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Misao Sasaki

Publications and source records attributed to Misao Sasaki.

At least 55 records · Page 3Linked to original sources
arXiv2024

Testing Gravity with Frequency-Dependent Overlap Reduction Function in Pulsar Timing Array

The positive evidence of a nano-hertz gravitational wave background recently found by several pulsar timing array (PTA) collaborations opened up a window to test modified gravity theories in a unique frequency band in parallel to other gravitational wave detection experiments. In particular, the overlap reduction function (ORF) in PTA observation is sensitive to the phase velocity of gravitational waves. In this work, we provide analytical expressions for the coefficients of the multipole moments in the ORF, and utilize these analytical results to study constraints on the phase velocity from the frequency dependent overlap reduction function obtained from the Chinese PTA (CPTA) data. While the data contain large error bars yet, interesting constraints are found in the frequency-dependent ORF in the case of subluminal phase velocity. This makes us expect that the nano-hertz band gravitational wave background will become one of the important arenas for exploring modified gravity theories.

astro-ph.CO↗
arXiv2024

Probing Primordial Black Hole Scenarios with Terrestrial Gravitational Wave Detectors

It is possible that primordial black holes consitute (or consituted) a significant fraction of the energy budget of our universe. Terrestrial gravitational wave detectors offer the opportunity to test the existence of primordial black holes in two different mass ranges, from $10^2\,{\rm g}-10^{16}\,{\rm g}$ to $10^{-6}\,M_\odot-100 \,M_\odot$. The first mass window is open via induced gravitational waves and the second one by gravitational waves from binary mergers. In this review, we outline and explain the different gravitational wave signatures of primordial black holes that may be probed by terrestrial gravitational wave detectors, such as the current LIGO/Virgo/KAGRA and future ones like Einstein Telescope and Cosmic Explorer. We provide rough estimates for the frequency and amplitude of the associated GW background signals. We also discuss complementary probes for these primordial black hole mass ranges.

gr-qc↗
arXiv2023

Universal Gravitational Waves from Interacting and Clustered Solitons

Causal soliton formation (e.g. oscillons, Q-balls) in the primordial Universe is expected to give rise to a universal gravitational wave (GW) background, at frequencies smaller than scales of nonlinearity. We show that modifications of the soliton density field, driven by soliton interactions or initial conditions, can significantly enhance universal GWs. Gravitational clustering of solitons naturally leads to generation of correlations in the large-scale soliton density field. As we demonstrate for axion-like particle (ALP) oscillons, the growing power spectrum amplifies universal GW signals, opening new avenues for probing the physics of the early Universe with upcoming GW experiments. Our results are applicable to variety of scenarios, such as solitons interacting through a long range Yukawa-like fifth force.

astro-ph.CO↗
arXiv2023

Primordial Black Hole Formation in Non-Minimal Curvaton Scenario

In the curvaton scenario, the curvature perturbation is generated after inflation at the curvaton decay, which may have a prominent non-Gaussian effect. For a model with a non-trivial kinetic term, an enhanced curvature perturbation on a small scale can be realized, which can lead to copious production of primordial black holes (PBHs) and induce secondary gravitational waves (GWs). Using the probability distribution function (PDF) which takes full nonlinear effects into account, we calculate the PBH formation. We find that under the assumption that thus formed PBHs would not overclose the universe, the non-Gaussianity of the curvature perturbation can be well approximated by the local quadratic form, which can be used to calculate the induced GWs. In this model the limit of large non-Gaussianity can be reached when the curvaton energy fraction $r$ is small at the moment of curvaton decay. We also show that in the $r\to1$ limit the PDF is similar to that of ultraslow-roll inflation.

astro-ph.CO↗
arXiv2023

Highly asymmetric probability distribution from a finite-width upward step during inflation

We study a single-field inflation model in which the inflaton potential has an upward step between two slow-roll regimes by taking into account the finite width of the step. We calculate the probability distribution function (PDF) of the curvature perturbation $P[{\cal{R}}]$ using the $δN$ formalism. The PDF has an exponential-tail only for positive ${\cal{R}}$ whose slope depends on the step width. We find that the tail may have a significant impact on the estimation of the primordial black hole abundance. We also show that the PDF $P[{\cal{R}}]$ becomes highly asymmetric on a particular scale exiting the horizon before the step, at which the curvature power spectrum has a dip. This asymmetric PDF may leave an interesting signature in the large scale structure such as voids.

astro-ph.CO↗
arXiv2023

Yang-Mills instantons as the end point of black hole evaporation

Non-perturbative contributions of the Euclidean path integral are important to understand the information loss paradox. In this paper, we revisit the Yang-Mills instantons in the Einstein-Yang-Mills theory. There exists a globally regular solution that is known as the Bartnik-McKinnon solution and a black hole solution. The regular and the black hole solutions are smoothly connected in the small horizon limit. Their Euclidean action is solely characterized by the ADM mass, and the transition probability follows the usual Bekenstein-Hawking entropy formula. Therefore, the Yang-Mills instantons provide a non-perturbative channel to the black hole evaporation, which competes effectively with perturbative processes, and becomes dominant toward the end of evaporation. We show that these instantons provide a smooth transition mechanism from a black hole to regular spacetime.

