history, including the inflationary and DE epochs. Towards the unified interpretation of dynamical DE all over the cosmic history in the class of $R^2$-corrected DE models, we explore the universal features of the scalaron dynamics in the radiation-dominated epoch, along with the chameleon mechanism, by keeping our eyes on the inflationary and DE epochs. We show that the scalaron evolution does not follow a \textit{surfing solution} and is mostly adiabatic before big bang nucleosynthesis (BBN), even properly including the \textit{kick} by the nonperturbative QCD phase transition, hence a catastrophic consequence claimed in the literature is not applied to this class of DE models. This is due to the presence of the gigantic scale hierarchy between $R^2$ correction and DE, so is the universal feature for the class of $R^2$-corrected DE models. The prospects for the post- or onset-inflationary epoch would be pretty different from what the standard $R^2$ inflationary scenario undergoes due to the presence of the chameleon mechanism.

gr-qc↗

arXiv2022

Static and spherically symmetric solutions in f(Q) gravity

f(Q) gravity is the extension of symmetric teleparallel general relativity (STGR), in which both curvature and torsion vanish, and gravity is attributed to nonmetricity. This work performs theoretical analyses of static and spherically symmetric solutions with an anisotropic fluid for general f(Q) gravity. We find that the off-diagonal component of the field equation due to a coincident gauge leads to stringent restrictions on the functional form of f(Q) gravity. In addition, although the exact Schwarzschild solution only exists in STGR, we obtain Schwarzschild-like solutions in nontrivial f(Q) gravity and study its asymptotic behavior and deviation from the exact one.

gr-qc↗