arXiv subjects

Misao Sasaki

Publications and source records attributed to Misao Sasaki.

At least 235 records · Page 13Linked to original sources

arXiv2005

A New delta N Formalism for Multi-Component Inflation

The delta N formula that relates the final curvature perturbation on comoving slices to the inflaton perturbation on flat slices after horizon crossing is a powerful and intuitive tool to compute the curvature perturbation spectrum from inflation. However, it is customarily assumed further that the conventional slow-roll condition is satisfied, and satisfied by all components, during horizon crossing. In this paper, we develop a new delta N formalism for multi-component inflation that can be applied in the most general situations. This allows us to generalize the idea of general slow-roll inflation to the multi-component case, in particular only applying the general slow-roll condition to the relevant component. We compute the power spectrum of the curvature perturbation in multi-component general slow-roll inflation, and find that under quite general conditions it is invertible.

astro-ph↗

arXiv2005

Post-inflationary behavior of adiabatic perturbations and tensor-to-scalar ratio

We explain why it is so difficult and perhaps even impossible to increase the cosmological tensor-to-scalar perturbation ratio during the post-inflationary evolution of the universe. Nevertheless, contrary to some recent claims, tensor perturbations can be relatively large in the simplest inflationary models which do not violate any rules of modern quantum field theory.

astro-ph↗

arXiv2005

Constraining Cosmological Parameters by the Cosmic Inversion Method

We investigate the question of how tightly we can constrain the cosmological parameters by using the ``cosmic inversion'' method in which we directly reconstruct the power spectrum of primordial curvature perturbations, $P(k)$, from the temperature and polarization spectra of the cosmic microwave background (CMB). In a previous paper, we suggested that it may be possible to constrain the cosmological parameters using the fact that the reconstructed $P(k)$ does not depend on how many polarization data we incorporate in our inversion procedure if and only if the correct values of the cosmological parameters are used. The advantage of this approach is that we need no assumption regarding the functional form of $P(k)$. In this paper, we estimate typical errors in the determination of the cosmological parameters when our method is applied to the PLANCK observation. We investigate constraints on $h$, $Ω_b$, $Ω_m$, and $Ω_Λ$ through Monte Carlo simulations.

astro-ph↗

arXiv2005

Vacuum destabilization from Kaluza-Klein modes in an inflating brane

We discuss the effects from the Kaluza-Klein modes in the brane world scenario when an interaction between bulk and brane fields is included. We focus on the bulk inflaton model, where a bulk field $Ψ$ drives inflation in an almost $AdS_5$ bulk bounded by an inflating brane. We couple $Ψ$ to a brane scalar field $ϕ$ representing matter on the brane. The bulk field $Ψ$ is assumed to have a light mode, whose mass depends on the expectation value of $ϕ$. To estimate the effects from the KK modes, we compute the 1-loop effective potential $V_\eff(ϕ)$. With no tuning of the parameters of the model, the vacuum becomes (meta)stable -- $V_\eff(ϕ)$ develops a true vacuum at a nonzero $ϕ$. In the true vacuum, the light mode of $Ψ$ becomes heavy, degenerates with the KK modes and decays. We comment on some implications for the bulk inflaton model. Also, we clarify some aspects of the renormalization procedure in the thin wall approximation, and show that the fluctuations in the bulk and on the brane are closely related.

hep-th↗

arXiv2005

A general proof of the conservation of the curvature perturbation

Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale. The equations are simple, resembling closely the first-order equations, and they lead to results which generalise those already proven in linear perturbation theory and (in part) in second-order perturbation theory. In particular, the curvature perturbation is conserved provided that the pressure is a unique function of the energy density.

