arXiv subjects

Keisuke Inomata

Publications and source records attributed to Keisuke Inomata.

At least 19 recordsLinked to original sources

arXiv2026

Cancellation of one-loop time dependence in superhorizon curvature perturbations from all scales

We show the conservation of the superhorizon curvature perturbations at one-loop level in spatially-flat gauge, including contributions from loop wavenumbers on all scales. In contrast to previous works, we do not assume a hierarchy $k \gg q$ between the wavenumber of the loop integral, $k$, and that of the power spectrum, $q$, and we explicitly include the regime $k \lesssim q$. Taking into account the nonlinear relation between the inflaton fluctuation and the curvature perturbation with the $\delta N$ formalism, we show that the apparent time dependence of the one-loop curvature power spectrum cancels once all contributions, including boundary terms, are combined consistently.

astro-ph.CO↗

arXiv2026

Note on pulsar timing array correlation functions induced by peculiar velocities

Several papers have recently calculated the contribution to pulsar timing array overlap reduction functions (ORFs) induced by our peculiar velocity with respect to the rest frame of the stochastic gravitational-wave background. Here we show that a harmonic-space calculation confirms the most recent result. We note that, with the harmonic-space calculation, the ORFs for spin-1 GWs and the correlations with astrometry measurements are also easily obtained.

astro-ph.CO↗

arXiv2025

Adding electromagnetic birefringence to pulsar timing and astrometry to detect gravitational waves

It was recently shown that the time variation of the polarization of electromagnetic waves from pulsars can be used, in cross-correlation with pulsar timing, to probe the chirality of an isotropic gravitational wave background. Here, we show that the expression for the cross-correlation is derived efficiently with the total-angular-momentum formalism and use this framework to extend the formulation to cross-correlation with astrometry. We do so for spin-1 gravitational waves (that may arise in alternative-gravity theories) as well as the general-relativistic spin-2 gravitational waves.

astro-ph.CO↗

arXiv2025

The poltergeist mechanism -- Enhancement of scalar-induced gravitational waves with early matter-dominated era

Gravitational waves induced by primordial density perturbations provide a powerful probe of the Universe's thermal history, which may include an early matter-dominated (eMD) era predicted by well-motivated particle-physics models. The induced GWs can be significantly enhanced when the Universe undergoes a sudden transition from an eMD era to an era with pressure, such as a radiation or kination era. This enhancement arises from the growth of density perturbations during the eMD era and their rapid oscillations during the era with pressure. This phenomenon is called the poltergeist mechanism. In this review, we explain the essence of the poltergeist mechanism and explore concrete scenarios in which such an enhancement can occur.

astro-ph.CO↗

arXiv2025

Anisotropic Gravitational Waves from Anisotropic Axion Rotation

Gravitational waves (GWs) provide a powerful probe of the early universe due to their ability to free-stream across cosmic history. We study GW production in a compelling scenario where a rotating axion(-like) field becomes relevant for a brief period in the early universe before transitioning into a kination fluid and rapidly dissipating its energy through cosmic expansion. During this short epoch, the curvature perturbation can be predominantly sourced by the rotating axion and may significantly exceed the adiabatic component. Moreover, axion field perturbations grow on superhorizon scales during this phase. These effects can generate a strong stochastic background of induced GWs. This GW background also exhibits a pronounced large-scale anisotropy inherited from the axion fluctuations, serving as a distinctive signature of the scenario. Importantly, the transient nature of axion relevance enables this scenario to evade stringent bounds on large-scale perturbations. We analyze various observational constraints and find that both the amplitude and anisotropy of the resulting GW signal could be accessible to future detectors.

hep-ph↗

arXiv2025

Role of the counterterms in the conservation of superhorizon curvature perturbations at one loop

Recently, several papers have claimed that superhorizon curvature perturbations are not conserved at the one-loop level in single-field inflation models if there is a transient ultra-slow-roll period. In this work, we point out that the contributions from the counterterms were overlooked in the recent papers. We show that the counterterm contributions play a crucial role in canceling the one-loop power spectrum of superhorizon curvature perturbations in the comoving gauge.

astro-ph.CO↗

arXiv2025

Conservation of superhorizon curvature perturbations at one loop: Backreaction in the in-in formalism and Renormalization

