Updated on 2023/02/05

FUJITA, Tomohiro

Scopus Paper Info
Paper Count: 0  Citation Count: 0  h-index: 22

Citation count denotes the number of citations in papers published for a particular year.

Affiliation
Affiliated organization, Waseda Institute for Advanced Study
Job title
Assistant Professor(non-tenure-track)

Concurrent Post

• Faculty of Science and Engineering   School of Advanced Science and Engineering

Papers

• Tomohiro Fujita, Yuto Minami, Maresuke Shiraishi, Shuichiro Yokoyama

2022.08

View Summary

Recently, the cross-correlation between $E$- and $B$-mode polarization of the
cosmic microwave background (CMB), which is well explained by cosmic
birefringence with rotation angle $\beta\approx 0.3$ deg, has been found in CMB
polarization data. We carefully investigate the possibility of explaining the
observed $EB$ correlation by the primordial chiral gravitational waves (CGWs),
which can be generated in the parity-violating theories in the primordial
Universe. We found that the CGWs scenario does not work due to the
overproduction of the $BB$ auto-correlation which far exceeds the observed one
by SPTPol and POLARBEAR.

• Tomohiro Fujita, Kyohei Mukaida, Yuichiro Tada

2022.06

View Summary

We develop the stochastic formalism for $\mathrm{U}(1)$ gauge fields that has
the Chern-Simons coupling to a rolling pseudo-scalar field during inflation.
The Langevin equations for the physical electromagnetic fields are derived and
the analytic solutions are studied. Using numerical simulation we demonstrate
that the electromagnetic fields averaged over the Hubble scale continuously
change their direction and their amplitudes fluctuate around the analytically
obtained expectation values. Though the isotropy is spontaneously broken by
picking up a particular local Hubble patch, each Hubble patch is understood
independent and the isotropy is conserved globally by averaging all the Hubble
patches.

• Matteo Fasiello, Tomohiro Fujita, Zvonimir Vlah

2022.05

View Summary

We derive exact analytic solutions for density and velocity fields to all
orders in Eulerian perturbation theory for $\Lambda$CDM cosmology. In
particular, we show that density and velocity field kernels can be written in a
separable form in time and momenta at each perturbative order. The kernel
solutions are built from an analytic basis of momentum operators and their
time-dependent coefficients, which solve a set of recursive differential
equations. We also provide an exact closed perturbative solution for such
coefficients, expanding around the (quasi-)EdS approximation. We find that the
perturbative solution rapidly converges towards the numerically obtained
solutions and its leading order result suffices for any practical requirements.
To illustrate our findings, we compute the exact two-loop dark matter density
and velocity power spectra in $\Lambda$CDM cosmology. We show that the
difference between the exact $\Lambda$CDM and the (quasi-)EdS approximated
result can reach the level of several percent. This deviation can be partially
mitigated by exploiting the degeneracy with the EFT counterterms. As an
additional benefit of our algorithm for the solutions of time-dependent
coefficients, the computational complexity of power spectra loops in
$\Lambda$CDM is reduced to the same level of the EdS case. In performing the
two-loop computation, we devise an explicit method to implement the so-called
IR cancellations, as well as the cancellations arising as a consequence of mass
and momentum conservation.

• Hiromasa Nakatsuka, Soichiro Morisaki, Tomohiro Fujita, Jun'ya Kume, Yuta Michimura, Koji Nagano, Ippei Obata

2022.05

View Summary

Ultralight bosonic particles are fascinating candidates of dark matter (DM).
It behaves as classical waves in our Galaxy due to its large number density.
There have been various methods proposed to search for the wave-like DM, such
as methods utilizing interferometric gravitational-wave detectors.
Understanding the characteristics of DM signals is crucial to extract the
properties of DM from data. While the DM signal is nearly monochromatic with
the angular frequency of its mass, the amplitude and phase are gradually
changing due to the velocity dispersion of DMs in our Galaxy halo. The
stochastic amplitude and phase should be properly taken into account to
accurately constrain the coupling constant of DM from data. Previous works
formulated a method to obtain the upper bound on the coupling constant
incorporating the stochastic effects. One of these works compared the upper
bound with and without the stochastic effect in a measurement time that is much
shorter than the variation time scale of the amplitude and phase. In this
paper, we extend their formulation to arbitrary measurement time and evaluate
the stochastic effects. Moreover, we investigate the velocity-dependent signal
for dark photon DM including an uncertainly of the velocity. We demonstrate
that our method accurately estimates the upper bound on the coupling constant
with numerical simulations. We also estimate the expected upper bound of the
coupling constant of axion DM and dark photon DM from future experiments in a
semi-analytic way. The stochasticity especially affects constraints on a small
mass region. Our formulation offers a generic treatment of the ultralight
bosonic DM signal with the stochastic effect.

• Tomohiro Fujita, Jun'ya Kume, Kyohei Mukaida, Yuichiro Tada

2022.04

View Summary

The axionic inflaton with the Chern-Simons coupling may generate U(1) gauge
fields and charged particles simultaneously. In order to incorporate the
backreaction from the charged particles on the gauge fields, we develop a
procedure to obtain an equilibrium solution for the gauge fields by treating
the induced current as effective electric and magnetic conductivities.
Introducing mean field approximation, and numerically solving self-consistency
equations, we find that the gauge field amplitudes are drastically suppressed.
Interestingly, as the production becomes more efficient, the charged particles
gain a larger part of the transferred energy from the inflaton and eventually
dominate it. Our formalism offers a basis to connect this class of inflationary
models to a rich phenomenology such as baryogenesis and magnetogenesis.

