arXiv subjects

Yuko Urakawa

Publications and source records attributed to Yuko Urakawa.

At least 19 recordsLinked to original sources

arXiv2026

Constraining Axion-like Particles through Multi-epoch Monitoring of Strong Gravitational Lenses

We present new constraints on ultralight axion-like particles (ALPs) through multi-epoch measurements of differential birefringence induced due to a coupling ($g_{aγ}$) between the ALP and electromagnetic fields. Broadband polarimetric observations in the 2-8 GHz range of the gravitationally lensed system CLASS B1152+199 were carried out over five epochs spanning three months with a cadence of roughly 20 days, and the differential birefringence angle ($Δ\,θ_{a,{\rm lens}}$) between the lensed images were estimated. We also combined an archival observation that effectively increases the span to 9.5 yr to probe the effect of an oscillating ALP field imprinted as oscillating $Δ\,θ_{a,{\rm lens}}$ over time. Here we present a new technique for combining multi-epoch measurements of $Δ\,θ_{a,{\rm lens}}$ by considering the coherence of the ALP field, such that, $Δ\,θ_{a,{\rm lens}}$ over these observations are related. The time scale of coherence depends on the mass of the ALP field ($m_a$). Our results are consistent with non-detection and we constrain $g_{aγ} \leq 7.8\times 10^{-12} \,\left( {ρ_{a,\text{em}}}/{20 \text{ GeV cm}^{-3}} \right)^{-1/2}\;\mathrm{GeV}^{-1}$ to $\leq 3.2\times 10^{-8} \,\left( {ρ_{a,\text{em}}}/{20 \text{ GeV cm}^{-3}} \right)^{-1/2}\;\mathrm{GeV}^{-1}$ at 95% confidence for $m_a$ between $1.6\times 10^{-22}\;\mathrm{eV}$ and $3.8\times 10^{-18}\;\mathrm{eV}$, where $ρ_{a,{\rm em}}$ is the density of the ALP field at emission. This improves over the constraint provided by the CERN Axion Solar Telescope by up to an order of magnitude in the $m_a$ range $1.6\times 10^{-22}\;\mathrm{eV}$ to $3\times 10^{-21}$ eV.

astro-ph.CO↗

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

Nonlinear Lattice Framework for Inflation: Bridging stochastic inflation and the $δ{N}$ formalism

Understanding when inflationary perturbations become genuinely nonlinear near the horizon crossing requires methods that go beyond both linear perturbation theory and the gradient expansion. In this work, we introduce a nonlinear lattice framework for single-field inflation based on a shear-free, locally Friedmann-Lemaître-Robertson-Walker geometry. This approach captures inhomogeneous local expansion rates, curvature contributions to the local Friedmann equation, and proper-volume weighting at a fraction of the computational cost of full numerical relativity. We construct fully nonlinear $δN$ observables on uniform-density slices, together with other practical time-dependent estimators for the curvature perturbations. After validating the framework in a standard slow-roll regime, we apply it to Starobinsky's linear-potential model featuring an intermittent ultra-slow-roll (USR) phase and a sharp potential transition. During this non-attractor USR regime, the lattice captures the separation of curvature perturbation estimators, the growth and subsequent stabilisation of non-Gaussianity, and a transient weakening of the shear-free approximation when the inflaton velocity becomes very small. Our framework provides a practical intermediate approach between rigid background lattice simulations and full numerical relativity, offering a nonlinear bridge between lattice methods, the $δN$ formalism, and the stochastic inflation formalism as perturbations transition beyond the linear regime.

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

Potential Surge Preheating: enhanced resonance from potential features

We investigate the effects of local features in the inflationary potential on the preheating dynamics after inflation. We show that a small feature in the potential can enhance the resonance and bring the radiation-like state equation during preheating despite the inflationary potential being a quadratic one. Such localized features may naturally arise due to various physical effects without altering the large-scale predictions of the original model for cosmic microwave background (CMB) observables. We demonstrate that these features effectively introduce localized higher-power terms in the potential, significantly influencing the preheating dynamics $\unicode{x2013}$ a phenomenon we term potential surge preheating. We outline the resulting modifications in energy distribution among different components. We further show that these small-scale features leave detectable imprints in the form of gravitational wave signals. These signals influence CMB measurements of the effective number of relativistic species, $N_{\mathrm{eff}}$, offering a way to reconstruct the shape of the inflaton potential at small scales. Finally, we argue that these modifications to the scalar potential provide a framework to explore preheating dynamics and the fragmentation of scalar fields using simple scalar potentials.

astro-ph.CO↗

arXiv2024

Statistical anisotropy of primordial gravitational waves from generalized $δN$ formalism

In this letter, we demonstrate how to use the generalized $δN$ formalism, which enables us to compute the evolution of all the large scale fluctuations, including gravitational waves, solely by solving the evolution of the background homogeneous Universe. Using the Noether charge density, we derive an analytic formula which describes the mapping between the fluctuations at the horizon crossing and the sourced gravitational waves at the end of inflation. This formula can apply also to an inflation model with an anisotropic background. Using this formula, we discuss the condition for the non-vanishing linear polarization and the qualitative difference between single- and multi-gauge field models.

