arXiv subjects

Misao Sasaki

Publications and source records attributed to Misao Sasaki.

At least 73 records · Page 4Linked to original sources

arXiv2022

Highly non-Gaussian tails and primordial black holes from single-field inflation

For primordial perturbations, deviations from Gaussian statistics on the tail of the probability distribution can be associated with non-perturbative effects of inflation. In this paper, we present some particular examples in which the tail of the distribution becomes highly non-Gaussian although the statistics remains almost Gaussian in the perturbative regime. We begin with an extension of the ultra-slow-roll inflation that incorporates a transition process, where the inflaton climbs up a tiny potential step at the end of the non-attractor stage before it converges to the slow-roll attractor. Through this example, we identify the key role of the off-attractor behaviour for the upward-step transition, and then extend the analysis to another type of the transition with two slow-roll stages connected by a tiny step. We perform both the perturbative and non-perturbative analyses of primordial fluctuations generated around the step in detail, and show that the tiny but nontrivial transition may affect large perturbations in the tail of the distribution, while the perturbative non-Gaussianity remains small. Our result indicates that the non-Gaussian tails can have rich phenomenology which has been overlooked in conventional analyses. We also study the implications of this non-Gaussian tail for the formation of primordial black holes, and find that their mass fraction can be parametrically amplified by several orders of magnitudes in comparison with the case of the Gaussian distribution. Additionally, we also discuss a mechanism of primordial black holes formation for this upward step inflation model by trapping the inflaton in the bottom of the step.

astro-ph.CO↗

arXiv2022

One-loop tensor power spectrum from an excited scalar field during inflation

We present a consistent one-loop calculation for the inflationary tensor power spectrum in the presence of an excited spectator scalar field using the in-in formalism. We find that the super-horizon primordial power spectrum of the tensor mode can be scale-invariantly enhanced or reduced by the loop effects of a subhorizon scalar field. Our calculation also includes the scalar-induced gravitational wave spectrum classically computed in the previous literature, which is significant only near the scales where the scalar field is amplified. The super-horizon enhancement is a higher-order effect of the interaction Hamiltonian, which can be understood as a Bogoliubov transformation introduced by nonlinear interactions. On the other hand, the scale-invariant reduction of the tensor power spectrum may occur due to the fourth-order scalar-scalar-tensor-tensor coupling. This phenomenon can be understood as the evolution of an anisotropic Bianchi type-I background in the separate universe approach. Our result suggests that large-scale measurements may indirectly test the dramatic effects of small-scale cosmological perturbations through loop corrections. This possibility opens a new ground in probing the small-scale physics of the primordial Universe through gravitational wave detectors of cosmological scales.

astro-ph.CO↗

arXiv2022

Non-Gaussianity effects on the primordial black hole abundance for sharply-peaked primordial spectrum

We perturbatively study the effect of non-Gaussianities on the mass fraction of primordial black holes (PBHs) at the time of formation by systematically taking its effect into account in the one-point probability distribution function of the primordial curvature perturbation. We focus on the bispectrum and trispectrum and derive formulas that describe their effects on the skewness and kurtosis of the distribution function. Then considering the case of narrowly peaked spectra, we obtain simple formulas that concisely express the effect of the bi- and trispectra. In particular, together with the $g_{\rm NL}$ and $τ_{\rm NL}$ parameters of the trispectrum, we find that non-Gaussianity parameters for various types of the bispectrum are linearly combined to give an effective parameter, $f_{\rm NL}^{\rm eff}$, that determines the PBH mass fraction in the narrow spectral shape limit.

astro-ph.CO↗

arXiv2022

One Small Step for an Inflaton, One Giant Leap for Inflation: a novel non-Gaussian tail and primordial black holes