astro-ph↗

arXiv2005

Gauss-Bonnet dark energy

We propose the Gauss-Bonnet dark energy model inspired by string/M-theory where standard gravity with scalar contains additional scalar-dependent coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom (or quintessence) phase of late universe may occur in the presence of such term when the scalar is phantom or for non-zero potential (for canonical scalar). However, with the increase of the curvature the GB term may become dominant so that phantom phase is transient and $w=-1$ barrier may be passed. Hence, the current acceleration of the universe may be caused by mixture of scalar phantom and (or) potential/stringy effects. It is remarkable that scalar-Gauss-Bonnet coupling acts against the Big Rip occurence in phantom cosmology.

hep-th↗

arXiv2005

A new analytical method for self-force regularization II. Testing the efficiency for circular orbits

In a previous paper, based on the black hole perturbation approach, we formulated a new analytical method for regularizing the self-force acting on a particle of small mass $μ$ orbiting a Schwarzschild black hole of mass $M$, where $μ\ll M$. In our method, we divide the self-force into the $\tilde S$-part and $\tilde R$-part. All the singular behaviors are contained in the $\tilde S$-part, and hence the $\tilde R$-part is guaranteed to be regular. In this paper, focusing on the case of a scalar-charged particle for simplicity, we investigate the precision of both the regularized $\tilde S$-part and the $\tilde R$-part required for the construction of sufficiently accurate waveforms for almost circular inspiral orbits. For the regularized $\tilde S$-part, we calculate it for circular orbits to 18 post-Newtonian (PN) order and investigate the convergence of the post-Newtonian expansion. We also study the convergence of the remaining $\tilde{R}$-part in the spherical harmonic expansion. We find that a sufficiently accurate Green function can be obtained by keeping the terms up to $\ell=13$.

gr-qc↗

arXiv2005

Quantum Fluctuations for de Sitter Branes in Bulk AdS(5)

The vacuum expectation value of the square of the field fluctuations of a scalar field on a background consisting of {\it two} de Sitter branes embedded in an anti-de Sitter bulk are considered. We apply a dimensional reduction to obtain an effective lower dimensional de Sitter space equation of motion with associated Kaluza-Klein masses and canonical commutation relations. The case of a scalar field obeying a restricted class of mass and curvature couplings, including massless, conformal coupling as a special case, is considered. We find that the local behaviour of the quantum fluctuations suffers from surface divergences as we approach the brane, however, if the field is {\it constrained} to its value on the brane from the beginning then surface divergences disappear. The ratio of $<ϕ^2>$ between the Kaluza-Klein spectrum and the lowest eigenvalue mode is found to vanish in the limit that one of the branes goes to infinity.

hep-th↗

arXiv2005

Kaluza-Klein gravitons are negative energy dust in brane cosmology

We discuss the effect of Kaluza-Klein (KK) modes of bulk metric perturbations on the second Randall-Sundrum (RS II) type brane cosmology, taking the possible backreaction in the bulk and on the brane into account. KK gravitons may be produced via quantum fluctuations during a de Sitter (dS) inflating phase of our brane universe. In an effective 4-dimensional theory in which one integrates out the extra-dimensional dependence in the action, KK gravitons are equivalent to massive gravitons on the brane with masses $m>3H/2$, where $H$ represents the expansion rate of a dS brane. Thus production of even a tiny amount of KK gravitons may eventually have a significant impact on the late-time brane cosmology. As a first step to quantify the effect of KK gravitons on the brane, we calculate the effective energy density and pressure for a single KK mode. Surprisingly, we find that a KK mode behaves as cosmic dust with a negative energy density on the brane. We note that the bulk energy density of a KK mode is positive definite and there occurs no singular phenomenon in the bulk.

gr-qc↗

arXiv2004

Self-force Regularization in the Schwarzschild Spacetime

We discuss the gravitational self-force on a particle in a black hole space-time. For a point particle, the full (bare) self-force diverges. The metric perturbation induced by a particle can be divided into two parts, the direct part (or the S part) and the tail part (or the R part), in the harmonic gauge, and the regularized self-force is derived from the R part which is regular and satisfies the source-free perturbed Einstein equations. But this formulation is abstract, so when we apply to black hole-particle systems, there are many problems to be overcome in order to derive a concrete self-force. These problems are roughly divided into two parts. They are the problem of regularizing the divergent self-force, i.e., ``subtraction problem'' and the problem of the singularity in gauge transformation, i.e., ``gauge problem''. In this paper, we discuss these problems in the Schwarzschild background and report some recent progress.

