arXiv subjects

Takahiro Tanaka

Publications and source records attributed to Takahiro Tanaka.

At least 19 recordsLinked to original sources

arXiv2026

The Challenge of Detecting Quantum Nature of Gravitational Waves

We investigate whether squeezing can provide an observable signature of quantum gravitational waves. Because a realistic detector couples only to a particular wave-packet mode, squeezing in global source modes need not remain observable. We show that inflationary two-mode squeezing reduces to an unsqueezed thermal state in the accessible one-mode sector, phase incoherence washes out squeezing in stochastic backgrounds, and the limited coverage of the solid angle of detectors strongly suppresses squeezing from isolated sources. We then show that source squeezing is not essential, {\it i.e.}, a quantized gravitational wave can generate a positive squeezing witness if the detector state is initially prepared in a squeezed state, whereas a classical external gravitational field cannot, producing only a displacement. However, the resulting signal is bounded by the extremely small graviton--detector coupling. Thus, detector squeezing can remove the need for squeezed incident waves, but not the suppression caused by weak gravitational interaction.

hep-ph↗

arXiv2026

How Dilatation Invariance Suppresses Loop Corrections to Curvature Perturbations on CMB Scales

Departures from standard slow-roll (SR) inflation have attracted increasing interest in recent years. In particular, scenarios that strongly enhance the power spectrum of the curvature perturbation are often proposed as a mechanism for producing primordial black holes, which could account for part or the totality of dark matter. An ongoing debate in these models is whether non-linear interactions of cosmological perturbations on small scales, namely loop corrections, can become sufficiently large to backreact on CMB scales. In this work, we adopt a non-linear framework, dropping spatial-gradient terms, to study loop corrections at super-Hubble scales. Our focus is to clarify the role of spatial-diffeomorphism invariance, especially dilatation invariance, which is the symmetry under overall rescaling of spatial coordinates, in demonstrating the suppression of loop corrections to the CMB power spectrum. This analysis is valid for any inflaton potential, both with smooth and sharp transitions, and at any loop order, as long as the CMB scales cross the horizon during an SR phase. We compare this result with the different explanations proposed in the literature using various gauge choices, and show our analysis is consistent both in the comoving and in the spatially-flat gauges. In particular, we show that in the spatially-flat gauge, the cubic interaction contains a term that diverges in the sharp transition limit. However, this contribution is exactly cancelled by the quartic interaction.

astro-ph.CO↗

arXiv2026

Gravitationally Induced Quantum Decoherence of Macroscopic Objects

We formulate the gravitationally induced quantum decoherence of a massive object prepared in a spatial superposition. Starting from linearized gravity coupled to a massive system particle and an environmental scalar field, we derive a closed-time-path influence functional governing the reduced system dynamics. In the nonrelativistic and quasi-static regime, the decoherence exponent can be written as a bilinear functional of the difference of the system stress-energy tensors and an effective noise kernel obtained by dressing the environmental stress-energy tensor correlator with graviton propagators. We then apply this framework to the Newtonian long-range gravitational interaction and evaluate the resulting decoherence function for a dilute nonrelativistic gas modeled by finite wave packets and coarse-grained in time and space. By performing controlled approximations, we obtain analytic expressions for the cumulative decoherence function and show that the dominant contribution is accumulated logarithmically over a broad range of distances, while remaining subdominant to conventional collisional decoherence under realistic conditions.

gr-qc↗

arXiv2026

Locality in effective field theory for inflationary soft modes

The gradient expansion and the separate universe approach provide an effective description of inflationary soft modes after coarse-graining shorter-wavelength degrees of freedom. We formulate a locality condition on the quantum state, requiring that the hard-mode state in each local universe depend on the soft modes only through the local soft-mode values in the same patch. When this condition is satisfied, the coarse-grained soft-mode dynamics remains local, and loop corrections from hard modes to superhorizon correlators of the adiabatic curvature perturbation are perturbatively suppressed. This provides a model-independent diagnosis of when enhanced corrections due to hard modes can invalidate the gradient expansion. We further show that the same locality condition implies a generalized soft theorem, from which the standard consistency relations follow under additional assumptions. This formulation clarifies the origin of possible deviations from the standard consistency relations in multi-field systems or in a non-attractor background. We also show that the locality condition guarantees the absence of infrared divergences for the correlators of operators invariant under a large gauge transformation. Thus, locality of the hard-mode state provides a unified criterion for the effective description of inflationary soft modes, generalized soft theorems, the suppression of hard-mode loop corrections, and the infrared regularity of observable correlators.

gr-qc↗

arXiv2025

The Possibility of Formation of Compact Boson Stars via Cosmological Evolution of a Background Scalar Field

