# Jun'ichi Yokoyama

Last updated: Nov 18, 2019 at 16:47

Name Jun'ichi Yokoyama The University of Tokyo Graduate School of Science Research Center for The Early Universe 50212303

## Awards & Honors

Mar 2014
Paper Award, Physical Society of Japan

Feb 2013
Inoue Award, Inoue Foundation

Mar 1997
JPS Paper Award, Physical Society of Japan

Feb 1993
Inoue Research Promotion Award, Inoue Foundation

## Published Papers

B. J. Carr, Kazunori Kohri, Yuuiti Sendouda, Jun'ichi Yokoyama
Phys. Rev. D 94, 044029 (2016)      Apr 2016
The fraction of the Universe going into primordial black holes (PBHs) with
initial mass M_* \approx 5 \times 10^{14} g, such that they are evaporating at
the present epoch, is strongly constrained by observations of both the
extragalactic and Gala...
B. J. Carr, Kazunori Kohri, Yuuiti Sendouda, Jun'ichi Yokoyama
Phys.Rev.D81:104019,2010      Dec 2009
We update the constraints on the fraction of the Universe going into
primordial black holes in the mass range 10^9--10^17 g associated with the
effects of their evaporations on big bang nucleosynthesis and the extragalactic
photon background. We i...

## Misc

Nov 2019
We revisit the gravitational leptogenesis scenario in which the inflaton is
coupled to gravity by the Chern-Simons term and the lepton asymmetry is
generated through the gravitational anomaly in the lepton number current during
inflation. We const...
Soichiro Hashiba, Jun'ichi Yokoyama
Phys. Lett. B 798, 135024 (2019)      May 2019
Incorporating three generations of right-handed Majorana neutrinos to
quintessential inflation, we construct a model which simultaneously explains
inflation, dark energy, dark matter and baryogenesis. These neutrinos have
hierarchical masses \$M_3 ...
Soichiro Hashiba, Jun'ichi Yokoyama
Phys. Rev. D 99, 043008 (2019)      Dec 2018
The purely gravitational dark matter (PGDM) which interacts with the standard
model particles only by gravitational interaction has recently been discussed.
Due to its feeble interaction, PGDM may be produced mainly by the gravitational
particle c...
Tomohiro Nakama, Jun'ichi Yokoyama
Phys. Rev. D 99, 061303 (2019)      Nov 2018
High energy collisions of particles may have created tiny black holes in the
early Universe, which might leave stable remnants instead of fully evaporating
as a result of Hawking radiation. If the reheating temperature was sufficiently
close to th...
Soichiro Hashiba, Jun'ichi Yokoyama
JCAP 01 (2019) 028      Sep 2018
We calculate the number density and the energy density of a massive scalar
particle conformally coupled to gravity produced by gravitational particle
creation in the case the kination stage follows inflation. In this model, mode
functions are deri...
Minxi He, Alexei A. Starobinsky, Jun'ichi Yokoyama
Apr 2018
We analyze a two-field inflationary model consisting of the Ricci scalar
squared () term and the standard Higgs field non-minimally coupled to
gravity in addition to the Einstein term. Detailed analysis of the power
spectrum of this model...
Tomohiro Nakama, Jun'ichi Yokoyama
PTEP 2019 (2019) no.3, 033E02      Mar 2018
It is shown that a positive non-minimal coupling of the Higgs field to
gravity can solve the two problems in inflation models in which
postinflationary universe is dominated by an energy with stiff equation of
state such as a kination, namely, ove...
Koichiro Horiguchi, Kiyotomo Ichiki, Jun'ichi Yokoyama
May 2017
While the observed nearly scale-invariant initial power spectrum is regarded
as one of the favorable evidence of the standard inflationary cosmology,
precision observations of the Cosmic Microwave Background (CMB) anisotropies
also suggest possibl...
Hiroaki W. H. Tahara, Jun'ichi Yokoyama
PTEP 2018 (2018) no.1, 013E03      Apr 2017
Gravitational waves from inflation induce polarization patterns in the cosmic
microwave background (CMB). It is known that there are only two types of
non-Gaussianities of the gravitaional waves in the most general scalar field
theories having sec...