gr-qc↗

arXiv2024

g$δN$ formalism

The $δN$ formalism has been the major computational tool to study the superhorizon evolution of the scalar type perturbation sourced by scalar fields. Recently, this formalism was generalized to compute an arbitrary scalar, vector, and tensor type perturbations, including the gravitational waves (GWs), sourced by an arbitrary bosonic fields. In this paper, we explain how to use the generalized $δN$ formalism (the g$δN$ formalism), considering a model with U(1) gauge fields as a concrete example. Several new findings on this model and prospects on future gravitational wave experiments are also discussed, including the condition for the two linear polarizations of GWs to have different amplitudes. This paper provides a detailed explanation of our previous paper published in Physical Review Letters. We also discuss the Weinberg's adiabatic mode for an anisotropic background, showing a qualitative difference from the one for the FLRW background.

gr-qc↗

arXiv2022

New scenario of QCD axion clump formation I: Linear analysis

The QCD axion acquires the potential through the non-perturbative effect of the QCD matters around the QCD phase transition. During this period, the direct interaction between the axion and the QCD matters sets in. Focusing on the impact of this direct interaction, we propose two scenarios where the fluctuation of the axion can rapidly grow, potentially leading to the formation of axion miniclusters even if the Peccei-Quinn (PQ) symmetry was already broken during inflation. The first scenario assumes that the primordial curvature perturbation at the horizon scale during the QCD epoch was significantly enhanced and the second one assumes that the initial misalignment was tuned around the hilltop of the potential.

astro-ph.CO↗

arXiv2021

Galaxy imaging surveys as spin-sensitive detector for cosmological colliders

Galaxy imaging surveys provide us with information on both the galaxy distribution and their shapes. In this paper, we systematically investigate the sensitivity of galaxy shapes to new physics in the initial conditions. For this purpose, we decompose the galaxy shape function into spin components, and compute the contributions to each spin component from both intrinsic alignment and weak lensing. We then consider the angular-dependent primordial non-Gaussianity, which is generated by a non-zero integer spin particle when active during inflation, and show that a galaxy imaging survey essentially functions as a spin-sensitive detector of such particles in the early universe. We also perform a forecast of the PNG generated from a higher spin particle, considering a Rubin Observatory LSST-like galaxy survey.

astro-ph.CO↗

arXiv2021

Anisotropic separate universe and Weinberg's adiabatic mode

In the separate universe approach, an inhomogeneous universe is rephrased as a set of glued numerous homogeneous local patches. This is the essence of the gradient expansion and the $δN$ formalism, which have been widely used in solving a long wavelength evolution of the universe. In this paper, we show that the separate universe approach can be generically used, as long as a theory under consideration is local and preserves the spatial diffeomorphism invariance. Focusing on these two conditions, we also clarify the condition for the existence of the so-called Weinberg's adiabatic mode. Remarkably, the separate universe approach and the $δN$ formalism turn out to be applicable also to models with shear on large scales and also to modified theories of gravity, accepting violation of four-dimensional diffeomorphism invariance. The generalized $δN$ formalism enables us to calculate all the large scale fluctuations, including gravitational waves. We also argue several implications on anisotropic inflation and ultra slow-roll inflation.

astro-ph.CO↗

arXiv2020

Nano-Hz gravitational wave signature from axion dark matter

We calculate the accurate spectrum of the stochastic gravitational wave background from U(1) gauge fields produced by axion dark matter. The explosive production of gauge fields soon invalidates the applicability of the linear analysis and one needs nonlinear schemes. We make use of numerical lattice simulations to properly follow the nonlinear dynamics such as backreaction and rescattering which gives important contributions to the emission of gravitational waves. It turns out that the axion with the decay constant $f \sim 10^{16}$ GeV which gives the correct dark matter abundance predicts the circularly polarized gravitational wave signature detectable by SKA. We also show that the resulting gravitational wave spectrum has a potential to explain NANOGrav 12.5 year data.

astro-ph.CO↗

arXiv2020

Searching for axion-like particles under strong gravitational lenses

We establish strong gravitational lens systems as robust probes of axion-like particles (ALPs) -- a candidate for dark matter. A tiny interaction of photons with ALPs induces birefringence. Multiple images of gravitationally lensed polarised objects allow differential birefringence measurement, alleviating systematics and astrophysical dependencies. We apply this novel method to the lens system CLASS B1152+199 and constrain ALP-photon coupling $\le 9.2\times 10^{-11}\, {\rm GeV}^{-1} \textrm{ to } 7.7\times 10^{-8}\, {\rm GeV}^{-1}$ ($95\%$ C.L.) for ALP mass between $3.6\times 10^{-21} \,{\rm eV}$ and $4.6 \times 10^{-18} \,{\rm eV}$. A larger sample will improve the constraints.

astro-ph.CO↗

arXiv2020

Can axion clumps be formed in a pre-inflationary scenario?

