arXiv subjects

Teruaki Suyama

Publications and source records attributed to Teruaki Suyama.

At least 19 recordsLinked to original sources

arXiv2026

A universal connection between lens density profiles and low-frequency wave optics in gravitational-wave lensing

We investigate the low-frequency behavior of the amplification factor in gravitational lensing and explore how it encodes information about the density profile of the lensing object. We derive the low-frequency expansion of the amplification factor under the Born approximation for a broad class of projected density profiles. For spherically symmetric profiles that decay faster than any power law at large distances, we derive a systematic expansion of the amplification factor in powers of frequency, with the logarithmic dependence appearing only in the leading term, and show that each expansion coefficient is determined by a finite set of moments of the density profile. We then extend the analysis to profiles with power-law tails and demonstrate that such profiles induce additional non-analytic frequency dependences, including fractional powers and logarithmic terms, which directly reflect the asymptotic behavior of the density distribution. Furthermore, we investigate the effects of non-sphericity and show that contributions from the quadrupole moment appear only as higher-order corrections relative to the spherically symmetric component in the low-frequency regime. Finally, we investigate the validity of the Born approximation in the low-frequency expansion. We derive a criterion for the maximum order of the low-frequency expansion up to which the Born approximation remains dominant over the post-Born corrections.

astro-ph.CO↗

arXiv2026

Initial clustering of primordial black holes: A general formulation for arbitrary local non-Gaussianity

Initial spatial clustering of primordial black holes (PBHs) induced by local-type non-Gaussianity (LNG) can substantially modify cosmological constraints on PBH abundance. Several inflationary scenarios that enhance curvature perturbations at small scales relevant to PBH formation predict LNG that is not necessarily perturbative. Therefore, it is crucial to establish a theoretical framework capable of investigating the initial clustering induced by arbitrary LNG. Here, we present a general analytical formulation for the PBH two-point correlation function applicable to arbitrary LNGs in the alternative approach. Under the assumptions that PBHs form only at the large peaks of perturbations, and that large-scale modes weakly modulate the local variance of small-scale perturbations, we derive an analytic expression for the PBH bias parameter, directly connecting initial clustering to the primordial trispectrum in the collapsed limit. We demonstrate the versatility of our formula by computing the bias parameters in the ultra-slow-roll inflation, curvaton, and modulated reheating scenarios. We also formally generalize the framework to broad power spectra to account for correlations across different PBH mass scales. Because our formulation does not rely on weak or perturbative non-Gaussianity assumptions, our result provides a universal theoretical basis for evaluating initial clustering impact on PBH observables.

astro-ph.CO↗

arXiv2026

Consistency relations of amplitude and phase fluctuations of gravitational waves magnified by strong gravitational lensing

We discuss the amplitude and phase fluctuations of gravitational waves due to wave optics lensing in the presence of both a strong lens and cosmological weak lenses. By applying the geometric optics approximation to the strong lens and treating the weak lensing potential perturbatively, we obtain the amplification factor up to the second order in the weak lensing potential. Additionally, we establish a methodology to systematically evaluate the weak lensing effects based on diagrammatic rules. Based on the derived amplification factor, we evaluate the statistics of the fluctuations and demonstrate that the consistency relations originally established in the absence of a strong lens still hold in exactly the same form when a strong lens is present. The physical origin of these relations is also discussed. Furthermore, we demonstrate that for the mean of the weak lensing signal, both the magnification of the signal and the shift of the Fresnel scale to larger scales occur, consistent with the behavior observed in the variance.

astro-ph.CO↗

arXiv2026

Graviton Floor

It has been observed that the Universe is permeated by the cosmic photon background, ranging from radio waves to gamma rays. We investigate the conversion of the photon background into gravitons in the presence of background magnetic fields in the Milky Way Galaxy and in blazar jets. We find that the resulting graviton background is dominated by the contribution generated in blazar jets. Importantly, this graviton background constitutes a graviton floor for high-frequency gravitational wave detectors searching for new physics, analogous to the neutrino floor.

