Improvement of the Target Sensitivity in DECIGO by Optimizing Its Parameters for Quantum Noise Including the Effect of Diffraction Loss

Tomohiro Ishikawa,Rika Yamada,Tomotada Akutsu,Yuta Michimura,Taigen Morimoto,Kentaro Komori,Seiji Kawamura,Takeo Naito,Masaki Ando,Koji Nagano,Izumi Watanabe,Shoki Iwaguchi,Mitsuru Musha

doi:10.3390/galaxies9010014

Abstract

The DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is the future Japanese, outer space gravitational wave detector. We previously set the default design parameters to provide a good target sensitivity to detect the primordial gravitational waves (GWs). However, the updated upper limit of the primordial GWs by the Planck observations motivated us toward further optimization of the target sensitivity. Previously, we had not considered optical diffraction loss due to the very long cavity length. In this paper, we optimize various DECIGO parameters by maximizing the signal-to-noise ratio (SNR) of the primordial GWs to quantum noise, including the effects of diffraction loss. We evaluated the power spectrum density for one cluster in DECIGO utilizing the quantum noise of one differential Fabry–Perot interferometer. Then we calculated the SNR by correlating two clusters in the same position. We performed the optimization for two cases: the constant mirror-thickness case and the constant mirror-mass case. As a result, we obtained the SNR dependence on the mirror radius, which also determines various DECIGO parameters. This result is the first step toward optimizing the DECIGO design by considering the practical constraints on the mirror dimensions and implementing other noise sources.

Highlights

The existence of gravitational waves (GWs) was predicted by Einstein’s theory of general relativity and verified recently by LIGO and Virgo [1,2]
Among the various origins of GWs, inflation in the early universe could have produced a stochastic background of primordial GWs through the quantum fluctuations in spacetime [4]
We evaluate the relationship of the power spectrum density (PSD) of GWs between interferometer X and the linear combination of three interferometers A

Summary

Introduction

The existence of gravitational waves (GWs) was predicted by Einstein’s theory of general relativity and verified recently by LIGO and Virgo [1,2]. To detect the primordial GWs, we designed the DECi-hertz Interferometer Gravitationalwave Observatory (DECIGO) [5] R = 0.5 m, cavity length L = 1000 km, finesse F = 10, laser wavelength λ = 515 nm, and laser power P0 = 10 W These parameters, together with the correlation of the two clusters in the same position, were employed to provide a good target sensitivity to detect the primordial GWs assuming Ωgw = 2 × 10−15. Since the original design study, the upper limit of the primordial GWs was updated to be Ωgw = 1 × 10−16 from observations by the Planck satellite [8], and other electromagnetic observations [9] This motivated us to improve the target sensitivity [10]. Once we establish the method to optimize the parameters for the quantum noise, we can do the optimization by including other noise sources in this method

Theory

Optimization of the DECIGO Parameters

Result

Same Radius as That of the Default DECIGO Parameter

Mirror Radius as a Free Parameter

Conclusions