gr-qc↗
arXiv2023

Primordial-tensor-induced stochastic gravitational waves

Cosmological stochastic gravitational waves (GWs) induced by a spectator field are usually expected to have an amplitude very small compared with those generated by the curvature perturbation, or equivalently by a field dominating the universe. On the contrary to this expectation, we show that a spectator field that provides a tensor perturbation, on top of the metric tensor perturbation, can generate a significant amount of GWs. The amplitude and frequency of the generated GWs may lie within the sensitivity range of future GW detectors. In particular, if the sound velocities of the two tensor perturbations coincide, the induced GW amplitude may become very large due to resonance by forced oscillation, even in the limit of small coupling between them. A distinct feature of this scenario is that, since tensor modes can hardly lead to the formation of primordial black holes (PBHs), we expect no presence of PBHs, in contrast to the usual scalar-induced case, in which the detection of strong enough induced GWs suggests the existence of PBHs.

gr-qc↗
arXiv2023

Extra-tensor-induced origin for the PTA signal: No primordial black hole production

Recently, pulsar timing array (PTA) collaborations announced evidence for an isotropic stochastic gravitational wave (GW) background. The origin of the PTA signal can be astrophysical or cosmological. In the latter case, the so-called secondary scalar-induced GW scenario is one of the viable explanations, but it has a potentially serious issue of the overproduction of primordial black holes (PBHs) due to the enhanced curvature perturbation. In this letter, we present a new interpretation of the PTA signal. Namely, it is originated from an extra spectator tensor field that exists on top of the metric tensor perturbation. As the energy density of the extra tensor field is always subdominant, it cannot lead to the formation of PBHs. Thus our primordial-tensor-induced scenario is free from the PBH overproduction issue.

astro-ph.CO↗
arXiv2023

Hamiltonian approach to 2nd order gauge invariant cosmological perturbations

In view of growing interest in tensor modes and their possible detection, we clarify the definition of tensor modes up to 2nd order in perturbation theory within the Hamiltonian formalism. Like in gauge theory, in cosmology the Hamiltonian is a suitable and consistent approach to reduce the gauge degrees of freedom. In this paper we employ the Faddeev-Jackiw method of Hamiltonian reduction. An appropriate set of gauge invariant variables that describe the dynamical degrees of freedom may be obtained by suitable canonical transformations in the phase space. We derive a set of gauge invariant variables up to 2nd order in perturbation expansion and for the first time we reduce the 3rd order action without adding gauge fixing terms. In particular, we are able to show the relation between the uniform-$ϕ$ and Newtonian slicings, and study the difference in the definition of tensor modes in these two slicings.

gr-qc↗
arXiv2023

Tunneling between multiple histories as a solution to the information loss paradox

The information loss paradox associated with black hole Hawking evaporation is an unresolved problem in modern theoretical physics. In a recent brief essay, we revisited the the evolution of the black hole entanglement entropy via the Euclidean path integral (EPI) of the quantum state and allow for the branching of semi-classical histories along the Lorentzian evolution. We posited that there exist at least two histories that contribute to EPI, where one is an information-losing history while the other is information-preserving. At early times, the former dominates EPI, while at late times the latter becomes dominant. By so doing we recovered the essence of the Page curve and thus the unitarity, albeit with the turning point, i.e., the Page time, much shifted toward the late time. In this full-length paper, we fill in the details of our arguments and calculations to strengthen our notion. One implication of this modified Page curve is that the entropy bound may thus be violated. We comment on the similarity and difference between our approach and that of the replica wormholes and the islands conjectures.

gr-qc↗
arXiv2023

Gravitational waves from rapid structure formation on microscopic scales before matter-radiation equality

The existence of scalar fields can be probed by observations of stochastic gravitational waves. Scalar fields mediate attractive forces, usually stronger than gravity, on the length scales shorter than their Compton wavelengths, which can be non-negligible in the early Universe, when the horizon size is small. These attractive forces exhibit an instability similar to the gravitational instability, only stronger. They can, therefore, lead to the growth of structures in some species. We identify a gravitational waves signature of such processes and show that it can be detected by future gravitational waves experiments.

astro-ph.CO↗
arXiv2023

Logarithmic Duality of the Curvature Perturbation

We study the comoving curvature perturbation $\mathcal{R}$ in the single-field inflation models whose potential can be approximated by a piecewise quadratic potential $V(φ)$ by using the $δN$ formalism. We find a general formula for $\mathcal{R}(δφ, δπ)$, consisting of a sum of logarithmic functions of the field perturbation $δφ$ and the velocity perturbation $δπ$ at the point of interest, as well as of $δπ_*$ at the boundaries of each quadratic piece, which are functions of ($δφ, δπ$) through the equation of motion. Each logarithmic expression has an equivalent dual expression, due to the second-order nature of the equation of motion for $φ$. We also clarify the condition under which $\mathcal{R}(δφ, δπ)$ reduces to a single logarithm, which yields either the renowned ``exponential tail'' of the probability distribution function of $\mathcal{R}$ or a Gumbel-distribution-like tail.

