Testing the spatial geometry of the Universe with TianQin: Prospect of using supermassive black hole binaries

Abstract

Context. The determination of the spatial geometry of the Universe plays an important role in modern cosmology. Any deviation from the cosmic curvature ΩK = 0 would have a profound impact on the primordial inflation paradigm and fundamental physics. Aims. In this paper, we carry out a systematic study of the prospect of measuring the cosmic curvature with the inspiral signal of supermassive black hole binaries (SMBHBs) that could be detected with TianQin. Methods. The study is based on a method that is independent of cosmological models. It extended the application of gravitational wave (GW) standard sirens in cosmology. By comparing the distances from future simulated GW events and simulated H(z) data, we evaluated whether TianQin produced robust constraints on the cosmic curvature parameter Ωk. More specifically, we considered three-year to ten-year observations of supermassive black hole binaries with total masses ranging from 103 M⊙ to 107 M⊙. Results. Our results show that in the future, with the synergy of ten-year high-quality observations, we can tightly constrain the curvature parameter at the level of 1σ Ωk = −0.002 ± 0.061. Moreover, our findings indicate that the total mass of SMBHB does influence the estimation of cosmic curvature, as implied by the analysis performed on different subsamples of gravitational wave data. Conclusions. Therefore, TianQin is expected to provide a more powerful and competitive probe of the spatial geometry of the Universe, compared to future spaced-based detectors such as DECIGO.