gr-qc↗

arXiv2026

Quantum description of gravitational waves generated by a classical source

We investigate the quantum properties of gravitational waves (GWs) generated by a classical energy-momentum tensor. Treating the GW field as a quantum field coupled to a classical source, we evaluate the expectation value of the GW operator. We demonstrate that this expectation value exactly reproduces the classical retarded solution. Furthermore, we show that the mean and variance of the number of emitted gravitons are equal. This suggests that the graviton emission is a Poisson process, as expected for a coherent state. We establish a quantitative criterion for the validity of the classical wave description. By applying this criterion, we find that the classical approximation is remarkably accurate for astrophysical sources, but laboratory-scale systems may reside in a regime where the discrete nature of graviton emission becomes significant.

gr-qc↗

arXiv2026

Revisiting Constraints on Primordial Curvature Power Spectrum from PBH Abundances

Primordial black holes (PBHs) can form in the early Universe, for instance during radiation domination, from the collapse of large-amplitude density perturbations shortly after horizon re-entry. This mechanism establishes an approximate one-to-one correspondence between the PBH mass and the scale of the peak in the primordial curvature perturbations. Consequently, the constraints on PBH abundances can be translated into upper limits on the amplitude of the primordial curvature power spectrum, thereby providing an indirect probe of the last e-folds of inflation corresponding to these smaller scales. We derive constraints on the amplitude of primordial curvature power spectra with both narrow and broad peaks using the most up-to-date bounds on PBH abundances. Given the theoretical uncertainties in PBH formation, we systematically compare the constraints obtained using the Press-Schechter (PS) formalism and peak theory, accounting for the nonlinear relation between curvature perturbations and density contrast. We quantify the impact of spherical versus non-spherical collapse criteria and show that including non-sphericity significantly increases the inferred amplitude of the primordial power spectrum, reflecting the larger threshold density contrast required for PBH formation. We also find that whereas the constraints obtained using the PS formalism and peak theory remain largely similar for the monochromatic case, they differ significantly toward smaller scales in the case of a broad primordial power spectrum. This discrepancy underscores that current constraints remain sensitive to the choice of statistical formalism. Our consistent treatment of monochromatic and extended mass functions provides a systematic mapping based on existing methodologies, while highlighting that reducing these theoretical uncertainties is a crucial step toward probing the early Universe through PBHs.

astro-ph.CO↗

arXiv2026

Dark Matter Induced Scalarization as a Possible Solution to the Hyperon Puzzle

We investigate the properties of neutron stars when a massive scalar field, which could comprise all dark matter, is non-minimally coupled to the Ricci scalar. This coupling generates additional contributions to the field's effective mass, leading to tachyonic instabilities inside neutron stars and giving rise to rich phenomenology. Within this framework, we obtain neutron-star configurations with maximum masses exceeding 2 $M_\odot$, even when hyperons, which typically soften the equation of state and significantly lower the maximum mass, are included. Furthermore, we find that larger coupling strengths lead to multiple solutions for the scalar-field configuration. We analyze the structure of the corresponding effective potential responsible for this behavior. We also investigate how the inclusion of a scalar self-interaction term, in addition to the non-minimal coupling, modifies the resulting neutron-star properties.

gr-qc↗

arXiv2026

Excursion Set Approach to Primordial Black Holes: Cloud-in-Cloud and Mass Function Revisited