• Tomohiro Fujita, Kaname Imagawa, Kai Murai

2022.03

View Summary

Chromo-natural inflation (CNI) is an inflationary model where an axion
coupled with SU$(2)$ gauge fields acts as the inflaton. In CNI, the gauge
fields have nonzero vacuum expectation values (VEVs), which results in the
enhancement of gravitational waves (GWs). The original CNI is ruled out by the
Planck observations due to the overproduction of GWs. In this work, we consider
an inflationary model where the gauge fields acquire nonzero VEVs after the CMB
modes exit the horizon. Moreover, we add to the model another field that
dominates the universe and drives inflation after the axion starts to oscillate
and the gauge field VEVs vanish. By performing numerical simulations, we find a
parameter space where the enhanced GWs do not violate the CMB constraints and
can be detected by the future GWs observations such as BBO and ET.

1
Citation
(Scopus)
• Tomohiro Fujita, Kai Murai, Ryo Namba

2022.03

View Summary

We prove the universality of predictions for linear perturbations from the
entire class of models of inflation driven by a pseudo-scalar field coupled to
an SU($N$) gauge boson, where SU($2$) subgroups in the SU($N$) crossed with the
background spatial SO($3$) spontaneously break into a single SO($3$). The
effect of which SU($2$) subgroup in SU($N$) acquires a VEV through spontaneous
symmetry breaking can be quantified by a single parameter $\lambda$, which
always appears in combination with the gauge coupling constant $g$. In the
linear perturbations, as well as the background system, the same dynamics and
predictions as in the chromo-natural inflation hold for its SU($N$) extension
by replacing $g \to g\lambda$. The latter models thereby draw the same
prediction curve on the $n_s$-$r$ plane as the former at the tree level as long
as $g \lambda$ stays constant during inflation. We briefly discuss possible
transitions from one value of $\lambda$ to another during inflation and the
observational prospects.

1
Citation
(Scopus)
• So Okano, Tomohiro Fujita

Journal of Cosmology and Astroparticle Physics   2022 ( 03 ) 040 - 040  2022.03

View Summary

Abstract

When the inflaton couples to photons and amplifies electric fields, charged particles produced via the Schwinger effect can dominate the universe after inflation, which is dubbed as the Schwinger preheating. Using the hydrodynamic approach for the Boltzmann equation, we numerically study two cases, the Starobinsky inflation model with the kinetic coupling and the Watanabe-Kanno-Soda inflation model. The Schwinger preheating is not observed in the latter model but occurs for a sufficiently large inflaton-photon coupling in the first model. We analytically address its condition and derive a general attractor solution of the electric fields. The occurrence of the Schwinger preheating in the first model is determined by whether the electric fields enter the attractor solution during inflation or not.

• Tomohiro Fujita, Hiromasa Nakatsuka, Ippei Obata, Sam Young

2022.02

View Summary

Antisymmetric tensor field (two-form field) is a ubiquitous component in
string theory and generally couples to the scalar sector through its kinetic
term. In this paper, we propose a cosmological scenario that the particle
production of two-form field, which is triggered by the background motion of
the coupled inflaton field, occurs at the intermediate stage of inflation and
generates the sizable amount of primordial black holes as dark matter after
inflation. We also compute the secondary gravitational waves sourced by the
curvature perturbation and show that the resultant power spectra are testable
with the future space-based laser interferometers.

• Tomohiro Fujita, Kai Murai, Ippei Obata, Maresuke Shiraishi

Journal of Cosmology and Astroparticle Physics   2022 ( 01 ) 007 - 007  2022.01

View Summary

<jats:title>Abstract</jats:title>
<jats:p>We study the trispectrum of the gravitational waves (GWs) generated through the dynamics of an axionic spectator field and SU(2) gauge fields during inflation.
In non-Abelian gauge theory, the gauge fields have four-point self-interactions, which induce the tree-level GW trispectrum.
We formulate this type of the GW trispectrum including the non-dynamical contributions and evaluate it in the equilateral limit as a unique signal of this model.
We find that the ratio of the GW trispectrum to the cube of the scalar power spectrum can be as large as 𝒪(10<jats:sup>6</jats:sup>) in the viable parameter space, which could be captured in the CMB observations.</jats:p>

3
Citation
(Scopus)
• Yuta Michimura, Tomohiro Fujita, Jun’ya Kume, Soichiro Morisaki, Koji Nagano, Hiromasa Nakatsuka, Atsushi Nishizawa, Ippei Obata

Journal of Physics: Conference Series   2156 ( 1 ) 012071 - 012071  2021.12

View Summary

Abstract

Among various dark matter candidates, bosonic ultralight fields with masses below 1eV are well motivated. Recently, a number of novel approaches have been put forward to search for ultralight dark matter candidates using laser interferometers at various scales. Those include our proposals to search for axion-like particles (ALPs) and vector fields with laser interferometric gravitational wave detectors. ALPs can be searched for by measuring the oscillating polarization rotation of laser light. Massive vector fields weakly coupled to the standard model sector can also be searched for by measuring the oscillating forces acting on the suspended mirrors of the interferometers. In this paper, the current status of the activities to search for such ultralight dark matter candidates using a gravitational wave detector in Japan, KAGRA, is reviewed. The analysis of data from KAGRA’s observing run in 2020 to search for vector dark matter, and the installation of polarization optics to the arm cavity transmission ports of the interferometer to search for ALPs in future observing runs are underway.

