1. Bridging the Gap between Terrestrial Detectors and LISA Elba 2002 May 24, 2002 Seiji Kawamura National Astronomical Observatory of Japan

2. Contents 1.Introduction 2.What is DECIGO? 3.Science obtained by DECIGO 4.Sensitivity of DECIGO 5.Current Status of DECIGO 6.Summary

3. Gap between Terrestrial Detectors and LISA 10 -18 10 -24 10 -22 10 -20 10 -4 10 4 10 2 10 0 10 -2 Frequency [Hz] Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) Gap

4. 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 Completely new sources could be detected Completely new science could be obtained

5. How to bridge the gap? Improve the low-frequency sensitivity of terrestrial detectors Improve the high-frequency sensitivity of LISA Build a detector suitable for the gap frequency  DECIGO

6. What is DECIGO? Deci-hertz Interferometer Gravitational Wave Observatory Space Antenna with short arm length 10 -18 10 -24 10 -22 10 -20 10 -4 10 4 10 2 10 0 10 -2 Frequency [Hz] Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) DECIGO (Sensitivity: Arbitrary)

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

8. Relationship between Sensitivity and Arm Length 10 -18 10 -21 10 -19 10 -20 Frequency [Hz] Strain [Hz -1/2 ] LISA 10 -4 10 2 10 0 10 -2 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

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

10. 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, 221103 (2001)

11. Ultimate Sensitivity of DECIGO Frequency [Hz] 10 -4 10 4 10 2 10 0 10 -2 Strain [Hz -1/2 ] LISA Terrestrial Detectors (e.g. LCGT) Ultimate DECIGO 10 -18 10 -24 10 -22 10 -20 10 -26 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)

12. In the Paper, we said… The ultimate sensitivity of a space antenna in the far future could be, however, 3  10 -27 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.

13. 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 Solve confusion limiting noise problem

14. DECIGO Working Group NAOJ does not have a definitive plan after ALMA. Convened in 2002 as one of WGs to study the future project candidates for NAOJ 1 st Meeting held on May 9, 2002 80 members currently involved To be discussed in the 2 nd Symposium (June 4-5, 2002) for NAOJ future projects

15. Summary It is important to bridge the gap between terrestrial detectors and LISA. DECIGO is a candidate for the Japanese space antenna. There is no WD-WD confusion limiting noise above 0.1 Hz. DECIGO could measure the acceleration of expansion of the Universe. DECIGO-WG has just started investigating the possibility.