The abundance and mass function of primordial black holes (PBHs) are often estimated using the Press-Schechter (PS) formalism. In the case of halo formation, the PS formalism suffers from the miscounting of regions collapsing into halos, known as the cloud-in-cloud problem, which is usually corrected by introducing a multiplicative `fudge factor 2'. By analogy, this factor has sometimes been applied to PBH calculations, although its validity has remained unsettled. We reformulate the PS approach for PBHs (forming during radiation-dominated epoch) within the excursion-set framework, where the smoothed density contrast undergoes a stochastic random walk as the smoothing scale varies and collapse is identified with the first threshold crossing. While the halo case is described by a Markovian process, we show that the PBH case is non-Markovian, even when the sharp-k filter Window function is adopted. Decomposing the total collapse probability into two distinct components of the stochastic motion, we numerically confirm that their contributions are exactly equal in the case of halo formation, justifying the fudge factor. For PBHs, however, we demonstrate that this equality no longer holds, and consistent inclusion of both contributions is essential to ensure a positive-definite mass function. Our results clarify the origin of the ambiguity surrounding the fudge factor and establish a robust theoretical foundation for PBH abundance calculations.

astro-ph.CO↗

arXiv2026

An upper limit on cosmological chiral gravitational wave background

Within the standard framework in which electroweak sphaleron processes relate lepton and baryon number, we derive an upper limit on the amplitude of a chiral gravitational wave background produced prior to the electroweak epoch. This bound is independent of the production time of chiral GWs for superhorizon modes, while it becomes sensitive to the production time for subhorizon modes. For sufficiently high reheating temperatures, the bound becomes significantly more stringent than the conventional big bang nucleosynthesis constraints at frequencies above the MHz scale, thereby providing a powerful and \emph{model-independent} probe of parity-violating physics in the early Universe.

hep-ph↗

arXiv2026

Nonlocal Corrections to Scalar Field Effective Action in de Sitter spacetime

We investigate the one-loop effective action for a test scalar field in a general Friedmann-Lemaître-Robertson-Walker (FLRW) background, specifically focusing on quantum corrections up to the second order in the interaction strength. By employing the Schwinger-Keldysh formalism, we derive the equation of motion for the field expectation value, which incorporates not only the standard local radiative corrections but also novel nonlocal features: a memory term and a stochastic noise term. We identify all ultraviolet divergent structures within these nonlocal terms and provide a consistent renormalization procedure. To analyze the physical impact of these terms, we apply a local approximation under the assumption of slowly-varying fields, by which the memory term acts as a negative contribution to the drift coefficient. As a concrete application, we consider a massive $ϕ^4$ theory and show that these one-loop corrections lead to a suppression of the field variance in the infrared regime compared to the tree-level results.

gr-qc↗

arXiv2025

Novel Quantity for Probing Matter Perturbations Below the Fresnel Scale in Gravitational Lensing of Gravitational Waves

Gravitational lensing of gravitational waves provides a powerful probe of the mass density distribution in the universe. Wave optics effects, such as diffraction, make the lensing effect sensitive to the structure around the Fresnel scale, which depends on the gravitational wave frequency and is typically sub-Galactic for realistic observations. Contrary to this common lore, we show that wave optics can, in principle, probe matter perturbations even below the Fresnel scale. This is achieved by introducing a new quantity derived from the amplification factor, which characterizes the lensing effect, and analyzing its correlation function. Our results demonstrate that this quantity defines an effective Fresnel scale: a characteristic scale that can be arbitrarily small, even when observational frequencies are bounded. In practice, the effective Fresnel scale is constrained by the observation time $T$ and is suppressed by a factor of $1/\sqrt{fT}$ relative to the standard Fresnel scale at frequency $f$. Nevertheless, it remains significantly smaller than the conventional Fresnel scale for $fT \gg 1$; for instance, in one-year observations of mHz GWs, the effective Fresnel scale can be as small as 1 pc. This approach opens new avenues for probing the fine-scale structure of the universe and the nature of dark matter.

astro-ph.CO↗

arXiv2025

Towards model-independent identification of lensed gravitational waves using Kramers-Kronig relation

