Updated on 2022/05/25

写真a

 
FUJITA, Tomohiro
 
Affiliation
Affiliated organization, Waseda Institute for Advanced Study
Job title
Assistant Professor(non-tenure-track)
Mail Address
メールアドレス
 

Papers

  • When does the Schwinger preheating occur?

    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.

    DOI

  • Gravitational wave trispectrum in the axion-SU(2) model

    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>

    DOI

  • Axion dark matter search using arm cavity transmitted beams of gravitational wave detectors

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

    Physical Review D   104 ( 6 )  2021.09

    DOI

  • Primordial tensor non-gaussianity from massive gravity

    Fujita, T., Mizuno, S., Mukohyama, S.

    Journal of Cosmology and Astroparticle Physics   2020 ( 1 )  2020

    DOI

  • Generation of chiral asymmetry via helical magnetic fields

    Schober, J., Fujita, T., Durrer, R.

    Physical Review D   101 ( 10 )  2020

    DOI

  • Scale-invariant helical magnetic fields from inflation

    Fujita, T., Durrer, R.

    Journal of Cosmology and Astroparticle Physics   2019 ( 9 )  2019

    DOI

  • Hunting Axion Dark Matter with Protoplanetary Disk Polarimetry

    Fujita, T., Tazaki, R., Toma, K.

    Physical Review Letters   122 ( 19 )  2019

    DOI

  • Tensor spectra templates for axion-gauge fields dynamics during inflation

    Fujita, T., Sfakianakis, E.I., Shiraishi, M.

    Journal of Cosmology and Astroparticle Physics   2019 ( 5 )  2019

    DOI

  • Axion dark matter search with interferometric gravitational wave detectors

    Nagano, K., Fujita, T., Michimura, Y., Obata, I.

    Physical Review Letters   123 ( 11 )  2019

    DOI

  • Mixed non-gaussianity from axion-gauge field dynamics

    Fujita, T., Namba, R., Obata, I.

    Journal of Cosmology and Astroparticle Physics   2019 ( 4 )  2019

    DOI

  • Hunting for statistical anisotropy in tensor modes with B -mode observations

    Hiramatsu, T., Yokoyama, S., Fujita, T., Obata, I.

    Physical Review D   98 ( 8 )  2018

    DOI

  • Erratum: Critical constraint on inflationary magnetogenesis (Journal of Cosmology and Astroparticle Physics (2014) (013))

    Fujita, T., Yokoyama, S.

    Journal of Cosmology and Astroparticle Physics   2014 ( 5 )  2014

    DOI

▼display all

Misc

  • Blue-tilted Primordial Gravitational Waves from Massive Gravity

    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.

    DOI

  • Footprint of Two-Form Field: Statistical Anisotropy in Primordial Gravitational Waves

    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.

    DOI

  • Optical Ring Cavity Search for Axion Dark Matter

    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.

    DOI

  • Quantum Entanglement in Multi-field Inflation

    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.

    DOI

  • Tensor Non-Gaussianity from Axion-Gauge-Fields Dynamics : Parameter Search

    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.

    DOI

  • Statistically Anisotropic Tensor Modes from Inflation

    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.

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  • Does Anisotropic "Inflation" Produce a Small Statistical Anisotropy?

    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 $99.999\%$.

    DOI

  • Finding the chiral gravitational wave background of an axion-SU(2) inflationary model using CMB observations and laser interferometers

    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.

    DOI

  • Large Tensor Non-Gaussianity from Axion-Gauge Fields Dynamics

    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.

    DOI

  • Does the detection of primordial gravitational waves exclude low energy inflation?

    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.

    DOI

  • Very Massive Tracers and Higher Derivative Biases

    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.

    DOI

  • Primordial Gravitational Waves from Axion-Gauge Fields Dynamics

    Emanuela Dimastrogiovanni, Matteo Fasiello, Tomohiro Fujita

    Journal of Cosmology and Astroparticle Physics   2017 ( 1 )  2016.08

     View Summary

    Inspired by the chromo-natural inflation model of Adshead&Wyman, we reshape
    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.

    DOI

  • Hubble induced mass after inflation in spectator field models

    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.

    DOI

  • Fermionic Schwinger effect and induced current in de Sitter space

    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.

    DOI

  • Pre-reheating Magnetogenesis in the Kinetic Coupling Model

    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.

    DOI

  • Large-scale magnetic fields can explain the baryon asymmetry of the Universe

    Tomohiro Fujita, Kohei Kamada

    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.

    DOI

  • Spatially covariant theories of gravity: disformal transformation, cosmological perturbations and the Einstein frame

    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.

    DOI

  • Consistent generation of magnetic fields in axion inflation models

    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.

    DOI

  • Can a spectator scalar field enhance inflationary tensor mode?

    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.

    DOI

  • Probing Beyond Standard Model via Hawking Radiated Gravitational Waves

    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.

  • Non-perturbative approach for curvature perturbations in stochastic-$δN$ formalism

    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.

    DOI

  • Curvaton in large field inflation

    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.

    DOI

  • Is there supercurvature mode of massive vector field in open inflation?

    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.

    DOI

  • Critical constraint on inflationary magnetogenesis

    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.

    DOI

  • Void magnetic field and its primordial origin in inflation

    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.

  • Baryon Asymmetry, Dark Matter, and Density Perturbation from PBH

    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.

    DOI

  • A new algorithm for calculating the curvature perturbations in stochastic inflation

    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.

    DOI

  • Large Scale Cosmic Perturbation from Evaporation of Primordial Black Holes

    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.

    DOI

  • Higher order statistics of curvature perturbations in IFF model and its Planck constraints

    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.

    DOI

  • Universal upper limit on inflation energy scale from cosmic magnetic field

    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.

    DOI

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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

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