Boson stars, hypothetical astrophysical objects bound by the self-gravity of a scalar field, have been widely studied as a type of exotic compact object that is horizonless and provides a testing ground for physics beyond the Standard Model. In particular, many previous works have demonstrated methods for distinguishing compact boson stars from black holes in general relativity through gravitational wave observations. However, the formation scenario of compact boson stars within the age of the universe remains unclear. In this paper, we explore a possible scenario for the formation of compact boson stars. The model we consider requires two coupled scalar fields: a complex scalar field that forms a boson star and a spatially homogeneous background field, as formation of a compact boson star cannot be achieved in a single filed model. Using the adiabatic approximation, we show that non-relativistic boson clouds can evolve into compact boson stars through the cosmological time-evolution of the background field. In our model the background field evolves to increase the effective mass of the scalar field, and as a result compact boson stars can form within the cosmological timescale, if the variation of the background field is as large as the Planck scale. However, further investigation is required because the required initial states are not the configurations that can be described by the well-studied Schrödinger-Poisson system.

gr-qc↗

arXiv2025

Quantum Droplets in Curved Space

This Letter investigates the formation of quantum droplets in curved spacetime, highlighting the significant influence of curvature on the formation and properties of these objects. While our computations encompass various dimensions, we primarily focus on two dimensions. Our findings reveal a novel class of curvature-driven quantum effects leading to the formation of quasistable liquid droplets, suggesting a feasible pathway for experimental observation, particularly in microgravity environments.

hep-th↗

arXiv2025

Signature of polarized ultralight vector dark matter in pulsar timing arrays

We investigate observational signatures of ultralight vector dark matter with masses $m \sim 10^{-24}$-$10^{-22}$ eV in pulsar timing arrays, taking into account general polarization states of the vector field. We find that vector dark matter induces pulsar timing residuals with nontrivial directional dependence, reflecting the anisotropic property and polarization structure specific to vector dark matter, unlike scalar dark matter. We also derive angular correlation curves of the timing residuals. Intriguingly, circular polarization of the vector dark matter enhances the quadrupole nature of the correlation curve, resulting in a more notable bending of the Hellings-Downs curve. The derived correlation curves offer a useful means to distinguish gravitational wave and dark matter contributions and to probe the nature of dark matter.

astro-ph.CO↗

arXiv2025

Quantum Treatment of Black Hole Superradiance

Rotating black holes can form dense boson clouds through superradiant instability, making Kerr black holes a powerful probe of ultralight massive bosons. Previous studies of black hole superradiance have often treated bosonic fields classically, leaving open questions about how particles are produced and how the clouds grow over time. In this work, we canonically quantize a massive scalar field around a Kerr black hole, providing a fully quantum description of black hole superradiance. We show that the evolution of the particle number in the cloud, as well as the energy and angular momentum of the scalar field, can be consistently explained within the standard framework of quantum field theory in curved spacetime. Furthermore, we prove that the growth of the cloud occurs independently of the choice of initial state. We also explore several phenomena related to a massive scalar field in a rotating black hole spacetime, including Hawking radiation, adiabatic backreaction on the black hole spin, and the direction of level transitions in the presence of self-interactions of the field. Our analysis provides a consistent quantum-mechanical perspective that includes all these phenomena.

gr-qc↗

arXiv2025

CO Multi-line Imaging of Nearby Galaxies (COMING). XI. Azimuthally averaged star formation rate and stellar mass relation with molecular gas amount

This study investigated the relation between the surface density of star formation rate (SFR) ($Σ_{\mathrm{SFR}}$), stellar mass ($Σ_{M_{\ast}}$), and molecular gas mass ($Σ_{M_\mathrm{mol}}$) on nearly 1 kpc scales averaged over concentric tilted rings using the $^{12}$CO $J=1-0$ mapping data of 92 nearby galaxies obtained in the CO Multi-line Imaging of Nearby Galaxies (COMING) project. We categorized these galaxies into three groups based on the deviation of each global SFR from the star-forming main sequence (MS), $Δ$MS: upper MS (UMS), MS, and lower MS (LMS). UMS galaxies tend to be less massive or barred spiral galaxies, exhibiting molecular gas fraction ($f_{\mathrm{gas}}$) comparable to those of MS galaxies but higher star formation efficiency (SFE). In contrast, the LMS galaxies tend to be massive or active galaxies hosting an active galactic nucleus (AGN). Their $f_{\mathrm{gas}}$ values are lower than those of MS galaxies, and their SFEs are slightly lower or comparable to those of MS galaxies in the inner region. These trends indicate that enhanced SFE contributes to higher $Δ$MS values, whereas reduced $f_{\mathrm{gas}}$ results in lower $Δ$MS values. The less prominent bulge or the presence of a bar structure in UMS galaxies induces disk-wide star formation, consequently increasing the SFE. In LMS galaxies, the molecular gas is exhausted, and their star formation activity is low. Environmental effects, such as tidal gas stripping, may also reduce gas supply from the outer regions. Furthermore, our sample galaxies show that both the specific star formation rate (sSFR) and $f_{\mathrm{gas}}$ decrease in the central region in LMS galaxies but did not change in the same region in UMS galaxies. These results seem to support the inside-out quenching of star formation although the dominant cause of depletion remains uncertain.