gr-qc↗

arXiv2004

Zeta Functions in Brane World Cosmology

We present a calculation of the zeta function and of the functional determinant for a Laplace-type differential operator, corresponding to a scalar field in a higher dimensional de Sitter brane background, which consists of a higher dimensional anti-de Sitter bulk spacetime bounded by a de Sitter section, representing a brane. Contrary to the existing examples, which all make use of conformal transformations, we evaluate the zeta function working directly with the higher dimensional wave operator. We also consider a generic mass term and coupling to curvature, generalizing previous results. The massless, conformally coupled case is obtained as a limit of the general result and compared with known calculations. In the limit of large anti-de Sitter radius, the zeta determinant for the ball is recovered in perfect agreement with known expressions, providing an interesting check of our result and an alternative way of obtaining the ball determinant.

hep-th↗

arXiv2004

On Detection of Black Hole Quasi-Normal Ringdowns: Detection Efficiency and Waveform Parameter Determination in Matched Filtering

Gravitational radiation from a slightly distorted black hole with ringdown waveform is well understood in general relativity. It provides a probe for direct observation of black holes and determination of their physical parameters, masses and angular momenta (Kerr parameters). For ringdown searches using data of gravitational wave detectors, matched filtering technique is useful. In this paper, we describe studies on problems in matched filtering analysis in realistic gravitational wave searches using observational data. Above all, we focus on template constructions, matches or signal-to-noise ratios (SNRs), detection probabilities for Galactic events, and accuracies in evaluation of waveform parameters or black hole hairs. We have performed matched filtering analysis for artificial ringdown signals which are generated with Monte-Carlo technique and injected into the TAMA300 observational data. It is shown that with TAMA300 sensitivity, the detection probability for Galactic ringdown events is about 50% for black holes of masses greater than $20 M_{\odot}$ with SNR $> 10$. The accuracies in waveform parameter estimations are found to be consistent with the template spacings, and resolutions for black hole masses and the Kerr parameters are evaluated as a few % and $\sim 40 %$, respectively. They can be improved up to $< 0.9 %$ and $< 24 %$ for events of ${\rm SNR} \ge 10$ by using fine-meshed template bank in the hierarchical search strategy.

gr-qc↗

arXiv2004

Reconstructing the Primordial Spectrum with CMB Temperature and Polarization

We develop a new method to reconstruct the power spectrum of primordial curvature perturbations, $P(k)$, by using both the temperature and polarization spectra of the cosmic microwave background (CMB). We test this method using several mock primordial spectra having non-trivial features including the one with an oscillatory component, and find that the spectrum can be reconstructed with a few percent accuracy by an iterative procedure in an ideal situation in which there is no observational error in the CMB data. In particular, although the previous ``cosmic inversion'' method, which used only the temperature fluctuations, suffered from large numerical errors around some specific values of $k$ that correspond to nodes in a transfer function, these errors are found to disappear almost completely in the new method.

astro-ph↗

arXiv2004

Holography in Einstein Gauss-Bonnet Gravity

We investigate a holographic relation between Einstein Gauss-Bonnet gravity in $n$ dimensions and its dual field theory in ($n-1$) dimensions. We briefly review the AdS/CFT correspondence for the entropy in the $n$-dimensional Einstein gravity and consider its extension to the case of the $n$-dimensional Einstein Gauss-Bonnet gravity. We show that there is a holographic relation between entropies of an Einstein Gauss-Bonnet black hole in the bulk and the corresponding radiation on the brane in the high temperature limit. In particular, we find that the Hubble entropy evaluated when the brane crosses the horizon also coincides with the black hole entropy in the high temperature limit.

