Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.106.042007

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Cosmology and Astroparticle Physics  

In this work, we have performed numerical simulations of primordial black hole (PBH) formation in the Friedman-Lemaître-Robertson-Walker universe filled by radiation fluid, introducing the local-type non-Gaussianity to the primordial curvature fluctuation. We have compared the numerical results from simulations with previous analytical estimations on the threshold value for PBH formation done in the previous paper [1], particularly for negative values of the non-linearity parameter f NL. Our numerical results show the existence of PBH formation of (the so-called) type I also in the case f NL ≤2-0.336, which was not found in the previous analytical expectations using the critical averaged compaction function. In particular, although the universal value for the averaged critical compaction function ō o = 2/5 found previously in the literature is not satisfied for all the profiles considered in this work, an alternative direct analytical estimate has been found to be roughly accurate to estimate the thresholds, which gives the value of the critical averaged density with a few % deviation from the numerical one for f NL ≤3-1.

DOI： 10.1088/1475-7516/2022/05/012

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/galaxies10010025

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Scopus

Publisher：Proceedings of the 20th Workshop on General Relativity and Gravitation in Japan, JGRG 2010  

Modulated reheating scenario is one of the most attractive models that predict possible detections of primordial non-Gaussianity through future CMB observations such as the Planck satellite. We study the baryonic-isocurvature fluctuations in the Affleck- Dine baryogenesis with the modulated reheating scenario. We show that the simple Affleck-Dine baryogenesis would be incompatible with the modulated reheating scenario with respect to the current observational constraint on the baryonic-isocurvature fluctuations, like a gravitino dark matter scenario.

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstract

Since the discovery of the accelerated expansion of the present Universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality data, allowing us to explore gravity on cosmological scales. To bridge the recent theoretical developments and observations, we present an overview of a variety of modified theories of gravity and the cosmological observables in the cosmic microwave background and large-scale structure, supplemented with a summary of predictions for cosmological observables derived from cosmological perturbations and sophisticated numerical studies. We specifically consider scalar-tensor theories in the Horndeski and DHOST family, massive gravity/bigravity, vector-tensor theories, metric-affine gravity, and cuscuton/minimally-modified gravity, and discuss the current status of those theories with emphasis on their physical motivations, validity, appealing features, the level of maturity, and calculability. We conclude that the Horndeski theory is one of the most well-developed theories of modified gravity, although several remaining issues are left for future observations. The paper aims to help to develop strategies for testing gravity with ongoing and forthcoming cosmological observations.

DOI： 10.1093/ptep/ptad052

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstract

In the most distant reaches of the Universe, the 21 cm hyperfine transition in neutral hydrogen provides one of the only available tracers of large-scale structure. A number of instruments have been working or are planned to measure the 21 cm line signals, and in particular, the Experiment to Detect the Global EoR Signature (EDGES) has recently reported the first detection of an absorption signal, which corresponds to the 21 cm line global signal at the epoch of reionization (EoR). The future large radio telescope, the Square Kilometre Array (SKA) will be able to deliver the high-precision measurement of the 21 cm line emission/absorption signals. In this paper, we review the current status for the 21 cm line global and fluctuation signals from EoR to the dark ages, and then summarize the recent studies of how we probe the primordial Universe particularly motivated by the recent EDGES result and future observations by SKA. We focus on two applications for constraining cosmology with the EDGES result: constraints on the primordial magnetic fields and those on the primordial power spectrum. We also discuss the potential of future SKA for probing the inflationary Universe, by discussing expected constraints on the primordial power spectrum, its adiabaticity, and primordial non-Gaussianities from future observations of 21 cm fluctuations.

DOI： 10.1093/pasj/psac015

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Motivated by the result of Planck+BICEP/Keck recently released, we investigate the consistency of the multi-field inflation models in terms of the spectral index n_s and the tensor-to-scalar ratio r. In this study, we focus on double-inflaton models with and without a spectator field.We find that inflaton with a quadratic potential can become viablewhen three fields with a specific hierarchical mass spectrum are realized such that two fields act as inflatons and the other one is the spectator. We also discuss the conditions to avoid the fine-tuning, by careful study of how the prediction depends on the background trajectory in the inflaton-field space.

DOI： 10.1088/1475-7516/2022/07/042

DOI： 10.1088/1475-7516/2022/07/042

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2022/07/042/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1475-7516/2021/10/053

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2021/10/053/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevD.103.L101303

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Scopus

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.103.L101303/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We develop a possibility of generating tensor non-Gaussianity in a kind of anisotropic inflation, where a (1) gauge field is kinetically coupled to a spectator scalar field.Owing to this coupling, the coherent mode of the electric field appears and softly breaks the isotropy of the Universe.We compute the bispectrum of linearly-polarized tensor perturbations sourced by the gauge field and find that it is strongly red-tiltedand has distinctive statistical anisotropies including higher-order multipole moments.Interestingly, the tensor bispectra with the specific combinations of linear polarization modes are dominant, and their amplitudes depend on the different sets of multipole moments.This new type of statistically-anisotropic tensor non-Gaussianity can be potentially testable with the upcoming cosmic microwave background B-mode polarization experiments.

DOI： 10.1088/1475-7516/2021/03/047

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2021/03/047/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1475-7516/2021/01/071

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Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2021/01/071

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevD.102.103505

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Scopus

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.102.103505/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1209/0295-5075/132/39001

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Scopus

Other Link： https://iopscience.iop.org/article/10.1209/0295-5075/132/39001/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevD.102.043524

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Scopus

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.102.043524/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Primordial magnetic field (PMF) is one of the feasible candidates to explain
observed large-scale magnetic fields, for example, intergalactic magnetic
fields. We present a new mechanism that brings us information about PMFs on
small scales based on the abundance of primordial black holes (PBHs). The
anisotropic stress of the PMFs can act as a source of the super-horizon
curvature perturbation in the early universe. If the amplitude of PMFs is
sufficiently large, the resultant density perturbation also has a large
amplitude, and thereby, the PBH abundance is enhanced. Since the anisotropic
stress of the PMFs is consist of the square of the magnetic fields, the
statistics of the density perturbation follows the non-Gaussian distribution.
Assuming Gaussian distributions and delta-function type power spectrum for
PMFs, based on a Monte-Carlo method, we obtain an approximate probability
density function of the density perturbation, and it is an important piece to
relate the amplitude of PMFs with the abundance of PBHs. Finally, we place the
strongest constraint on the amplitude of PMFs as a few hundred nano-Gauss on
$10^{2}\;{\rm Mpc}^{-1} \leq k\leq 10^{18}\;{\rm Mpc}^{-1}$ where the typical
cosmological observations never reach.

DOI： 10.1088/1475-7516/2020/05/039

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Scopus

arXiv

Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2020/05/039

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Progress of Theoretical and Experimental Physics  

DOI： 10.1093/ptep/ptaa011

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Scopus

In the presence of the local-type primordial non-Gaussianity, it is known
that the clustering of primordial black holes (PBHs) emerges even on
super-horizon scales at the formation time. This effect has been investigated
in the high-peak limit of the PBH formation in the radiation-dominated epoch in
the literature. There is another possibility that the PBH formation takes place
in the early matter-dominated epoch. In this scenario, the high-peak limit is
not applicable because even initially small perturbations grow and can become a
PBH. We first derive a general formula to estimate the clustering of PBHs with
primordial non-Gaussianity without assuming the high-peak limit, and then apply
this formula to a model of PBH formation in a matter-dominated epoch.
Clustering is less significant in the case of the PBH formation in the
matter-dominated epoch than that in the radiation-dominated epoch.
Nevertheless, it is much larger than the Poisson shot noise in many cases.
Relations to the constraints of the isocurvature perturbations by the cosmic
microwave background radiation are quantitatively discussed.

DOI： 10.1103/PhysRevD.100.123544

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Scopus

arXiv

Computations of the primordial black hole (PBH) mass function discussed in
the literature have conceptual issues. They stem from that the mass function is
a differential quantity and the standard criterion of the PBH formation from
the seed primordial fluctuations cannot be directly applied to the computation
of the differential quantities. We propose a new criterion of the PBH formation
which is an addition of one extra condition to the existing one. By doing this,
we derive a formal expression of the PBH mass function without introducing any
ambiguous interpretations which exist in the previous studies. Once the
underlying primordial fluctuations are specified, the PBH mass function can be
in principle determined by the new formula. As a demonstration of our
formulation, we compute the PBH mass function analytically for the case where
the perturbations are Gaussian and the space is one dimension.

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Spatially fluctuating primordial magnetic fields (PMFs) inhomogeneously
reheat the Universe when they dissipate deep inside the horizon before
recombination. Such an energy injection turns into an additional photon
temperature perturbation. We investigate secondary cosmic microwave background
(CMB) temperature anisotropies originated from this mechanism, which we call
{\it inhomogeneous magnetic reheating}. We find that it can bring us
information about non-linear coupling between PMFs and primordial curvature
perturbations parametrized by $b_{\rm NL}$, which should be important for
probing the generation mechanism of PMFs. In fact, by using current CMB
observations, we obtain an upper bound on the non-linear parameter as $\log
(b_{\rm NL} (B_{\lambda}/{\rm nG})^2) \lesssim {-36.5n_{B} - 94.0}$ with
$B_{\lambda}$ and $n_{\rm B}$ being a magnetic field amplitude smoothed over
$\lambda=1\; {\rm Mpc}$ scale and a spectral index of the PMF power spectrum,
respectively. Our constraints are far stronger than a previous forecast based
on the future CMB spectral distortion anisotropy measurements because
inhomogeneous magnetic reheating covers a much wider range of scales, i.e.,
$1\; {\rm Mpc}^{-1} \lesssim k\lesssim 10^{15}\; {\rm Mpc}^{-1}$.

DOI： 10.1093/mnras/stz2882

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Scopus

arXiv

We formulate the two-point correlation function of primordial black holes
(PBHs) at their formation time, based on the functional integration approach
which has often been used in the context of halo clustering. We find that PBH
clustering on super-Hubble scales could never be induced in the case where the
initial primordial fluctuations are Gaussian, while it can be enhanced by the
so-called local-type trispectrum (four-point correlation function) of the
primordial curvature perturbations.

