SYSTEMS

The Hubble parameter is one of the central parameters in modern cosmology. Their values inferred from the late-time observations are systematically higher than those from the early-time measurements by about 10%. This is called the "Hubble tension". To come to a robust conclusion, independent probes with accuracy at percent levels are crucial. With the self-calibration by the theory of general relativity, gravitational waves from compact binary coalescence open a completely novel observational window for Hubble parameter determination. Hence, it can shed some light on the Hubble tension. Depends on whether being associated with electromagnetic counterparts or not, gravitational wave events can be categorized into bright sirens and dark sirens. The future space-borne gravitational wave observatory network, such as the LISA-Taiji network, will be able to measure the gravitational wave signals in the Millihertz bands with unprecedented accuracy. This advantage could help the measurement of the Hubble constant. 

A group of Chinese scientists from Beijing and Shenzhen forecasts the ability to constrain the Hubble parameter by using GW sirens data from the future space-borne gravitational-wave observatories, such as LISA (ESA/NASA space mission) and Taiji (Chinese space project). The signals are generated by the inspirals and mergers of the massive black hole binary. Astronomers believe each galaxy host a central massive black hole, whose mass is about one part per thousand of their bulge mass. However, there are outliers. These galaxies just experienced a very violent collision with their neighbors. In the long history of the universe, these galaxy merger events happened frequently and will trigger star formation. Besides that, the central massive black holes associated with the galaxies will also merge with each other and emit significant GW radiation in the very final phase of coalescence. There is a time delay between the massive black hole merger and the galaxy merger. In the view of the cosmic time scale, this delay is an instant. However, from the point of view of a human lifetime, this delay is about a million years. Hence, some galaxies will host two or three massive black holes. If the separation of these massive black holes is less than 0.001 parsec, the gravitational wave emission will dominate the radiation transfer process and drive the hardening of the black hole binaries. Thanks to the excellent sensitivity of space gravitational wave observatories, we will be able to measure this tiny signal. And more importantly, this gravitational wave signal carries fruitful cosmological information.

By including several statistical and instrumental noises, the Chinese team shows that "within 5 years operation time, the LISA-Taiji network is able to constrain the Hubble parameter within 1% accuracy, and possibly beats the scatters down to 0.5% or even better". Besides of that, they also calculate the averaged event numbers for different massive black hole formation models and several observation times. After 5-year network observation, for the optimistic heavy seed model, the average event number with the Hubble parameter accuracy better than 1% could reach 0.9, and its 95% confidence interval will up-cross unity. "We will very probably capture one gold or diamond event after 5-year network observation." the scientists forecast.

See the article: Renjie Wang, Wen-Hong Ruan, Qing Yang, Zong-Kuan Guo, Rong-Gen Cai, Bin Hu Hubble parameter estimation via dark sirens with the lisa-taiji network Natl Sci Rev https://doi.org/10.1093/nsr/nwab054