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 [3] and terrestrial detectors such as LCGT [4], somewhat similarly with BBO [5]. 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 [6]. 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
Ω_GW=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 [1]. 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) [7] and pre-DECIGO (See Fig. 3). DPF tests the key technologies for DECIGO just as LISA pathfinder [8] 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.

References:
[1] “Possibility of Direct Measurement of the Acceleration of the Universe Using 0.1 Hz Band Laser Interferometer Gravitational Wave Antenna in Space”,
N. Seto, S. Kawamura, and T. Nakamura, Phys. Rev. Lett, 87 (2001) 221103. Abstract
[2] “The Japanese space gravitational wave antenna – DECIGO”, S Kawamura, et al.,
Journal of Physics: Conference Series 122 (2008) 012006. Abstract.
[3] “LISA: System and Technology Study Report”, ESA document ESA-SCI (2000)
[4] “Japanese large-scale interferometers”, K Kuroda, et al.,
Class. Quantum Grav. 19 (2002) 1237-1245. Abstract.
[5] Phinney E S, et al., “The Big Bang Observer NASA Mission Concept Study” (2003)
[6] “The gravitational wave background from cosmological compact binaries”,
Alison J. Farmer, E. S. Phinney, Mon. Not. R. Astron. Soc. 346 (2003) 1197. Abstract.
[7] “DECIGO pathfinder”, M. Ando, et al,
Journal of Physics: Conference Series 120 (2008) 032005. Abstract.
[8] “LISA Pathfinder”, P McNamara, S Vitale and K Danzmann (on behalf of the LISA Pathfinder Science Working Team), Class. Quantum Grav. 25 (2008) 114034. Abstract

Asymmetry in Early Universe : What Happened Before the Big Bang ?

Marc Kamionkowski [image courtesy: Caltech]

In a paper published in December 16 issue of the journal Physical Review D, a team of physicists from California Institute of Technology has shed new light on the nature of asymmetry in Early Universe and the question of what came before the big bang.

"It's no longer completely crazy to ask what happened before the Big Bang," comments Prof. Marc Kamionkowski, who with graduate student Adrienne Erickcek and senior research associate in physics Sean Carroll, proposed a mathematical model explaining an anomaly in what is supposed to be a universe of uniformly distributed radiation and matter.

Their investigations turn on the inflationary scenerio of the early universe, first proposed in 1980, which posits that space expanded exponentially in the instant following the Big Bang. "Inflation starts the universe with a blank slate," Adrienne Erickcek describes. The hiccup in inflation, however, is that the universe is not as uniform as the simplest form of the theory predicts it to be. Some parts of it are more intensely varied than others.

Image: The Cosmic Microwave Background [courtesy NASA/WMAP Science Team]

Until recently, measurements of the Cosmic Microwave Background (CMB) radiation, a form of electromagnetic radiation that permeated the universe 400,000 years after the Big Bang, were consistent with inflation--miniscule fluctuations in the CMB seemed to be the same everywhere. But a few years ago, some researchers, including a group led by Krzysztof Gorski of NASA's Jet Propulsion Laboratory, which is managed by Caltech, scrutinized data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP). They discovered that the amplitude of fluctuations in the CMB is not the same in all directions.

"If your eyes measured radio frequency, you'd see the entire sky glowing. This is what WMAP sees," Kamionkowksi describes. WMAP depicts the CMB as an afterglow of light from shortly after the Big Bang that has decayed to microwave radiation as the universe expanded over the past 13.7 billion years. The probe also reveals more pronounced mottling--deviations from the average value--in the CMB in one half of the sky than the other.

"It's a certified anomaly," Kamionkowski remarks. "But since inflation seems to do so well with everything else, it seems premature to discard the theory." Instead, the team worked with the theory in their math addressing the asymmetry.

They started by testing whether the value of a single energy field thought to have driven inflation, called the inflaton, was different on one side of the universe than the other. It didn't work--they found that if they changed the mean value of the inflaton, then the mean temperature and amplitude of energy variations in space also changed. So they explored a second energy field, called the curvaton, which had been previously proposed to give rise to the fluctuations observed in the CMB. They introduced a perturbation to the curvaton field that turns out to affect only how temperature varies from point to point through space, while preserving its average value.

