Letter of Interest Numerical Relativity for Next-Generation Gravitational

Snowmass2021 - Letter of Interest

Numerical relativity for next-generation gravitational-wave probes of fundamental physics

Thematic Areas: (check all that apply /) (CompF1) Experimental Algorithm Parallelization (CompF2) Theoretical Calculations and Simulation (CompF3) Machine Learning (CompF4) Storage and processing resource access (Facility and Infrastructure R&D) (CompF5) End user analysis (CompF6) Quantum computing (CompF7) Reinterpretation and long-term preservation of data and code (Other) (CF7) Cosmic Probes of Fundamental Physics

Contact Information: Pablo Laguna (University of Texas at Austin) [[email protected]], Geoffrey Lovelace (California State University, Fullerton) [[email protected]], Helvi Witek (University of Illinois at Urbana-Champaign) [[email protected]]

Authors: (long author lists can be placed after the text)

Pablo Laguna, Geoffrey Lovelace, Helvi Witek, and additional authors after the text Abstract: (maximum 200 words) The next generation of gravitational-wave detectors, conceived to begin operation in the 2030s, will probe fundamental physics with unprecedented sensitivity. These observations will measure the equation of state of dense nuclear matter in the most extreme environments in the universe, reveal with exquisite ﬁdelity the nonlinear dynamics of warped spacetime, put general relativity to the strictest test in the most extreme conditions, and perhaps use black holes as cosmic particle detectors. Achieving each of these goals will require a new generation of numerical relativity simulations that will run at scale on the supercomputers of the 2030s to achieve the necessary accuracy, which far exceeds the capabilities of numerical relativity today.

1 Motivation. The next generation of gravitational-wave detectors (proposed detectors include LIGO Voy- ager [1], Cosmic Explorer [2], and Einstein Telescope [3], each with expected operations beginning in the 2030s) will use gravitational waves from sources throughout the cosmos to probe fundamental physics with unprecedented sensitivity. Their observations of coalescing binary neutron stars and black-hole/neutron-star binaries will measure the equation of state of dense nuclear matter in the most extreme environments in the universe, and their observations of gravitational waves from merging stellar-mass black holes which contain the strongest spacetime curvature in the universe will put general relativity to the strictest tests. Accurate theoretical models of gravitational waves are critical for inferring the properties and behav- iors of the waves’ sources. Near the time of coalescence, when the spacetime curvature and (if present) neutron-star matter are the most nonlinear and dynamic, all analytic approximations break down; then, the emitted gravitational waves can only be calculated with numerical relativity, that is, by numerically solving the equations of general relativity or, for simulations involving neutron stars, general relativistic magnetohydrodynamics (e.g., [4–8] and the references therein). These simulations are technically challenging, in part because the equations are strongly nonlinear and (if neutron-star matter is present) because the solutions contain shocks. They are also computationally expensive, requiring high-performance computing to achieve the necessary accuracy for applications to today’s observations. The accuracy required of numerical-relativity simulations increases with detector sensitivity. Achiev- ing accuracy sufﬁcient for next-generation detectors’ observations with the highest signal-to-noise-ratios (SNR)—i.e. the observations with the most potential to reveal new fundamental physics—will require a new generation of numerical-relativity software, designed to run at scale on the supercomputers that will be available in the 2030s. Meeting this goal will include developing new, open-source numerical-relativity codes that will produce publicly available catalogs of simulated gravitational waveforms for coalescing compact binaries (i.e., binary black holes, binary neutron stars, and black-hole/neutron-star binaries). Key applications of numerical relativity for probing fundamental physics include the following.

Nuclear physics and neutron stars When two neutron stars, or a black hole and a neutron star, coalesce, they emit gravitational waves that encode the behavior of the densest matter in the universe. But recovering this information from gravitational-wave observations requires an accurate theoretical understanding of the emitted waves, which requires numerical relativity simulations. These simulations are challenging and expensive, yet they must be sufficiently accurate to avoid intro- ducing systematic bias into the interpretation of gravitational-wave observations. The accuracy required increases with the square of the observation’s signal-to-noise ratio [9]. The first (and loudest) gravitational wave observation from coalescing binary neutron stars, GW170817 [10], had a signal-to-noise ratio (SNR) ∼ 30. Recent studies [11, 12] find that systematic uncertainties from inaccurate waveform models would be substantial at SNRs ∼ 90, which could be achieved if a signal as loud as GW170817 were observed in current-generation detectors when they achieve their design sensitivities. The tremendous sensitivity gains that future gravitational-wave detector concepts [1–3] would achieve means that they would observe a GW170817-like signal with an SNR in the thousands [13], requiring vastly more accurate theoretical waveform models. Meeting this challenge will require a new generation of numerical-relativity software employing novel methods that will enable high accuracy and performance on the supercomputers that will be available in the next decade. Several such codes are currently in development (e.g. [14, 15]), but none of them have yet matured to the point where they can calculate gravitational waves from merging neutron-star binaries.