DOI： 10.1093/ptep/ptz105

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Scopus

arXiv

Other Link： http://arxiv.org/pdf/1906.04958v2

In this paper, we fully investigate cosmological scenarios in which dark
matter is non-minimally coupled to an extra scalar degree of freedom. The
interaction is realized by means of conformal and disformal terms in the
transformed gravitational metric. Considering linear perturbation theory, we
show that the growth rate of dark matter differs from the uncoupled case and
that the well-known Kaiser formula undergoes modification. As a result,
redshift space distortion measurements cease to be a direct probe of the linear
growth rate of total matter, since the distortion factor has an extra,
coupling-dependent term. We study the effect of the coupling in three
cosmological models, two conformally and one disformally coupled, and forecast
the constraints on the coupling, and other cosmological parameters, from future
galaxy surveys.

DOI： 10.1088/1475-7516/2019/10/049

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Scopus

arXiv

We discuss the effect of local type non-Gaussianity on the abundance of
primordial black holes (PBH) based on the peak theory. We provide the PBH
formation criterion based on the so-called compaction function and use the peak
theory statistics associated with the curvature perturbation with the local
type non-Gaussianity. Providing a method to estimate the PBH abundance, we
demonstrate the effects of non-Gaussianity. It is explicitly shown that the
value of non-linear parameter $|f_{\rm NL}| \sim 1$ induces a similar effect to
a few factors of difference in the amplitude of the power spectrum.

DOI： 10.1088/1475-7516/2019/09/033

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Scopus

arXiv

We investigate a possibility of primordial black hole (PBH) formation with a
hierarchical mass spectrum in multiple phases of inflation. As an example, we
find that one can simultaneously realize a mass spectrum which has recently
attracted a lot of attention: stellar-mass PBHs ($\sim\mathcal{O}(10)M_\odot$)
as a possible source of binary black holes detected by LIGO/Virgo
collaboration, asteroid-mass ($\sim\mathcal{O}(10^{-12})M_\odot$) as a main
component of dark matter, and earth-mass ($\sim\mathcal{O}(10^{-5})M_\odot$) as
a source of ultrashort-timescale events in Optical Gravitational Lensing
Experiment microlensing data. The recent refined de Sitter swampland conjecture
may support such a multi-phase inflationary scenario with hierarchical mass
PBHs as a transition signal of each inflationary phase.

DOI： 10.1103/PhysRevD.100.023537

Web of Science

arXiv

We investigate the potential of cosmological observations, such as galaxy
surveys, for constraining degenerate higher-order scalar-tensor (DHOST)
theories, focusing in particular on the linear growth of the matter density
fluctuations. We develop a formalism to describe the evolution of the matter
density fluctuations during the matter dominated era and in the early stage of
the dark energy dominated era in DHOST theories, and give an approximate
expression for the gravitational growth index in terms of several parameters
characterizing the theory and the background solution under consideration. By
employing the current observational constraints on the growth index, we obtain
a new constraint on a parameter space of DHOST theories. Combining our result
with other constraints obtained from the Newtonian stellar structure, we show
that the degeneracy between the effective parameters of DHOST theories can be
broken without using the Hulse-Taylor pulsar constraint.

DOI： 10.1103/PhysRevD.99.104051

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Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We constrain the abundance of primordial black holes (PBH) using 2622
microlensing events obtained from 5-years observations of stars in the Galactic
bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of
microlensing events display a single or at least continuous population that has
a peak around the light curve timescale $t_{\rm E}\simeq 20~{\rm days}$ and a
wide distribution over the range $t_{\rm E}\simeq [1, 300]~{\rm days}$, while
the data also indicates a second population of 6 ultrashort-timescale events in
$t_{\rm E}\simeq [0.1,0.3]~{\rm days}$, which are advocated to be due to
free-floating planets. We confirm that the main population of OGLE events can
be well modeled by microlensing due to brown dwarfs, main sequence stars and
stellar remnants (white dwarfs and neutron stars) in the standard Galactic
bulge and disk models for their spatial and velocity distributions. Using the
dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic
bulge/disk models, we obtain the tightest upper bound on the PBH abundance in
the mass range $M_{\rm PBH}\simeq[10^{-6},10^{-3}]M_\odot$ (Earth-Jupiter mass
range), if we employ null hypothesis that the OGLE data does not contain any
PBH microlensing event. More interestingly, we also show that Earth-mass PBHs
can well reproduce the 6 ultrashort-timescale events, without the need of
free-floating planets, if the mass fraction of PBH to DM is at a per cent
level, which is consistent with other constraints such as the microlensing
search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our
result gives a hint of PBH existence, and can be confirmed or falsified by
microlensing search for stars in M31, because M31 is towards the MW halo
direction and should therefore contain a much less number of free-floating
planets, even if exist, than the direction to the MW center.

DOI： 10.1103/PhysRevD.99.083503

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Scopus

arXiv

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.99.083503/fulltext

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We investigate future constraints on primordial local-type non-Gaussianity
from 21 cm angular power spectrum from minihalos. We particularly focus on the
trispectrum of primordial curvature perturbations which are characterized by
the non-linearity parameters $\tau_{\rm NL}$ and $g_{\rm NL}$. We show that
future measurements of minihalo 21 cm angular power spectrum can probe these
non-linearity parameters with an unprecedented precision of $\tau_{\rm
NL}\sim30$ and $g_{\rm NL}\sim2\times10^3$ for Square Kilometre Array (SKA) and
$\tau_{\rm NL}\sim0.6$ and $g_{\rm NL}\sim8\times10^2$ for Fast Fourier
Transform Telescope (FFTT). These levels of sensitivity would give significant
implications for models of the inflationary Universe and the origin of cosmic
density fluctuations.

DOI： 10.1088/1475-7516/2019/02/033

Web of Science

Scopus

arXiv

We investigate a possibility of constraining statistical anisotropies of the
primordial tensor perturbations by using future observations for the Cosmic
Microwave Background (CMB) B-mode polarization. By parameterizing a
statistically-anisotropic tensor power spectrum as $P_h ({\boldsymbol{k } }) =
P_h (k) \sum_n g_n \cos^n \theta_{\boldsymbol{k } }$, where
$\theta_{\boldsymbol{k } }$ is an angle of the direction of
$\hat{k}={\boldsymbol{k } }/k$ from a preferred direction, we find that it would
be possible for future B-mode observations such as CMB-S4 to detect the tensor
statistical anisotropy at the level of $g_n \sim {\mathcal O} (0.1)$.

DOI： 10.1103/PhysRevD.98.083522

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1808.08044v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

Primordial magnetic fields (PMFs) can source gravitational wave background (GWB). In this paper, we investigate the possible constraints on small-scale PMF considering the ongoing and forthcoming direct detection observations of GWB. In contrast to the conventional cosmological probes, e.g., cosmic microwave background anisotropies, which are useful to investigate large-scale PMFs (> 1 Mpc), the direct detection experiments of GWB can explore small-scale PMFs whose scales correspond to the observed frequencies of GWB. We show that future ground-based or space-based interferometric gravitational wave detectors give a strong constraint of about 10(2) nG on much smaller scales of about k approximate to 10(12) Mpc(-1). We also demonstrate that pulsar timing arrays have a potential to strongly constrain PMFs. The current limits on GWB from pulsar timing arrays can put the tight constraint on the amplitude of the PMFs of about 30 nG whose coherent length is about k approximate to 10(6) Mpc(-1). The future experiments for the direct detection of GWB by the Square Kilometre Array could give much tighter constraints on the amplitude of PMFs of about 5 nG on k approximate to 10(6) Mpc(-1), on which scales it is difficult to reach by using the cosmological observations.

DOI： 10.1103/PhysRevD.98.083518

Web of Science

Scopus

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.98.083518/fulltext

DOI： 10.1103/PhysRevLett.121.059901

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

The Horndeski scalar-tensor theory and its recent extensions allow nonlinear derivative interactions of the scalar degree of freedom. We study the matter bispectrum of large scale structure as a probe of these modified gravity theories, focusing in particular on the effect of the terms that newly appear in the so-called "beyond Horndeski" theories. We derive the second-order solution for the matter density perturbations and find that the interactions beyond Horndeski lead to a new time-dependent coefficient in the second-order kernel which differs in general from the standard value of general relativity and the Horndeski theory. This can deform the matter bispectrum at the folded triangle configurations (k1+k2=k3), while it is never possible within the Horndeski theory.

DOI： 10.1103/PhysRevD.97.103517

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Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

It has been proposed that primordial black holes (PBHs) form binaries in the radiation dominated era. Once formed, some fraction of them may merge within the age of the universe by gravitational radiation reaction. We investigate the merger rate of the PBH binaries when the PBHs have a distribution of masses around O(10)M, which is a generalization of the previous studies where the PBHs are assumed to have the same mass. After deriving a formula for the merger time probability distribution in the PBH mass plane, we evaluate it under two different approximations. We identify a quantity constructed from the mass distribution of the merger rate density per unit cosmic time and comoving volume R(m1, m2), α = -(m1+ m2)2∂2 In R/∂m1∂m2, which universally satisfies 0.97≲α≲ 1.05 for all binary masses independently of the PBH mass function. This result suggests that the measurement of this quantity is useful for testing the PBH scenario.

DOI： 10.3847/1538-4357/aaa7f4

Scopus

arXiv

Language：English   Publisher：Institute of Physics Publishing  

This article reviews current understanding of primordial black holes (PBHs), with particular focus on those massive examples (≳1015 g) which remain at the present epoch, not having evaporated through Hawking radiation. With the detection of gravitational waves by LIGO, we have gained a completely novel observational tool to search for PBHs, complementary to those using electromagnetic waves. Taking the perspective that gravitational-wave astronomy will make significant progress in the coming decades, the purpose of this article is to give a comprehensive review covering a wide range of topics on PBHs. After discussing PBH formation, as well as several inflation models leading to PBH production, we summarize various existing and future observational constraints. We then present topics on formation of PBH binaries, gravitational waves from PBH binaries, and various observational tests of PBHs using gravitational waves.

DOI： 10.1088/1361-6382/aaa7b4

Scopus

arXiv

Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

Measurements of 21 cm line fluctuations from minihalos have been discussed as a powerful probe of a wide range of cosmological models. However, previous studies have taken into account only the pixel variance, where contributions from different scales are integrated. In order to sort out information from different scales, we formulate the angular power spectrum of 21 cm line fluctuations from minihalos at different redshifts, which can enhance the constraining power enormously. By adopting this formalism, we investigate expected constraints on parameters characterizing the primordial power spectrum, particularly focusing on the spectral index ns and its runnings αs and βs. We show that future observations of 21 cm line fluctuations from minihalos, in combination with cosmic microwave background, can potentially probe these runnings as αs ∼ (10-3) and βs ∼ (10-4). Its implications to the test of inflationary models are also discussed.