Yi-Peng Wu, Jun'ichi Yokoyama
JCAP05(2018)009      Apr 2017
We investigate loop corrections to the primordial fluctuations in the
single-field inflationary paradigm from spectator fields that experience a
smooth transition of their vacuum expectation values. We show that when the
phase transition involves ...
Hossein Bazrafshan Moghaddam, Robert Brandenberger, Jun'ichi Yokoyama
Phys. Rev. D 95, 063529 (2017)      Dec 2016
We study particle production at the end of inflation in kinetically driven
G-inflation model and show that, in spite of the fact that there are no
inflaton oscillations and hence no parametric resonance instabilities, the
production of matter part...
Naritaka Oshita, Jun'ichi Yokoyama
Prog. Theor. Exp. Phys. (2016) 051E02      Mar 2016
We revisit the derivation of the Hawking-Moss transition rate. Using the
static coordinates, we show that the Euclidean action is entirely determined by
the contribution of the entropy of de Sitter space which is proportional to the
surface area o...
Takahiro Hayashinaka, Tomohiro Fujita, Jun'ichi Yokoyama
Mar 2016
We explore Schwinger effect of spin 1/2 charged particles with static
electric field in 1+3 dimensional de Sitter spacetime. We analytically
calculate the vacuum expectation value of the spinor current which is induced
by the produced particles in...
Naritaka Oshita, Jun'ichi Yokoyama
Phys. Lett. B 785 (2018) 197-200      Jan 2016
We discuss a two-step mechanism to create a new inflationary domain beyond a
wormhole throat which is created by a phase transition around an evaporating
black hole. The first step is creation of a false vacuum bubble with a
thin-wall boundary by ...
Tomohiro Fujita, Xian Gao, Jun'ichi Yokoyama
Nov 2015
We investigate the cosmological background evolution and perturbations in a
general class of spatially covariant theories of gravity, which propagates two
tensor modes and one scalar mode. We show that the structure of the theory is
preserved unde...
Christophe Ringeval, Daisuke Yamauchi, Jun'ichi Yokoyama, Francois R. Bouchet
JCAP 1602: 033, 2016      Oct 2015
Cosmic strings formed during inflation are expected to be either diluted over
super-Hubble distances, i.e., invisible today, or to have crossed our past
light cone very recently. We discuss the latter situation in which a few
strings imprint their...
Sachiko Kuroyanagi, Takashi Hiramatsu, Jun'ichi Yokoyama
JCAP 02 (2016) 023      Sep 2015
We investigate the imprint of reheating on the gravitational wave spectrum
produced by self-ordering of multi-component scalar fields after a global phase
transition. The equation of state of the Universe during reheating, which
usually has differ...
Pisin Chen, Teruaki Suyama, Jun'ichi Yokoyama
Phys. Rev. D 92, 124016 (2015)      Aug 2015
We propose a new scalar-tensor model which induces significant deviation from
general relativity inside dense objects like neutron stars, while passing
solar-system and terrestrial experiments, extending a model proposed by Damour
and Esposito-Far...
Taro Kunimitsu, Teruaki Suyama, Yuki Watanabe, Jun'ichi Yokoyama
JCAP08(2015)044      Apr 2015
We systematically show that in potential driven generalized G-inflation
models, quantum corrections coming from new physics at the strong coupling
scale can be avoided, while producing observable tensor modes. The effective
action can be approxima...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Apr 2015
It has been pointed out that the null energy condition can be violated stably
in some non-canonical scalar-field theories. This allows us to consider the
Galilean Genesis scenario in which the universe starts expanding from Minkowski
spacetime and...
Kazufumi Takahashi, Jun'ichi Yokoyama
Phys. Rev. D 91, 084060 (2015)      Mar 2015
f(R) gravity is one of the simplest generalizations of general relativity,
which may explain the accelerated cosmic expansion without introducing a
cosmological constant. Transformed into the Einstein frame, a new scalar degree
of freedom appears ...
