Taiji Program: Gravitational-Wave Sources

Abstract: We review potential low-frequency gravitational-wave sources, which are expected to be detected by Taiji, a Chinese space-based gravitational-wave detector, estimate the detection rates of these gravitational-wave sources and present the parameter estimation of massive black hole binaries.

• The paper presents a rigorous analysis of low-frequency gravitational-wave sources, emphasizing MBHBs and employing numerical relativity for precise waveform modeling.
• It introduces a hybrid method combining effective-one-body dynamics with Teukolsky solutions to enhance the detection of EMRIs/IMRIs.
• The study evaluates Taiji's sensitivity in parameter estimation, providing key insights for testing general relativity and understanding early Universe phenomena.

Overview of "Taiji Program: Gravitational-Wave Sources"

This paper provides a comprehensive analysis of potential low-frequency gravitational-wave sources anticipated for detection by the Taiji program, a Chinese space-based gravitational-wave detector. The study outlines the expected detection rates of these sources and explores the parameter estimation of massive black hole binaries (MBHBs), one of the key targets for the Taiji mission. The authors present a rigorous evaluation of different gravitational-wave (GW) sources, including compact binary coalescences encompassing massive black hole binaries, extreme/intermediate mass ratio inspirals (EMRIs/IMRIs), compact binaries in the Milky Way, and the stochastic gravitational-wave backgrounds (SGWBs) generated in the early Universe.

Gravitational Waves from Compact Binary Coalescences

1. Massive Black Hole Binaries: The paper emphasizes the significance of MBHBs as prime targets, highlighting the detection of gravitational waveforms with distinct inspiral, merger, and ringdown phases. The derivation considers numerical relativity for precise template construction crucial for systems with mass ratios exceeding 1:20. The potential simultaneous observation of GWs and electromagnetic counterparts could provide critical insight into the dynamics of accretion and host galaxy co-evolution.
2. Extreme/Intermediate Mass Ratio Inspirals (EMRIs/IMRIs): These GW sources, characterized by the inspiral of compact objects into massive black holes, offer the potential for testing general relativity and probing spacetime around massive black holes. Challenges in detecting these sources arise due to the need for an extensive template bank for matched filtering. A hybrid methodology combining effective-one-body dynamics with Teukolsky equation solutions is suggested to increase the accuracy of waveform templates.
3. Compact Binaries in the Milky Way: Compact binary systems within the Milky Way represent a predominant class of GW signals detectable by space-based interferometers, particularly emphasizing double white dwarfs which create a confusion-limited signal at lower frequencies.

Gravitational Waves from the Early Universe

The paper reviews theoretical sources from the early Universe, such as inflation, reheating/preheating, and phase transitions. Inflationary models predict primordial tensor perturbations contributing to SGWBs. The parameters influencing the SGWB include tensor spectral indices and horizon-related dynamics, especially during preheating where significant SGWB can be generated through highly time-dependent anisotropic stress tensors.

1. Inflation: Analysis of slow-roll approximation yields predictions about the energy spectrum and the consistency relations crucial in understanding early Universe physics.
2. Reheating/Preheating: The paper discusses how these can lead to significant GWs which remain within the Hubble horizon and are detectable in principle, albeit with difficulty given the frequencies involved.
3. First-Order Phase Transition: The occurrence of these transitions can generate SGWBs from bubble nucleations and collisions in the early Universe, offering a pathway to probe new physics beyond the Standard Model if detectable by space-based interferometers.

The Taiji Program: Sensitivity and Capabilities

The Taiji program, akin to the LISA mission, aims to detect GWs in a space-based environment, theoretically achieving significant sensitivity in the frequency range of interest. The paper includes an evaluation of Taiji’s capacity to determine key parameters of MBHBs using the Fisher Information Matrix, with results indicative of comparable precision to that of LISA, notably in measuring chirp mass, systematic mass ratio, and luminosity distance.

Implications and Future Prospects

The implications of this research are vast, not only in enhancing our understanding of astrophysical processes and massive black hole evolution but also bridging observational cosmology and fundamental physics via insights from detected gravitational waves. The anticipated observations by Taiji could provide empirical evidence to test theories such as general relativity in strong-field regimes, explore cosmological parameters, and augment our comprehension of the Universe's expansion history and structure formation. Future development in precise waveform modeling and data analysis techniques will be crucial to fully leverage the scientific potential of upcoming gravitational wave observatories like Taiji.

This paper lays a foundation for future explorations in gravitational wave detection, highlighting the advanced theoretical and practical aspects that will contribute to the broader field of GW astronomy and accompanying phenomena.