• Hiroki Fujimoto, Yuka Oshima, Masaki Ando, Tomohiro Fujita, Yuta Michimura, Koji Nagano, Ippei Obata

2021.10

View Summary

Axion-like particles (ALPs) are undiscovered pseudo-scalar particles that are
candidates for ultralight dark matter. ALPs interact with photons slightly and
cause the rotational oscillation of linearly polarized light. Dark matter Axion
search with riNg Cavity Experiment (DANCE) searches for ALP dark matter by
amplifying the rotational oscillation with a bow-tie ring cavity. Simultaneous
resonance of linear polarizations is necessary to amplify both the carrier
field and the ALP signal, and to achieve the design sensitivity. The
sensitivity of the current prototype experiment DANCE Act-1 is less than
expectation by around three orders of magnitude due to the resonant frequency
difference between s- and p-polarization in the bow-tie ring cavity. In order
to tune the resonant frequency difference, the method of introducing an
auxiliary cavity was proposed. We designed an auxiliary cavity that can cancel
out the resonant frequency difference and realize simultaneous resonance,
considering optical loss. We also confirmed that the sensitivity of DANCE Act-1
with the auxiliary cavity can reach the original sensitivity.

1
Citation
(Scopus)
• Tomohiro Fujita, Kyohei Mukaida, Kai Murai, Hiromasa Nakatsuka

2021.10

View Summary

We study SU($N$) gauge fields that couple to the inflaton through the
Chern-Simons term. We provide a general procedure to construct homogeneous,
isotropic, and attractor solutions of the gauge fields during inflation. The
gauge fields develop various VEVs corresponding to different spontaneous
symmetry breaking patterns of SU($N$) where embedded SU($2$) subgroups are
broken with the spatial rotation SO($3$) symmetry. As specific examples, we
construct the stable solutions for $N = 3$ and $4$. We also numerically solve
the gauge field dynamics and confirm that our analytic solutions are complete
and attractor. It is straightforward to apply our procedure to the other simple
Lie groups.

• Koji Nagano, Hiromasa Nakatsuka, Soichiro Morisaki, Tomohiro Fujita, Yuta Michimura, Ippei Obata

Physical Review D   104 ( 6 )  2021.09

View Summary

Axion is a promising candidate for ultralight dark matter which may cause a
polarization rotation of laser light. Recently, a new idea of probing the axion
dark matter by optical linear cavities used in the arms of gravitational wave
detectors has been proposed [Phys. Rev. Lett. 123, 111301 (2019)]. In this
article, a realistic scheme of the axion dark matter search with the arm cavity
transmission ports is revisited. Since photons detected by the transmission
ports travel in the cavity for odd-number of times, the effect of axion dark
matter on their phases is not cancelled out and the sensitivity at low-mass
range is significantly improved compared to the search using reflection ports.
We also take into account the stochastic nature of the axion field and the
availability of the two detection ports in the gravitational wave detectors.
The sensitivity to the axion-photon coupling, $g_{a\gamma}$, of the
ground-based gravitational wave detector, such as Advanced LIGO, with 1-year
observation is estimated to be $g_{a\gamma} \sim 3\times10^{-12}$ GeV$^{-1}$
below the axion mass of $10^{-15}$ eV, which improves upon the limit achieved
by the CERN Axion Solar Telescope.

• Hiroki Fujimoto, Yuka Oshima, Masaki Ando, Tomohiro Fujita, Yuta Michimura, Koji Nagano, Ippei Obata

2021.05

View Summary

Axion-like particles (ALPs) are pseudo-scalar particles that are candidates
for ultralight dark matter. ALPs interact with photons slightly and cause the
rotational oscillation of linear polarization. DANCE searches for ALP dark
matter by enhancing the rotational oscillation in a bow-tie ring cavity. The
signal to noise ratio of DANCE can be improved by long-term observation, and we
are planning a year-long observation for the final DANCE. In this document, I
will report on the control systems of the ring cavity we developed for the
future long-term observation.

• Tomohiro Fujita, Ippei Obata, Takahiro Tanaka, Kei Yamada

Classical and Quantum Gravity   38 ( 4 ) 045010 - 045010  2021.02  [Refereed]

2
Citation
(Scopus)
• So Okano, Tomohiro Fujita

2020.05

View Summary

We investigate the gravitational wave production induced by the primordial
magnetic fields in a parity-violating magnetogenesis model. It is shown that
the gravitational waves detectable by LISA, DECIGO or BBO and the magnetic
fields strong enough to explain the blazar observation can be simultaneously
produced. The magnetic fields and the gravitational waves have the same
chirality and their amplitudes are related, which may also be tested by future
observations.

11
Citation
(Scopus)
• Yuta Michimura, Yuka Oshima, Taihei Watanabe, Takuya Kawasaki, Hiroki Takeda, Masaki Ando, Koji Nagano, Ippei Obata, Tomohiro Fujita

16TH INTERNATIONAL CONFERENCE ON TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS (TAUP 2019)   1468  2020  [Refereed]

View Summary

We have proposed a new approach to search for axion dark matter with an optical ring cavity [Phys. Rev. Lett. 121, 161301 (2018)]. The coupling of photons to axions or axion-like particles makes a modulated difference in the phase velocity between left- and right-handed photons. Our method is to measure this phase velocity difference with a ring cavity, by measuring the resonant frequency difference between two circular polarizations. Our estimation shows that the sensitivity to axion-photon coupling constant g(a gamma) for axion mass m less than or similar to 10(-10) eV can be improved by several orders of magnitude compared with the current best limits. In this paper, we present the principles of the Dark matter Axion search with riNg Cavity Experiment (DANCE) and the status of the prototype experiment, DANCE Act-1.

