Testing General Relativity with Low-Frequency, Space-Based Gravitational-Wave Detectors (1212.5575v2)

Abstract: We review the tests of general relativity that will become possible with space-based gravitational-wave detectors operating in the ~0.01mHz - 1Hz low-frequency band. The fundamental aspects of gravitation that can be tested include the presence of additional gravitational fields other than the metric; the number and tensorial nature of gravitational-wave polarization states; the velocity of propagation of gravitational waves; the binding energy and gravitational-wave radiation of binaries, and therefore the time evolution of binary inspirals; the strength and shape of the waves emitted from binary mergers and ringdowns; the true nature of astrophysical black holes; and much more. The strength of this science alone calls for the swift implementation of a space-based detector; the remarkable richness of astrophysics, astronomy, and cosmology in the low-frequency gravitational-wave band make the case even stronger.

• The paper demonstrates that low-frequency, space-based detectors enable rigorous tests of General Relativity through analyses of wave polarizations and propagation velocities.
• It applies advanced modeling and data analysis techniques on binary inspirals and black hole dynamics to evaluate deviations from GR's predictions.
• The research underscores LISA-like missions' potential to detect alternative gravitational fields and enhance our understanding of massive astrophysical phenomena.

Overview of "Testing General Relativity with Low-Frequency, Space-Based Gravitational-Wave Detectors"

The paper by Gair et al. presents a comprehensive review of the potential for testing General Relativity (GR) using future space-based gravitational-wave detectors operating in the low-frequency range of approximately $10^{-5}$ to $1$ Hz. These detectors, often exemplified by the Laser Interferometer Space Antenna (LISA) and its proposed variants, promise a new empirical regime for gravitational physics by directly observing wave emissions from massive astrophysical events, such as mergers of colossal black holes.

Key Areas of Investigation

The authors identify several fundamental aspects of gravitation that can be rigorously examined with such detectors, offering both confirmations and novel tests of GR:

1. Detection of Alternative Gravitational Fields: The potential to detect other gravitational fields outside of the metric tensor, as predicted by alternative theories like those involving scalar-tensor fields, is a significant aspect of this research.
2. Gravitational-Wave Polarizations: Contrary to GR, which predicts only two tensor polarizations for gravitational waves, alternative theories may predict additional forms, such as scalar or vector modes. The detection and analysis of these would provide substantial evidence for or against current theoretical extensions to GR.
3. Propagation Velocity of Gravitational Waves: Testing whether gravitational waves propagate at the speed of light, as predicted by GR, or at a different speed if they possess mass, characterizing a massive-graviton scenario, is another high-priority scientific objective.
4. Binding Energy and Evolution of Binary Systems: By evaluating the inspiral of binary systems, the paper aims to refine our understanding of energy losses due to gravitational radiation, which would have observable impacts on the phasing and amplitude evolution of the gravitational waves.
5. Structure of Black Holes: Space-based detectors will enable unprecedented tests of the so-called "no-hair" theorem of black holes, allowing researchers to ascertain whether these remain as simple, uniquely defined objects solely by their mass and spin.

Implications and Future Developments

Space-based gravitational wave observatories represent a unique opportunity to explore the universe in a gravitational spectrum largely obscured to other types of telescopes. The implications of successful observations are manifold: providing stringent tests of GR, constraining or even discovering new physics beyond GR, and offering a new window into the early universe and the dynamics of massive celestial objects.

The rich astrophysical science possible with these detectors further enhances their value, ranging from understanding the formation and growth of massive black holes, characterizing binary evolution in various cosmic environments, to potentially detecting a stochastic gravitational-wave background from the early universe.

Challenges and Considerations

The paper highlights several challenges that must be addressed to capitalize on these opportunities fully. Accurate theoretical models are needed to predict and interpret potential deviations from GR; multi-messenger observations (combining gravitational and electromagnetic data) are crucial for certain tests. Moreover, the extensive anticipated signal overlap in low-frequency bands necessitates sophisticated data analysis techniques, far beyond those used for current ground-based gravitational-wave observatories.

In summarizing, this research sets the stage for a transformative scientific endeavor. The successful deployment and operation of space-based gravitational-wave detectors could help resolve some of the most profound questions in physics and our understanding of the cosmos's gravitational behavior.