Takashi Hiramatsu, Yuhei Miyamoto, Jun'ichi Yokoyama
Dec 2014
The mechanism of thermal inflation, a relatively short period of accelerated
expansion after primordial inflation, is a desirable ingredient for a certain
class of particle physics models if they are not to be in contention with the
cosmology of t...
Nov 2014
We study reheating processes and its cosmological consequences in the
Starobinsky model embedded in the old-minimal supergravity. First, we consider
minimal coupling between the gravity and matter sectors in the higher curvature
theory, and transf...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
JCAP 09 (2015) 018      Nov 2014
We consider an inflationary scenario where the rate of inflaton roll defined
by remains constant. The rate of roll is small for
slow-roll inflation, while a generic rate of roll leads to the interesting case
of constant-rol...
Tomohiro Fujita, Jun'ichi Yokoyama, Shuichiro Yokoyama
Nov 2014
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 gravitat...
Sachiko Kuroyanagi, Kazunori Nakayama, Jun'ichi Yokoyama
Prog. Theor. Exp. Phys. (2015) 013E02      Oct 2014
If the tensor-to-scalar ratio of cosmological perturbations takes a large
value , which may be inferred by recent BICEP2 result, we can hope
to determine thermal history, in particular, the reheating temperature, ,
after inflat...
Kohei Kamada, Yuhei Miyamoto, Daisuke Yamauchi, Jun'ichi Yokoyama
Phys. Rev. D 90, 083502 (2014)      Jul 2014
The network of cosmic strings generated in a phase transition during
inflation enters the scaling regime later than that of usual strings. If it
occurs after the recombination, temperature anisotropies of the cosmic
microwave background (CMB) at h...
Teruaki Suyama, Yi-Peng Wu, Jun'ichi Yokoyama
Phys. Rev. D 90, 043514 (2014)      Jun 2014
Scalar field with generalized kinetic interactions metamorphoses depending on
its field value, ranging from cosmological constant to stiff matter. We show
that such a scalar field can give rise to temporal enhancement of the curvature
perturbation...
Phys. Rev. D 90, 103520 (2014)      May 2014
We consider the ghost-free higher order corrections to the Starobinsky model
in the old minimal supergravity. In general, higher order corrections cannot be
forbidden by symmetries, which likely violate the flatness of the scalaron
potential and m...
Tomohiro Nakama, Tomohiro Harada, A. G. Polnarev, Jun'ichi Yokoyama
Jan 2014
Primordial black holes (PBHs) are an important tool in cosmology to probe the
primordial spectrum of small-scale curvature perturbations that reenter the
cosmological horizon during radiation domination epoch. We numerically solve
the evolution of...
Tomohiro Nakama, Tomohiro Harada, A. G. Polnarev, Jun'ichi Yokoyama
JCAP01(2014)037      Oct 2013
Primordial black holes (PBHs) are an important tool in cosmology to probe the
primordial spectrum of small-scale curvature perturbations that reenter the
cosmological horizon during radiation domination epoch. We numerically solve
the evolution of...
Kohei Kamada, Tsutomu Kobayashi, Taro Kunimitsu, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys. Rev. D 88, 123518 (2013)      Sep 2013
Higgs G-inflation is a Higgs inflation model with a generalized Galileon term
added to the standard model Higgs field, which realizes inflation compatible
with observations. Recently, it was claimed that the generalized Galileon term
induces insta...
Yuhei Miyamoto, Hayato Motohashi, Teruaki Suyama, Jun'ichi Yokoyama
Phys. Rev. D 89, 085037 (2014)      Aug 2013
We study the dynamics of the oscillating gauged scalar field in a thermal
bath. A Langevin type equation of motion of the scalar field, which contains
both dissipation and fluctuation terms, is derived by using the real-time
finite temperature eff...
Yuki Watanabe, Jun'ichi Yokoyama
Mar 2013
Reheating after R^2 inflation proceeds through gravitational particle
production of conformally noninvariant fields. We argue that the nonvanishing
expectation value of flat directions generic in supersymmetric theories break
conformal invariance ...