9
Citation
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• Fujita, T., Mizuno, S., Mukohyama, S.

Journal of Cosmology and Astroparticle Physics   2020 ( 1 )  2020

6
Citation
(Scopus)
• Schober, J., Fujita, T., Durrer, R.

Physical Review D   101 ( 10 )  2020

10
Citation
(Scopus)
• Fujita, T., Durrer, R.

Journal of Cosmology and Astroparticle Physics   2019 ( 9 )  2019

25
Citation
(Scopus)
• Fujita, T., Tazaki, R., Toma, K.

Physical Review Letters   122 ( 19 )  2019

33
Citation
(Scopus)
• Fujita, T., Sfakianakis, E.I., Shiraishi, M.

Journal of Cosmology and Astroparticle Physics   2019 ( 5 )  2019

View Summary

SU(2) gauge fields can generate large gravitational waves during inflation, if they are coupled to an axion which can be either the inflaton or a spectator field. The shape of the produced tensor power spectrum h depends on the form of the axion potential. We derive analytic expressions and provide general templates for h for various types of the spectator axion potential. Furthermore, we explore the detectability of the oscillatory feature, which is present in h in the case of an axion monodromy model, by possible future CMB B-mode polarization observations.

11
Citation
(Scopus)
• Nagano, K., Fujita, T., Michimura, Y., Obata, I.

Physical Review Letters   123 ( 11 )  2019

40
Citation
(Scopus)
• Fujita, T., Namba, R., Obata, I.

Journal of Cosmology and Astroparticle Physics   2019 ( 4 )  2019

14
Citation
(Scopus)
• Hiramatsu, T., Yokoyama, S., Fujita, T., Obata, I.

Physical Review D   98 ( 8 )  2018

7
Citation
(Scopus)
• Fujita, T., Yokoyama, S.

Journal of Cosmology and Astroparticle Physics   2014 ( 5 )  2014

9
Citation
(Scopus)

Misc

• Tomohiro Fujita, Sachiko Kuroyanagi, Shuntaro Mizuno, Shinji Mukohyama

Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics   789  2018.08

View Summary

We study a theory of massive tensor gravitons which predicts blue-tilted and
largely amplified primordial gravitational waves. After inflation, while their
mass is significant until it diminishes to a small value, gravitons are diluted
as non-relativistic matter and hence their amplitude can be substantially
amplified compared to the massless gravitons which decay as radiation. We show
that such gravitational waves can be detected by interferometer experiments,
even if their signal is not observed on the CMB scales.

• Ippei Obata, Tomohiro Fujita

Physical Review D   99 ( 2 )  2018.07

View Summary

We study the observational signatures of two-form field in the inflationary
cosmology. In our setup a two-form field is kinetically coupled to a spectator
scalar field and generates sizable gravitational waves and smaller curvature
perturbation. We find that the sourced gravitational waves have a distinct
signature: they are always statistically anisotropic and their spherical
moments are non-zero for hexadecapole and tetrahexacontapole, while the
quadrupole moment vanishes. Since their amplitude can reach
$\mathcal{O}(10^{-3})$ in the tensor-to-scalar ratio, we expect this novel
prediction will be tested in the next generation of the CMB experiments.

• Ippei Obata, Tomohiro Fujita, Yuta Michimura

Physical Review Letters   121 ( 16 )  2018.05

View Summary

We propose a novel experiment to search for axion dark matter which
differentiates the phase velocities of the left and right-handed polarized
photons. Our optical cavity measures the difference of the resonant frequencies
between two circular-polarizations of the laser beam. The design of our cavity
adopts double-pass configuration to realize a null experiment and give a high
common mode rejection of environmental disturbances. We estimate the potential
sensitivity to the axion-photon coupling constant $g_{a\gamma}$ for the axion
mass $m \lesssim 10^{-10}$ eV. In a low mass range $m \lesssim 10^{-15}$ eV, we
can achieve $g_{a\gamma} \lesssim 3\times 10^{-16} ~\text{GeV}^{-1}$ which is
beyond the current bound by several orders of magnitude.

• Nadia Bolis, Tomohiro Fujita, Shuntaro Mizuno, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   2018 ( 9 )  2018.05

View Summary

We study the emergence of quantum entanglement in multi-field inflation. In
this scenario, the perturbations of one field contribute to the observable
curvature perturbation, while multi-field dynamics with the other fields affect
the curvature perturbation through particle production and entanglement. We
develop a general formalism which defines the quantum entanglement between the
perturbations of the multiple fields both in the Heisenberg and Schr\"odinger
pictures, and show that entanglement between different fields can arise
dynamically in the context of multi-field inflationary scenarios. We also
present a simple model in which a sudden change in the kinetic matrix of the
scalar fields generates entanglement and an oscillatory feature appears in the
power spectrum of the inflaton perturbation.