We report a novel prediction from single-field inflation that even a tiny step in the inflaton potential can change our perception of primordial non-Gaussianities of the curvature perturbation. Our analysis focuses on the tail of probability distribution generated by an upward step transition between two stages of slow-roll evolution. The nontrivial background dynamics with off-attractor behavior is identified. By using a non-perturbative $δN$ analysis, we explicitly show that a highly non-Gaussian tail can be generated by a tiny upward step, even when the conventional nonlinearity parameters $f_{NL}$, $g_{NL}$, etc. remain small. With this example, we demonstrate for the first time the sensitive dependence of non-perturbative effects on the tail of probability distribution. Our scenario has an inconceivable application to primordial black holes by either significantly boosting their abundance or completely forbidding their appearance.

astro-ph.CO↗

arXiv2022

Primordial black hole formation from massless scalar isocurvature

We numerically study the primordial black hole (PBH) formation by an isocurvature perturbation of a massless scalar field on super Hubble scales in the radiation-dominated universe. As a first step we perform simulations of spherically symmetric configurations. For the initial condition, we employ the spatial gradient expansion and provide the general form of the growing mode solutions valid up through the second order in this expansion. The initial scalar field profile is assumed to be Gaussian with a characteristic comoving wavenumber $k$; $\sim\exp(-k^2R^2)$, where $R$ is the radial coordinate. We find that a PBH is formed for a sufficiently large amplitude of the scalar field profile. Nevertheless, we find that the late time behavior of the gravitational collapse is dominated by the dynamics of the fluid but not by the scalar field, which is analogous to the PBH formation from an adiabatic perturbation in the radiation-dominated universe.

gr-qc↗

arXiv2022

Graviton non-gaussianity in $α$-vacuum

We compute the leading order non-Gaussianity, i.e., the bispectrum, of the tensor perturbation in the general $α$-vacuum on de Sitter space in general relativity. In addition to the well-known Bunch-Davies (BD) vacuum, there exits an infinite number of de Sitter invariant vacua represented by a real parameter $α$ and a phase $ϕ$, with $α=0$ being the BD vacuum. They are called $α$-vacua. In the standard slow-roll inflation, as de Sitter invariance no longer applies, the $α$-vacua lose its relevance in the rigorous sense. Nevertheless, if we assume that the parameter $α$ is only weakly dependent on the wavenumber with an appropriate UV cutoff, we may consider pseudo-$α$-vacua. In the case of false vacuum inflation where the background spacetime is pure de Sitter, a non-trivial (non-BD) $α$-vacuum could indeed be realized. We find an intriguing result that the bispectrum may be exponentially enhanced to be detectable by observation even if the spectrum is too small to be detected.

hep-th↗

arXiv2022

Primordial Black Holes from CDM Isocurvature Perturbations

We show that primordial black holes can be produced from the collapse of large isocurvature perturbations of the cold dark matter. We develop a novel procedure to compute the resulting black hole abundance by studying matched perturbations of matter-only universes, and we use our procedure to translate observational constraints on black hole abundances into model-independent constraints on cold dark matter isocurvature perturbations over a wide range of scales. The constraint on the typical amplitude of the primordial perturbations weakens slightly slower than linearly on small scales.

astro-ph.CO↗

arXiv2022

Establishing the Non-Primordial Origin of Black Hole-Neutron Star Mergers

Primordial black holes (PBHs) from the early Universe constitute an attractive dark matter candidate. First detections of black hole-neutron star (BH-NS) candidate gravitational wave events by the LIGO/Virgo collaboration, GW200105 and GW200115, already prompted speculations about non-astrophysical origin. We analyze, for the first time, the total volumetric merger rates of PBH-NS binaries formed via two-body gravitational scattering, finding them to be subdominant to the astrophysical BH-NS rates. In contrast to binary black holes, a significant fraction of which can be of primordial origin, either formed in dark matter halos or in the early Universe, PBH-NS rates cannot be significantly enhanced by contributions preceding star formation. Our findings imply that the identified BH-NS events are of astrophysical origin, even when PBH-PBH events significantly contribute to the GW observations.