Christophe Ringeval, Teruaki Suyama, Jun'ichi Yokoyama
Feb 2013
As additional perturbative degrees of freedom, it is known that magnetic
fields of inflationary origin can source curvature perturbations on
super-Hubble scales. By requiring the magnetic generated curvature to remain
smaller than its inflationary...
Takeshi Kobayashi, Jun'ichi Yokoyama
JCAP02(2013)005      Oct 2012
Cosmic inflation driven by branes wrapping the extra dimensions involves
Kaluza-Klein (KK) degrees of freedom in addition to the zero-mode position of
the brane which plays the role of the inflaton. As the wrapped brane passes by
localized sources...
Hayato Motohashi, Teruaki Suyama, Jun'ichi Yokoyama
Phys.Rev. D86 (2012) 123514      Oct 2012
We provide a general formalism to calculate the infrared correlators of
multiple interacting scalar fields in the de Sitter space by means of the
stochastic approach. These scalar fields are treated as test fields and hence
our result is applicabl...
Taro Kunimitsu, Jun'ichi Yokoyama
Phys. Rev. D 86, 083541 (2012)      Aug 2012
During inflation in the early universe, the Higgs field continuously acquires
long-wave quantum fluctuations. They accumulate to yield a non-vanishing value
with an exponentially large correlation length. We study consequences of such
Higgs conden...
Matthew Lake, Jun'ichi Yokoyama
JCAP 09 (2012) 030 [JCAP 08 (2013) E01]      Jul 2012
We consider a generalization of the Nielsen-Olesen ansatz, in an
abelian-Higgs model with externally coupled charge, which describes strings
with twisted magnetic flux lines in the vortex core. The solution does not
possess cylindrical symmetry, w...
Xian Gao, Tsutomu Kobayashi, Maresuke Shiraishi, Masahide Yamaguchi, Jun'ichi Yokoyama, Shuichiro Yokoyama
Jul 2012
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 fo...
Ryo Saito, Masahiro Nakashima, Yu-ichi Takamizu, Jun'ichi Yokoyama
Jun 2012
We investigate the possibility that a heavy scalar field, whose mass exceeds
the Hubble scale during inflation, could leave non-negligible signatures in the
Cosmic Microwave Background (CMB) temperature anisotropy power spectrum through
the parame...
A. G. Polnarev, Tomohiro Nakama, Jun'ichi Yokoyama
JCAP09(2012)027      Apr 2012
For an arbitrary strong, spherically symmetric super-horizon curvature
perturbation, we present analytical solutions of the Einstein equations in
terms of asymptotic expansion over the ratio of the Hubble radius to the
length-scale of the curvatur...
Teruaki Suyama, Jun'ichi Yokoyama
Apr 2012
There is an observational indication of extragalactic magnetic fields. No
known astrophysical process can explain the origin of such large scale magnetic
fields, which motivates us to look for their origin in primordial inflation. By
solving the l...
Kohei Kamada, Yuhei Miyamoto, Jun'ichi Yokoyama
Apr 2012
The cosmic string is a useful probe of the early Universe and may give us a
clue to physics at high energy scales where any artificial particle
accelerators cannot reach. Although one of the most promising tools is the
cosmic microwave background,...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Phys. Rev. Lett. 110, 121302 (2013)      Mar 2012
It is shown that the tension between recent neutrino oscillation experiments,
favoring sterile neutrinos with masses of the order of 1 eV, and cosmological
data which impose stringent constraints on neutrino masses from the free
streaming suppress...
Kohei Kamada, Kazunori Nakayama, Jun'ichi Yokoyama
Oct 2011
In F-term supergravity inflation models, scalar fields other than the
inflaton generically receive a Hubble induced mass, which may restore gauge
symmetries during inflation and phase transitions may occur during or after
inflation as the Hubble p...
Xian Gao, Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Aug 2011
We completely clarify the feature of primordial non-Gaussianities of tensor
perturbations in generalized G-inflation, i.e., the most general single-field
inflation model with second order field equations. It is shown that the most
general cubic ac...