DOI： 10.1088/1475-7516/2018/02/053

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press  

We provide a new bound on the amplitude of primordial magnetic fields (PMFs) by using a novel mechanism, magnetic reheating. The damping of the magnetohydrodynamics fluid motions in a primordial plasma brings the dissipation of the PMFs. In the early Universe with z ≳ 2 × 106, cosmic microwave background (CMB) photons are quickly thermalized with the dissipated energy and shift to a different Planck distribution with a new temperature. In other words, the PMF dissipation changes the baryon-to-photon number ratio, and we name such a process magnetic reheating. From the current baryon-to-photon number ratio obtained from the big bang nucleosynthesis and CMB observations, we put the strongest constraint on the PMFs on small scales which CMB observations cannot access, B0 ≲ 1.0 μG at the scales 104 &lt
k &lt
108 h Mpc-1.Moreover, when the PMF spectrum is given in a blue power-law type, the magnetic reheating puts a quite strong constraint, for example, B0 ≲ 10-17, 10-23, and 10-29 nG at 1 comoving Mpc for nB = 1.0, 2.0, and 3.0, respectively. This constraint would give an impact on generation mechanisms of PMFs in the early Universe.

DOI： 10.1093/mnrasl/slx195

Web of Science

Scopus

arXiv

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We consider the inflationary universe with a spectator scalar field coupled
to a $U(1)$ gauge field and calculate curvature perturbation and gravitational
waves (GWs). We find that the sourced GWs can be larger than the one from
vacuum fluctuation and they are statistically anisotropic as well as linearly
polarized. The GW power spectrum acquires higher multipole moments as
$\mathcal{P}_h \propto (1-\cos^2\theta+\cos^4\theta-\cos^6\theta)$ irrespective
of the model parameters.

DOI： 10.1088/1475-7516/2018/07/023

arXiv

Other Link： http://arxiv.org/pdf/1801.02778v1

Publisher：AMER PHYSICAL SOC  

We explore the use of galaxy bispectrum induced by the nonlinear
gravitational evolution as a possible probe to test general scalar-tensor
theories with second-order equations of motion. We find that time dependence of
the leading second-order kernel is approximately characterized by one
parameter, the second-order index, which is expected to trace the higher-order
growth history of the Universe. We show that our new parameter can
significantly carry new information about the non-linear growth of structure.
We forecast future constraints on the second-order index as well as the
equation-of-state parameter and the growth index.

DOI： 10.1103/PhysRevD.96.123516

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1709.03243v1

Language：English   Publisher：IOP PUBLISHING LTD  

We revisit the generation of dark matter isocurvature perturbations in the
curvaton model in greater detail, both analytically and numerically. As
concrete examples, we investigate the cases of thermally decoupled dark matter
and axionic dark matter. We show that the radiation produced by the decay of
the curvaton, which has not been taken into account in previous analytical
studies, can significantly affect the amplitude of isocurvature perturbations.
In particular, we find that they are drastically suppressed even when the dark
matter freeze-out (or the onset of the axion oscillations for axionic dark
matter) occurs before the curvaton decays, provided the freeze-out takes place
deep in the curvaton-dominated Universe. As a consequence, we show that the
current observational isocurvature constraints on the curvaton parameters are
not as severe as usually thought.

DOI： 10.1088/1475-7516/2017/12/042

Web of Science

Scopus

arXiv

We present an impact of coupling between dark matter and a scalar field,
which might be responsible for dark energy, on measurements of redshift-space
distortions. We point out that, in the presence of conformal and/or disformal
coupling, linearized continuity and Euler equations for total matter fluid
significantly deviate from the standard ones even in the sub-horizon scales. In
such a case, a peculiar velocity of total matter field is determined not only
by a logarithmic time derivative of its density perturbation but also by
density perturbations for both dark matter and baryon, leading to a large
modification of the physical interpretation of observed data obtained by
measurements of redshift-space distortions. We reformulate galaxy two-point
correlation function in the redshift space based on the modified continuity and
Euler equations. We conclude from the resultant formula that the true value of
the linear growth rate of large-scale structure cannot be necessarily
constrained by single-redshift measurements of the redshift-space distortions,
unless one observes the actual time-evolution of structure.

DOI： 10.1093/pasj/psy083

arXiv

Publisher：OXFORD UNIV PRESS  

We perform a Fisher analysis to estimate the expected constraints on the
Epoch of Reionization (EoR) model parameters (i.e., minimum virial temperature,
the ionizing efficiency and the mean free path of ionizing photons) taking into
account the thermal noise of the exisitting telescopes, MWA and LOFAR. We
consider how the inclusion of the 21cm bispectrum improves the constraints
compared to using the power spectrum alone. Assuming that we perfectly remove
the foreground, we found that the bispectrum, which is calculated by the
21cmFAST code, can constrain the EoR model parameters more tightly than the
power spectrum since the bispectrum is more sensitive to the EoR model
parameters than the power spectrum. We also found that degeneracy between the
EoR model parameters can be broken by combining the bispectrum with the power
spectrum.

DOI： 10.1093/mnras/stx530

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS INC  

We estimate the spin distribution of primordial black holes based on the
recent study of the critical phenomena in the gravitational collapse of a
rotating radiation fluid. We find that primordial black holes are mostly slowly
rotating.

DOI： 10.1093/ptep/ptx087

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/1704.06573v2

Publisher：OXFORD UNIV PRESS  

In the past several decades, the standard cosmological model has been
established and its parameters have been measured to a high precision, while
there are still many of the fundamental questions in cosmology; such as the
physics in the very early Universe, the origin of the cosmic acceleration and
the nature of the dark matter. The future world's largest radio telescope,
Square Kilometre Array (SKA), will be able to open the new frontier of
cosmology and will be one of the most powerful tools for cosmology in the next
decade. The cosmological surveys conducted by the SKA would have the potential
not only to answer these fundamental questions but also deliver the precision
cosmology. In this article we briefly review the role of the SKA from the view
point of the modern cosmology. The cosmology science led by the SKA-Japan
Consortium (SKA-JP) Cosmology Science Working Group is also discussed.

DOI： 10.1093/pasj/psw098

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We explore the use of galaxy bispectra with multitracer technique as a
possible probe of primordial non-Gaussianities. We forecast future constraints
on non-linearity parameters, $f_{\rm NL}^{\rm eq}$ and $f_{\rm NL}^{\rm orth}$,
which respectively characterize the equilateral- and orthogonal-types
primordial bispectra, and show that the multitracer analysis would be effective
with reducing the cosmic-variance noise if the number density of galaxies is
high enough. We find that the measurement of galaxy bispectrum by future galaxy
surveys can reach the constraints on the non-local type primordial
non-Gaussianities to the level severer than current one which has been obtained
by cosmic microwave background observations.

DOI： 10.1103/PhysRevD.95.063530

Web of Science

arXiv

Publisher：AMER PHYSICAL SOC  

We investigate the statistical power of higher-order statistics and
cross-correlation statistics to constrain the primordial non-Gaussianity from
the imaging surveys. In particular, we consider the local-type primordial non-
Gaussianity and discuss how well one can tightly constrain the higher-order
non-Gaussian parameters ($g_{\rm NL}$ and $\tau_{\rm NL}$) as well as the
leading order parameter $f_{\rm NL}$ from the halo/galaxy clustering and weak
gravitational lensing measurements. Making use of a strong scale-dependent
behavior in the galaxy/halo clustering, Fisher matrix analysis reveals that the
bispectra can break the degeneracy between non-Gaussian parameters ($f_{\rm
NL}$, $g_{\rm NL}$ and $\tau_{\rm NL}$) and this will give simultaneous
constraints on those three parameters. The combination of cross-correlation
statistics further improves the constraints by factor of 2. As a result,
upcoming imaging surveys like the Large Synoptic Survey Telescope have the
potential to improve the constraints on the primordial non-Gaussianity much
tighter than those obtained from the CMB measurement by Planck, giving us an
opportunity to test the single-sourced consistency relation, $\tau_{\rm NL} \ge
(36/25) f_{\rm NL}^2$.

DOI： 10.1103/PhysRevD.93.103537

Web of Science

Scopus

arXiv

Language：English   Publisher：AMER PHYSICAL SOC  

We investigate expected constraints on equilateral-type primordial
non-Gaussianities from future/ongoing imaging surveys, making use of the fact
that they enhance the halo/galaxy bispectrum on large scales. As model
parameters to be constrained, in addition to $f_{\rm NL}^{\rm equil}$, which is
related to the primordial bispectrum, we consider $g_{\rm NL}^{(\partial
\sigma)^4}$, which is related to the primordial trispectrum that appeared in
the effective field theory of inflation. After calculating the angular
bispectra of the halo/galaxy clustering and weak gravitational lensing based on
the integrated perturbation theory, we perform Fisher matrix analysis for three
representative surveys. We find that among the three surveys, the tightest
constraints come from Large Synoptic Survey Telescope ; its expected $1\sigma$
errors on $f_{\rm NL}^{\rm equil}$ and $g_{\rm NL}^{(\partial \sigma)^4}$ are
respectively given by $7.0 \times 10^2$ and $4.9 \times 10^7$. Although this
constraint is somewhat looser than the one from the current cosmic microwave
background observation, since we obtain it independently, we can use this
constraint as a cross check. We also evaluate the uncertainty with our results
caused by using several approximations and discuss the possibility to obtain
tighter constraint on $f_{\rm NL}^{\rm equil}$ and $g_{\rm NL}^{(\partial
\sigma)^4}$.

DOI： 10.1103/PhysRevD.94.043532

Web of Science

arXiv

Publisher：OXFORD UNIV PRESS  

Redshifted 21cm signal is a promising tool to investigate the state of
intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of
Reionization(EoR). In our previous work (Shimabukuro et al 2015), we studied
the variance and skewness to give a clear interpretation of 21cm power spectrum
and found that skewness is a good indicator of the epoch when X-ray heating
becomes effective. Thus, the non-Gaussian feature of the spatial distribution
of the 21cm signal is expected to be useful to investigate the astrophysical
effects in the CD and EoR. In this paper, in order to investigate such a
non-Gaussian feature in more detail, we focus on the bispectrum of the 21cm
signal. It is expected that the 21cm brightness temperature bispectrum is
produced by non-gaussianity due to the various astrophysical effects such as
Wouthysen-Field (WF) effect, X-ray heating and reionization. We study the
various properties of 21cm bispectrum such as scale dependence, shape
dependence and redshift evolution. And also we study the contribution from each
component of 21cm bispectrum. We find that the contribution from each component
has characteristic scale-dependent feature, and it is expected that we could
obtain more detailed information on the IGM in the CD and EoR by using the 21cm
bispectrum in the future experiments, combined with the power spectrum and
skewness.