Observations of microlensed gravitational waves (GWs) emanated by compact binary coalescences (CBCs) are essential for studying the mass density distribution in the universe, including black holes and dark matter halos. However, no confident detection of microlensed GWs have been reported to date. There are two important challenges in the identification of microlensed GWs. The first is that the source waveform and lens structure models are not known a-priori. The second is that certain classes of unlensed GWs could mimic microlensed GWs, resulting in undesirable false alarms. In this work, we propose to use the Kramers-Kronig relation for gravitational lensing systems. We argue that such systems are essentially linear response systems obeying causality, where KK relation must hold. The power of this method lies in the fact that microlensed GWs, regardless of the lens structure, must obey KK relation, while unlensed GW events are not in general expected to obey it. This, in principle, allows us to identify microlensed GWs while dismissing microlensing mimickers. We provide the first important steps towards a methodology that exploits KK relation, and test its usefulness under idealized conditions.

gr-qc↗

arXiv2025

Nearly Monochromatic Primordial Black Holes as total Dark Matter from Bubble Collapse

We propose a two-field model where the inflaton $χ$ is non-minimally coupled to the instanton $ϕ$. By choosing an appropriate coupling function, we realize the scenario where the difference of the values of potential between false vacuum (FV) and true vacuum (TV) is maximized during inflation. Most of the bubbles are created at this time. After inflation ends, the potential value of FV drops below that of TV so that these bubbles collapse to form primordial black holes (PBHs). By tuning the parameters of our model, we analyze the Coleman-de Luccia (CDL) and Hawking-Moss (HM) process, finding that the corresponding mass function of PBHs is sharply peaked, implying that we can realize either PBHs as cold dark matter, sub-solar PBHs, or supermassive PBHs in this scenario without enhancement of primordial curvature perturbations.

astro-ph.CO↗

arXiv2025

Dark matter from inflationary quantum fluctuations

We explore a scenario in which dark matter is a massive bosonic field, arising solely from quantum fluctuations generated during inflation. In this framework, dark matter exhibits primordial isocurvature perturbations with an amplitude of ${\cal O}(1)$ at small scales that are beyond the reach of current observations such as those from the CMB and large-scale structure. We derive an exact transfer function for the dark matter field perturbations during the radiation dominated era. Based on this result, we also derive approximate expressions of the transfer function in some limiting cases where we confirm that the exact transfer function reproduces known behaviors. Assuming a monochromatic initial power spectrum, we use the transfer function to identify the viable parameter space defined by the dark matter mass and the length scale of perturbations. A key prediction of this scenario is copious formation of subsolar mass dark matter halos at high redshifts. Observational confirmation of a large population of such low-mass halos will support for the hypothesis that dark matter originated purely from inflationary quantum fluctuations.

astro-ph.CO↗

arXiv2025

Gertsenshtein effect on the spacetime curved by background magnetic field with geometric optics

When electromagnetic (or gravitational) waves propagate in the presence of a background magnetic field, a portion of the waves converts into gravitational (or electromagnetic) waves. This phenomenon, known as the (inverse) Gertsenshtein effect, is typically analyzed in Minkowski spacetime, neglecting the spacetime curvature induced by the magnetic field itself. This paper investigates, for the first time, the influence of spacetime curvature on the (inverse) Gertsenshtein effect. To this end, we first determine the metric perturbation from Minkowski spacetime up to second order in the magnetic field strength, assuming cylindrical symmetry. We also discuss the ambiguities in the form of the metric perturbation arising from gauge freedom and boundary conditions. Using the geometric optics approximation, we then derive a set of coupled equations governing the propagation of electromagnetic and gravitational waves in the resulting curved spacetime. These equations are solved for two specific scenarios: a plane wave and a spherical wave. From the solutions, we compute the evolution of the wave amplitudes and the associated energy fluxes. Our analysis reveals that two competing effects govern the amplitude evolution: magnification due to the focusing of waves by spacetime curvature, and attenuation due to wave conversion via the Gertsenshtein effect. In the plane wave case, these effects precisely cancel, resulting in no net change in amplitude. In contrast, for the spherical wave, the Gertsenshtein effect dominates over focusing, leading to an overall reduction in amplitude.