High-precision gravitational-wave observations The next generation of gravitational-wave detectors– and also the LISA mission in space [16]—will yield some observations of coalescing binary black holes

2 with SNR in the thousands, enabling high-ﬁdelity observation of the behavior of the curved spacetime near stellar-mass black-hole horizons, the most strongly curved spacetime known. Gravitational wave signals will be so plentiful they will sometimes overlap. As they have for current observations [10, 17, 18], numerical-relativity simulations will play crucial roles in the detection and interpretation of gravitational waves from merging black holes and neutron stars. In particular, waveforms from these simulations have been used to construct and validate approximate, phenomenological models necessary for interpreting observations (since numerical relativity is too costly to produce every model waveform needed) [17, 19–23], have featured in direct analysis of observations [24], and have helped validate our methods for detecting faint gravitational waves in detector data [25]. But when next-generation detectors are operating, future studies will need to determine how the chal- lenges of extremely high SNR and overlapping signals will impact the accuracy required to prevent numerical- relativity simulations from biasing the interpretation of next-generation gravitational-wave observations [26]. These requirements will be critical for ensuring that our numerical and approximate, analytic waveform models do not bias our interpretation of observations.

Testing gravity in the nonlinear regime A consistent theory of quantum gravity is a major goal of modern physics. General relativity (GR) itself is not consistent with quantum mechanics, because it breaks down at high-energy scales: it is non-renormalizable and exhibits physical singularities, such as those inside black holes and at the big bang. Candidate quantum-gravity theories include well-motivated extensions of GR, typically involving additional fields, higher curvature corrections, or symmetry breaking [27–29]. The nonlinear regime of gravity that unfolds during the collision of compact objects is a particularly promising target to probe for extensions of GR both because new phenomena are expected to be most prominent in that case [30] and because candidate theories can be confronted with gravitational-wave observations [31–34]. However, current tests of gravity have either been limited to the weak-field regime or to null-tests against GR, because complete model waveforms that capture these truly nonlinear beyond-GR effects are lacking. Numerical relativity has produced first proof-of-principle simulations beyond GR [35–43] and of boson stars [44–48]. Enabling high precision tests of gravity and searches for signatures of new physics will require innovative theoretical avenues to devise well-posed formulations of beyond-GR theories, and their application to creating high-precision catalogs of simulated waveforms.

Black holes as cosmic particle detectors Although dark matter makes up more than 80% of all matter in the universe, its nature, composition and properties have remained elusive. Black holes might shed light on the dark matter question and also ultralight beyond-standard model particles in general. Massive bosonic fields scattering off rotating black holes may form condensates around them if the fields’ Compton wavelength is comparable to the black holes’ size [49, 50]. That is, astrophysical black holes in the mass 10 −21 −8 range 5M ... 10 M are sensitive to ultralight particles in the mass range 10 eV ... 10 eV [49, 51, 52]. This range includes popular dark matter candidates [53], the QCD axion [54] and axion-like particles of the string axiverse [55]. Because the underlying phenomenon of black hole superradiance only relies on gravitational interactions, it facilitates searches for new particles independently from their specific coupling to the standard model and thus complements traditional collider physics or direct detection experiments. The single black hole scenario has been studied extensively, and there are first computations of binary black-hole systems in the weak-field [56, 57] or extreme mass ratio regime [58, 59]. How these light fields impact the nonlinear dynamics of the late inspiral and coalescence of black-hole binaries endowed with scalar condensates and what its observational signatures are remain open questions. Addressing them will enable gravitational-wave based searches for new particles but will require significant advances in numerical relativity.

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Additional Authors: Ana Achucarro´ (Leiden University) [[email protected]], Michalis Agathos (University of Cambridge) [[email protected]], Anastasios Avgoustidis (University of Nottingham) [[email protected]], Dimitry Ayzenberg (University of Tubingen)¨ [[email protected]], Luca Baiotti (Osaka University) [[email protected]], Tessa Baker (Queen Mary University of London) [[email protected]],