The QCD axion and an axion-like particle (ALP) are compelling candidates of dark matter. For the QCD axion, it is known that when the Peccei-Quinn (PQ) symmetry is spontaneously broken after inflation, the large initial fluctuation can lead to axion clump formation. On the other hand, when the symmetry is already broken during inflation, it has been believed that the axion clump formation does not occur due to the small amplitude of the initial axion fluctuation. We revisit this prevailing understanding, considering both the QCD axion and an ALP. We find that for the QCD axion, the clump formation does not occur even if we consider an extremely fine-tuned initial condition. Meanwhile, it turns out that for an ALP which allows a more general potential form, the clump formation can take place through the tachyonic instability or/and the resonance instability, considering a multiple cosine potential.

astro-ph.CO↗

arXiv2020

Constraining the gravitational coupling of axion dark matter at LIGO

The axion-gravity Chern-Simons coupling is well motivated but is relatively weakly constrained, partly due to difficult measurements of gravity. We study the sensitivity of LIGO measurements of chirping gravitational waves (GWs) on such coupling. When the frequency of the propagating GW matches with that of the coherent oscillation of axion dark matter field, the decay of axions into gravitons can be stimulated, resonantly enhancing the GW. Such a resonance peak can be detected at LIGO as a deviation from the chirping waveform. Since all observed GWs will undergo similar resonant enhancement from the Milky-Way (MW) axion halo, LIGO O1+O2 observations can potentially provide the strongest constraint on the coupling, at least for the axion mass $m_a = 5 \times 10^{-13} - 5 \times 10^{-12}$ eV. Along the course, we also emphasize the relevance of the finite coherence of axion fields and the ansatz separating forward and backward propagations of GWs. As a result, the parity violation of the Chern-Simons coupling is not observable from chirping GWs.

hep-ph↗

arXiv2019

Resonant magnetogenesis from axions

We investigate the generation of seed magnetic field through the Chern-Simons coupling between the U(1) gauge field and an axion field that commences to oscillate at various epoch, depending on the mass scale. We address axions which begin oscillation during inflation, reheating, and also the radiation dominated era after the thermalization of the Universe. We study the resonant generation mechanisms and highlight that a small oscillation time scale with respect to that of the cosmic expansion can lead to an efficient generation of (hyper) magnetic field via resonant generation, even for ${\cal O}(1)$ coupling. In addition, we demonstrate that the generated field can be helical due to the tachyonic amplification phase prior to the onset of oscillation. Furthermore, it is shown that the parametric resonance during reheating can generate a circularly polarized (hyper) magnetic field in a void region with the present amplitude $B_0 =3\times 10^{-15}$Gauss and the coherent length $λ_0 = 0.3$pc without being plagued by the backreaction issue.

astro-ph.CO↗

arXiv2019

Efficient self-resonance instability from axions

It was recently shown that a coherent oscillation of an axion can cause an efficient parametric resonance, leading to a prominent emission of the gravitational waves (GWs). In this paper, conducting the Floquet analysis, we investigate the parametric resonance instability, which potentially triggers the emission of the GWs from axions. Such a resonance instability takes place, when the time evolution of the background field significantly deviates from the harmonic oscillation. Therefore, the resonance instability cannot be described by the Mathieu equation, whose stability/instability chart is well known. In this paper, introducing an explicitly calculable parameter $\tilde{q}$, which can be used to classify different types of the parametric resonance described by the general Hill's equation, we investigate the stability/instability chart for the general Hill's equation. This can also apply to the case where the background oscillation is anharmonic. We show that the flapping resonance instability, which takes place for $\tilde{q}=O(1)$, typically leads to the most significant growth of the inhomogeneous modes among the self-resonance instability. We also investigate whether the flapping resonance takes place for the cosine potential or not.

astro-ph.CO↗

arXiv2018

Cosmological imprints of string axions in plateau

We initiate a study on various cosmological imprints of string axions whose scalar potentials have plateau regions. In such cases, we show that a delayed onset of oscillation generically leads to a parametric resonance instability. In particular, for ultralight axions, the parametric resonance can enhance the power spectrum slightly below the Jeans scale, alleviating the tension with the Lyman $α$ forest observations. We also argue that a long-lasting resonance can lead to an emission of gravitational waves at the frequency bands which are detectable by gravitational wave interferometers and pulsar timing arrays.

astro-ph.CO↗

arXiv2018

Intrinsic galaxy alignment from angular dependent primordial non-Gaussianity

In this paper, we explore a detectable imprint of massive fields with integer spins $s \geq 2$, which may be predicted from string theory. It was shown that such a massive non-zero spin field can generate the squeezed primordial bispectrum which depends on the angle between the two wavenumbers. We show that considering the contribution from the massive spin-2 field, the angular dependent primordial non-Gaussianity (PNG) yields a strong scale dependence in the bias parameter for the galaxy alignment, which becomes prominent at small scales. As another example of an angular dependent PNG, we also consider the primordial bispectrum where the angular dependence was introduced by a vector field, while breaking the global rotational symmetry. As a consequence, we find that the B-mode cosmic shear and non-diagonal components do not vanish. These aspects provide qualitative differences from the PNG sourced by massive non-zero spin fields.

astro-ph.CO↗