We show that the superhorizon-limit curvature perturbations are conserved at one-loop level in single-field inflation models with a transient non-slow-roll period. We take the spatially-flat gauge, where the backreaction plays a crucial role for the conservation of superhorizon curvature perturbations unless the counter terms are tuned. We calculate the backreaction with the in-in formalism. In addition, we explicitly show the renormalization of the UV divergences with the counter terms.

astro-ph.CO↗

arXiv2024

Gravitational Waves from Particles Produced from Bubble Collisions in First-Order Phase Transitions

We discuss a new source of gravitational waves (GWs) from first-order phase transitions. The collisions of bubbles of the new phase can efficiently produce particles that couple to the background field undergoing the transition, thereby transferring a significant fraction of the released vacuum energy into a distribution of inhomogeneous and dynamic particle populations that persist long after the bubbles have disappeared. We study the GWs produced by such particle distributions, showing that GWs arise from the quadrupolar anisotropy in the radiation emitted from the bubble collisions, and present a semi-analytical calculation of the two-point correlation function for the associated energy distributions. We find that this new contribution can qualitatively modify the overall GW signal from such phase transitions, creating a distinct shift in the spectral slope at low frequencies that could be observed by future GW experiments. It is therefore important to take this new contribution into account for any transition where the background field has significant self-coupling or couplings to other fields that could lead to efficient particle production at bubble collision.

astro-ph.CO↗

arXiv2024

Constraints on the Sharpness of the Curvature Power Spectrum

Motivated by the fact that a sharply peaked curvature spectrum is often considered in the literature, we examine theoretical constraints on the sharpness of such a spectrum. In particular, we show that the sharply peaked curvature power spectrum, originating from the enhancement of subhorizon perturbations during inflation, is significantly constrained by energy conservation. While the constraints do not depend on the exact form of inflaton potential, we also study concrete inflaton potentials that realize a sharply peaked curvature spectrum and how theoretical limits are saturated in these cases.

astro-ph.CO↗

arXiv2024

Parity-breaking galaxy 4-point function from lensing by chiral gravitational waves

Recent searches for parity breaking in the galaxy four-point correlation function, as well as the prospects for greatly improved sensitivity to parity breaking in forthcoming surveys, motivate the search for physical mechanisms that could produce such a signal. Here we show that a parity-violating galaxy four-point correlation function may be induced by lensing by a chiral gravitational-wave background. We estimate the amplitude of a signal that would be detectable with a current galaxy survey, taking into account constraints to the primordial gravitational-wave-background amplitude. We find that this mechanism is unlikely to produce a signal large enough to be seen with a galaxy survey but note that it may come within reach with future 21cm observations.

astro-ph.CO↗

arXiv2024

Overlap reduction functions for pulsar timing arrays and astrometry

We present an efficient technique for calculating the angular two-point correlation functions (or ``overlap reduction functions'') induced by gravitational waves in both the pulse arrival times of pulsars and in the angular deflections of distant sources. In the most general case, there are six auto- and cross-correlations for the pulse arrival times and the two components of the angular deflection. We provide results for spin-2 (i.e., general-relativistic) gravitational waves as well as the spin-1 modes that may arise in alternative-gravity theories. These calculations can be easily implemented for future analysis or study, and we provide code to do so.

astro-ph.CO↗

arXiv2024

Inflationary Butterfly Effect: Non-perturbative Dynamics From Small-Scale Features

For the first time, we investigate the non-perturbative dynamics of single field inflation with a departure from slow-roll. Using simulations, we find that oscillatory features in the potential can drastically alter the course of inflation, with major phenomenological implications. In certain cases, the entire Universe gets trapped in a forever inflating de Sitter state. In others, only some regions get stuck in a false vacuum, offering an alternative channel for primordial black hole formation. Analogous to the flap of a butterfly, these results show that small-scale phenomena can have profound consequences on the evolution of the entire Universe. This demonstrates the necessity of a non-perturbative approach in the exploration of the small-scale physics of inflation, particularly in the regime relevant for gravitational-wave astronomy.

astro-ph.CO↗

arXiv2024

Superhorizon Curvature Perturbations Are Protected against One-Loop Corrections

We examine one-loop corrections from small-scale curvature perturbations to the superhorizon-limit ones in single-field inflation models, which have recently caused controversy. We consider the case where the Universe experiences transitions of slow-roll (SR) $\to$ intermediate period $\to$ SR. The intermediate period can be an ultra-slow-roll period or a resonant amplification period, either of which enhances small-scale curvature perturbations. We assume that the superhorizon curvature perturbations are conserved at least during each of the SR periods. Within this framework, we show that the superhorizon curvature perturbations during the first and the second SR periods coincide at one-loop level in the slow-roll limit.