The new model predicts more cold than hot spots in the CMB, Kamionkowski says. Erickcek adds that this prediction will be tested by the Planck satellite, an international mission led by the European Space Agency with significant contributions from NASA, scheduled to launch in April 2009.

For Erickcek, the team's findings hold the key to understanding more about inflation. "Inflation is a description of how the universe expanded," she adds. "Its predictions have been verified, but what drove it and how long did it last? This is a way to look at what happened during inflation, which has a lot of blanks waiting to be filled in."

But the perturbation that the researchers introduced may also offer the first glimpse at what came before the Big Bang, because it could be an imprint inherited from the time before inflation. "All of that stuff is hidden by a veil, observationally," Kamionkowski says. "If our model holds up, we may have a chance to see beyond this veil."

Reference
"A hemispherical power asymmetry from inflation"
Adrienne L. Erickcek, Marc Kamionkowski, and Sean M. Carroll,
Phys. Rev. D 78, 123520 (2008). Abstract

[We thank Media Relations, Caltech for materials used in this posting]

World-record Performance using a Silicon-based Avalanche Photodetector

Mario Paniccia [Photo courtesy: Intel]

In an article published today in online version of Nature Photonics, a team led by Intel researchers reported a path-breaking advancement in the field of Silicon Photonics by achieving world-record performance with a silicon-based Avalanche Photodetector (APD), a light sensor that gains superior sensitivity by detecting light and amplifying weak signals as light is directed onto silicon. This could lower costs and improve performance as compared to commercially available optical devices.

Silicon Photonics is an emerging technology using standard silicon to send and receive optical information among computers and other electronic devices. The technology aims to address future bandwidth needs of data-intensive computing applications such as remote medicine and lifelike 3-D virtual worlds.

The photodetector developed by the team is Ge/Si-based and has built-in amplification, which makes it much more useful in instances where very little light falls on the detector. It is called an avalanche photodetector because an avalanche process occurs inside the device. First, a negative and a positive charge (electrons and holes in semiconductor terminology) are created when the light strikes the detector. The electron is accelerated by an electric field until it attains a high enough energy to slam into a silicon atom and create another pair of positive and negative charges. Each time this happens the number of total electrons doubles, until this “avalanche” of charges are collected by the detection electronics.

This amplification effect (called gain) is the key to the device, and it serves as the motivation for why anyone would try to do this in silicon and not just continue to use traditional InP (Indium phosphide)-based APDs. The materials properties of silicon inherently led to lower noise and better performance in this avalanche process.

Image: A ladybug crawls across an experimental Avalanche Photodetector chip containing silicon optical devices that are only a fraction of a millimeter [Photo courtesy: Intel]

The APD device developed by the Intel team used silicon and CMOS processing to achieve a "gain-bandwidth product" of 340 GHz -- the best result ever measured for this key APD performance metric. This opens the door to lower the cost of optical links running at data rates of 40Gbps or higher and proves, for the first time, that a silicon photonics device can exceed the performance of a device made with traditional, more expensive optical materials such as indium phosphide (InP).

"This research result is another example of how silicon can be used to create very high-performing optical devices," said Dr. Mario Paniccia, Intel Fellow and director of the company's Photonics Technology Lab. "In addition to optical communication, these silicon-based APDs could also be applied to other areas such as sensing, imaging, quantum cryptography or biological applications."

Reference
"Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product"
Yimin Kang, Han-Din Liu, Mike Morse, Mario J. Paniccia, Moshe Zadka, Stas Litski, Gadi Sarid, Alexandre Pauchard, Ying-Hao Kuo, Hui-Wen Chen, Wissem Sfar Zaoui, John E. Bowers, Andreas Beling, Dion C. McIntosh, Xiaoguang Zheng & Joe C. Campbell,
Nature Photonics (7 December 2008 doi:10.1038/nphoton.2008.247). Abstract.