hep-th↗

arXiv2004

A new calculation of the mass fraction of primordial black holes

We revisit the calculation of the abundance of primordial black holes (PBHs) formed from primordial density perturbations, using a formation criterion derived by Shibata and Sasaki which refers to a metric perturbation variable rather than the usual density contrast. We implement a derivation of the PBH abundance which uses peaks theory, and compare it to the standard calculation based on a Press--Schechter-like approach. We find that the two are in reasonable agreement if the Press--Schechter threshold is in the range $Δ_{\rm th} \simeq 0.3$ to 0.5, but advocate use of the peaks theory expression which is based on a sounder theoretical footing.

astro-ph↗

arXiv2004

A new analytical method for self-force regularization I. scalar charged particle in Schwarzschild spacetime

We formulate a new analytical method for regularizing the self-force acting on a particle of small mass $μ$ orbiting a black hole of mass $M$, where $μ\ll M$. At first order in $μ$, the geometry is perturbed and the motion of the particle is affected by its self-force. The self-force, however, diverges at the location of the particle, and hence should be regularized. It is known that the properly regularized self-force is given by the tail part (or the $R$-part) of the self-field, obtained by subtracting the direct part (or the $S$-part) from the full self-field. The most successful method of regularization proposed so far relies on the spherical harmonic decomposition of the self-force, the so-called mode-sum regularization or mode decomposition regularization. However, except for some special orbits, no systematic analytical method for computing the regularized self-force has been given. In this paper, utilizing a new decomposition of the retarded Green function in the frequency domain, we formulate a systematic method for the computation of the self-force. Our method relies on the post-Newtonian (PN) expansion but the order of the expansion can be arbitrarily high. To demonstrate the essence of our method, in this paper, we focus on a scalar charged particle on the Schwarzschild background. The generalization to the gravitational case is straightforward, except for some subtle issues related with the choice of gauge (which exists irrespective of regularization methods).

gr-qc↗

arXiv2004

Reconstructing the Primordial Spectrum from WMAP Data by the Cosmic Inversion Method

We reconstruct the primordial spectrum of the curvature perturbation, $P(k)$, from the observational data of the Wilkinson Microwave Anisotropy Probe (WMAP) by the cosmic inversion method developed recently. In contrast to conventional parameter-fitting methods, our method can potentially reproduce small features in $P(k)$ with good accuracy. As a result, we obtain a complicated oscillatory $P(k)$. We confirm that this reconstructed $P(k)$ recovers the WMAP angular power spectrum with resolution up to $Δ\ell \simeq 5$. Similar oscillatory features are found, however, in simulations using artificial cosmic microwave background data generated from a scale-invariant $P(k)$ with random errors that mimic observation. In order to examine the statistical significance of the nontrivial features, including the oscillatory behaviors, therefore, we consider a method to quantify the deviation from scale-invariance and apply it to the $P(k)$ reconstructed from the WMAP data. We find that there are possible deviations from scale-invariance around $k \simeq 1.5\times10^{-2}$ and $2.6\times10^{-2}{\rm Mpc}^{-1}$.

astro-ph↗

arXiv2004

Linearized gravity on the de Sitter brane in the Einstein Gauss-Bonnet theory

We investigate the linearized gravity on a single de Sitter brane in the anti-de Sitter (AdS) bulk in the Einstein Gauss-Bonnet (EGB) theory. We find that the Einstein gravity is recovered for a high energy brane, i.e., in the limit of the large expansion rate, $H\ell\gg 1$, where $H$ is the de Sitter expansion rate and $\ell$ is the curvature radius of the AdS bulk. We also show that, in the short distance limit $r\ll \min\{\ell,H^{-1}\}$, the Brans-Dicke gravity is obtained, whereas in the large distance limit $r\gg\max\{\ell,H^{-1}\}$, the Brans-Dicke gravity is obtained for $H\ell=O(1)$, and the Einstein gravity is recovered both for $H\ell\gg1$ and $H\ell\ll1$. In the limit $H\ell\to0$, these results smoothly match to the results known for the Minkowski brane.

hep-th↗