In this perspective, we outline that a space borne gravitational wave detector network combining LISA and Taiji can be used to measure the Hubble constant with an uncertainty less than 0.5% in ten years, compared with the network of the ground based gravitational wave detectors which can measure the Hubble constant within a 2% uncertainty in the next five years by the standard siren method. Taiji is a Chinese space borne gravitational wave detection mission planned for launch in the early 2030 s. The pilot satellite mission Taiji-1 has been launched in August 2019 to verify the feasibility of Taiji. The results of a few technologies tested on Taiji-1 are presented in this paper.

Gravitational waves (GWs) from tens of millions of compact binaries in our Milky Way enter the millihertz band of space-based detection. The majority of them cannot be resolved individually, resulting in foreground confusion noise for the Laser Interferometer Space Antenna (LISA). The concept of the Taiji mission is similar to LISA’s with slightly better sensitivity, which means that Galactic GW signals will also affect detection with Taiji. Here, we generate GW signals from 29.8 million Galactic binaries for Taiji and subtract the “resolvable” sources. The confusion noise is estimated and fitted in an analytic form with 6-month, 1-year, 2-year, and 4-year observation times. We find that the full sensitivity curve is slightly lower for Taiji than for LISA at frequencies of $\le 0.8\mathrm{mHz}$ and around 2 mHz. For a 4-year lifetime, more than 29 thousand sources are resolvable with Taiji. Compared to LISA, Taiji can subtract $\sim 20\%$ more sources, and the distribution of them in our Milky Way is consistent with that of the resolvable sources with LISA. At frequencies around 2 mHz or with the chirp masses ranging from 0.2 to $0.4M_{\odot }$, more sources become resolvable with Taiji.

Extending the electromagnetic and gravitational radiations from binary black holes with electric and magnetic charges in circular orbits in Liu et al. (Phys. Rev. D 102:103520, 2020), we calculate the total emission rates of energy and angular momentum due to gravitational and electromagnetic radiations from dyonic binary black holes in precessing elliptical orbits. It is shown that the emission rates of energy and angular momentum due to gravitational and electromagnetic radiations have the same dependence on the conic angle for different orbits. Moreover, we obtain the evolutions of orbits and find that a circular orbit remains circular while an elliptic orbit becomes quasi-circular due to electromagnetic and gravitational radiations. Using the evolution of orbits, we derive the waveform models for dyonic binary black hole inspirals and show the amplitudes of the gravitational waves for dyonic binary black hole inspirals differ from those for Schwarzschild binary black hole inspirals, which can be used to test electric and magnetic charges of black holes.

The space-based laser interferometers, LISA, Taiji and TianQin, are targeting to observe milliHz gravitational waves (GWs) in the 2030s. The joint observations from multiple space-based detectors yield significant advantages. In this work, we recap the studies and investigations for the joint space-based GW detector networks to highlight: 1) the high precision of sky localization for the massive binary black hole (BBH) coalescences and the GW sirens in the cosmological implication, 2) the effectiveness to test the parity violation in the stochastic GW background observations, 3) the efficiency of subtracting galactic foreground, 4) the improvement in stellar-mass BBH observations. We inspect alternative networks by trading off massive BBH observations and stochastic GW background observation.

Space-based gravitational-wave detectors consist of a triangle of three spacecraft, which makes it possible to detect polarization modes of gravitational waves due to the motion of the detectors in space. In this paper we explore the ability of Taiji to detect the polarization modes in the parametrized post-Einsteinian framework. Assuming massive black hole binaries with the total mass of $M=4\times {10}^5{M}_{\odot }$ at redshift of $z=1$, we find that Taiji can measure the dipole and quadruple emission ($\mathrm{\Delta }{\alpha }_D/{\alpha }_D$ and $\mathrm{\Delta }{\alpha }_Q/{\alpha }_Q$) with the accuracy of up to $\sim 0.04\%$, with the fiducial value of ${\alpha }_D=0.001$, the scalar transverse and longitudinal modes ($\mathrm{\Delta }{\alpha }_B$ and $\mathrm{\Delta }{\alpha }_L$) up to $\sim 0.01$, and the vector modes ($\mathrm{\Delta }{\alpha }_V$) up to $\sim 0.0005$.

Coalescing massive black hole binaries (MBHBs) are one of primary sources for space-based gravitational wave (GW) observations. The mergers of these binaries are expected to give rise to detectable electromagnetic (EM) emissions with a narrow time window. The premerger detection of GW signals is vital for follow-up EM observations. The conventional approach for searching GW signals involves high computational costs. In this study, we present a deep learning model to search for GW signals from MBHBs. Our model is able to process 4.7 days of simulated data within 0.01 seconds and detect GW signals several hours to days before the final merger. The model provides the possibility of the coincident GW and EM detection of MBHBs.