DOI： 10.1093/mnras/stw482

Web of Science

Scopus

arXiv

Language：English   Publisher：AMER PHYSICAL SOC  

The conventional slow-roll approximation is broken in the so-called "ultra
slow-roll" models of inflation, for which the inflaton potential is exactly (or
extremely) flat. The interesting nature of (canonical) ultra slow-roll
inflation is that the curvature perturbation grows on superhorizon scales, but
has a scale-invariant power spectrum. We study the ultra slow-roll inflationary
dynamics in the presence of non-canonical kinetic terms of the scalar field,
namely ultra slow-roll G-inflation. We compute the evolution of the curvature
perturbation and show that the primordial power spectrum follows a broken power
law with an oscillation feature. It is demonstrated that this could explain the
lack of large-scale power in the cosmic microwave background temperature
anisotropies. We also point out that the violation of the null energy condition
is prohibited in ultra slow-roll G-inflation and hence a blue tensor tilt is
impossible as long as inflation is driven by the potential. This statement is,
however, not true if the energy density is dominated by the kinetic energy of
the scalar field.

DOI： 10.1103/PhysRevD.94.103515

Web of Science

arXiv

Language：English   Publisher：AMER PHYSICAL SOC  

We point out that the gravitational-wave event GW150914 observed by the LIGO
detectors can be explained by the coalescence of primordial black holes (PBHs).
It is found that the expected PBH merger rate would exceed the rate estimated
by the LIGO scientific Collaboration and Virgo Collaboration if PBHs were the
dominant component of dark matter, while it can be made compatible if PBHs
constitute a fraction of dark matter. Intriguingly, the abundance of PBHs
required to explain the suggested lower bound on the event rate, $> 2$ events
${\rm Gpc}^{-3} {\rm yr}^{-1}$, roughly coincides with the existing upper limit
set by the nondetection of the cosmic microwave background spectral distortion.
This implies that the proposed PBH scenario may be tested in the
not-too-distant future.

DOI： 10.1103/PhysRevLett.117.061101

Web of Science

PubMed

arXiv

Language：English   Publisher：OXFORD UNIV PRESS  

The redshifted 21cm line signal from neutral hydrogens is a promising tool to
probe the cosmic dawn and the epoch of reionization (EoR). Ongoing and future
low-frequency radio experiments are expected to detect its fluctuations,
especially through the power spectrum. In this paper, we give a physical
interpretation of the time evolution of the power spectrum of the 21cm
brightness temperature fluctuations, which can be decomposed into dark matter
density, spin temperature and neutral fraction of hydrogen fluctuations. From
the one-point statistics of the fluctuations, such as variance and skewness, we
find that the peaks and dips in the time evolution are deeply related to X-ray
heating of the intergalactic gas, which controls the spin temperature. We
suggest the skewness of the brightness temperature distribution is a key
observable to identify the onset of X-ray heating.

DOI： 10.1093/mnras/stv965

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Primordial black holes (PBHs) are theoretical black holes which can be formed during the radiation dominant era through the gravitational collapse of radiational overdensities. It has been well known that in the context of the structure formation in our Universe such collapsed objects, e.g., halos/galaxies, could be considered as bias tracers of underlying matter fluctuations and the halo/galaxy bias has been studied well. Applying such a biased tracer picture to PBHs, we investigate the large scale clustering of PBHs and obtain an almost mass-independent constraint to the scenario that the dark matter (DM) consists of PBHs. We focus on the case where the statistics of the primordial curvature perturbations is almost Gaussian, but with small local-type non-Gaussianity. If PBHs account for the DM abundance, such a large scale clustering of PBHs behaves as nothing but the matter isocurvature perturbation which is strictly constrained by the observations of cosmic microwave backgrounds (CMBs). From this constraint, we show that, in the case where a certain single field causes both CMB temperature perturbations and PBH formation, the PBH-DM scenario is excluded even with quite small local-type non-Gaussianity, |fNL|∼O(0.01).

DOI： 10.1103/PhysRevD.91.123534

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We investigate the effect of equilateral-type primordial trispectrum on the halo/galaxy bispectrum. We consider three types of equilateral primordial trispectra which are generated by quartic operators naturally appearing in the effective field theory of inflation and can be characterized by three nonlinearity parameters, gNLσ4, gNLσ2(∂σ)2, and gNL(∂σ)4. Recently, constraints on these parameters have been investigated from Cosmic Microwave Background (CMB) observations by using WMAP9 data. In order to consider the halo/galaxy bispectrum with the equilateral-type primordial trispectra, we adopt the integrated perturbation theory in which the effects of primordial non-Gaussianity are wholly encapsulated in the linear primordial polyspectrum for the evaluation of the biased polyspectrum. We show the shapes of the halo/galaxy bispectrum with the equilateral-type primordial trispectra and find that the primordial trispectrum characterized by gNLσ4 provides the same scale dependence as the gravity-induced halo/galaxy bispectrum. Hence, it would be difficult to obtain the constraint on gNLσ4 from the observations of the halo/galaxy bispectrum. On the other hand, the primordial trispectra characterized by gNLσ2(∂σ)2 and gNL(∂σ)4 provide the common scale dependence which is different from that of the gravity-induced halo/galaxy bispectrum on large scales. Hence, future observations of the halo/galaxy bispectrum would give constraints on the nonlinearity parameters, gNLσ2(∂σ)2 and gNL(∂σ)4 independently from CMB observations.

DOI： 10.1103/PhysRevD.91.123521

Scopus

Language：English   Publisher：IOP PUBLISHING LTD  

We calculate the CMB $\mu$-distortion and the angular power spectrum of its
cross-correlation with the temperature anisotropy in the presence of the
non-Gaussian neutrino isocurvature density (NID) mode. While the pure Gaussian
NID perturbations give merely subdominant contribution to $<\mu>$ and vanishing
$< \mu T>$, the latter quantity can be large enough to be detected in the
future when the NID perturbations $\mathcal S(\mathbf x)$ are proportional to
the square of a Gaussian field $g(\mathbf x)$, i.e. $\mathcal S({\mathbf
x})\propto g^2({\mathbf x})$. In particular, large $< \mu T>$ can be realized
since Gaussian-squared perturbations can yield a relatively large bispectrum,
satisfying the constraints from the power spectrum of CMB anisotropies, i.e.
$\mathcal{P}_\mathcal{SS}(k_0) \sim\mathcal{P}_g^2(k_0)\lesssim10^{-10}$ at
$k_0=0.05$ Mpc$^{-1}$. We also forecast constraints from the CMB temperature
and E-mode polarisation bispectra, and show that
$\mathcal{P}_g(k_0)\lesssim10^{-5}$ would be allowed from Planck data. We find
that $< \mu >$ and $|l(l+1)C^{\mu T}_l|$ can respectively be as large as
$10^{-9}$ and $10^{-14}$ with uncorrelated scale-invariant NID perturbations
for $\mathcal{P}_g(k_0)=10^{-5}$. When the spectrum of the Gaussian field is
blue-tilted (with spectral index $n_g \simeq 1.5$), $< \mu T>$ can be enhanced
by an order of magnitude.

DOI： 10.1088/1475-7516/2015/03/013

Web of Science

arXiv

Language：English   Publisher：OXFORD UNIV PRESS INC  

We consider the possibility of enhancing the inflationary tensor mode by
introducing a spectator scalar field with a small sound speed which induces
gravitational waves as a second order effect. We analytically obtain the power
spectra of gravitational waves and curvature perturbation induced by the
spectator scalar field. We found that the small sound speed amplifies the
curvature perturbation much more than the tensor mode and the current
observational constraint forces the induced gravitational waves to be
negligible compared with those from the vacuum fluctuation during inflation.

DOI： 10.1093/ptep/ptv037

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/1411.3658v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Although the cosmological paradigm based on cold dark matter and adiabatic,
nearly scale-invariant primordial fluctuations is consistent with a wide
variety of existing observations, it has yet to be sufficiently tested on
scales smaller than those of massive galaxies, and various alternatives have
been proposed that differ significantly in the consequent small-scale power
spectrum (SSPS) of large-scale structure. Here we show that a powerful probe of
the SSPS at $k\gtrsim 10$ Mpc$^{-1}$ can be provided by the 21 cm forest, that
is, systems of narrow absorption lines due to intervening, cold neutral
hydrogen in the spectra of high-redshift background radio sources in the cosmic
reionization epoch. Such features are expected to be caused predominantly by
collapsed gas in starless minihalos, whose mass function can be very sensitive
to the SSPS. As specific examples, we consider the effects of neutrino mass,
running spectral index (RSI) and warm dark matter (WDM) on the SSPS, and
evaluate the expected distribution in optical depth of 21 cm absorbers out to
different redshifts. Within the current constraints on quantities such as the
sum of neutrino masses $\sum m_\nu$, running of the primordial spectral index
$d n_s/d \ln k$ and WDM particle mass $m_{\rm WDM}$, the statistics of the 21
cm forest manifest observationally significant differences that become larger
at higher redshifts. In particular, it may be possible to probe the range of
$m_{\rm WDM} \gtrsim 10$ keV that may otherwise be inaccessible. Future
observations of the 21 cm forest by the Square Kilometer Array may offer a
unique and valuable probe of the SSPS, as long as radio sources such as quasars
or Population III gamma-ray bursts with sufficient brightness and number exist
at redshifts of $z \gtrsim$ 10 - 20, and the astrophysical effects of
reionization and heating can be discriminated.

DOI： 10.1103/PhysRevD.90.083003

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

Matter isocurvature perturbations are strictly constrained from cosmic
microwave background observations. We study a sneutrino curvaton model where
both cold dark matter (CDM)/baryon isocurvature perturbations are generated. In
our model, total matter isocurvature perturbations are reduced since the
CDM/baryon isocurvature perturbations compensate for each other. We show that
this model can not only avoid the stringent observational constraints but also
suppress temperature anisotropies on large scales, which leads to improved
agreement with observations.

