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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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