The Japanese Space Gravitational Wave Antenna: DECIGO
[This is an invited article. Interested readers may also check out the following article in our archive: "Interferometric Detection of Gravitational Waves: 5 Needed Breakthroughs -- Seiji Kawamura" -- 2Physics.com]
Author: Seiji Kawamura
Affiliation: TAMA Project, National Astronomical Observatory of Japan
DECIGO is the future Japanese space gravitational wave antenna. It stands for DECi-hertz Interferometer Gravitational wave Observatory [1,2]. The goal of DECIGO is to detect gravitational waves mainly between 0.1 Hz and 10 Hz coming from various sources including those from the beginning of the universe, and open a new window of observation for gravitational wave astronomy.
DECIGO will bridge the frequency gap between LISA  and terrestrial detectors such as LCGT , somewhat similarly with BBO . It can play a role of follow-up for LISA by observing inspiral sources that have moved above the LISA band, and can also play a role of predictor for terrestrial detectors by observing inspiral sources that have not yet moved into the terrestrial detector band.
The more important advantage of DECIGO specializing in this frequency band is that the confusion limiting noise caused by irresolvable gravitational wave signals from many compact binaries is expected to be very low above 0.1 Hz . Therefore, DECIGO can reach an extremely high sensitivity.
Fig.1. Pre-conceptual design of DECIGO
The pre-conceptual design of DECIGO consists of three drag-free spacecraft, whose relative displacements are measured by a differential Fabry–Perot (FP) Michelson interferometer (see Fig. 1). The arm length was chosen to be 1,000 km in order to realize a finesse of 10 with a 1 m diameter mirror and 0.5 mm laser light. The mass of the mirror is 100 kg and the laser power is 10 W. Three sets of such interferometers sharing the mirrors as arm cavities comprise one cluster of DECIGO. The constellation of DECIGO is composed of four clusters of DECIGO located separately in the heliocentric orbit with two of them nearly at the same position.
The FP configuration requires the distance between two mirrors, thus, the distance between two spacecraft to be constant during continuous operations. This makes DECIGO very different from a possible counterpart with the transponder-type detector (e.g. LISA), where the spacecraft, which are much farther apart, are freely falling according to their local gravitational field. We adopted the FP configuration because it can provide a better shot-noise-limited sensitivity than the transponder configuration due to the enhanced gravitational wave signals.
The sensitivity goal of DECIGO, as shown in Fig. 2, is limited by the radiation pressure noise below 0.15 Hz, and by the shot noise above 0.15 Hz. In order to realize this goal, all the practical noise should be suppressed well below this level. This imposes stringent requirements for the subsystems of DECIGO. We anticipate that extremely rigorous investigations are required to attain the requirements especially in the acceleration noise and frequency noise.
Fig. 2. Sensitivity goal of DECIGO and expected gravitational wave signals.
Nevertheless, accomplishing the goal sensitivity of DECIGO will ensure a variety of fruitful sciences to be obtained.
(1) Verification and characterization of inflation: DECIGO can detect stochastic background corresponding to
ΩGW=2 X 10-16 by correlating the data from the two clusters of DECIGO, which are placed nearly at the same position, for three years. According to the standard inflation model, it is expected that we could detect gravitational waves produced at the inflation period of the universe with DECIGO. This is extremely significant because gravitational waves are the only means which make it possible to directly observe the inflation of the universe.
(2) Characterization of dark energy: DECIGO can detect gravitational waves coming from neutron star binaries at z=1 for five years prior to coalescences. It is expected that within this range about 50,000 neutron star binaries will coalesce every year. Therefore, DECIGO will detect gravitational waves coming from a large number of neutron star binaries at the same time. By analyzing the waveforms of these gravitational wave signals precisely, it is possible to determine the acceleration of the expansion of the universe . The acceleration of the expansion of the universe can be also measured by finding host galaxies of each binary, which is possible with the expected angular resolution of about 1 arcsec, and determining their red shifts optically. This will lead to better characterization of dark energy.
(3) Formation mechanism of supermassive black holes in the center of galaxies : DECIGO can detect gravitational waves coming from coalescences of intermediate-mass black hole binaries with an extremely high fidelity. For example the coalescences of black hole binaries of 1,000 solar masses at z=1 give a signal to noise ratio of 6,000. This will make it possible to collect numerous data about the relationship between the mass of the black holes and the frequency of the coalescences, which will reveal the formation mechanism of supermassive black holes in the center of galaxies.
We plan to launch two missions before DECIGO: DECIGO pathfinder (DPF)  and pre-DECIGO (See Fig. 3). DPF tests the key technologies for DECIGO just as LISA pathfinder  does for LISA. We expect that it will be launched in 2012. Pre-DECIGO is supposed to detect gravitational waves with minimum specifications. We hope that it will be launched in 2018. Finally DECIGO will be launched in 2024 to open a new window of observation for gravitational wave astronomy.
Fig. 3 Roadmap to DECIGO
As shown in Fig. 4, DPF will employ a small drag-free spacecraft that contains two freely falling masses, whose relative displacement is measured with a Fabry–Perot interferometer, which is illuminated by the frequency-stabilized laser light. The masses are clamped tightly for the launch and released gently in space. DPF will be delivered in the geocentric sun-synchronous orbit with an altitude of 500km. The strain sensitivity of DPF will be ~10-15 around the frequency band of 0.1-1Hz. The primary objective of DPF is to test the drag-free system, the FP cavity measurement system in space, frequency-stabilized laser in space, and the clamp release system. The scientific objective of DPF is to detect rather unlikely events of intermediate-mass black hole inspirals in our galaxy; it is possible to detect such events with the aimed sensitivity of DPF.
DPF was identified as one of the mission candidates for the small science-spacecraft mission series which had been recently initiated by the Japanese space agency, JAXA/ISAS. This mission series are expected to reduce the cost of missions significantly compared with the conventional large-spacecraft missions. The reduction of the cost also relies on the development of a satellite bus that is common to any mission. We are now in the process of refining the design of DPF.
Fig. 4. Pre-conceptual design of DECIGO pathfinder.
In order to realize DPF and then DECIGO, international collaboration is essential. Actually we have recently had the 1st International LISA-DECIGO Workshop in Japan (http://tamago.mtk.nao.ac.jp/decigo/LISA-DECIGO.html), which I hope can be a good start for a long and fruitful collaboration between DECIGO and LISA.
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Labels: Gravitational Waves