Teruaki Suyama, Jun'ichi Yokoyama
Jun 2011
If more than one curvaton dominate the Universe at different epochs from each
other, curvature perturbations can be temporarily enhanced to a value much
larger than the observed one 10^{-5}. The traces of the enhancement may be left
as higher orde...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Prog. Theor. Phys. 126 (2011), 511-529      May 2011
We study generalized Galileons as a framework to develop the most general
single-field inflation models ever, Generalized G-inflation, containing yet
further generalization of G-inflation, as well as previous examples such as
k-inflation, extended...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:103524,2011      Mar 2011
We present a comprehensive study of primordial fluctuations generated from
G-inflation, in which the inflaton Lagrangian is of the form with . The Lagrangian still gives
rise to second-order g...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
JCAP 1106 (2011) 006      Jan 2011
It is known that scalar-tensor theory of gravity admits regular crossing of
the phantom divide line for dark energy, and existing viable
models of present dark energy for its particular case -- gravity --
possess one such cross...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Int.J.Mod.Phys. D20 (2011) 1347-1355      Jan 2011
We have investigated the evolution of a homogeneous isotropic background of
the Universe and inhomogeneous subhorizon matter density perturbations in
viable models of present dark energy and cosmic acceleration
analytically and numerically....
Takashi Hiramatsu, Masahiro Kawasaki, Toyokazu Sekiguchi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:123531, 2011      Dec 2010
Cosmological evolution of axionic string network is analyzed in terms of
field-theoretic simulations in a box of 512^3 grids, which are the largest
ever, using a new and more efficient identification scheme of global strings.
The scaling parameter...
Kohei Kamada, Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:083515,2011      Dec 2010
A new class of inflation models within the context of G-inflation is
proposed, in which the standard model Higgs boson can act as an inflaton thanks
to Galileon-like non-linear derivative interaction. The generated primordial
density perturbation ...
Yu-ichi Takamizu, Jun'ichi Yokoyama
Phys.Rev.D83:043504,2011      Nov 2010
We clarify the behavior of curvature perturbations in a nonlinear theory in
case the inflaton temporarily stops during inflation. We focus on the evolution
of curvature perturbation on superhorizon scales by adopting the spatial
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.Lett.105:231302,2010      Aug 2010
We propose a new class of inflation model, G-inflation, which has a
Galileon-like nonlinear derivative interaction of the form in the Lagrangian with the resultant equations of
motion being of second order. It is ...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Prog. Theor. Phys. 124 (2010), 541-546      May 2010
The effect of massive neutrinos on matter power spectrum is discussed in the
context of gravity. It is shown that the anomalous growth of density
fluctuations on small scales due to the scalaron force can be compensated by
free streaming of...
Larissa Lorenz, Jerome Martin, Jun'ichi Yokoyama
Phys.Rev.D82:023515,2010      Apr 2010
Stochastic effects during inflation can be addressed by averaging the quantum
inflaton field over Hubble-patch sized domains. The averaged field then obeys a
Langevin-type equation into which short-scale fluctuations enter as a noise
term. We solv...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Prog. Theor. Phys. 123 (2010), 887-902      Feb 2010
Evolution of a background space-time metric and sub-horizon matter density
perturbations in the Universe is numerically analyzed in viable models
of present dark energy and cosmic acceleration. It is found that viable models
generically exh...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Proceedings of the 19nth Workshop on General Relativity and Gravitation in Japan, 239-242 (2010) http://www2.rikkyo.ac.jp/web/jgrg19/index.html      Feb 2010
We present numerical calculation of the evolution of a background space-time
metric and sub-horizon matter density perturbations in viable gravity
models of present dark energy and cosmic acceleration. We found that viable
models genericall...
Ryo Saito, Jun'ichi Yokoyama
Prog.Theor.Phys.123:867-886,2010      Dec 2009
We investigate features of Gravitational Waves (GWs) induced by primordial
density fluctuations with a large amplitude peak associated with formation of
Primordial Black Holes (PBHs). It is shown that the spectrum of induced GW is
insensitive to t...
