## Toward inspiral-merger-ringdown gravitational waveforms beyond general relativity

#### June 14 – 16, 2023

The rapidly growing field of gravitational wave (GW) astronomy finally allows us to test general relativity (GR) in the dynamical, strong-field regime of a coalescing compact binary system. The detection and parameter estimation of these events crucially relies on providing accurate waveform templates to the LIGO-Virgo-KAGRA Collaboration, or to its successors in the next decade (such as the Einstein Telescope, the Cosmic Explorer and LISA). Yet, libraries of such waveform templates only exist in GR to this day.

Despite the successes of GR, there are strong motivations to modify it, both from observational cosmology (e.g., dark energy and dark matter) and from theoretical puzzles in quantum gravity (e.g., non-renormalizability and the information loss paradox). Deriving gravitational waveforms in modified gravity theories is technically challenging. So far, existing works based on the post-Newtonian formalism, the effective-one-body framework, black hole perturbation theory, and numerical relativity provide only partial waveforms covering either the early or late inspiral, merger, or ringdown stages of a compact binary coalescence.

The key focus of this workshop is to “connect the dots”, and pave the way toward complete and accurate inspiral-merger-ringdown (IMR) waveform templates in modified gravity theories. This goal will be achieved by gathering key experts in the field to identify the synergy between their own activities, foster new collaborations and bring novel ideas on the following issues:

- Which modified gravity theories are best motivated?
- What are the limitations of current tests of GR with GW observations?
- What is the state of the art in the modeling of the inspiral, merger and ringdown beyond GR?
- How to fit these developments together and obtain complete IMR waveforms beyond GR?
- Can we test gravity in new ways with such waveforms, and are there new beyond-GR effects?
- What are the challenges for detecting beyond-GR physics with next-generation detectors?