astro-ph.CO↗

arXiv2022

Resolving information loss paradox with Euclidean path integral

The information loss paradox remains unresolved ever since Hawking's seminal discovery of black hole evaporation. In this essay, we revisit the entanglement entropy via Euclidean path integral (EPI) and allow for the branching of semi-classical histories during the Lorentzian evolution. We posit that there exist two histories that contribute to EPI, where one is information-losing that dominates at early times, while the other is information-preserving that dominates at late times. By so doing we recover the Page curve and preserve the unitarity, albeit with the Page time shifted significantly towards the late time. One implication is that the entropy bound may thus be violated. We compare our approach with string-based islands and replica wormholes concepts.

gr-qc↗

arXiv2021

Beating the Lyth bound by parametric resonance during inflation

We propose a novel mechanism for enhancing the primordial gravitational waves without significantly affecting the curvature perturbations produced during inflation. This is achieved due to non-linear sourcing of resonantly amplified scalar field fluctuations. Our result is an explicit scale-dependent counter-example of the famous Lyth bound, which opens up a promising perspective of producing detectable inflationary tensor modes with low-scale inflation and a sub-Planckian field excursion. We explicitly demonstrate the testability of our mechanism with upcoming Cosmic Microwave Background B-mode observations.

astro-ph.CO↗

arXiv2021

Exploring Evaporating Primordial Black Holes with Gravitational Waves

Primordial black holes (PBHs) from the early Universe have been connected with the nature of dark matter and can significantly affect cosmological history. We show that coincidence dark radiation and density fluctuation gravitational wave signatures associated with evaporation of $\lesssim10^9$ g PBHs can be used to explore and obtain important hints about the formation mechanisms of spinning and non-spinning PBHs spanning orders of magnitude in mass-range, which is challenging to do otherwise.

astro-ph.CO↗

arXiv2021

Multi-field dark energy: cosmic acceleration on a steep potential

We argue that dark energy with multiple fields is theoretically well-motivated and predicts distinct observational signatures, in particular when cosmic acceleration takes place along a trajectory that is highly non-geodesic in field space. Such models provide novel physics compared to $Λ$CDM and quintessence by allowing cosmic acceleration on steep potentials. From the theoretical point of view, these theories can easily satisfy the conjectured swampland constraints and may in certain cases be technically natural, potential problems which are endemic to standard single-field dark energy. Observationally, we argue that while such multi-field models are likely to be largely indistinguishable from the concordance cosmology at the background level, dark energy perturbations can cluster, leading to an enhanced growth of large-scale structure that may be testable as early as the next generation of cosmological surveys.

astro-ph.CO↗

arXiv2021

Lost in the Log-Polynomial Expansion: Comment on arXiv:2101.08278

In [1] we highlighted the fact that the log polynomial expansion employed in Nature Astron. 3, no.3, 272-277 (2019) [2] is a poor approximation to flat $Λ$CDM, so using it to infer deviations from flat $Λ$CDM is not well-motivated. The "orthogonalized logarithmic polynomials" recently presented in arXiv:2101.08278 [3] are an attempt to respond to the earlier criticism [1]. Here we demonstrate that this new technique [3] - interesting though it may be - fails to address the fundamental problem raised in [1]. Unfortunately, the claim made in [2] may still be lost in the expansion.

astro-ph.CO↗

arXiv2021

On Problems with Cosmography in Cosmic Dark Ages

Quasars show considerable promise as standard candles in a high-redshift window beyond Type Ia supernovae. Recently, Risaliti, Lusso \& collaborators \cite{Risaliti:2018reu, Lusso:2019akb, Lusso:2020pdb} have succeeded in producing a high redshift Hubble diagram ($ z \lesssim 7$) that supports "a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat $Λ$CDM model up to $z \sim 1.5-2$, but strong deviations emerge at higher redshifts". This conclusion hinges upon a log polynomial expansion for the luminosity distance. In this note, we demonstrate that this expansion (or "improvements" thereof) typically can only be trusted up to $z \sim 1.5-2$. As a result, a breakdown in the validity of the expansion may be misinterpreted as a (phantom) deviation from flat $Λ$CDM. We further illustrate the problem through mock data examples.