Kiyotomo Ichiki, Ryo Nagata, Jun'ichi Yokoyama
Phys.Rev.D81:083010,2010      Nov 2009
As a test of the standard inflationary cosmology, which generically predicts
nearly scale-invariant spectrum of primordial curvature fluctuations, we
perform Markov-Chain Monte-Carlo analysis to search for possible modulations in
the power spectru...
Masahiro Nakashima, Kazuhide Ichikawa, Ryo Nagata, Jun'ichi Yokoyama
JCAP 1001:030,2010      Oct 2009
We investigate constraints on the time variation of the fine structure
constant between the recombination epoch and the present epoch,
\Delta\alpha/\alpha \equiv (\alpha_{rec} - \alpha_{now})/\alpha_{now}, from
cosmic microwave background (CMB) ta...
Kazunori Nakayama, Jun'ichi Yokoyama
JCAP 1001:010,2010      Oct 2009
We study observational implications of the stochastic gravitational wave
background and a non-Gaussian feature of scalar perturbations on the curvaton
mechanism of the generation of density/curvature fluctuations, and show that
they can determine ...
Hayato Motohashi, Alexei A. Starobinsky, Jun'ichi Yokoyama
Int.J.Mod.Phys.D18:1731-1740,2009      May 2009
For a class of viable cosmological models in gravity which deviation
from the Einstein gravity decreases as a inverse power law of the Ricci scalar
for large , an analytic solution for density perturbations in the matter
component du...
Martin Lemoine, Jerome Martin, Jun'ichi Yokoyama
Phys.Rev.D80:123514,2009      Apr 2009
We set constraints on moduli cosmology from the production of dark matter --
decay, assuming the modulus remains light during inflation. We find that the
moduli problem be...
Martin Lemoine, Jerome Martin, Jun'ichi Yokoyama
Europhys.Lett.89:29001,2010      Mar 2009
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuation...
Ryo Nagata, Jun'ichi Yokoyama
Phys.Rev. D79 (2009) 043010      Dec 2008
The primordial curvature fluctuation spectrum is reconstructed by the maximum
likelihood reconstruction method using the five-year Wilkinson Microwave
Anisotropy Probe data of the cosmic microwave background temperature
anisotropy. We apply the co...
Ryo Saito, Jun'ichi Yokoyama
Phys.Rev.Lett.102:161101,2009      Dec 2008
Formation of significant number of primordial black holes (PBHs) is realized
if and only if primordial density fluctuations have a large amplitude, which
means that tensor perturbations generated from these scalar perturbations as a
second order e...
Masahiro Nakashima, Ryo Nagata, Jun'ichi Yokoyama
Prog.Theor.Phys.120:1207-1215,2008      Oct 2008
The constraints on the time variation of the fine structure constant at
recombination epoch relative to its present value, , are
obtained from the an...
Ryo Nagata, Jun'ichi Yokoyama
Phys.Rev.D78:123002,2008      Sep 2008
The primordial curvature fluctuation spectrum is reconstructed by the cosmic
inversion method using the five-year WMAP data of the cosmic microwave
background temperature anisotropy. We apply the covariance matrix analysis and
decompose the recons...
Ryo Saito, Jun'ichi Yokoyama, Ryo Nagata
JCAP 0806:024,2008      Apr 2008
We show a text-book potential for single-field inflation, namely, the
Coleman-Weinberg model can induce double inflation and formation of primordial
black holes (PBHs), because fluctuations that leave the horizon near the end of
first inflation ar...
Kazunori Nakayama, Shun Saito, Yudai Suwa, Jun'ichi Yokoyama
JCAP 0806:020,2008      Apr 2008
Thermal history of the universe after big-bang nucleosynthesis (BBN) is well
understood both theoretically and observationally, and recent cosmological
observations also begin to reveal the inflationary dynamics. However, the epoch
between inflati...
Nemanja Kaloper, Lorenzo Sorbo, Jun'ichi Yokoyama
Phys.Rev.D78:043527,2008      Mar 2008
We revisit inflation in induced gravity. Our focus is on models where the low
scale Planck mass is completely determined by the breaking of the scaling
symmetry in the field theory sector. The Higgs-like field which breaks the
symmetry with a GUT-...