DOI： 10.1088/1475-7516/2014/10/068

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/1409.1669v1

Language：English   Publisher：IOP PUBLISHING LTD  

We study the impact of a temperature-dependent curvaton decay rate on the
primordial curvature perturbation generated in the curvaton scenario. Using the
familiar sudden decay approximation, we obtain an analytical expression for the
curvature perturbation after the decay of the curvaton. We then investigate
numerically the evolution of the background and of the perturbations during the
decay. We first show that the instantaneous transfer coefficient, related to
the curvaton energy fraction at the decay, can be extended into a more general
parameter, which depends on the net transfer of the curvaton energy into
radiation energy or, equivalently, on the total entropy ratio after the
complete curvaton decay. We then compute the curvature perturbation and compare
this result with the sudden decay approximation prediction.

DOI： 10.1088/1475-7516/2014/10/032

Web of Science

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We investigate constraints on the spectral index of primordial gravitational
waves (GWs), paying particular attention to a blue-tilted spectrum. Such
constraints can be used to test a certain class of models of the early
Universe. We investigate observational bounds from LIGO+Virgo, pulsar timing
and big bang nucleosynthesis, taking into account the suppression of the
amplitude at high frequencies due to reheating after inflation and also
late-time entropy production. Constraints on the spectral index are presented
by changing values of parameters such as reheating temperatures and the amount
of entropy produced at late time. We also consider constraints under the
general modeling approach which can approximately describe various scenarios of
the early Universe. We show that the constraints on the blue spectral tilt
strongly depend on the underlying assumption and, in some cases, a highly
blue-tilted spectrum can still be allowed.

DOI： 10.1088/1475-7516/2015/02/003

Web of Science

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We investigate the degeneracy of the isocurvature perturbations and the
primordial gravitational waves, by using recent observations of the cosmic
microwave background (CMB) reported by Planck and BICEP2 collaborations. We
show that the tension in the bound on the tensor-to-scalar ratio $r$ between
Planck and BICEP2 can be resolved by introducing the anti-correlated
isocurvature perturbations. Quantitatively, we find that with the
anti-correlated isocurvature perturbations the constraints on $r$ from Planck
alone and BICEP2 results can be consistent at 68 % C.L.

DOI： 10.1088/1475-7516/2014/08/043

Web of Science

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We comprehensively explore the quadratic curvaton models in the chaotic
inflation. In the light of the BICEP2 result $r \approx 0.2$, all model
parameters and relevant observables are computed. It is found the curvaton
field value is constrained into a narrow range, $\sigma_* =
\mathcal{O}(10^{-2}$-$10^{-1})$ and the running of the spectral index is $n_s'
\gtrsim -10^{-3}$. We show that if the curvaton is added, the models are
heavily degenerated on the $n_s$ - $r$ plane. However, introducing a new plane,
the degeneracy can be resolved. To distinguish the curvaton models, precise
measurements of not only $r$ but also $n_s'$ and the tensor tilt $n_T$ are
required.

DOI： 10.1088/1475-7516/2014/09/015

Web of Science

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

Recently, BICEP2 has reported the large tensor-to-scalar ratio
$r=0.2^{+0.07}_{-0.05}$ from the observation of the cosmic microwave background
(CMB) B-mode at degree-scales. Since tensor modes induce not only CMB B-mode
but also the temperature fluctuations on large scales, to realize the
consistent temperature fluctuations with the Planck result we should consider
suppression of scalar perturbations on corresponding large scales. To realize
such a suppression, we consider anti-correlated iso-curvature perturbations
which could be realized in the simple curvaton model.

DOI： 10.1088/1475-7516/2014/05/046

Web of Science

arXiv

Publisher：AMER PHYSICAL SOC  

We investigate the power spectrum of the distortion of Cosmic Microwave
Background (CMB) due to the decay of the primordial magnetic fields. It is
known that there are two-types of the CMB distortions, so-called \mu- and
y-types and we find that the signal of the y-type distortion becomes larger
than that of the \mu-type one. We also discuss cross power spectra between the
CMB distortions and the CMB temperature anisotropy, which are naturally
generated due to the existence of the primordial magnetic fields. We find that
such cross power spectra have small amplitudes compared with the auto-power
spectra of the CMB distortions because of the Silk damping effect of the
temperature anisotropy. We also investigate the possibility of detecting such
signal in the future CMB experiments, including not only absolutely calibrated
experiments such as PIXIE but also relatively calibrated experiments such as
LiteBIRD and CMBpol.

DOI： 10.1103/PhysRevD.89.063508

Web of Science

Scopus

arXiv

Publisher：Physical Review D - Particles, Fields, Gravitation and Cosmology  

DOI： 10.1103/PhysRevD.89.043524

Scopus

DOI： 10.1103/PhysRevD.89.043524

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

We derive a formula for the halo/galaxy bispectrum on the basis of the integrated perturbation theory (iPT). In addition to the gravity-induced non-Gaussianity, we consider the non-Gaussianity of the primordial curvature perturbations and investigate in detail the effect of such primordial non-Gaussianity on the large-scale halo/galaxy bispectrum. In iPT, the effects of primordial non-Gaussianity are wholly encapsulated in the linear (primordial) polyspectra, and we systematically calculate the contributions to the large-scale behaviors arising from the three types of primordial bispectrum (local, equilateral, and orthogonal types), and primordial trispectrum of the local-type non-Gaussianity. We find that the equilateral- and orthogonal-type non-Gaussianities show distinct scale-dependent behaviors which can dominate the gravity-induced non-Gaussianity at very large scales. For the local-type non-Gaussianity, higher-order loop corrections are found to give a significantly large contribution to the halo/galaxy bispectrum of the squeezed shape and eventually dominate over the other contributions on large scales. A diagrammatic approach based on the iPT helps us to systematically investigate an impact of such higher-order contributions to the large-scale halo/galaxy bispectrum. © 2014 American Physical Society.

DOI： 10.1103/PhysRevD.89.043524

Scopus

Language：English   Publisher：IOP PUBLISHING LTD  

Recently, there are several reports that the cosmic magnetic fields on Mpc
scale in void region is larger than $\sim 10^{-15}$G with an uncertainty of a
few orders from the current blazar observations. On the other hand, in
inflationary magnetogenesis models, additional primordial curvature
perturbations are inevitably produced from iso-curvature perturbations due to
generated electromagnetic fields. We explore such induced curvature
perturbations in a model independent way and obtained a severe upper bound for
the energy scale of inflation from the observed cosmic magnetic fields and the
observed amplitude of the curvature perturbation, as $\rho_{\rm inf}^{1/4} <
30{\rm GeV} \times (B_{\rm obs}/10^{-15}{\rm G})^{-1}$ where $B_{\rm obs}$ is
the strength of the magnetic field at present. Therefore, without a dedicated
low energy inflation model or an additional amplification of magnetic fields
after inflation, inflationary magnetogenesis on Mpc scale is generally
incompatible with CMB observations.

DOI： 10.1088/1475-7516/2014/03/013

Web of Science

arXiv

Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We show that a very light scalar field experiencing quantum fluctuations during primordial inflation can explain the current cosmic acceleration. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the universe only recently. Assuming this scenario to be correct, and using supernovae data, the model predicts the energy scale of primordial inflation to be around a few TeV and suggests that it has lasted for an extremely long period. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale.

DOI： 10.1088/1742-6596/485/1/012023

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

DOI： 10.1088/1475-7516/2014/05/E02

Scopus

Language：English   Publisher：AMER PHYSICAL SOC  

We derive a formula for the halo/galaxy bispectrum on the basis of the
integrated Perturbation Theory (iPT). In addition to the gravity-induced
non-Gaussianity, we consider the non-Gaussianity of the primordial curvature
perturbations, and investigate in detail the effect of such primordial
non-Gaussianity on the large-scale halo/galaxy bispectrum. In iPT, the effects
of primordial non-Gaussianity are wholly encapsulated in the linear
(primordial) polyspectra, and we systematically calculate the contributions to
the large-scale behaviors arising from the three types of primordial bispectrum
(local-, equilateral-, and orthogonal-types), and primordial trispectrum of the
local-type non-Gaussianity. We find that the equilateral- and orthogonal-type
non-Gaussianities show distinct scale-dependent behaviors which can dominate
the gravity-induced non-Gaussianity at very large scales. For the local-type
non-Gaussianity, higher-order loop corrections are found to give a
significantly large contribution to the halo/galaxy bispectrum of the squeezed
shape, and eventually dominate over the other contributions on large scales. A
diagrammatic approach based on the iPT helps us to systematically investigate
an impact of such higher-order contributions to the large-scale halo/galaxy
bispectrum.

DOI： 10.1103/PhysRevD.89.043524

Web of Science

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

We compute the power spectrum ζ and non-linear parameters fNL and τNL of the curvature perturbation induced during inflation by the electromagnetic fields in the kinetic coupling model (IFF model). By using the observational result of ζ,fNL and τNL reported by the Planck collaboration, we study the constraint on the model comprehensively. Interestingly, if the single slow-rolling inflaton is responsible for the observed ζ, the constraint from τNL is most stringent. We also find a general relationship between fNL and τNL generated in this model. Even if fNL ∼ (1), a detectable τNL can be produced. © 2013 IOP Publishing Ltd and Sissa Medialab srl.

DOI： 10.1088/1475-7516/2013/09/009

Scopus

arXiv

Anisotropic stress perturbations induced by primordial Gaussian vector fields
create non-Gaussianity in curvature perturbations. We found that such
non-Gaussianity closely resembles the local-type non-Gaussianity parametrized
by $f_{\rm NL}$, and generates scale-dependent bias in large-scale structures.
We also found a simple relationship between the scale-dependent bias and the
power spectrum of the vector fields. When the vector fields are interpreted as
primordial magnetic fields, the effective $f_{\rm NL}$ is shown to be always
negative. The scale-dependent bias provides a new approach to probing
primordial vector fields.

DOI： 10.1093/mnras/stt594

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We investigate the gravitational wave background induced by the first order
scalar perturbations in the curvaton models. We consider the quadratic and
axion-like curvaton potential which can generate the blue-tilted power spectrum
of curvature perturbations on small scales and derive the maximal amount of
gravitational wave background today. We find the power spectrum of the induced
gravitational wave background has a characteristic peak at the frequency
corresponding to the scale reentering the horizon at the curvaton decay, in the
case where the curvaton does not dominate the energy density of the Universe.
We also find the enhancement of the amount of the gravitational waves in the
case where the curvaton dominates the energy density of the Universe. Such
induced gravitational waves would be detectable by the future space-based
gravitational wave detectors or pulsar timing observations.

DOI： 10.1088/1475-7516/2013/08/042

Web of Science

arXiv

We compute the full bispectra, namely both auto- and cross- bispectra, of
primordial curvature and tensor perturbations in the most general single-field
inflation model whose scalar and gravitational equations of motion are of
second order. The formulae in the limits of k-inflation and potential-driven
inflation are also given. These expressions are useful for estimating the full
bispectra of temperature and polarization anisotropies of the cosmic microwave
background radiation.

