藤田 智弘 (フジタ トモヒロ)

写真a

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附属機関・学校 高等研究所

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講師(任期付)

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  • Detection of isotropic cosmic birefringence and its implications for axion-like particles including dark energy

    Tomohiro Fujita, Kai Murai, Hiromasa Nakatsuka, Shinji Tsujikawa

       2020年11月

     概要を見る

    We investigate the possibility that axion-like particles (ALPs) with various
    potentials account for the isotropic birefringence recently reported by
    analyzing the Planck 2018 polarization data. For the quadratic and cosine
    potentials, we obtain lower bounds on the mass, coupling constant to photon
    $g$, abundance and equation of state of the ALP to produce the observed
    birefringence. Especially when the ALP is responsible for dark energy, it is
    possible to probe the tiny deviation of dark energy equation of state from $-1$
    through the cosmic birefringence. We also explore ALPs working as early dark
    energy (EDE), which alleviates the Hubble tension problem. Since the other
    parameters are limited by the EDE requirements, we narrow down the ALP-photon
    coupling to $10^{-19}\, {\rm GeV}^{-1}\lesssim g\lesssim 10^{-16}\, {\rm
    GeV}^{-1}$ for the decay constant $f=M_\mathrm{pl}$. Therefore, the Hubble
    tension and the isotropic birefringence imply that $g$ is typically the order
    of $f^{-1}$, which is a non-trivial coincidence.

    DOI

  • Improved sensitivity of interferometric gravitational wave detectors to ultralight vector dark matter from the finite light-traveling time

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

       2020年11月

     概要を見る

    Recently several studies have pointed out that gravitational-wave detectors
    are sensitive to ultralight vector dark matter and can improve the current best
    constraints given by the Equivalence Principle tests. While a
    gravitational-wave detector is a highly precise measuring tool of the length
    difference of its arms, its sensitivity is limited because the displacements of
    its test mass mirrors caused by vector dark matter are almost common. In this
    Letter we point out that the sensitivity is significantly improved if the
    effect of finite light-traveling time in the detector's arms is taken into
    account. This effect enables advanced LIGO to improve the constraints on the
    $U(1)_{B-L}$ gauge coupling by an order of magnitude compared with the current
    best constraints. It also makes the sensitivities of the future
    gravitational-wave detectors overwhelmingly better than the current ones. The
    factor by which the constraints are improved due to the new effect depends on
    the mass of the vector dark matter, and the maximum improvement factors are
    $470$, $880$, $1600$, $180$ and $1400$ for advanced LIGO, Einstein Telescope,
    Cosmic Explorer, DECIGO and LISA respectively. Including the new effect, we
    update the constraints given by the first observing run of advanced LIGO and
    improve the constraints on the $U(1)_B$ gauge coupling by an order of magnitude
    compared with the current best constraints.

    DOI

  • Resonant gravitational waves in dynamical Chern-Simons-axion gravity

    Tomohiro Fujita, Ippei Obata, Takahiro Tanaka, Kei Yamada

       2020年08月

     概要を見る

    In this paper, we consider dynamical Chern-Simons gravity with the
    identification of the scalar field coupled though the Pontryagin density with
    the axion dark matter, and we discuss the effects of the parametric resonance
    on gravitational waves (GWs). When we consider GWs in a coherently oscillating
    axion cloud, we confirm that significant resonant amplification of GWs occurs
    in a narrow frequency band, and the amplification is restricted to the late
    epoch after the passage of the incident waves. We also identify the condition
    that an axion cloud spontaneously emits GWs. Once we take into account the
    randomness of the spatial phase distribution of the axion oscillations, we find
    that the amplification is suppressed compared with the coherent case, but
    significant amplification of GWs can still occur. We also examine whether or
    not the amplification of GWs is possible in the present universe, taking into
    account the history of the universe. We find that resonant amplification is
    difficult to be tested from GW observations in the standard scenario of the
    axion DM model, in which the axion is the dominant component of DM. However,
    there is some parameter window in which the resonant amplification of GWs might
    be observed, if the axion is subdominant component of DM, and the axion cloud
    formation is delayed until the Hubble rate becomes much smaller than the axion
    mass.

    DOI

  • Ultralight vector dark matter search with auxiliary length channels of gravitational wave detectors

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

       2020年08月

     概要を見る

    Recently, a considerable amount of attention has been given to the search for
    ultralight dark matter by measuring the oscillating length changes in the arm
    cavities of gravitational wave detectors. Although gravitational wave detectors
    are extremely sensitive for measuring the differential arm length changes, the
    sensitivity to dark matter is largely attenuated, as the effect of dark matter
    is mostly common to arm cavity test masses. Here, we propose to use auxiliary
    length channels, which measure the changes in the power and signal recycling
    cavity lengths and the differential Michelson interferometer length. The
    sensitivity to dark matter can be enhanced by exploiting the fact that
    auxiliary interferometers are more asymmetric than two arm cavities. We show
    that the sensitivity to $U(1)_{B-L}$ gauge boson dark matter with masses below
    $7\times 10^{-14}$ eV can be greatly enhanced when our method is applied to a
    cryogenic gravitational wave detector KAGRA, which employs sapphire test masses
    and fused silica auxiliary mirrors. We show that KAGRA can probe more than an
    order of magnitude of unexplored parameter space at masses around $1.5 \times
    10^{-14}$ eV, without any modifications to the existing interferometer.

    DOI

  • Probing Axion-like Particles via CMB Polarization

    Tomohiro Fujita, Yuto Minami, Kai Murai, Hiromasa Nakatsuka

       2020年08月

     概要を見る

    Axion-like particles (ALPs) rotate the linear polarization of photons through
    the ALP-photon coupling and convert the cosmic microwave background (CMB)
    $E$-mode to the $B$-mode. We derive the relation between the ALP dynamics and
    the rotation angle by assuming that the ALP $\phi$ has a quadratic potential,
    $V=m^2\phi^2/2$. We compute the current and future sensitivities of CMB
    observations to the ALP-photon coupling $g$, which can reach $g=4\times
    10^{-21}\,\mathrm{GeV}^{-1}$ for $10^{-32}\,\mathrm{eV}\lesssim m\lesssim
    10^{-28}\,\mathrm{eV}$ and extensively exceed the other searches for any mass
    $m\lesssim 10^{-25}\,\mathrm{eV}$. We find that the fluctuation of the ALP
    field at the observer, which has been neglected in previous studies, can induce
    significant isotropic rotation of the CMB polarization. The measurements of
    isotropic and anisotropic rotation allow us to put bounds on relevant
    quantities such as the ALP mass $m$ and the ALP density parameter
    $\Omega_\phi$. In particular, if LiteBIRD detects anisotropic rotation, we
    obtain the lower bound on the tensor-to-scalar ratio as $r > 5 \times 10^{-9}$.

    DOI

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