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...

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...

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 ...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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....

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...

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 ...

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
gradient expansion...

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...

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...

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...

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...

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...

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...

We set constraints on moduli cosmology from the production of dark matter --
radiation and baryon -- radiation isocurvature fluctuations through modulus
decay, assuming the modulus remains light during inflation. We find that the
moduli problem be...

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...

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...

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...

The constraints on the time variation of the fine structure constant at
recombination epoch relative to its present value, , are
obtained from the an...

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...

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...

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...

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-...

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 ...

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...

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...

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...

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...

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 ...

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...

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...

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 ...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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....

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...

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 ...

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...