DECIGO – Japanese Space Gravitational Wave Detector

International Workshop on GPS Meteorology

January 17, 2003

@ 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

Gravitational Wave

Mirror

Mirror

Laser

Beamsplitter

Photodetector

BH binary, NS binary,

Streching

Shrinking

Supernova, Big Bang

Longer arm gives larger signals!

Laser

Photodetector

Mirror

Mirror

Laser

PhotodetectorMirror

Mirror

Large-scale GW Detectorsin the World

LIGO (4km)

LIGO (4km)

VIRGO (3km)

GEO (600m) TAMA (300m)

AIGO (?km)

LCGT (3km)

Achievements of TAMA300

Sep. 2000

Sep. 1999

Feb. 1999

Jun. 2001

Frequency　[Hz]

10 100 1k 10k 100k

10- 19

10 -21

10 -17

10 -15

10- 13

Aug. 2002

2000：　World-best sensitivity obtained2001：　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 improvedat lower frequencies

LISALaser 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

10-18

10-24

10-22

10-20

10-4 10410210010-2

Frequency [Hz]

Str

ain

[H

z-1/2] LISA

Terrestrial Detectors(e.g. LCGT)Gap

Importance of Bridgingthe Gap

New window brings new science! Observe inspiral sources that have moved abov

e the LISA band Observe inspiral sources that have not yet mov

ed 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

Inspiral Merger Ringdown

What is DECIGO? Deci-hertz Interferometer Gravitational Wave Observatory Space Antenna with shorter arm length

10-18

10-24

10-22

10-20

10-4 10410210010-2

Frequency [Hz]

Str

ain

[H

z-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

10-18

10-21

10-19

10-20

Frequency [Hz]

Str

ain

[H

z-1/2] LISA

10-4 10210010-2

DECIGO

Shot Noise (f<f0): XSN/L P∝ -1/2/L (L∝ -2) -1/2/L=L/L

(Arm Length:

x100

f0 f01/100 of LISA)

Force Noise:

x100

XFN/L 1/L∝ f0:1/L

f1f-2

Advantages of DECIGONo 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

Str

ain

Template (No Acceleration)

Real Signal ?Phase Delay～ 1sec (10 years)

Seto, Kawamura, Nakamura, PRL 87, 221103 (2001)

Ultimate Sensitivity of DECIGO

Frequency [Hz]10-4 10410210010-2

Str

ain

[H

z-1/2]

LISATerrestrial Detectors(e.g. LCGT)

Ultimate DECIGO

10-18

10-24

10-22

10-20

10-26 M=100kg, L=5×108m)(Quantum noise limited

DECIGO(LISA Technology

L=5×107m)

Ultimate DECIGO × 1000(Necessary for

acceleration measurement)

In the Paper, we said… The ultimate sensitivity of a space antenna in th

e far future could be, however, 310-27 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.

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

1st Meeting held on May 9, 2002 83 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.