DOI： 10.1093/ptep/ptt031

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We discuss implications of Planck results for models with local type
non-Gaussianity. In light of the recent results of the Planck satellite, we
constrain model parameters of several representative models and give the
prediction of trispectrum, in particular, gNL. We also consider interesting
possibilities that trispectrum appears as the first signature of the
non-Gaussianities of the curvature perturbations, that is, fNL is small while
gNL can be significantly large.

DOI： 10.1088/1475-7516/2013/06/012

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/1303.5374v3

Language：English   Publisher：IOP PUBLISHING LTD  

We provide a general formula for calculating correlators of arbitrary
function of a Gaussian field. This work extends the standard leading-order
approximation based on the delta N formalism to the case where truncation of
the delta N at some low order does not yield the correct answer. As an
application of this formula, we investigate 2, 3 and 4-point functions of the
primordial curvature perturbation generated in the massless preheating model by
approximating the mapping between the curvature perturbation and the Gaussian
field as a sum of the many spiky normal distribution functions as suggested by
lattice calculations. We also discuss observational consequences of this case
and show that trispectrum would be a key observable to search signature of
preheating in the CMB map. It is found the forms of the curvature correlation
functions for any delta N, at the leading order in the correlator of the
Gaussian field, coincide with the standard local type ones. Within this
approximation, it is also found that the standard formula for the non-linearity
parameters given by the product of the derivatives of the e-folding number
still holds after we replace the bare e-folding number appearing in the
original delta N expansion with the one smoothed in the field space with a
Gaussian window function.

DOI： 10.1088/1475-7516/2013/06/018

Web of Science

arXiv

Language：English   Publisher：AMER PHYSICAL SOC  

We analytically derive a more accurate formula for the power spectrum of the
biased objects with the primordial non-Gaussianity parameterized not only by
the non-linearity parameter fNL, but also by gNL and tauNL which characterize
the trispectrum of the primordial curvature perturbations. We adopt the
integrated perturbation theory which was constructed in Matsubara (2011). We
discuss an inequality between fNL and tauNL in the context of the
scale-dependent bias, by introducing a stochasticity parameter. We also mention
higher order loop corrections into the scale-dependency of the bias parameter.

DOI： 10.1103/PhysRevD.87.023525

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1210.2495v1

We study temperature and polarization anisotropies of the cosmic microwave
background (CMB) radiation sourced from primordial cross-bispectra between
metric perturbations and vector fields, which are generated from the inflation
model where an inflaton and a vector field are coupled. In case the vector
field survives after the reheating, both the primordial scalar and tensor
fluctuations can be enhanced by the anisotropic stress composed of the vector
fields during radiation dominated era. We show that through this enhancement
the primordial cross-bispectra generate not only CMB bispectra but also CMB
power spectra. In general, we can expect such cross-bispectra produce the
non-trivial mode-coupling signals between the scalar and tensor fluctuations.
However, we explicitly show that such mode-coupling signals do not appear in
CMB power spectra. Through the numerical analysis of the CMB scalar-mode power
spectra, we find that although signals from these cross-bispectra are smaller
than primary non-electromagnetic ones, these have some characteristic features
such as negative auto-correlations of the temperature and polarization modes,
respectively. On the other hand, signals from tensor modes are almost
comparable to primary non-electromagnetic ones and hence the shape of observed
$B$-mode spectrum may deviate from the prediction in the non-electromagnetic
case. The above imprints may help us to judge the existence of the coupling
between the scalar and vector fields in the early Universe.

DOI： 10.1088/1475-7516/2012/11/046

Web of Science

Scopus

arXiv

We investigate bounds on the strength of the primordial magnetic field (PMF)
from the cosmic microwave background (CMB) bispectra of the intensity
(temperature) modes induced from the auto- and cross-correlated bispectra of
the scalar and tensor components of the PMF anisotropic stress. At first, we
construct a general formula for the CMB intensity and polarization bispectra
from PMFs composed of any type of perturbation. Then we derive an approximate
expression which traces the exact shape of the CMB bispectrum in order to
reduce the computation time with respect to a large number of the multipole
configurations, and also show that the non-Gaussian structure coming from PMFs
is classified as the local-type configuration. Computing the signal-to-noise
ratio on the basis of the approximate formula with the information of the
instrumental noises and resolutions, we find expected upper bounds on the
magnetic field strength, when the magnetic spectrum is nearly scale invariant
($n_B = -2.9$), smoothed on $1 {\rm Mpc}$ scale at 95% confidence level from
the WMAP and PLANCK experiments as $B_{1 \rm Mpc} < 4.0 - 6.7 {\rm nG}$ and
$3.8 - 6.5 {\rm nG}$, respectively, depending on the energy scale of the
magnetic field production from $10^{14} {\rm GeV}$ to $10^3 {\rm GeV}$. Our new
consequences imply slight overestimations by the previous rough discussions.

DOI： 10.1088/1475-7516/2012/03/041

Web of Science

Scopus

arXiv

Language：English  

Publisher：Progress of Theoretical Physics Supplement  

We discuss generation of non-Gaussianity in primordial density perturbation through the super-horizon evolution during inflation by using the so-called δN formalism. In order to discuss the difficulties of generating large local-type non-Gaussianity during inflation, we discuss Kadota-Stewart model. This model gives an example in which we have to choose rather unnatural initial conditions even if large non-Gaussianity can be generated.

Scopus

Publisher：IOP PUBLISHING LTD  

A possible origin of the anomalous dip and bump in the primordial power
spectrum, which are reconstructed from WMAP data corresponding to the multipole
$\ell=100\sim 140$ by using the inversion method, is investigated as a
consequence of modification of scalar field dynamics in the inflation era.
Utilizing an analytic formula to handle higher order corrections to the
slow-roll approximation, we evaluate the relation between a detailed shape of
inflaton potential and a fine structure in the primordial power spectrum. We
conclude that it is unlikely to generate the observed dip and bump in the power
spectrum by adding any features in the inflaton potential. Though we can make a
fine enough shape in the power spectrum by controlling the feature of the
potential, the amplitude of the dip and bump becomes too small in that case.

DOI： 10.1088/1475-7516/2011/12/008

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We investigate the halo bias in the case where the primordial curvature
fluctuations, $\Phi$, are sourced from both a Gaussian random field and a
Gaussian-squared field, as $\Phi({\bf x}) = \phi({\bf x}) + \psi({\bf x})^2 -
<\psi({\bf x})^2>$, so-called "ungaussiton model". We employ the
peak-background split formula and find a new scale-dependence in the halo bias
induced from the Gaussian-squared field.

DOI： 10.1088/1475-7516/2011/11/001

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1108.5569v1

Web of Science

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the cosmic microwave background (CMB) bispectra of the intensity (temperature) and polarization modes induced by the graviton non-Gaussianities, which arise from the parity-conserving and parity-violating Weyl cubic terms with time-dependent coupling. By considering the time-dependent coupling, we find that even in the exact de Sitter space time, the parity violation still appears in the three-point function of the primordial gravitational waves and could become large. Through the estimation of the CMB bispectra, we demonstrate that the signals generated from the parity-conserving and parity-violating terms appear in completely different configurations of multipoles. For example, the parityconserving non-Gaussianity induces the nonzero CMB temperature bispectrum in the configuration with Σ3n=1 ln = even and, while due to the parity-violating non-Gaussianity, the CMB temperature bispectrum also appears for Σ3n=1 ln = odd. This signal is just good evidence of the parity violation in the non-Gaussianity of primordial gravitational waves. We find that the shape of this non-Gaussianity is similar to the so-called equilateral one and the amplitudes of these spectra at large scale are roughly estimated as |blll| ~ l-4 × 3.2 × 10-2 (GeV/Λ)2 (r/0.1)4, where Λ is an energy scale that sets the magnitude of the Weyl cubic terms (higher derivative corrections) and r is a tensor-to-scalar ratio. Taking the limit for the nonlinearity parameter of the equilateral type as f eqNL &lt
300, we can obtain a bound as Λ &gt
3 × 106 GeV, assuming r = 0.1.

DOI： 10.1143/PTP.126.937

Scopus

arXiv

Publisher：Publication Office, Progress of Theoretical Physics  

We discuss generation of non-Gaussianity in primordial density perturbation through the super-horizon evolution during inflation by using the so-called δN formalism. In order to discuss the difficulties of generating large local-type non-Gaussianity during inflation, we discuss Kadota-Stewart model. This model gives an example in which we have to choose rather unnatural initial conditions even if large non-Gaussianity can be generated.

DOI： 10.1143/PTPS.190.135

Language：English   Publishing type：Research paper (scientific journal)  

We study the scale-dependent bias of the halo power spectrum arising from primordial non-Gaussianity. We present an analytic result of the halo bias including up to the trispectrum contributions. We find that the scale-dependent bias opens a new possibility of probing the relation between the non-linearity parametersfNL and τNL. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

DOI： 10.1111/j.1745-3933.2011.01124.x

Web of Science

Scopus

Language：English   Publisher：OXFORD UNIV PRESS  

We study the halo mass function in the presence of the kurtosis type of
primordial non-Gaussianity. The kurtosis corresponds to the trispectrum as
defined in Fourier space. The primordial trispectrum is commonly characterized
by two parameters, $\tau_{\rm NL}$ and $g_{\rm NL}$. As applications of the
derived non-Gaussian mass function, we consider the effect on the abundance of
void structure, the effect on early star formation and on formation of the most
massive object at high redshift. We show that by comparing the effects of
primordial non-Gaussianity on cluster abundance with that on void abundance, we
can distinguish between the skewness and the kurtosis types of primordial
non-Gaussianity. As for early star formation, we show that the kurtosis type of
primordial non-Gaussianity seems not to affect the reionization history of the
Universe on average. However, at high redshifts (up to $z\simeq 20$) such
non-Gaussianity does somewhat affect the early stages of reionization.

DOI： 10.1111/j.1365-2966.2011.19323.x

Web of Science

Scopus

arXiv

We investigate a statistical anisotropy on the Cosmic Microwave Background
(CMB) bispectrum, which can be generated from the primordial non-Gaussianity
induced by quantum fluctuations of a vector field. We find new configurations
in the multipole space of the CMB bispectrum given by $\ell_1 = \ell_2 + \ell_3
+ 2, |\ell_2 - \ell_3| - 2$ and their permutations, which violate the
rotational invariance, such as an off-diagonal configuration in the CMB power
spectrum. We also find that in a model presented by Yokoyama and Soda (2008),
the amplitude of the statistically anisotropic bispectrum in the above
configurations becomes as large as that in other configurations such as $\ell_1
= \ell_2 + \ell_3$. As a result, it might be possible to detect these
contributions in future experiments, which would give us novel information
about the physics of the early Universe.