• Aniket Agrawal, Tomohiro Fujita, Eiichiro Komatsu

Journal of Cosmology and Astroparticle Physics   2018 ( 6 )  2018.02

View Summary

We calculate the bispectrum of scale-invariant tensor modes sourced by
spectator SU(2) gauge fields during inflation in a model containing a scalar
inflaton, a pseudoscalar axion and SU(2) gauge fields. A large bispectrum is
generated in this model at tree-level as the gauge fields contain a tensor
degree of freedom, and its production is dominated by self-coupling of the
gauge fields. This is a unique feature of non-Abelian gauge theory. The shape
of the tensor bispectrum is approximately an equilateral shape for $3\lesssim m_Q\lesssim 4$, where $m_Q$ is an effective dimensionless mass of the SU(2)
field normalised by the Hubble expansion rate during inflation. The amplitude
of non-Gaussianity of the tensor modes, characterised by the ratio $B_h/P^2_h$,
is inversely proportional to the energy density fraction of the gauge field.
This ratio can be much greater than unity, whereas the ratio from the vacuum
fluctuation of the metric is of order unity. The bispectrum is effective at
constraining large $m_Q$ regions of the parameter space, whereas the power
spectrum constrains small $m_Q$ regions.

• Tomohiro Fujita, Ippei Obata, Takahiro Tanaka, Shuichiro Yokoyama

Journal of Cosmology and Astroparticle Physics   2018 ( 7 )  2018.01

View Summary

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.

• Tomohiro Fujita, Ippei Obata

Journal of Cosmology and Astroparticle Physics   2018 ( 1 )  2017.11

View Summary

Anisotropic inflation is an interesting model with an U(1) gauge field and it
predicts the statistical anisotropy of the curvature perturbation characterized
by a parameter $g_*$. However, we find that the background gauge field does not
follow the classical attractor solution due to the stochastic effect. We
develop the stochastic formalism of a vector field and solve Langevin and
Fokker-Planck equations. It is shown that this model is excluded by the CMB
constraint $g_*\le 10^{-2}$ with a high probability about $99.999\%$.

• Ben Thorne, Tomohiro Fujita, Masashi Hazumi, Nobuhiko Katayama, Eiichiro Komatsu, Maresuke Shiraishi

Physical Review D   97 ( 4 )  2017.07

View Summary

A detection of B-mode polarization of the Cosmic Microwave Background (CMB)
anisotropies would confirm the presence of a primordial gravitational wave
background (GWB). In the inflation paradigm this would be an unprecedented
probe of the energy scale of inflation as it is directly proportional to the
power spectrum of the GWB. However, similar tensor perturbations can be
produced by the matter fields present during inflation, breaking this simple
relationship. It is therefore important to be able to distinguish between
different generation mechanisms of the GWB. In this paper, we analyse the
detectability of a new axion-SU(2) gauge field model using its chiral,
scale-dependent tensor spectrum. We forecast the detectability of the resulting
CMB TB and EB cross-correlations by the LiteBIRD satellite, considering the
effects of residual foregrounds, gravitational lensing, and for the first time
assess the ability of such an experiment to jointly detect primordial TB and EB
spectra and self-calibrate its polarimeter. We find that LiteBIRD will be able
to detect the chiral signal for $r_*>0.03$ with $r_*$ denoting the
tensor-to-scalar ratio at the peak scale, and that the maximum signal-to-noise
for $r_*<0.07$ is $\sim 2$. We go on to consider an advanced stage of a
LISA-like mission, and find that such experiments would complement CMB
observations by providing sensitivity to GWB chirality on scales inaccessible
to the CMB. We conclude that in order to use the CMB to distinguish this model
from a conventional vacuum fluctuation model two-point statistics provide some
power, but to achieve high statistical significance we would require higher
order statistics which take advantage of the model's non-Gaussianity. On the
other hand, in the case of a spectrum peaked at very small scales, inaccessible
to the CMB, a highly significant detection could be made using space-based
laser interferometers.

• Aniket Agrawal, Tomohiro Fujita, Eiichiro Komatsu

Physical Review D   97 ( 10 )  2017.07

View Summary

We show that an inflation model in which a spectator axion field is coupled
to an SU(2) gauge field produces a large three-point function (bispectrum) of
primordial gravitational waves, $B_{h}$, on the scales relevant to the cosmic
microwave background experiments. The amplitude of the bispectrum at the
equilateral configuration is characterized by
$B_{h}/P_h^2=\mathcal{O}(10)\times \Omega_A^{-1}$, where $\Omega_A$ is a
fraction of the energy density in the gauge field and $P_h$ is the power
spectrum of gravitational waves produced by the gauge field.

• Tomohiro Fujita, Ryo Namba, Yuichiro Tada

Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics   778  2017.05

View Summary

We show that a detectable tensor-to-scalar ratio $(r\ge 10^{-3})$ on the CMB
scale can be generated even during extremely low energy inflation which
saturates the BBN bound $\rho_{\rm inf}\approx (30 {\rm MeV})^4$. The source of
the gravitational waves is not quantum fluctuations of graviton but those of
$SU(2)$ gauge fields, energetically supported by coupled axion fields. The
curvature perturbation, the backreaction effect and the validity of
perturbative treatment are carefully checked. Our result indicates that
measuring $r$ alone does not immediately fix the inflationary energy scale.