astro-ph.CO↗

arXiv2021

Possible resolution of a spacetime singularity with field transformations

It is widely believed that classical gravity breaks down and quantum gravity is needed to deal with a singularity. We show that there is a class of spacetime curvature singularities which can be resolved with metric and matter field transformations. As an example, we consider an anisotropic power-law inflation model with scalar and gauge fields in which a space-like curvature singularity exists at the beginning of time. First, we provide a transformation of the metric to the flat geometry, i.e. the Minkowski metric. The transformation removes the curvature singularity located at the origin of the time. An essential difference from previous work in the literature is that the origin of time is not sent to past infinity by the transformation but it remains at a finite time in the past. Thus the geometry becomes extendible beyond the singularity. In general, matter fields are still singular in their original form after such a metric transformation. However, we explicitly show that there is a case in which the singular behavior of the matter fields can be completely removed by a re-definition of matter fields. Thus, for the first time, we have resolved a class of initial cosmic singularities and successfully extended the spacetime beyond the singularity in the framework of classical gravity.

gr-qc↗

arXiv2021

Approximate gauge independence of the induced gravitational wave spectrum

Gravitational waves (GWs) induced by scalar curvature fluctuations are an important source of the cosmological GW background and a crucial counterpart of the primordial black hole scenario. However, doubts have been cast on the theoretically predicted induced GW spectrum due to its seeming gauge dependence. In this paper, we shed light on the gauge dependence issue of the induced GW spectrum in general cosmological backgrounds. First, inspired by the Hamiltonian formalism we provide very simple formulas for the tensor modes at second order in cosmological perturbation theory. We also emphasize the difference between observable and gauge invariant variables. Second, we argue that the Newton (or shear-free) gauge is suitable for both the calculation of induced GWs and the physical interpretation. We then show that, most notably, the induced GW spectrum is invariant under a set of reasonable gauge transformations, i.e. physically well behaved on small scales, once the source term has become inactive. This includes the commonly used flat, constant Hubble and synchronous gauges but excludes the comoving slicing gauge. We also show that a particular solution of the GW equation in a dust dominated universe while the source term is active can be gauged away by a small change of gauge.

gr-qc↗

arXiv2021

Cosmology of strongly interacting fermions in the early universe

In view of growing interest in long range scalar forces in the early universe to generate primordial black holes, we study in detail the general relativistic formulation of a Fermi gas interacting with a scalar field in cosmology. Our main finding is that the Yukawa interaction leads to a solution where the scalar field oscillates around zero fermion mass and all energy densities decay as radiation. On one hand, we show that if the Fermi gas starts relativistic, it could stay relativistic. On the other hand, if the fermions are initially non-relativistic, they remain non-relativistic for all practical purposes. We find that in both cases the energy density of the fermions and the scalar field decays as radiation. In the non-relativistic case, this is due to an oscillating and decaying effective mass. Such background dynamics questions whether there is a substantial enhancement of the fermion density fluctuations in the non-relativistic case. Our work can be easily extended to more general field dependent fermion mass. The analysis of the cosmological perturbations will be presented in a follow-up work.

hep-th↗

arXiv2021

Testing Stochastic Gravitational Wave Signals from Primordial Black Holes with Optical Telescopes

Primordial black holes (PBHs) can constitute the predominant fraction of dark matter (DM) if PBHs reside in the currently unconstrained "sublunar" mass range. PBHs originating from scalar perturbations generated during inflation can naturally appear with a broad spectrum in a class of models. The resulting stochastic gravitational wave (GW) background generated from such PBH production can account for the recently reported North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array data signal, and will be testable in future GW observations by interferometer-type experiments such as Laser Interferometer Space Antenna (LISA). We show that the broad mass function of such PBH DM is already being probed by Subaru Hyper Suprime-Cam (HSC) microlensing data and is consistent with a detected candidate event. Upcoming observations of HSC will be able to provide an independent definitive test of the stochastic GW signals originating from such PBH DM production scenarios.

astro-ph.CO↗