Research On Detecting Stochastic Gravitational-wave Background With TianQin

There will be a large number of gravitational waves(GWs)cannot be detected individually by detectors.Their incoherent superposition thus will form a stochastic gravitational-wave background(SGWB).In particular,SGWB is regarded as confusion noise,or so-called foreground when exceeding the noise level of the detector.Depending on the generation of the GW sources,the SGWB can be either astrophysical or cosmological in origin.The astrophysical-origin mainly contains inspiralling or coalescing compact binaries,core-collapse supernovae,rotating neutron stars,etc.On the other hand,the cosmological-origin is linked to the physical processes of the early Universe,such as inflation,first-order phase transitions,and cosmic defects.For the SGWB detection,three detector configurations of Tian Qin,Tian Qin I+II and Tian Qin+LISA are considered in this dissertation.Tian Qin is a space-borne GW observatory composed of three identical geocentric orbit satellites with an orbital radius of approximately 10~5 km,which is expected to be launched around 2035 aiming for GW detection in the milli-Hertz range.The three satellites will form an equilateral triangle such that the armlength of Tian Qin will be about 1.7×10~5 km.As the nominal working mode of Tian Qin is set to“three months on+three months off”,Tian Qin II is proposed to bridge the detection gap.The orbital planes of Tian Qin and Tian Qin II with the same orbital radius will be perpendicular to each other,and both detectors will form the Tian Qin I+II configuration.Additionally,the Laser Interferometer Space Antenna(LISA)is the other space-borne detector with an operating period and detection frequency band similar to Tian Qin.It is therefore meaningful to discuss the collaboration of the Tian Qin+LISA network.For the different detector configurations,corresponding detection methods are adopted in this dissertation:the null channel method is used for the independent detection of Tian Qin,while the cross-correlation method is used for the joint detection of Tian Qin I+II and Tian Qin+LISA.Furthermore,assuming that the armlengths of the detector are all equal,the Michelson and the time delay interferometer(TDI)channels in the above detector configurations are employed to explore the SGWB detection in this dissertation.First,this dissertation calculates the foreground of Tian Qin produced by Galactic double white dwarfs.Since some GWs will be detected individually by Tian Qin after one year of operation,the foreground of Tian Qin could be reduced below the detector noise of Tian Qin.When combining Tian Qin with LISA,it could take only half a year of accumulating observations to make the foreground lower than the detector noise of Tian Qin.Consequently,the impact of foreground on the GW detection could be reduced by increasing the operating time of the detector and combining multiple detectors.This dissertation also calculates the signal-to-noise ratio(SNR)of the SGWB.Regardless of the effect of the foreground,Tian Qin will detect the SGWB of compact binaries,including stellar-mass binary black holes through the null channel method with one year of operation.By selecting specific model parameters,it is also possible for Tian Qin to detect the SGWB of the cosmological-origin,such as first-order phase transitions.On the other hand,the aforementioned SGWBs will also be detected through the cross-correlation method adopted in Tian Qin I+II and Tian Qin+LISA.Furthermore,this dissertation assesses the detection capabilities of the three detector configurations.Compared with the cross-correlation method adopted in Tian Qin I+II or Tian Qin+LISA,the null channel method adopted in Tian Qin will provide better detection capability.However,when using the null channel method,one must rely strongly on the detailed understanding of the detector noise.Hence,the cross-correlation method is more reliable.We observe that the senstivitiy curves for the Michelson channel and the TDI channel are the same,which indicates that the operation of TDI does not affect the sensitivity towards SGWB detection.Finally,constraints on the physical model and parameters are addressed herein.The result is as follows:the formation of stellar-mass binary black holes could be determined through the SGWB spectrum;the event rate of extragalactic double white dwarfs could be limited by the SNR calculation;by detecting the primordial GW generated during inflation,the graviton mass could be limited;last but not least,the duration and the temperature of first-order phase transitions could be restricted by the SNR calculation.