astro-ph.CO↗
arXiv2023

New shape of parity-violating graviton non-Gaussianity

We show that the general vacuum states that respect the de Sitter symmetry, known as the α-vacua, can introduce non-vanishing parity-violating tensor non-Gaussianities. This is due to the mixing by the Bogoliubov transformation of the positive and negative frequency modes of the Bunch-Davies vacuum. We calculate explicitly the bispectra of tensor perturbations and show that the amplitude can be exponentially enhanced for certain choices of the squeezing parameter α and the phase ϕ of the α-vacua. We find a new shape for the parity-violating tensor bispectrum which peaks in the flattened configuration

hep-th↗
arXiv2023

Super-horizon resonant magnetogenesis during inflation

We propose a novel mechanism for significantly enhancing the amplitude of primordial electromagnetic fields during inflation. Similar to existing proposals, our idea is based on parametric resonance effects due to conformal-symmetry-breaking coupling of a gauge field and the inflaton. Our proposed scenario, however, significantly differs from previously studied models, and avoids their shortcomings. We, particularly, construct a viable system where the gauge field is exponentially amplified on super-horizon scales, therefore evading the no-go theorem formulated on the basis of widely encountered drastic back-reaction of the magnetic field energy on the inflationary background. We argue that in order for the resonant scenario to work with a bounded and positive-definite coupling function, a mass term for the gauge sector is required. We compute the spectrum of the produced magnetic fields and demonstrate the compatibility with current observational constraints. We demonstrate that while the magnetic fields do not noticeably back-react on the inflationary background, the non-zero mass term can contribute significantly to the total energy-momentum tensor. We point out the parameter space where the latter issue is absent.

astro-ph.CO↗
arXiv2023

Gravitational wave hints black hole remnants as dark matter

The end state of Hawking evaporation of a black hole is uncertain. Some candidate quantum gravity theories, such as loop quantum gravity and asymptotic safe gravity, hint towards Planck sized remnants. If so, the Universe might be filled with remnants of tiny primordial black holes, which formed with mass $M<10^9\,{\rm g}$. A unique scenario is the case of $M\sim 5\times10^5\,{\rm g}$, where tiny primordial black holes reheat the universe by Hawking evaporation and their remnants dominate the dark matter. Here, we point out that this scenario leads to a cosmological gravitational wave signal at frequencies $\sim 100{\rm Hz}$. Finding such a particular gravitational wave signature with, e.g. the Einstein Telescope, would suggest black hole remnants as dark matter.

gr-qc↗
arXiv2023

Threshold of Primordial Black Hole Formation against Velocity Dispersion in Matter-Dominated Era

We study the effects of velocity dispersion on the formation of primordial black holes~(PBHs) in a matter-dominated era. The velocity dispersion is generated through the nonlinear growth of perturbations and has the potential to impede the gravitational collapse and thereby the formation of PBHs. To make discussions clear, we consider two distinct length scales. The larger one is where gravitational collapse occurs which could lead to PBH formation, and the smaller one is where the velocity dispersion develops due to nonlinear interactions. We estimate the effect of the velocity dispersion on the PBH formation by comparing the free-fall timescale and the timescale for a particle to cross the collapsing region. As a demonstration, we consider a log-normal power spectrum for the initial density perturbation with the peak value $σ_0^2$ at a scale that corresponds to the larger scale. We find that the threshold value of the density perturbation $\tilde δ_{\rm th}$ at the horizon entry for the PBH formation scales as $\tilde δ_{\rm th}\propto σ_0^{2/5}$ for $σ_0\ll1$.

astro-ph.CO↗
arXiv2022

Scale-invariant enhancement of gravitational waves during inflation

The inflationary 1-loop tensor power spectrum from an excited spectator scalar field is calculated. Recent studies on primordial black holes suggest that the inflationary curvature perturbation may be huge on small scales. An enhanced curvature perturbation may arise from a drastic enhancement of spectator scalar field fluctuations. In this letter, using the in-in formalism, we calculate 1-loop quantum corrections to primordial gravitational waves by such an excited spectator field with a sharp peak in momentum space. We find scale-invariant loop corrections in this full quantum setup, in contrast to the sharply peaked corrections in the previously calculated scalar-induced tensor modes. Especially, on super Hubble scales, the primordial gravitational waves are also amplified, which can be understood as a Bogolyubov transformation of the vacuum due to the excited scalar field. This mechanism allows us to probe the scalar field properties on extremely short-distance scales with the current and future cosmic microwave background and gravitational wave experiments, opening a novel window for inflationary cosmology.

astro-ph.CO↗