Phys.Rev.D78:043502,2008      Mar 2008
We investigate the Affleck-Dine mechanism when multiple flat directions have
large values simultaneously. We consider in detail the case when both
and flat directions are operative with a non-renormalizable
superpotential. In case ...
Kazunori Nakayama, Shun Saito, Yudai Suwa, Jun'ichi Yokoyama
Phys.Rev.D77:124001,2008      Feb 2008
It is shown that space-based gravitational wave detectors such as DECIGO
and/or Big Bang Observer (BBO) will provide us with invaluable information on
the cosmic thermal history after inflation and they will be able to determine
the reheat tempera...
Jerome Martin, Jun'ichi Yokoyama
JCAP 0801:025,2008      Nov 2007
We consider the generation of large-scale magnetic fields in slow-roll
inflation. The inflaton field is described in a supergravity framework where
the conformal invariance of the electromagnetic field is generically and
naturally broken. For each...
Prog.Theor.Phys.122:969-986,2010      Jun 2009
We study the fine-tuning problem on the initial condition of inflation in the
minimal supersymmetric standard model (MSSM), associated with the narrowness of
the slow-roll region. We consider two cases before the onset of the MSSM
inflation, namel...
Toshihiro Kawaguchi, Masahiro Kawasaki, Tsutomu Takayama, Masahide Yamaguchi, Jun'ichi Yokoyama
Mon.Not.Roy.Astron.Soc.388:1426-1432,2008      Nov 2007
Formation of primordial black holes (PBHs) on astrophysical mass scales is a
natural consequence of inflationary cosmology if the primordial perturbation
spectrum has a large and negative running of the spectral index as
observationally inferred t...
Takeshi Chiba, Yoshiaki Himemoto, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D76:043516,2007      May 2007
We calculate the signal-to-noise ratio (SNR) of the stochastic
gravitational-wave background in an extreme case that its spectrum has a sharp
falloff with its amplitude close to the detection threshold. Such a spectral
feature is a characteristic ...
Kiyotomo Ichiki, Masahide Yamaguchi, Jun'Ichi Yokoyama
Phys.Rev.D75:084017,2007      Nov 2006
Effects of neutrino free streaming is evaluated on the primordial spectrum of
gravitational radiation taking both neutrino chemical potential and masses into
account. The former or the lepton asymmetry induces two competitive effects,
namely, to i...
Takeshi Chiba, Tatsuo Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D75:043516,2007      Oct 2006
Recent astrophysical observations indicate that the proton-electron mass
ratio and the fine structure constant have gone through nontrivial time
evolution. We discuss their time variation in the context of a dilaton runaway
scenario with gauge cou...
Bo Feng, Jun-Qing Xia, Jun'ichi Yokoyama
JCAP 0705:020,2007      Aug 2006
We probe the scale dependence of the primordial spectrum in the light of the
three-year WMAP (WMAP3) alone and WMAP3 in combination with the other
cosmological observations such as galaxy clustering and Type Ia Supernova
(SNIa). We pay particular ...
Masahiro Kawasaki, Tsutomu Takayama, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D74:043525,2006      May 2006
We numerically investigate density perturbations generated in the smooth
hybrid new inflation model, a kind of double inflation model that is designed
to reproduce the running spectral index suggested by the WMAP results. We
confirm that this mode...
Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev. D74 (2006) 043523      Dec 2005
Any one-field inflation is actually realized in a multifield configuration
because the inflaton must have couplings with other fields to reheat the
universe and is coupled to all other fields at least gravitationally. In all
single inflaton models...
Phys.Rev.D73:043507,2006      Dec 2005
We discuss a D-term inflation scenario where a right-handed sneutrino can be
an inflaton field leading to a viable inflation and leptogenesis, with a
minimal form of Kähler potential. The decay of an inflaton sneutrino can
non-thermally create l...
Osamu Seto, Jun'ichi Yokoyama
Phys.Rev. D73 (2006) 023508      Aug 2005
The influence of higher-order terms in the Kähler potential of the
supergravity D-term inflation model on the density perturbation is studied. We
show that these terms can make the inflaton potential flatter, which lowers the
energy scale of i...