• Tomohiro Fujita, Valentin Mauerhofer, Leonardo Senatore, Zvonimir Vlah, Raul Angulo

Journal of Cosmology and Astroparticle Physics   2020 ( 1 )  2016.09

View Summary

Most of the upcoming cosmological information will come from analyzing the
clustering of the Large Scale Structures (LSS) of the universe through LSS or
CMB observations. It is therefore essential to be able to understand their
behavior with exquisite precision. The Effective Field Theory of Large Scale
Structures (EFTofLSS) provides a consistent framework to make predictions for
LSS observables in the mildly non-linear regime. In this paper we focus on
biased tracers. We argue that in calculations at a given order in the dark
matter perturbations, highly biased tracers will underperform because of their
larger higher derivative biases. A natural prediction of the EFTofLSS is
therefore that by simply adding higher derivative biases, all tracers should
perform comparably well. We implement this prediction for the halo-halo and the
halo-matter power spectra at one loop, and the halo-halo-halo,
halo-halo-matter, and halo-matter-matter bispectra at tree-level, and compare
with simulations. We find good agreement with the prediction: for all tracers,
we are able to match the bispectra up to $k\simeq0.17\,h/$Mpc at $z=0$ and the
power spectra to a higher wavenumber.

• Emanuela Dimastrogiovanni, Matteo Fasiello, Tomohiro Fujita

Journal of Cosmology and Astroparticle Physics   2017 ( 1 )  2016.08

View Summary

its scalar content to relax the tension with current observational bounds.
Besides an inflaton, the setup includes a spectator sector in which an axion
and SU(2) gauge fields are coupled via a Chern-Simons-type term. The result is
a viable theory endowed with an alternative production mechanism for
gravitational waves during inflation. The gravitational wave signal sourced by
the spectator fields can be much larger than the contribution from standard
vacuum fluctuations, it is distinguishable from the latter on the basis of its
chirality and, depending on the theory parameters values, also its tilt. This
production process breaks the well-known relation between the tensor-to-scalar
ratio and the energy scale of inflation. As a result, even if the Hubble rate
is itself too small for the vacuum to generate a tensor amplitude detectable by
upcoming experiments, this model still supports observable gravitational waves.

• Tomohiro Fujita, Keisuke Harigaya

Journal of Cosmology and Astroparticle Physics   2016 ( 12 )  2016.07

View Summary

Spectator field models such as the curvaton scenario and the modulated
reheating are attractive scenarios for the generation of the cosmic curvature
perturbation, as the constraints on inflation models are relaxed. In this
paper, we discuss the effect of Hubble induced masses on the dynamics of
spectator fields after inflation. We pay particular attention to the Hubble
induced mass by the kinetic energy of an oscillating inflaton, which is
generically unsuppressed but often overlooked. In the curvaton scenario, the
Hubble induced mass relaxes the constraint on the property of the inflaton and
the curvaton, such as the reheating temperature and the inflation scale. We
comment on the implication of our discussion for baryogenesis in the curvaton
scenario. In the modulated reheating, the predictions of models e.g. the
non-gaussianity can be considerably altered. Furthermore, we propose a new
model of the modulated reheating utilizing the Hubble induced mass which
realizes a wide range of the local non-gaussianity parameter.

• Takahiro Hayashinaka, Tomohiro Fujita, Jun'ichi Yokoyama

Journal of Cosmology and Astroparticle Physics   2016 ( 7 )  2016.03

View Summary

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 the electric field. The renormalization is
performed with the adiabatic subtraction scheme. We find that the current
becomes negative, namely it flows in the direction opposite to the electric
field, if the electric field is weaker than a certain threshold value depending
on the fermion mass, which is also known to happen in the case of scalar
charged particles in 1+3 de Sitter spacetime. Contrary to the scalar case,
however, the IR hyperconductivity is absent in the spinor case.

• Tomohiro Fujita, Ryo Namba

Physical Review D   94 ( 4 )  2016.02

View Summary

Recent blazar observations provide growing evidence for the presence of
magnetic fields in the extragalactic regions. While a natural speculation is to
associate the production to inflationary physics, it has been known that
magnetogenesis solely from inflation is quite challenging. We therefore study a
model in which a non-inflaton field $\chi$ coupled to the electromagnetic field
through its kinetic term, $-I^2(\chi) F^2 /4$, continues to move after
inflation until the completion of reheating. This leads to a post-inflationary
amplification of the electromagnetic field. We compute all the relevant
contributions to the curvature perturbation, including gravitational
interactions, and impose the constraints from the CMB scalar fluctuations on
the strength of magnetic fields. We, for the first time, explicitly verify both
the backreaction and CMB constraints in a simple yet successful magnetogenesis
scenario without invoking a dedicated low-scale inflationary model in the
weak-coupling regime of the kinetic coupling model.

Physical Review D   93 ( 8 )  2016.02

View Summary

Helical hypermagnetic fields in the primordial Universe can produce the
observed amount of baryon asymmetry through the chiral anomaly without any
ingredients beyond the standard model of particle physics. While they generate
no $B-L$ asymmetry, the generated baryon asymmetry survives the spharelon
washout effect, because the generating process remains active until the
electroweak phase transition. Solving the Boltzmann equation numerically and
finding an attractor solution, we show that the baryon asymmetry of our
Universe can be explained, if the present large-scale magnetic fields indicated
by the blazar observations have a negative helicity and existed in the early
Universe before the electroweak phase transition. We also derive the upper
bound on the strength of the helical magnetic field, which is tighter than the
cosmic microwave background constraint, to avoid the overproduction of baryon
asymmetry.

• Tomohiro Fujita, Xian Gao, Jun'ichi Yokoyama

Journal of Cosmology and Astroparticle Physics   2016 ( 2 )  2015.11

View Summary

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 under the disformal transformation. We also evaluate the primordial
spectra for both the gravitational waves and the curvature perturbation, which
are invariant under the disformal transformation. Due to the existence of
higher spatial derivatives, the quadratic Lagrangian for the tensor modes
itself cannot be transformed to the form in the Einstein frame. Nevertheless,
there exists a one-parameter family of frames in which the spectrum of the
gravitational waves takes the standard form in the Einstein frame.