astro-ph.GA↗

arXiv2025

Understanding Driving Risks using Large Language Models: Toward Elderly Driver Assessment

This study investigates the potential of a multimodal large language model (LLM), specifically ChatGPT-4o, to perform human-like interpretations of traffic scenes using static dashcam images. Herein, we focus on three judgment tasks relevant to elderly driver assessments: evaluating traffic density, assessing intersection visibility, and recognizing stop signs recognition. These tasks require contextual reasoning rather than simple object detection. Using zero-shot, few-shot, and multi-shot prompting strategies, we evaluated the performance of the model with human annotations serving as the reference standard. Evaluation metrics included precision, recall, and F1-score. Results indicate that prompt design considerably affects performance, with recall for intersection visibility increasing from 21.7% (zero-shot) to 57.0% (multi-shot). For traffic density, agreement increased from 53.5% to 67.6%. In stop-sign detection, the model demonstrated high precision (up to 86.3%) but a lower recall (approximately 76.7%), indicating a conservative response tendency. Output stability analysis revealed that humans and the model faced difficulties interpreting structurally ambiguous scenes. However, the model's explanatory texts corresponded with its predictions, enhancing interpretability. These findings suggest that, with well-designed prompts, LLMs hold promise as supportive tools for scene-level driving risk assessments. Future studies should explore scalability using larger datasets, diverse annotators, and next-generation model architectures for elderly driver assessments.

cs.CV↗

arXiv2025

Post-adiabatic dynamics and waveform generation in self-force theory: an invariant pseudo-Hamiltonian framework

Gravitational waveform modeling in self-force theory has reached a mature stage in recent years, with fast and accurate models emerging at both adiabatic (0PA) and first post-adiabatic (1PA) orders in a multiscale expansion. Here, we provide a gauge-invariant 1PA waveform-generation framework that involves no direct calculation of the (gauge-dependent) self-force. To achieve this, we recast the multiscale framework in a pseudo-Hamiltonian form, working on the six-dimensional phase space intrinsic to the multiscale expansion. We characterize the gauge freedom on phase space and show how a localization procedure avoids nonlocal-in time effects in the 1PA dynamics. We find a conservative Hamiltonian structure can be naturally embedded into the complete, dissipative 1PA pseudo-Hamiltonian dynamics, giving rise to natural definitions of the conserved energy, angular momentum, and radial and polar actions. As a byproduct, we clarify that the on-shell value of the conservative Hamiltonian is equal to the mechanical energy historically predicted by the first law of binary black hole mechanics.

gr-qc↗

arXiv2025

Extended $δN$ formalism: Nonspatially Flat Separate Universe Approach

The $δN$ formalism is a powerful approach to compute non-linearly the large-scale evolution of the comoving curvature perturbation $ζ$. It assumes a set of FLRW patches that evolve independently, but in doing so, all the gradient terms are discarded, which are not negligibly small in models beyond slow-roll. In this Letter, we extend the formalism to capture these gradient corrections by encoding them in a homogeneous-spatial-curvature contribution assigned to each FLRW patch. For a concrete example, we apply this formalism to the ultra-slow-roll inflation, and find that it can correctly describe the large-scale evolution of the comoving curvature perturbation from the horizon exit. We also briefly discuss non-Gaussianities in this context.

astro-ph.CO↗

arXiv2025

Quantum decoherence of gravitational waves

The quantum nature of gravity remains an open question in fundamental physics, lacking experimental verification. Gravitational waves (GWs) provide a potential avenue for detecting gravitons, the hypothetical quantum carriers of gravity. However, by analogy with quantum optics, distinguishing gravitons from classical GWs requires the preservation of quantum coherence, which may be lost due to interactions with the cosmic environment causing decoherence. We investigate whether GWs retain their quantum state by deriving the reduced density matrix and evaluating decoherence, using an environmental model where a scalar field is conformally coupled to gravity. Our results show that quantum decoherence of GWs is stronger at lower frequencies and higher reheating temperatures. We identify a model-independent amplitude threshold below which decoherence is negligible, providing a fundamental limit for directly probing the quantum nature of gravity. In the standard cosmological scenario, the low energy density of the universe at the end of inflation leads to complete decoherence at the classical amplitude level of inflationary GWs. However, for higher energy densities, decoherence is negligible within a frequency window in the range $100\ {\rm Hz} \text{-} 10^8\ {\rm Hz}$, which depends on the reheating temperature. In a kinetic-dominated scenario, the dependence on reheating temperature weakens, allowing GWs to maintain quantum coherence above $10^7\ {\rm Hz}$.

