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• 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.

• 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.

• 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.

• 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.

• 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 $r > 5 \times 10^{-9}$.