Noriyuki Kogo, Misao Sasaki, Jun'ichi Yokoyama
Prog.Theor.Phys.114:555-572,2005      Apr 2005
We investigate the question of how tightly we can constrain the cosmological
parameters by using the `cosmic inversion'' method in which we directly
reconstruct the power spectrum of primordial curvature perturbations, ,
from the temperatur...
Kazuharu Bamba, J. Yokoyama
Feb 2005
Generation of large-scale magnetic fields is studied in dilaton
electromagnetism in noncommutative inflationary cosmology, taking into account
the effects of the spacetime uncertainty principle motivated by string theory.
We show that it is possib...
Tomo Takahashi, Masahide Yamaguchi, Jun'ichi Yokoyama, Shuichiro Yokoyama
Phys.Lett.B678:15-19,2009      May 2009
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 hav...
Kazunori Kohri, Masahiro Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D72:083510,2005      Feb 2005
We propose a new scenario of non-thermal production of neutralino cold dark
matter, in which the overproduction problem of lightest supersymmetric
particles (LSPs) in the standard thermal history is naturally solved. The
mechanism requires a heavy...
Kazuharu Bamba, J. Yokoyama
Phys.Rev. D70 (2004) 083508      Sep 2004
The generation of large-scale magnetic fields is studied in dilaton
electromagnetism in noncommutative inflationary cosmology taking into account
the effects of the spacetime uncertainty principle motivated by string theory.
We show that it is pos...
Jun'ichi Yokoyama
Phys.Rev.Lett.96:171301,2006      Jan 2006
It is shown that the coherent field oscillation of moduli fields with weak or
TeV scale masses can dissipate its energy efficiently if they have a derivative
coupling to standard bosonic fields in a thermal state. This mechanism may
provide a new ...
Noriyuki Kogo, Misao Sasaki, Jun'ichi Yokoyama
Phys.Rev.D70:103001,2004      Sep 2004
We develop a new method to reconstruct the power spectrum of primordial
curvature perturbations, , by using both the temperature and polarization
spectra of the cosmic microwave background (CMB). We test this method using
several mock primor...
Kazunori Kohri, Masahiro Yamaguchi, Jun'ichi Yokoyama
Phys.Rev. D70 (2004) 043522      Mar 2004
We study the cosmological consequences of generic scalar fields like moduli
which decay only through gravitationally suppressed interactions. We consider a
new production mechanism of gravitinos from moduli decay, which might be more
effective tha...
Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D70:023513,2004      Feb 2004
It is shown that in a smooth hybrid inflation model in supergravity adiabatic
fluctuations with a running spectral index with on a large scale and
on a smaller scale can be naturally generated, as favored by the
first-year data o...
Kazuharu Bamba, J. Yokoyama
Phys.Rev.D69:043507,2004      Oct 2003
The generation of large-scale magnetic fields is studied in dilaton
electromagnetism in inflationary cosmology, taking into account the dilaton's
evolution throughout inflation and reheating until it is stabilized with
possible entropy production....
Jun'ichi Yokoyama
Phys.Lett. B635 (2006) 66-71      Oct 2005
We calculate the dissipation rate of a coherently oscillating scalar field in
a thermal environment using nonequilibrium quantum field theory and apply it to
the reheating stage after cosmic inflation. It is shown that the rate is
nonvanishing eve...
Jun'ichi Yokoyama
Phys.Rev. D70 (2004) 103511      Jun 2004
Relaxation process of a coherent scalar field oscillation in the thermal bath
is investigated using nonequilibrium quantum field theory. The Langevin-type
equation of motion is obtained which has a memory term and both additive and
multiplicative ...
Noriyuki Kogo, Makoto Matsumiya, Misao Sasaki, Jun'ichi Yokoyama
Astrophys.J.607:32-39,2004      Sep 2003
We reconstruct the primordial spectrum of the curvature perturbation, ,
from the observational data of the Wilkinson Microwave Anisotropy Probe (WMAP)
by the cosmic inversion method developed recently. In contrast to conventional
parameter-f...