DECIGO – Japanese Space Gravitational Wave Detector International Workshop on GPS Meteorology January 17, Tsukuba Center for Institutes Seiji Kawamura* and DECIGO-WG (*) National Astronomical Observatory of Japan

What is Gravitational Wave? Predicted by A. Einstein in the general relativity Not yet detected directly Emitted by accelerating masses Causing a time-variant tidal strain in space Propagating with the speed of light

Detection of Gravitational Waves Using Laser Interferometer

Longer arm gives larger signals! Laser Photodetector Mirror Laser Photodetector Mirror

Large-scale GW Detectors in the World LIGO (4km) VIRGO (3km) GEO (600m) TAMA (300m) AIGO (?km) LCGT (3km)

Achievements of TAMA ： World-best sensitivity obtained 2001 ： World-longest 1000 hours of data obtained

Laser Interferometer in Space Signal increased - Due to longer interaction between GW and light - Cancellation of signals at higher frequencies Noise reduced - Lower seismic noise and gravity gradient noise Sensitivity improved at lower frequencies

LISA Laser Interferometer Space Antenna Joint Project by NASA and ESA Aiming at 0.1mHz - 100mHz To be launched in 2001

Gap between Terrestrial Detectors and LISA Frequency [Hz] Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) Gap

Importance of Bridging the Gap New window brings new science! Observe inspiral sources that have moved above the LISA band Observe inspiral sources that have not yet moved into the ground-based detector band Cosmological background could be detected Completely new sources could be detected Completely new science could be obtained

GWs from Inspirals

What is DECIGO? Deci-hertz Interferometer Gravitational Wave Observatory Space Antenna with shorter arm length Frequency [Hz] Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) DECIGO (Sensitivity: Arbitrary)

DECIGO Named by T. Nakamura in the PRL paper by N. Seto, S. Kawamura, and T. Nakamura Potential candidate for the future Japanese space GW antenna Study of DECIGO just started; still primitive

Relationship between Sensitivity and Arm Length Frequency [Hz] Strain [Hz -1/2 ] LISA DECIGO Shot Noise (f<f 0 ): X SN /L ∝ P -1/2 /L ∝ (L -2 ) -1/2 /L=L/L (Arm Length: x100 f0f0 f0f0 1/100 of LISA) Force Noise: x100 X FN /L ∝ 1/L f 0 :1/L f1f1 f -2

Advantages of DECIGO No confusion limiting noise above 0.1Hz (From the LISA report)

Acceleration of Expansion of the Universe NS-NS (z ～ 1) GW DECIGO Output Expansion ＋ Acceleration? Time Strain Template (No Acceleration) Real Signal ? Phase Delay ～ 1sec (10 years) Seto, Kawamura, Nakamura, PRL 87, (2001)

Ultimate Sensitivity of DECIGO Frequency [Hz] Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) Ultimate DECIGO M=100kg, L=5×10 8 m) (Quantum noise limited DECIGO (LISA Technology L=5×10 7 m) Ultimate DECIGO × 1000 (Necessary for acceleration measurement)

In the Paper, we said… The ultimate sensitivity of a space antenna in the far future could be, however, 3  around 0.1 Hz in terms of strain, assuming the quantum limit sensitivity for a 100 kg mass and an arm length of 1/10 of LISA. We name this detector DECIGO. This requires an enormous amount of effective laser power, and also requires that the other noise sources, such as gravity gradient noise, thermal noise, practical noise, etc. should be all suppressed below the quantum noise. Here we assume that such an antenna may be available by the end of this century.

Necessary Technologies for DECIGO Solar Radiation Deflection of Light Gravity Gradient Three Satellite Formation Flight Drag-free Satellite Heterodyne Detection Reflection with phase- locking

How to Improve Sensitivity? Increase effective power - Increase the laser power - Increase the diameter of mirror Use shorter wavelength? Reduce other sensing noise Reduce force noise

DECIGO Working Group Convened in 2002 as one of WGs to study the future project candidates for NAOJ 1 st Meeting held on May 9, members currently involved R&D experiments to be started very soon

Two R&D Experiments to be Started Very Soon Collaboration work between NASA- Goddard (LISA) and NAOJ (DECIGO) on ground testing of space interferometry Constructing a measurement system for Earth environmental monitoring by CRL, Niigata Univ., and NAOJ

Conclusions It is important to bridge the gap between terrestrial detectors and LISA. DECIGO is a candidate for the Japanese space GW antenna. DECIGO-WG has just started investigating the possibility. R&D experiments will be started very soon.