gr-qc↗

arXiv2025

Resonant DM scattering in the galactic center under the influence of EMRI

Dark matter (DM) search is one of the greatest challenges in physics. If DM consists of particles, it may form a spike around supermassive black holes (BH) prevalent in galaxy centers. This spike could be potentially observed by altering the orbits of Extreme Mass Ratio Inspirals (EMRIs), one of LISA's main targets. Meanwhile, the effects of EMRI on the DM spike have also been explored. In this study, we focus on the tidal resonances between DM particles and EMRI secondary. As the EMRI orbit evolves via gravitational wave backreaction, each DM particle experiences a significant number of resonances. Although the effect of each individual resonance is small, their cumulative impact might significantly alter the DM particle's orbit. To examine this possibility, we explicitly derive the interaction Hamiltonian for tidal resonances and conducted numerical calculations.

gr-qc↗

arXiv2025

Gravitational-wave constraints on the GWTC-2 events by measuring the tidal deformability and the spin-induced quadrupole moment

Gravitational waves from compact binary coalescences provide a unique laboratory to test properties of compact objects. As alternatives to the ordinary black holes in general relativity, various exotic compact objects have been proposed. Some of them have largely different values of the tidal deformability and spin-induced quadrupole moment from those of black holes, and their binaries could be distinguished from the binary black hole by using gravitational waves emitted during their inspiral regime, excluding the highly model-dependent merger and ringdown regimes. We reanalyze gravitational waves from low-mass merger events in the GWTC-2, detected by the Advanced LIGO and Advanced Virgo. Focusing on the influence of tidal deformability and spin-induced quadrupole moment in the inspiral waveform, we provide model-independent constraints on deviations from the standard binary black hole case. We find that all events that we have analyzed are consistent with the waveform of the binary black hole in general relativity. Bayesian model selection shows that the hypothesis that the binary is composed of exotic compact objects is disfavored by all events.

gr-qc↗

arXiv2025

Remarks on overestimating the effects of inhomogeneities on the Hubble constant

The Hubble constant is one of the most important parameters in cosmology. Discrepancies in values of the Hubble constant estimated from various measurements, the so-called Hubble tension, are a serious problem. In this paper, we study the effects of small-scale inhomogeneities of structure formation on the measurement of the Hubble constant using the luminosity distance-redshift relation. By adopting the adhesion model in Newtonian cosmology as the model of structure formation, we investigate whether or not the effects of inhomogeneities can be sufficiently large to affect the current observations of the Hubble constant. We show that inappropriate treatment of the effects of inhomogeneities can cause a large deviation of the measured value of the Hubble constant from the background value, whose magnitude is comparable with the Hubble tension. Our main message is the importance of adopting an appropriate model of structure formation to investigate the effects of inhomogeneities. We also add discussion on the spatial averaging approach used to estimate the measured Hubble constant in the inhomogeneous universe.

gr-qc↗

arXiv2024

Self-interacting axion clouds around rotating black holes in binary systems

Superradiant instability can form clouds around rotating black holes (BHs) composed of ultralight bosonic fields, such as axions. A BH with such a cloud in a binary system exhibits rich phenomena, and gravitational waves (GWs) from the BH merger provide a means to probe axions. For the first time, we study the evolution of axion clouds in a binary system during the inspiral phase, including axion self-interaction effects. When the self-interaction is significant, unlike in the negligible case, two types of clouds coexist through mode coupling. We examine the evolution of the system considering the effects of dissipation caused by both self-interaction and tidal interaction. For tidal interaction, in addition to the processes of emission to infinity and absorption by the BH, indirect emission via transitions (both resonant and off-resonant) is also considered as a second-order perturbation. Our results demonstrate that the signatures of axion self-interaction are imprinted in the modification of the GW phase. Furthermore, we find the possibility of a dynamical instability called bosenova during the binary inspiral phase.

gr-qc↗

arXiv2024

Extending Hamiltonian Formulation of Particle Motion in Perturbed Kerr Spacetime to various time parameterizations

The Hamiltonian formulation with action-angle variables is very useful when considering the motion of particles undergoing a self-force reaction due to gravitational wave emission. Using the proper time as a parameter along the trajectory is considered to be appropriate when developing formal arguments, but these arguments can easily be extended to arguments with other time variables. The aim of this paper is to clarify the relations between the schemes with various parameterizations of the orbit.

gr-qc↗