DOI： 10.1143/PTP.126.923

Web of Science

Scopus

arXiv

Publisher：IOP PUBLISHING LTD  

For the local-type primordial perturbation, it is known that there is an
inequality between the bispectrum and the trispectrum. By using the
diagrammatic method, we develop a general formalism to systematically construct
the similar inequalities up to any order correlation function. As an
application, we explicitly derive all the inequalities up to six and
eight-point functions.

DOI： 10.1088/1475-7516/2011/07/033

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1105.5851v2

We present a detailed calculation of our previous short paper [M. Shiraishi,
D. Nitta, S. Yokoyama, K. Ichiki, and K. Takahashi, Phys. Rev. D 82, 121302
(2010).] in which we have investigated a constraint on the magnetic field
strength through comic microwave background temperature bispectrum of vector
modes induced from primordial magnetic fields. By taking into account full
angular dependence of the bispectrum with spin spherical harmonics and Wigner
symbols, we explicitly show that the cosmic microwave background bispectrum
induced from the statistical-isotropic primordial vector fluctuations can be
also described as an angle-averaged form in the rotationally invariant way. We
also study the cases with different spectral indices of the power spectrum of
the primordial magnetic fields.

DOI： 10.1103/PhysRevD.83.123523

Web of Science

Scopus

arXiv

Publishing type：Research paper (scientific journal)  

If the seed magnetic fields exist in the early Universe, tensor components of
their anisotropic stresses are not compensated prior to neutrino decoupling and
the tensor metric perturbations generated from them survive passively.
Consequently, due to the decay of these metric perturbations after
recombination, the so-called integrated Sachs-Wolfe effect, the large-scale
fluctuations of CMB radiation are significantly boosted. This kind of CMB
anisotropy is called the "tensor passive mode." Because these fluctuations
deviate largely from the Gaussian statistics due to the quadratic dependence on
the strength of the Gaussian magnetic field, not only the power spectrum but
also the higher-order correlations have reasonable signals. With these motives,
we compute the CMB bispectrum induced by this mode. When the magnetic spectrum
obeys a nearly scale-invariant shape, we obtain an estimation of a typical
value of the normalized reduced bispectrum as $\ell_1(\ell_1 +
1)\ell_3(\ell_3+1)|b_{\ell_1\ell_2\ell_3}| \sim (130-6) \times 10^{-16} (B_{1
\rm Mpc} / 4.7 {\rm nG})^6$ depending on the energy scale of the magnetic field
production from $10^{14}$GeV to $10^3$GeV. Here, $B_{1 {\rm Mpc } }$ is the
strength of the primordial magnetic field smoothed on $1 {\rm Mpc}$. From the
above estimation and the current observational constraint on the primordial
non-Gaussianity, we get a rough constraint on the magnetic field strength as
$B_{1 {\rm Mpc } } < 2.6 - 4.4 {\rm nG}$.

DOI： 10.1103/PhysRevD.83.123003

Web of Science

Scopus

arXiv

Web of Science

Publisher：IOP PUBLISHING LTD  

Modulated reheating scenario is one of the most attractive models that
predict possible detections of not only the primordial non-Gaussianity but also
the tensor fluctuation through future CMB observations such as the Planck
satellite, the PolarBeaR and the LiteBIRD satellite experiments. We study the
baryonic-isocurvature fluctuations in the Affleck-Dine baryogenesis with the
modulated reheating scenario. We show that the Affleck-Dine baryogenesis can be
consistent with the modulated reheating scenario with respect to the current
observational constraint on the baryonic-isocurvature fluctuations.

DOI： 10.1088/1475-7516/2011/01/027

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1008.1450v2

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PHYSREVD.83.029901

Web of Science

Language：English  

Publisher：Physical Review D - Particles, Fields, Gravitation and Cosmology  

DOI： 10.1103/PhysRevD.82.121302

Web of Science

Scopus

We present an all-sky formalism for the Cosmic Microwave Background (CMB)
bispectrum induced by the primordial non-Gaussianities not only in scalar but
also in vector and tensor fluctuations. We find that the bispectrum can be
formed in an explicitly rationally invariant way by taking into account the
angular and polarization dependences of the vector and tensor modes. To
demonstrate this and present how to use our formalism, we consider a specific
example of the correlation between two scalars and a graviton as the source of
non-Gaussianity. As a result, we show that the CMB reduced bispectrum of the
intensity anisotropies is evaluated as a function of the multipole and the
coupling constant between two scalars and a graviton denoted by $g_{tss}$;
$|b_{\ell \ell \ell}| \sim \ell^{-4} \times 8 \times 10^{-18} |g_{tss}|$. By
estimating the signal-to-noise ratio, we find that the constraint as $|g_{tss}|
< 6$ will be expected from the PLANCK experiment.

DOI： 10.1143/PTP.125.795

Web of Science

Scopus

arXiv

Publisher：Proceedings of the 20th Workshop on General Relativity and Gravitation in Japan, JGRG 2010  

Scopus

Language：English   Publisher：IOP PUBLISHING LTD  

We classify models generating large local-type non-Gaussianity into some
categories by using some "consistency relations" among the non-linearity
parameters f_{NL}^{local}, \tau_{NL}^{local} and g_{NL}^{local}, which
characterize the size of bispectrum for the former and trispectrum for the
later two. Then we discuss how one can discriminate models of large local-type
non-Gaussianity with such relations. We first classify the models by using the
ratio of \tau_{NL}^{local}/(6f_{NL}^{local}/5)^2, which is unity for
"single-source" models and deviates from unity for "multi-source" ones. We can
make a further classification of models in each category by utilizing the
relation between f_{NL}^{local} and g_{NL}^{local}. Our classification suggests
that observations of trispectrum would be very helpful to distinguish models of
large non-Gaussianity and may reveal the generation mechanism of primordial
fluctuations.

DOI： 10.1088/1475-7516/2010/12/030

Web of Science

Scopus

arXiv

We present a complete set of formulae for calculating the bispectra of CMB
temperature and polarization anisotropies generated from non-Gaussianity in the
vector and tensor mode perturbations. In the all-sky analysis it is found that
the bispectrum formulae for the tensor and vector-mode non-Gaussianity formally
take complicated forms compared to the scalar mode one because the photon
transfer functions in the tensor and vector modes depend on the azimuthal angle
between the direction of the wave number vector of the photon's perturbation
and that of the line of sight. We demonstrate that flat-sky approximations
remove this difficulty because this kind of azimuthal angle dependence
apparently vanishes in the flat-sky limit. Through the flat-sky analysis, we
also find that the vector or tensor bispectrum of $B$-mode polarization
vanishes in the squeezed limit, unless the cosmological parity is violated at
the nonlinear level.

DOI： 10.1103/PhysRevD.82.103505

Web of Science

Scopus

arXiv

Publisher：AMER PHYSICAL SOC  

We show that current cosmic acceleration can be explained by an almost
massless scalar field experiencing quantum fluctuations during primordial
inflation. Provided its mass does not exceed the Hubble parameter today, this
field has been frozen during the cosmological ages to start dominating the
universe only recently. By using supernovae data, completed with baryonic
acoustic oscillations from galaxy surveys and cosmic microwave background
anisotropies, we infer the energy scale of primordial inflation to be around a
few TeV, which implies a negligible tensor-to-scalar ratio of the primordial
fluctuations. Moreover, our model suggests that inflation lasted for an
extremely long period. Dark energy could therefore be a natural consequence of
cosmic inflation close to the electroweak energy scale.

DOI： 10.1103/PhysRevLett.105.121301

Web of Science

Scopus

PubMed

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We discuss generation of non-Gaussianity in density perturbation through the
super-horizon evolution during inflation by using the so-called $\delta N$
formalism. We first provide a general formula for the non-linearity parameter
generated during inflation. We find that it is proportional to the slow-roll
parameters, multiplied by the model dependent factors that may enhance the
non-gaussianity to the observable ranges. Then we discuss three typical
examples to illustrate how difficult to generate sizable non-Gaussianity
through the super-horizon evolution. First example is the double inflation
model, which shows that temporal violation of slow roll conditions is not
enough for the generation of non-Gaussianity. Second example is the ordinary
hybrid inflation model, which illustrates the importance of taking into account
perturbations on small scales. Finally, we discuss Kadota-Stewart model. This
model gives an example in which we have to choose rather unnatural initial
conditions even if large non-Gaussianity can be generated.

DOI： 10.1088/0264-9381/27/12/124003

Web of Science

arXiv

Publisher：IOP PUBLISHING LTD  

We revisit ghost dark matter, the possibility that ghost condensation may
serve as an alternative to dark matter. In particular, we investigate the
Friedmann-Robertson-Walker (FRW) background evolution and the large-scale
structure (LSS) in the $\Lambda$GDM universe, i.e. a late-time universe
dominated by a cosmological constant and ghost dark matter. The FRW background
of the $\Lambda$GDM universe is indistinguishable from that of the standard
$\Lambda$CDM universe if $M\gtrsim 1 {\rm eV}$, where $M$ is the scale of
spontaneous Lorentz breaking. From the LSS we find a stronger bound: $M\gtrsim
10 {\rm eV}$. For smaller $M$, ghost dark matter would have non-negligible
sound speed after the matter-radiation equality, and thus the matter power
spectrum would significantly differ from observation. These bounds are
compatible with the phenomenological upper bound $M\lesssim 100 {\rm GeV}$
known in the literature.

DOI： 10.1088/1475-7516/2010/05/007

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

In this talk, we have shown that current cosmic acceleration can be explained by an almost massless scalar field experiencing quantum fluctuations during primordial inflation. Provided its mass does not exceed the Hubble parameter today, this field has been frozen during the cosmological ages to start dominating the universe only recently. By using supernovae data, completed with baryonic acoustic oscillations from galaxy surveys and cosmic microwave background anisotropies, we infer the energy scale of primordial inflation to be around a few TeV, which implies a negligible tensor-to-scalar ratio of the primordial fluctuations. Moreover, our model suggests that inflation lasted for an extremely long period. Dark energy could therefore be a natural consequence of cosmic inflation close to the electroweak energy scale.