• Tomohiro Fujita, Ryo Namba, Yuichiro Tada, Naoyuki Takeda, Hiroyuki Tashiro

Journal of Cosmology and Astroparticle Physics   2015 ( 5 )  2015.03

View Summary

There has been a growing evidence for the existence of magnetic fields in the
extra-galactic regions, while the attempt to associate their origin with the
inflationary epoch alone has been found extremely challenging. We therefore
take into account the consistent post-inflationary evolution of the magnetic
fields that are originated from vacuum fluctuations during inflation. In the
model of our interest, the electromagnetic (EM) field is coupled to a
pseudo-scalar inflaton $\phi$ through the characteristic term $\phi F\tilde F$,
breaking the conformal invariance. This interaction dynamically breaks the
parity and enables a continuous production of only one of the polarization
states of the EM field through tachyonic instability. The produced magnetic
fields are thus helical. We find that the dominant contribution to the observed
magnetic fields in this model comes from the modes that leave the horizon near
the end of inflation, further enhanced by the tachyonic instability right after
the end of inflation. The EM field is subsequently amplified by parametric
resonance during the period of inflaton oscillation. Once the thermal plasma is
formed (reheating), the produced helical magnetic fields undergo a turbulent
process called inverse cascade, which shifts their peak correlation scales from
smaller to larger scales. We consistently take all these effects into account
within the regime where the perturbation of $\phi$ is negligible and obtain
$B_{\rm eff} \sim 10^{-19}$G, indicating the necessity of additional mechanisms
to accommodate the observations.

• Tomohiro Fujita, Jun'ichi Yokoyama, Shuichiro Yokoyama

Progress of Theoretical and Experimental Physics   2015 ( 4 )  2014.11

View Summary

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.

• Tomohiro Fujita

2014.07

View Summary

We propose a novel technique to probe the beyond standard model (BSM) of
particle physics. The mass spectrum of unknown BSM particles can be scanned by
observing gravitational waves (GWs) emitted by Hawking radiation of black
holes. This is because information on the radiation of the BSM particles is
imprinted in the spectrum of the GWs. We fully calculate the GW spectrum from
evaporating black holes taking into account the greybody factor. As an
observationally interesting application, we consider primordial black holes
which evaporate in the very early universe. In that case, since the frequencies
of GWs are substantially redshifted, the GWs emitted with the BSM energy scales
become accessible by observations.

• Tomohiro Fujita, Masahiro Kawasaki, Yuichiro Tada

Journal of Cosmology and Astroparticle Physics   2014 ( 10 )  2014.05

View Summary

In our previous paper, we have proposed a new algorithm to calculate the
power spectrum of the curvature perturbations generated in inflationary
universe with use of the stochastic approach. Since this algorithm does not
need the perturbative expansion with respect to the inflaton fields on
super-horizon scale, it works even in highly stochastic cases. For example,
when the curvature perturbations are very large or the non-Gaussianities of the
curvature perturbations are sizable, the perturbative expansion may break down
but our algorithm enables to calculate the curvature perturbations. We apply it
to two well-known inflation models, chaotic and hybrid inflation, in this
paper. Especially for hybrid inflation, while the potential is very flat around
the critical point and the standard perturbative computation is problematic, we
successfully calculate the curvature perturbations.

• Tomohiro Fujita, Masahiro Kawasaki, Shuichiro Yokoyama

Journal of Cosmology and Astroparticle Physics   2014 ( 9 )  2014.04

View Summary

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.

• Daisuke Yamauchi, Tomohiro Fujita, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   2014 ( 3 )  2014.02

View Summary

We investigate the Euclidean vacuum mode functions of a massive vector field
in a spatially open chart of de Sitter spacetime. In the one-bubble open
inflationary scenario that naturally predicts a negative spatial curvature
after a quantum tunneling, it is known that a light scalar field has the
so-called supercurvature mode, i.e. an additional discrete mode which describes
fluctuations over scales larger than the spatial curvature scale. If such
supercurvature modes exist for a vector field with a sufficiently light mass,
then they would decay slower and easily survive the inflationary era. However,
the existence of supercurvature mode strongly depends on details of the system.
To clarify whether a massive vector field has supercurvature modes, we consider
a U(1) gauge field with gauge and conformal invariances spontaneously broken
through the Higgs mechanism, and present explicit expressions for the Euclidean
vacuum mode functions. We find that, for any values of the vector field mass,
there is no supercurvature mode. In the massless limit, the absence of
supercurvature modes in the scalar sector stems from the gauge symmetry.

• Tomohiro Fujita, Shuichiro Yokoyama

Journal of Cosmology and Astroparticle Physics   2014 ( 3 )  2014.02

View Summary

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.

• Tomohiro Fujita

2014.01

View Summary

Since magnetic fields in galaxies, galactic clusters and even void regions
are observed, theoretical attempts to explain their origin are strongly
motivated. It is interesting to consider that inflation is responsible for the
origin of the magnetic fields as well as the density perturbation. However, it
is known that inflationary magnetogenesis suffers from several problems. We
explore these problems by using a specific model, namely the kinetic coupling
model, and show how the model is constrained. Model independent arguments are
also discussed.