gr-qc↗

arXiv2025

On the Phase-Magnitude Relation in Gravitational Lensing: Reformulation and Applications of the Kramers-Kronig relation

It is known that the amplification factor, defined as the ratio of the lensed to the unlensed waveform in the frequency domain, satisfies the Kramers-Kronig (KK) relation, which connects the real and imaginary parts of the amplification factor for any lensing signal. In this work, we reformulate the KK relation in terms of the magnitude and phase of the amplification factor. Unlike the original formulation, the phase cannot be uniquely determined from the magnitude alone due to the possible presence of a Blaschke product. While this ambiguity does not arise in the case of a point-mass lens, it can appear in more complex lens models, such as those with an NFW lens profile. As an application of our formulation, we demonstrate that the leading-order behavior of the phase in the low-frequency regime is completely determined by the leading-order behavior of the magnitude in the same regime. This reproduces known results from the literature, derived via low-frequency expansions for specific lens models. Importantly, our result does not rely on any particular lens model, highlighting a universal feature that the low-frequency behavior of the amplification factor is tightly constrained by the KK relation. As a further application, we present two examples in which the phase is constructed from a given analytic form of the magnitude using the newly derived KK relation. In particular, the second example allows for an analytic evaluation of the KK integral, yielding an explicit expression for the phase. This study offers a potentially powerful method for applying the KK relation in model-agnostic searches for lensing signals.

gr-qc↗

arXiv2025

Spectrum of high-frequency gravitational waves from graviton bremsstrahlung by the decay of inflaton: case with polynomial potential

We study the generation of high-frequency gravitational waves (GWs) through graviton bremsstrahlung during the decay of inflaton in the post-inflationary universe, focusing on scenarios with a polynomial inflaton potential. Two main reheating channels are considered: decays into bosons (spin 0) and fermions (spin $\frac{1}{2}$). We compute the resulting GW spectra from three-body decays, where the inflaton decays into a pair of daughter particles and a graviton. We numerically compute the GW spectra for various polynomial exponents by employing two distinct approaches: one treating the inflaton as a collection of rest particles and the other treating it as a coherently oscillating classical field. In the former approach, only gravitons with energies below half the inflaton mass are produced, while the latter allows for the production of gravitons with arbitrarily high energies when the potential exponent is 4 or greater. This difference arises because the inflaton's oscillations are no longer described by a single harmonic mode but instead consist of infinitely many harmonic modes with different frequencies. As a result, the GW spectrum exhibits multiple peaks, with these peaks being less pronounced for higher powers of the potential. We also examine the dependence of the GW spectrum on the coupling constant between the inflaton and daughter particles. Our findings suggest that future GW detectors targeting GWs in the GHz band, such as resonant cavities, may have the capability to detect these signals, offering potential insights into the reheating phase of the early universe.

astro-ph.CO↗

arXiv2024

Constraining small-scale primordial magnetic fields from the abundance of primordial black holes

The presence of magnetic fields in the early universe affects the cosmological processes, leading to the distinct signature, which allows constraining their properties and the genesis mechanisms. In this study, we revisit the method to constrain the amplitude of the magnetic fields on small scales in the radiation-dominated era from the abundance of primordial black holes. Constraints in the previous work were based on the fact that the density perturbations sourced by stronger magnetic fields become large enough to gravitationally collapse to form PBHs. However, we demonstrate that this picture is incomplete because magnetic fields also increase the threshold value of the density contrast required for PBH formation. The increase in threshold density contrast is more pronounced on smaller scales, and in extreme cases, it might even prevent PBH production despite the presence of significant magnetic field. Taking into account the relevant physical effects on the magnetized overdense region, we establish an upper-limit on the amplitude of comoving magnetic fields, approximately $0.13-0.15 {\rm μG}$ at a scale of $10^{17} {\rm Mpc}^{-1}$. Additionally, we compare our constraints with various small-scale probes.

astro-ph.CO↗