DOI： 10.1088/1742-6596/259/1/012082

Web of Science

Scopus

Publishing type：Research paper (international conference proceedings)  

We calculate CMB bispectrum of vector modes induced from primordial magnetic fields. We take into account the full angular dependence of the bispectrum and discuss the amplitude and also the shape of the bispectrum. In the squeezed limit, we estimate a typical values of the normalized reduced bispectrum as ℓ11(ℓ1+1) ℓ3(ℓ 3+1) |bℓ1ℓ2ℓ3|∼2 × 10 -19, for the strength of the primordial magnetic field smoothed on 1Mpc scale B1Mpc = 4.7nG assuming nearly scale-invariant spectrum of magnetic fields. We find that a new constraint on the magnetic field strength will be placed as B1Mpc < 10nG if PLANCK will place a limit on the nonlinearity parameter of local-type configuration as |flocalNL | < 5.

Scopus

Publisher：Proceedings of the 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009  

Scopus

Publisher：IOP PUBLISHING LTD  

We consider primordial fluctuations in thermal inflation scenario. Since the
thermal inflation drives about 10 $e$-folds after the standard inflation, the
time of horizon-exit during inflation corresponding to the present
observational scale shifts toward the end of inflation. It generally makes the
primordial power spectrum more deviated from a scale-invariant one and hence
renders some models inconsistent with observations. We present a mechanism of
generating the primordial curvature perturbation at the end of thermal
inflation utilizing a fluctuating coupling of a flaton field with the fields in
thermal bath. We show that, by adopting the mechanism, some inflation models
can be liberated even in the presence of the thermal inflation. We also discuss
non-Gaussianity in the mechanism and show that large non-Gaussianity can be
generated in this scenario.

DOI： 10.1088/1475-7516/2009/12/012

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

We investigate primordial non-Gaussianity and dark matter isocurvature fluctuations in the modulated reheating and the curvaton scenarios. In these scenarios, a large non-Gaussianity can be generated
on the other hand, depending on how dark matter is produced, too large isocurvature fluctuations can also arise, which is inconsistent with current observations. In this paper, we study this issue in a mixed scenario where the curvature fluctuations can also be produced from the inflaton fluctuations as well as those from a light scalar field such as the modulus and the curvaton. We show that primordial fluctuations can be highly non-Gaussian without conflicting with the current constraint on isocurvature fluctuations for such mixed scenarios. However, if the constraint on isocurvature fluctuations becomes severer as expected by the Planck satellite, fNL, a nonlinearity parameter for adiabatic fluctuations, should be very small as fNL3, which would give interesting implications for the generation mechanism of dark matter. Non-Gaussianity from isocurvature fluctuations is also discussed in these scenarios. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevD.80.063524

Web of Science

Scopus

Language：English   Publisher：ELSEVIER SCIENCE BV  

We show that, assuming the dispersion relation proposed recently by Horava in
the context of quantum gravity, radiation energy density exhibits a peculiar
dependence on the scale factor; the radiation energy density decreases
proportional to a^{-6}. This simple scaling can have an impact on cosmology. As
an example, we show that the resultant baryon asymmetry as well as the
stochastic gravity waves can be enhanced. We also discuss current observational
constraint on the dispersion relation.

DOI： 10.1016/j.physletb.2009.07.005

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0905.0055v2

Publisher：ELSEVIER SCIENCE BV  

We investigate density fluctuations in a scenario with gravitino dark matter
in the framework of modulated reheating, which is known to generate large
non-Gaussianity. We show that gravitino dark matter is disfavored if primordial
fluctuations have large local-type non-Gaussianity in this framework. We also
briefly discuss the case with the curvaton mechanism and some other possible
dark matter scenarios.

DOI： 10.1016/j.physletb.2009.06.007

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0905.0240v2

Publisher：IOP PUBLISHING LTD  

Recently it was shown that an inflationary background can be realized by any
$p$-form field non-minimally coupled to gravity. In this paper, we study
gravitational waves generated during p-form inflation. Even though the
background evolution is identical to that in conventional scalar field
inflation, the behavior of gravitational waves is different in p-form
inflation. In particular, we find that the propagation speed of gravitational
waves differs from unity in 2- and 3-form inflationary models. We point out
that the squared speed becomes negative in the large field models. The small
field models are free from pathologies and the correction to the spectrum of
gravitational waves turns out to be very small.

DOI： 10.1088/1475-7516/2009/05/004

Web of Science

Scopus

arXiv

Publisher：IOP PUBLISHING LTD  

We present a new efficient method for computing the non-linearity parameters
of the higher order correlation functions of local type curvature perturbations
in inflation models having a $\cal N$-component scalar field, focusing on the
non-Gaussianity generated during the evolution on super-horizon scales. In
contrast to the naive expectation that the number of operations necessary to
compute the $n$-point functions is proportional to ${\cal N}^n$, it grows only
linearly in ${\cal N}$ in our formalism. Hence, our formalism is particularly
powerful for the inflation models composed of a multi-component scalar field.
Explicit formulas obtained by applying our method are provided for $n=2,3,4$
and 5, which correspond to power-, bi-, tri- and {\it quad}-spectra,
respectively. We also discuss how many parameters we need to parameterize the
amplitude and the shape of the higher order correlation functions of local
type.

DOI： 10.1088/1475-7516/2009/02/012

Web of Science

Scopus

arXiv

Publisher：Proceedings of the 18th Workshop on General Relativity and Gravitation in Japan, JGRG 2008  

Scopus

Publisher：IOP PUBLISHING LTD  

We study an inflationary scenario with a vector impurity. We show that the
universe undergoes anisotropic inflationary expansion due to a preferred
direction determined by the vector. Using the slow-roll approximation, we find
a formula to determine anisotropy of the inflationary universe. We discuss
possible observable predictions of this scenario. In particular, it is stressed
that primordial gravitational waves can be induced from curvature
perturbations. Hence, even in low scale inflation, a sizable amount of
primordial gravitational waves may be produced during inflation.

DOI： 10.1088/1475-7516/2008/08/034

Web of Science

Scopus

arXiv

Language：English   Publisher：IOP PUBLISHING LTD  

We present a new mechanism for generating primordial statistical anisotropy
of curvature perturbations. We introduce a vector field which has a non-minimal
kinetic term and couples with a waterfall field in hybrid inflation model. In
such a system, the vector field gives fluctuations of the end of inflation and
hence induces a subcomponent of curvature perturbations. Since the vector has a
preferred direction, the statistical anisotropy could appear in the
fluctuations. We present the explicit formula for the statistical anisotropy in
the primordial power spectrum and the bispectrum of curvature perturbations.
Interestingly, there is the possibility that the statistical anisotropy does
not appear in the power spectrum but does appear in the bispectrum. We also
find that the statistical anisotropy provides the shape dependence to the
bispectrum.

DOI： 10.1088/1475-7516/2008/08/005

Web of Science

Scopus

arXiv

Language：English   Publisher：AMER PHYSICAL SOC  

We give a concise formula for the non-Gaussianity of the primordial curvature
perturbation generated on super-horizon scales in multi-scalar inflation model
without assuming slow-roll conditions. This is an extension of our previous
work. Using this formula, we study the generation of non-Gaussianity for the
double inflation models in which the slow-roll conditions are temporarily
violated after horizon exit, and we show that the non-linear parameter $f_{NL}$
for such models is suppressed by the slow-roll parameters evaluated at the time
of horizon exit.

DOI： 10.1103/PhysRevD.77.083511

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/0711.2920v3

Language：English   Publisher：IOP PUBLISHING LTD  

We analyze the non-Gaussianity for primordial curvature perturbations
generated in multi-scalar slow-roll inflation model including the model with
non-separable potential by making use of $\delta N$ formalism. Many authors
have investigated the possibility of large non-Gaussianity for the models with
separable potential, and they have found that the non-linear parameter,
$f_{NL}$, is suppressed by the slow-roll parameters. We show that for the
non-separable models $f_{NL}$ is given by the product of a factor which is
suppressed by the slow-roll parameters and a possible enhancement factor which
is given by exponentials of quantities of O(1).

DOI： 10.1088/1475-7516/2007/07/013

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/0705.3178v1

Language：English  

We show that the primordial curvature perturbations may originate not from
the quantum fluctuations of inflaton during inflation but from isocurvature
perturbations which are amplified during preheating after inflation. We
consider a simple preheating model, whose potential is given by $V(\phi,
\chi)={1/4}\lambda \phi^4+{1/2}g^2 \phi^2 \chi^2$ with $g^2/\lambda=2$, as a
possible realization of generating curvature perturbations during preheating.
We make use of the $\delta N$ formalism which requires only knowledge of the
homogeneous background solutions in order to evaluate the evolution of
curvature perturbations on super-horizon scales. We solve the background
equations numerically and find that the amplitude and the spectral index of
curvature perturbations originating from preheating can be tuned to the
observed values if the isocurvature perturbations at the end of inflation is
not suppressed on super-horizon scales. We also point out that the tensor to
scalar ratio in ${1/4}\lambda \phi^4$ inflation model can be significantly
lowered, hence letting the ${1/4}\lambda \phi^4$ model, which is ruled out by
the Wilkinson Microwave Anisotropy Probe (WMAP) data combined with SDSS data,
get back into the observationally allowed region.

DOI： 10.1088/0264-9381/24/6/015

arXiv

Other Link： http://arxiv.org/pdf/astro-ph/0606228v2

Language：English   Publisher：IOP PUBLISHING LTD  

We present a new formulation for the evaluation of the primordial spectrum of
curvature perturbations generated during inflation, using the fact that the
Wronskian of the scalar field perturbation equation is constant. In the
literature, there are many works on the same issue focusing on a few specific
aspects or effects. Here we deal with the general multi-component scalar field,
and show that our new formalism gives a method to evaluate the final amplitude
of the curvature perturbation systematically and economically. The advantage of
the new method is that one only has to solve a single mode of the scalar field
perturbation equation backward in time from the end of inflation to the stage
at which the perturbation is within the Hubble horizon, at which the initial
values of the scalar field perturbations are given. We also clarify the
relation of the new method with the new delta N formalism recently developed in
Lee et al.(2005).

DOI： 10.1088/1475-7516/2006/06/020

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/astro-ph/0605021v2

Language：English   Publisher：IOP PUBLISHING LTD  

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.

DOI： 10.1088/1475-7516/2005/10/004

Web of Science

arXiv

Other Link： http://arxiv.org/pdf/astro-ph/0506262v2

Event date： 2020.11

Language：English   Presentation type：Poster presentation  

Country：Japan