• Tomohiro Fujita, Keisuke Harigaya, Masahiro Kawasaki, Ryo Matsuda

Physical Review D - Particles, Fields, Gravitation and Cosmology   89 ( 10 )  2014.01

View Summary

We investigate the consistency of a scenario in which the baryon asymmetry,
dark matters, as well as the cosmic density perturbation are generated
simultaneously through the evaporation of primordial black holes (PBHs). This
scenario can explain the coincidence of the dark matter and the baryon density
of the universe, and is free from the isocurvature perturbation problem. We
show that this scenario predicts the masses of PBHs, right-handed neutrinos and
dark matters, the Hubble scale during inflation, the non-gaussianity and the
running of the spectral index. We also discuss the testability of the scenario
by detecting high frequency gravitational waves from PBHs.

• Tomohiro Fujita, Masahiro Kawasaki, Yuichiro Tada, Tomohiro Takesako

Journal of Cosmology and Astroparticle Physics   2013 ( 12 )  2013.08

View Summary

We propose a new approach for calculating the curvature perturbations
produced during inflation in the stochastic formalism. In our formalism, the
fluctuations of the e-foldings are directly calculated without perturbatively
expanding the inflaton field and they are connected to the curvature
perturbations by the $\delta N$ formalism. The result automatically includes
the contributions of the higher order perturbations because we solve the
equation of motion non-perturbatively. In this paper, we analytically prove
that our result (the power spectrum and the nonlinearity parameter) is
consistent with the standard result in single field slow-roll inflation. We
also describe the algorithm for numerical calculations of the curvature
perturbations in more general inflation models.

• Tomohiro Fujita, Keisuke Harigaya, Masahiro Kawasaki

Physical Review D - Particles, Fields, Gravitation and Cosmology   88 ( 12 )  2013.06

View Summary

We present a novel mechanism to generate the cosmic perturbation from
evaporation of primordial black holes. A mass of a field is fluctuated if it is
given by a vacuum expectation value of a light scalar field because of the
quantum fluctuation during inflation. The fluctuated mass causes variations of
the evaporation time of the primordial black holes. Therefore provided the
primordial black holes dominate the universe when they evaporate, primordial
cosmic perturbations are generated. We find that the amplitude of the large
scale curvature perturbation generated in this scenario can be consistent with
the observed value. Interestingly, our mechanism works even if all fields which
are responsible for inflation and the generation of the cosmic perturbation are
decoupled from the visible sector except for the gravitational interaction. An
implication to the running spectral index is also discussed.

• Tomohiro Fujita, Shuichiro Yokoyama

Journal of Cosmology and Astroparticle Physics   2013 ( 9 )  2013.06

View Summary

We compute the power spectrum P_\zeta, and non-linear parameters f_nl and
\tau_nl of the curvature perturbation induced during inflation by the
electromagnetic fields in the kinetic coupling model (IFF model). By using the
observational result of P_\zeta, f_nl and \tau_nl 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 P_\zeta, the constraint from \tau_nl is most stringent. We also find a
general relationship between f_nl and \tau_nl generated in this model. Even if
f_nl \sim O(1), a detectable \tau_nl can be produced.

• Tomohiro Fujita, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   2012 ( 10 )  2012.05

View Summary

Recently observational lower bounds on the strength of cosmic magnetic fields
were reported, based on gamma-ray flux from distant blazars. If inflation is
responsible for the generation of such magnetic fields then the inflation
energy scale is bounded from above as rho_{inf}^{1/4} < 2.5 times 10^{-7}M_{Pl}
times (B_{obs}/10^{-15}G)^{-2} in a wide class of inflationary magnetogenesis
models, where B_{obs} is the observed strength of cosmic magnetic fields. The
tensor-to-scalar ratio is correspondingly constrained as r< 10^{-19} times
(B_{obs}/10^{-15}G)^{-8}. Therefore, if the reported strength B_{obs} \geq
10^{-15}G is confirmed and if any signatures of gravitational waves from
inflation are detected in the near future, then our result indicates some
tensions between inflationary magnetogenesis and observations.

Awards

• 若手奨励賞（宇宙線・宇宙物理領域）

2022.03   日本物理学会   ゲージ場の宇宙論

Presentations

• 日本物理学会 第16回 若手奨励賞 受賞記念講演「ゲージ場の宇宙論」

藤田智弘  [Invited]

日本物理学会

Presentation date： 2022.03

Event date：
2022.03

• SU(N)-natural inflationにおける原始重力波生成

藤田智弘

第１回「素粒子と重力波」研究会

Presentation date： 2022.02

Event date：
2022.02

• SU(N) gauge-natural inflation

Tomohiro Fujita

Yukawa International Seminar 2022a "Gravity - The Next Generation-"

Presentation date： 2022.02

Event date：
2022.02

• Cosmic Birefringence and Axion Dark Energy

Tomohiro Fujita  [Invited]

8th Korea-Japan workshop on Dark Energy

Presentation date： 2021.10

Event date：
2021.10

• SU(N) chromo-natural inflation

藤田智弘, 中塚洋佑, 向田享平, 村井開

日本物理学会

Presentation date： 2021.09

Event date：
2021.09

Syllabus

• School of Fundamental Science and Engineering

2022   fall quarter

• School of Fundamental Science and Engineering

2022   fall quarter

• School of Creative Science and Engineering

2022   fall quarter

• School of Creative Science and Engineering

2022   fall quarter

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall quarter

• School of Advanced Science and Engineering

2022   fall quarter