Fundamental Physics and Cosmology with TianQin

Abstract: The exploration of the surrounding world and the universe is an important theme in the legacy of humankind. The detection of gravitational waves is adding a new dimension to this grand effort. What are the fundamental physical laws governing the dynamics of the universe? What is the fundamental composition of the universe? How has the universe evolved in the past and how will it evolve in the future? These are the basic questions that press for answers. The space-based gravitational wave detector TianQin will tune in to gravitational waves in the millihertz frequency range ($10^{-4} \sim 1$ Hz, to be specific), opening a new gravitational wave spectrum window to explore many of the previously hidden sectors of the universe. TianQin will discover many astrophysical systems, populating the universe at different redshifts: some will be of new types that have never been detected before, some will have very high signal-to-noise ratios, and some will have very high parameter estimation precision. The plethora of information collected will bring us to new fronts on which to search for the breaking points of general relativity, the possible violation of established physical laws, the signature of possible new gravitational physics and new fundamental fields, and to improve our knowledge on the expansion history of the universe. In this white paper, we highlight the advances that TianQin can bring to fundamental physics and cosmology.

Overview of Fundamental Physics and Cosmology with TianQin

The paper "Fundamental Physics and Cosmology with TianQin" explores the prospective capabilities of the TianQin mission in advancing our understanding of both fundamental physics and cosmology. TianQin is a planned space-based gravitational wave (GW) detector that aims to detect GWs primarily in the millihertz frequency range ($10^{-4} \sim 1$ Hz). The mission's scientific objectives cover a broad spectrum, promising significant contributions to gravitational physics and cosmological research.

Key Scientific Potential of TianQin

Gravitational Physics

1. Testing General Relativity (GR): TianQin will provide a unique opportunity to test the predictions of GR in the strong-field regime. By observing the dynamics of massive black hole binaries (MBHBs) and extreme mass-ratio inspirals (EMRIs), TianQin can test the validity of GR and probe for potential deviations. The mission could detect higher-order and non-linear gravitational wave (GW) modes, thereby enhancing our understanding of the non-linearity of Einstein's equations.
2. Detection of Novel Gravitational Effects: The mission is equipped to search for GW memory effects and explore the Kerr nature of astrophysical black holes. The memory effect, a non-linear feature predicted by GR, remains elusive and its detection would reaffirm our understanding of gravitational phenomena. Moreover, by examining Kerr black holes, TianQin could test hypotheses about black holes, which predict their complete characterization by mass and spin alone.
3. Exploration of Modified Gravity Theories: TianQin will investigate various modified theories of gravity (MGTs) that predict deviations from GR in terms of GW polarizations, dispersion relations, and generation mechanisms. This involves testing theories like Einstein-Aether and Chern-Simons gravity, which introduce additional gravitational interactions or modify existing ones.

Cosmological Insights

TianQin extends its research focus to cosmic phenomena and the universe's large-scale structure:

1. Dark Energy and Expansion History: Utilizing GW standard sirens like MBHBs, TianQin will allow scientists to measure the cosmos's expansion rate across vast time scales. This can improve constraints on the Hubble constant and offer insights into the dark energy equation of state, addressing unsolved issues like the Hubble tension—a discrepancy in the measured expansion rate of the universe.
2. Primordial Universe and Dark Matter: The mission has the capacity to detect stochastic GW backgrounds generated by phenomena such as cosmic strings and phase transitions in the early universe. These observations could provide clues about new particles and interactions beyond the Standard Model of Particle Physics, including potential dark matter candidates.
3. Primordial Black Holes (PBHs): TianQin will explore the existence of PBHs, which could account for some fraction of dark matter and offer a unique probe into the conditions of the early universe when density fluctuations may have led to their formation.

Implications and Future Directions

The paper outlines the role of TianQin in potentially transforming our understanding of fundamental physics and cosmology. By exploring new frontiers in gravitational physics and providing alternative methods for cosmological studies, TianQin could address longstanding issues such as the quantum nature of gravity and the expansion history of the universe. The synergy with upcoming GW detectors, like LISA and third-generation ground-based observatories, will enhance its scientific yield by enabling joint observations and data integration across different frequency bands.

TianQin is set to commence a new age of precision tests in the millihertz GW frequency range. It marks a pivotal step toward unraveling complex physics, offering a window into the universe's most enigmatic phenomena. The mission will likely stimulate further theoretical developments and shape the trajectory of research in cosmology and gravitational physics, providing a basis for breakthroughs in these fields.