Summary of the Town Hall Meeting on “Gravitational Wave Theory and Simulations in the Era of Detections”

Gravitational wave astronomy is finally a reality. The first multi-messenger observation of a binary neutron star merger (GW170817) was one of the biggest science stories of 2017, and binary black hole detections are becoming routine. However there is still a lot of work needed to coordinate the efforts of instrumentalists, data analysts, astrophysicists and gravitational theorists.

Gravitational-wave theorists, numerical relativists, astrophysicists and LIGO/Virgo members should work together as a single community to make the best of the wealth of data that will be collected during LIGO/Virgo’s O3 run. For this reason, Beverly Berger and Manuela Campanelli proposed to organize a Town Hall Meeting on “Gravitational Wave Theory and Simulations in the Era of Detections.”

The Town Hall Meeting, sponsored by DGRAV and co-sponsored by DAP, took place immediately after the DGRAV and DAP Business Meetings on Monday, April 16 at the APS April Meeting in Columbus, OH. Slides from both meetings are available in this post.

Laura Cadonati (on behalf of the LIGO Scientific Collaboration) presented the anticipated evolution of noise power spectral densities for LIGO/Virgo, as well as current plans for KAGRA, in the next few years (view slides here). Advanced LIGO (aLIGO) is expected to reach target sensitivity around 2019. The proposed A+ (if funded by NSF) could be operating by 2024. The expected range for a BNS (1.4+1.4 solar masses) would go from 96 Mpc (O2) to 173 Mpc (aLIGO) and 325 Mpc (A+). The expected range for a BBH (30+30 solar masses) would go from 983 Mpc (O2) to 1606 Mpc (aLIGO) and 2563 Mpc (A+).

Laura and Lisa Barsotti discussed current plans to update the Virgo detector. Finally, Laura presented a table with the current list of priorities from the LSC-Virgo White Paper on Gravitational Wave Data Analysis and Astrophysics.

Laura’s presentation was followed by 7 short presentations (view slides here) on astrophysics science targets and numerical relativity tools (or results) that could be relevant for O3.

  • Carl Rodriguez showed some results from simulations of BBH mergers in clusters, suggesting that such mergers could produce signals with nonzero eccentricity in the LIGO band.
  • Davide Gerosa pointed out the importance of measuring precessional effects (beyond the “effective spin” parameter) to extract astrophysics from the observations.
  • Ilias Cholis asked whether merging BHs in globular clusters could experience runaway collisions leading to formation of intermediate mass BHs; if so, LIGO observations could be used to derive limits on the occupation fraction of intermediate mass BHs in globular clusters.
  • Zach Etienne pointed out that Moore’s Law is slowing (at least for CPU-based codes – a point raised in discussions by Maurice van Putten and Kai Staats, among others) and that more efficient numerical relativity algorithms are necessary. He also pointed out the importance of writing software documentation for the benefit of students and postdocs.
  • Roland Haas gave an introduction to the Einstein Toolkit and its applications.
  • Antonios Tsokaros presented results on sequences of spinning BNS which suggest that corotating sequences can have low spin (<0.3) even for close binaries.
  • Milton Ruiz presented simulations of BNS in GRMHD which imply (for causal EOS) that the maximum mass of the remnant should be below ~2.16 solar masses.

After these presentations, Bangalore Sathyaprakash gave an introduction (view slides here) to current plans for “Third Generation” (3G) detectors on behalf of the GWIC 3G Committee and 3G Science Case Team. He made the argument that LIGO and Virgo both have facility-imposed limits on sensitivity (implying at most a factor ~3 improvement in strain sensitivity, with gravity gradient noise limiting sensitivity below 10 Hz). Therefore there is a compelling case to build detectors that can observe deeper into the cosmos, with facilities that will be good ~30-40 years after construction. One of the GWIC charges is to “commission a study of ground-based gravitational wave science from the global scientific community, investigating potential science vs. architecture vs. network configuration vs. cost trade-offs”. The GWIC subcommittee has constituted five 3G subcommittees: (1) Science Case Team (3G-SCT), (2) R&D Coordination, (3) Governance, (4) Agency Interfacing, (5) Community Networking. The Science Case will be developed by an international consortium of scientists under the leadership of the 3G-SCT, which consists of 18 members. For more details, see

The 3G Science Case consortium is open to anyone who wishes to contribute to the development of the science case for 3G. If you are interested, please send a one-page CV and research interests relevant to 3G to either B.S. Sathyaprakash <> or Vicky Kalogera <>.

Emanuele Berti (on behalf of the Executive Committee of the APS Division of Gravitational Physics) coordinated a lively discussion on how theorists could help LVC members and vice versa.

The discussion concerned the following main points:

  1. The astrophysics/GR communities need the full posteriors to optimize the science return of LVC observations. How quickly should the LVC release these posteriors? Most participants agreed that the answer to this question is complex.
    Bangalore Sathyaprakash explained that careful data analysis is necessary before releasing posteriors. Jolien Creighton argued that it is important for the LVC to retain some core science in order to maintain a vibrant collaboration, which in turn is vital to the larger community. A discussion involving various people (Will Farr, Emanuele Berti, Chad Galley, Davide Gerosa, and many others) followed.
    It was argued that the current model for open data release is tuned to the (urgent) needs of observational astronomers. However, astrophysicists and theorists could benefit from a quicker release of the bulk properties of detected events. For example, Chad Galley pointed out that in other fields it is common to release data sets of lower quality which can be improved over time.
    As the number of detections increases, one may consider releasing estimates of masses/distance/effective spins and other “simple” properties of each event for use by astronomers and modelers, and then refining parameter estimation for more in-depth applications (such as tests of general relativity).
  2. Emanuele Berti also proposed that the collaboration between theorists and LVC members could follow the particle-physics model in two ways:
    1. At CERN, experimental physicists work in close collaboration with a “theory division”. The level of involvement, access to the data, and authorship details for the members of this “theory division” could and should be discussed on a case-by-case basis. It would be healthy to start discussions of this possibility within the LVC. Similar discussions in the recent past led to a more active LVC involvement of numerical relativists.
    2. A key reference in particle physics is the “particle data group”, which is updated annually to reflect the state of the art in the knowledge of particle properties, physical constants, etcetera. Various people (including Leo Stein, Emanuele Berti, Vitor Cardoso, Pedro Ferreira and Thomas Sotiriou, among others) have been discussing the possibility of creating and maintaining a “gravity data group” that could list the best known limits on (say) the mass of the graviton, Lorentz violation in gravity etcetera, and current constraints on various proposed modifications of general relativity. This effort requires manpower to maintain a website (or a similar “living” resource) and a white paper – that could be circulated on the arXiv – explaining how to interpret the constraints and pointing to relevant references.
      Leo Stein suggested that the theory community also needs to agree on what type of science they want to do with LVC data. This would inform the LVC on what is really needed to enable such contributions.

Beverly Berger proposed the idea to organize a 2-3 day workshop in order to follow up on the Town Hall discussions.