Prospects for Fundamental Physics with LISA (2001.09793v3)

Abstract: In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA, we present here a sample of what we view as particularly promising directions, based in part on the current research interests of the LISA scientific community in the area of fundamental physics. We organize these directions through a "science-first" approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.

• The paper demonstrates LISA’s potential to detect deviations from general relativity, such as modified dispersion relations and equivalence principle violations.
• It employs precise gravitational wave observation techniques to probe the fundamental nature of black holes and explore dark energy dynamics.
• The study advocates model-independent tests using waveform analyses that could reveal new physics beyond the standard models.

Prospects for Fundamental Physics with LISA

The paper, "Prospects for Fundamental Physics with LISA," delineates the promising potential of the Laser Interferometer Space Antenna (LISA) in exploring fundamental physics. This overview focuses on the key areas where LISA is expected to make substantial contributions, particularly in addressing profound questions in gravitational physics, as well as in scrutinizing the various facets of gravitational wave (GW) observations.

Gravitational waves offer a unique and significant avenue for testing the frontiers of our understanding of gravity and fundamental physics. LISA, with its capability to detect long-wavelength GWs, presents an untapped opportunity to explore theories beyond the standard model of particle physics and general relativity (GR). The paper categorizes these prospects into several thematic areas, each corresponding to different sources and types of physical phenomena LISA will explore.

Key Thematic Areas in Fundamental Physics with LISA

1. Modified Dispersion Relations: The notion that GWs might propagate through space with different dispersion relations than predicted by GR is a tantalizing prospect, which LISA is uniquely equipped to explore. Given its sensitivity to the low-frequency domain, LISA can probe anomalies related to variations in the speed of gravity or modified gravitational dispersion relations, which in turn could indicate theories beyond GR, such as massive gravity or Lorentz-violating frameworks.
2. Testing the Equivalence Principle: LISA offers a platform to probe violations of the Equivalence Principle (EP), a cornerstone principle in GR. Such tests include detecting any anomalous gravitational coupling that might hint at additional fields or interactions that are not accounted for in GR. These investigations are also crucial for understanding modifications in theories like scalar-tensor or vector-tensor gravity.
3. Nature of Black Holes: One of LISA's most profound contributions will be in testing the "no-hair" theorem of black holes, which postulates that all black hole solutions of GR can be completely characterized by three parameters: mass, charge, and angular momentum. With precise measurements of quasinormal modes in black hole ringdowns, LISA can ascertain potential deviations which could suggest new physics or the presence of scalar fields.
4. Dark Energy Investigations: LISA provides a new avenue for exploring the dynamics of dark energy by using gravitational wave standard sirens to map the expansion history of the universe across different epochs. This could help test theories that suggest deviations from the cosmological constant model of dark energy.
5. Probing Dark Matter and Primordial Black Holes: The potential of LISA to detect intermediate mass black hole binaries and extreme mass ratio inspirals (EMRIs) opens up the possibility to explore scenarios in which dark matter could form boson clouds through superradiance around black holes. This suggests novel pathways to test theories pertaining to axions or other light bosons as constituents of dark matter.
6. Model-Independent Tests: LISA can perform robust tests that do not rely on specific models, such as searching for deviations in the waveform generation or propagation that indicates departures from GR. The exploration of stochastic gravitational wave backgrounds could also serve as a probe of early universe cosmology and phase transitions.

Practical Implications and Future Outlook

The implications of LISA's findings can extend into various realms of physics. They could fundamentally reshape our understanding of gravity, either consolidating GR's predictions across a wider parameter space or indicating profound physics operating at scales currently inaccessible. Moreover, the potential detection of GWs that exhibit features unforeseen by GR could provide insights into the quantum nature of black holes or the existence of extra dimensions.

The anticipation surrounding LISA’s mission underscores the importance of continuing theoretical and computational advances in gravitational physics. As LISA's launch approaches, further development in waveform modeling, data analysis techniques, and complementary electromagnetic observations will be crucial in maximizing the scientific return of this observatory.

LISA represents a significant stride in gravitational astrophysics, with its prospects promising to usher in a new era of exploration into some of the most compelling puzzles in modern physics.