General Relativity (GR) has passed all current tests in the weak field regime. The Gravitational Universe will permit unprecedented measurements of general relativity in the **strong field regime**.

The Gravitational Universe will explore relativistic gravity in the strong field, non-linear regime. It seems unlikely that any other methods will achieve the sensitivity of LISA to deviations of strong-field gravity by 2028. Unlike the ground-based instruments, LISA will have sufficient sensitivity to notice even small corrections to Einstein gravity.

LISA will **map the spacetime around astrophysical black holes**, yielding a battery of precision tests of GR in an entirely new regime. These have the potential to uncover hints about the nature of quantum gravity, as well as enabling measurements of properties of the universe on the largest scales.

The nature of gravity in the strong-field limit is so far largely unconstrained, leaving open several outstanding questions.

- Does gravity travel at the speed of light?
- Does the graviton have mass?
- How does gravitational information propagate: Are there more than two transverse modes of propagation?
- Does gravity couple to other dynamical fields, e.g., massless or massive scalars?
- What is the structure of spacetime just outside astrophysical black holes?
- Do their spacetimes contain horizons?
- Are astrophysical black holes described by the Kerr metric, as predicted by GR?

**An outstanding way to answer these questions** and learn about the fundamental nature of gravity is by observing the vibrations of the fabric of spacetime itself, for which coalescing binary black holes and Extreme Mass Ratio Inspirals are superior probes.