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The 111-120th REM talks

The 114th REM: June 1st, 2023, Thursday

• Time: 11:00-12:00
• Speaker: Hiroki Takeda (Kyoto Unv.)

  • Title: "Probing scalar polarizations in gravitational waves"

    • Abstract:

      Scalar polarization modes of gravitational waves, which are often predicted in the context of the viable extension of gravity, have been actively searched. However, couplings of the scalar modes to the matter are strongly constrained by the fifth-force experiments. Thus, the amplitude of scalar modes in the observed gravitational-wave signal is expected to be significantly suppressed compared to that of the tensor modes. In this talk, we will discuss the implications of the experiments in the solar system on the detectability of scalar modes in gravitational waves from compact binary coalescences. We first claim that the energy carried by the scalar modes at the generation is, at most, that of the tensor modes from the observed phase evolution of the inspiral gravitational waves. Next, we formulate general gravitational-wave propagation with full polarizations and point out that the energy flux hardly changes through propagation as long as the background changes slowly compared to the wavelength of the propagating waves. Finally, we show that the possible magnitude of scalar polarization modes, which can be probed by the ground-based gravitational-wave telescopes, is already severely constrained by the existing gravity tests in the solar system.

The 113rd REM: April 27th, 2023, Thursday

• Time: 11:00-12:00
• Speaker: Chihiro Hirose (Niigata Unv.)

  • Title: "Alignment Sensing and Control in KAGRA"

    • Abstract:

      In international joint observation of gravitational waves, it is necessary to increase the simultaneous operation rate of gravitational wave telescopes to measure the direction of arrival of gravitational waves. In Japan's KAGRA, the mirror is suspended by a pendulum for vibration isolation, but the tilt of the mirror causes drift. To stabilize the optical axis, Alignment Sensing and Control (ASC) needs to be introduced. Wavefront sensing is effective in detecting the difference between the resonator axis and the incident optical axis. Also, KAGRA's mirrors are cooled down to 20 K to reduce thermal noise. Therefore, the mirrors are made of sapphire, which has low mechanical loss even at low temperatures. I investigated the effect of birefringence on ASC by measuring Y-arm error signal. I also actually introduced ASC to the arm. In this gravitational exchange meeting I will talk about the effect of birefringence on ASC, and also about ASC for Observation 4 now.

The 112nd REM: Febrary 28th, 2023, Tuseday

• Time: 17:00-18:00
• Speaker: Kohei Yamamoto (AEI)

  • Title: "LISA metrology system: Clocks, ranging, and communication"

    • Abstract:

      The Laser Interferometer Space Antenna (LISA) aims to observe gravitational waves in the mHz regime over its 10-year mission time. LISA will operate laser interferometers between three spacecraft. Each spacecraft will utilize independent clocks, which determine the sampling times of onboard devices called phasemeter to extract the interferometric phases and, ultimately, gravitational wave signals. To suppress limiting laser frequency noise due to time-varying unequal arm lengths, phase signals sampled by each phasemeter must be combined in postprocessing to synthesize virtual equal-arm interferometers with the technique called time-delay interferometry (TDI). The synthesis, in turn, requires a synchronization of the independent clocks in postprocessing. For such on-ground data analysis, additional onboard measurements will be introduced on top of primary phase extractions: intersatellite clock-tone transfer and absolute ranging. The ranging technique also enables intersatellite data communication to ease communication between a ground station and LISA. This talk introduces recent experimental studies of these technologies using a hexagonal optical testbed at AEI Hannover.

The 111th REM: January 26th, 2023, Thursday

• Time: 11:00-12:00
• Speaker: Hiroki Fujimoto (UTokyo)

  • Title: "Axion dark matter search with laser interferometry"

    • Abstract:

      Researches on cosmology and astrophysics have revealed that more than 80% of the matter in the universe consists of an unknown substance, or dark matter. The nature of dark matter is still unknown and many searches have been conducted for various dark matter candidates. Axion-like particles (ALPs) are undiscovered particles that are well-motivated candidates for ultralight dark matter. The interaction between ALPs and photons slightly causes the rotational oscillation of linearly polarized light, and therefore ALP dark matter can be detected by the measurement of the polarization rotation of a laser beam.
      Recently, some ALP dark matter search experiments using the laser interferometer have been proposed. The basic idea is to use an optical cavity, which can enhance the effective light path and also the duration of the interaction with the ALP dark matter. With this technique, the polarization rotation can be amplified and high-sensitive ALP dark matter search can be conducted.
      In this talk, I will introduce our interferometric ALP dark matter search experiment: DANCE and the search using the gravitational wave detectors.