astro-ph.CO↗

arXiv2023

Axion Curvaton Model for the Gravitational Waves Observed by Pulsar Timing Arrays

The stochastic gravitational wave background (SGWB) recently detected by the PTA collaborations could be the gravitational waves (GWs) induced by curvature perturbations. However, primordial black holes (PBHs) might be overproduced if the SGWB is explained by the GWs induced by the curvature perturbations that follow the Gaussian distribution. This motivates models associated with the non-Gaussianity of the curvature perturbations that suppress the PBH production rate. In this work, we show that the axion curvaton model can produce the curvature perturbations that induce GWs for the detected SGWB while preventing the PBH overproduction with the non-Gaussianity.

astro-ph.CO↗

arXiv2023

Induced Gravitational Waves with Kination Era for Recent Pulsar Timing Array Signals

The evidence of the stochastic gravitational-wave background around the nano-hertz frequency range was recently found by worldwide pulsar timing array (PTA) collaborations. One of the cosmological explanations is the gravitational waves induced by enhanced curvature perturbations, but the issue of primordial black hole (PBH) overproduction in this scenario was pointed out in the literature. Motivated by this issue and the $\Omega_\text{GW} \sim f^2$ scaling suggested by the data, we study the gravitational waves induced in a cosmological epoch dominated by a stiff fluid ($w=1$) and find that they can safely explain the PTA data well without PBH overproduction.

astro-ph.CO↗

arXiv2023

The Detected Stochastic Gravitational Waves and Subsolar-Mass Primordial Black Holes

Multiple pulsar timing array (PTA) collaborations recently announced the evidence of common-spectral processes caused by gravitational waves (GWs). These can be the stochastic GW background and its origin may be astrophysical and/or cosmological. We interpret it as the GWs induced by the primordial curvature perturbations and discuss their implications on primordial black holes (PBHs). We show that the newly released data suggest PBHs much lighter than the Sun ($\mathcal{O}(10^{-4}) \, M_\odot$ for the delta-function curvature spectrum; $< \mathcal{O}(10^{-2})\, M_\odot$ more generally) in contrast to what was expected from the previous PTA data releases.

astro-ph.CO↗

arXiv2023

Axion Poltergeist

Rotations of axion fields in the early universe can produce dark matter and the matter-antimatter asymmetry of the universe. We point out that the rotation can generate an observable amount of a stochastic gravitational-wave (GW) background. It can be doubly enhanced in a class of models in which the equation of state of the rotations rapidly changes from a non-relativistic matter-like one to a kination-like one by 1) the so-called poltergeist mechanism and 2) slower redshift of GWs compared to the axion kination fluid. In supersymmetric UV completion, future GW observations can probe the supersymmetry-breaking scale up to $10^7\,$GeV even if the axion does not directly couple to the Standard Model fields.

hep-ph↗

arXiv2023

Synchronizing the Consistency Relation

We study the $N$-point function of the density contrast to quadratic order in the squeezed limit during the matter-dominated (MD) and radiation-dominated (RD) eras in synchronous gauge. Since synchronous gauge follows the free-fall frame of observers, the equivalence principle dictates that in the gradient approximation for the long-wavelength mode there is only a single, manifestly time-independent consistency relation for the $N$-point function. This simple form is dictated by the initial mapping between synchronous and local coordinates, unlike Newtonian gauge and its correspondingly separate dilation and Newtonian consistency relations. Dynamical effects only appear at quadratic order in the squeezed limit and are again characterized by a change in the local background, also known as the separate universe approach. We show that for the 3-point function the compatibility between these squeezed-limit relations and second-order perturbation theory requires both the initial and dynamical contributions to match, as they do in single-field inflation. This clarifies the role of evolution or late-time projection effects in establishing the consistency relation for observable bispectra, which is especially important for radiation acoustic oscillations and for establishing consistency below the matter-radiation equality scale in the MD era. Defining an appropriate angle and time average of these oscillations is also important for making separate universe predictions of spatially varying local observables during the RD era, which can be useful for a wider range of cosmological predictions beyond $N$-point functions.

astro-ph.CO↗