7 Cosimo Bambi (Fudan University) [[email protected]], Leor Barack (University of Southampton) [[email protected]], Enrico Barausse (SISSA Scuola Internazionale Superiore di Studi Avanzati) [[email protected]] Nicola Bellomo (University of Barcelona) [[email protected]], Laura Bernard (Observatoire de Paris) [[email protected]] Daniele Bertacca (University of Padova and INFN Sezione di Padova, Italy) [[email protected]], Emanuele Berti (Johns Hopkins University) [[email protected]], Gianfranco Bertone (U. of Amsterdam) [[email protected]], Ofek Birnholtz (Bar-Ilan University) [[email protected]], Beatrice´ Bonga (Radboud University Nijmegen) [[email protected]], Alexander Bonilla (Universidade Federal de Juiz de Fora) [abonilla@fisica.ufjf.br], Elisa Bortolas (University of Zurich)¨ [[email protected]], Richard Brito (Sapienza University of Rome) [[email protected]], Marco Bruni (Institute of Cosmology and Gravitation, Portsmouth, UK, and INFN Sezione di Trieste, Italy) [[email protected]], Alessandra Buonanno (Max Planck Institute for Gravitational Physics) [[email protected]], Adam Burrows (Princeton University) [[email protected]] Pedro R. Capelo (University of Zurich) [[email protected]], Carmelita Carbone (Istituto di Astrofisica Spaziale e Fisica cosmica Milano and INFN Sezione di Milano, Italy) [[email protected]], Vitor Cardoso (Institute Superior Tecnico)´ [[email protected]], Marco Cavaglia (Missouri University of Science and Technology) [[email protected]] Jose A. R. Cembranos (Universidad Complutense de Madrid) [cembra@fis.ucm.es], S. V. Chaurasia (Stockholm University) [[email protected]], Federico Cipolletta (Rochester Institute of Technology) [[email protected]], Gerald B. Cleaver (Baylor University) [gerald [email protected]], Sebastien Clesse (University of Louvain) [[email protected]], Geoffrey Compere` (Universite´ Libre de Bruxelles) [[email protected]], Katy Clough (University of Oxford) [[email protected]], Lucas Gardai Collodel (University of Tubingen)¨ [[email protected]], Isabel Cordero-Carrion´ (University of Valencia, Spain) [[email protected]], Saurya Das (University of Lethbridge) [[email protected]], Kyriakos Destounis (University of Tubingen)¨ [[email protected]], Tim Dietrich (Institute for Physics and Astronomy, University of Potsdam) [[email protected]], Konstantinos Dimopoulos (Lancaster University) [[email protected]], Daniela Doneva (University of Tubingen)¨ [[email protected]] Roberto Emparan (Universitat de Barcelona) [[email protected]], Zachariah B. Etienne (West Virginia University) [[email protected]] Deborah Ferguson (University of Texas at Austin) [[email protected]], Pedro G. Ferreira (University of Oxford) [[email protected]], Scott E. Field (University of Massachusetts at Dartmouth) [sfi[email protected]] Pau Figueras (Queen Mary University of London) [p.fi[email protected]], Giacomo Fragione (Northwestern University) [[email protected]], Noemi Frusciante (Instituto de Astrof´ısica e Cienciasˆ do Espaço, Universidade de Lisboa) [[email protected]] Juan Garc´ıa-Bellido (Universidad Autonoma´ de Madrid) [[email protected]], Davide Gerosa (University of Birmingham) [[email protected]], Archisman Ghosh (Ghent University) [[email protected]], Shrobana Ghosh (Cardiff University) [[email protected]],

8 Bruno Giacomazzo (University of Milano-Bicocca) [[email protected]], Chris Gordon (University of Canterbury) [[email protected]], Leonardo Gualtieri (Sapienza University of Rome) [[email protected]], Mark Hannam (Cardiff University) [[email protected]] Carlos Herdeiro (Aveiro University) [[email protected]] Lavinia Heisenberg (ETH Zurich) [[email protected]], Thomas Helfer (Johns Hopkins University) [[email protected]] Eric W. Hirschmann (Brigham Young University) [[email protected]], Eliu A. Huerta (University of Illinois at Urbana-Champaign) [[email protected]], Sascha Husa (University of the Balearic Islands) [[email protected]] Karan Jani (Vanderbilt University) [[email protected]], Philippe Jetzer (University of Zurich,¨ Switzerland) [[email protected]], Cristian Joana (University of Louvain), [[email protected]], Yonatan Kahn (University of Illinois at Urbana-Champaign) [[email protected]], Hassan Khalvati (Sharif University of Technology) [[email protected]], Lawrence E. Kidder (Cornell Univeristy) [[email protected]], Kostas Kokkotas (University of Tubingen)¨ [[email protected]], Joachim Kopp (CERN) [[email protected]], Savvas M. Koushiappas (Brown University) [[email protected]], Christian Kruger¨ (University of Tubingen)¨ [[email protected]], Baojiu Li (Durham University) [[email protected]], Steven L. Liebling (Long Island University) [[email protected]], Eugene A. Lim (King’s College London) [[email protected]], Nicole Lloyd-Ronning (Los Alamos Lab; University of New Mexico, Los Alamos) [[email protected]], Giuseppe Lodato (Universita` degli Studi di Milano, Italy) [[email protected]], Lucas Lombriser (University of Geneva) [[email protected]], Carlos O. Lousto (Rochester Institute of Technology) [[email protected]], Georgios Loukes-Gerakopoulos (Astronomical Institute of the Czech Academy of Sciences) [[email protected]], Elisa Maggio (Sapienza University of Rome) [[email protected]], Ilya Mandel (Monash University) [[email protected]], Charalampos Markakis (University of Cambridge & Queen Mary University of London) [[email protected]], Irvin Mart´ınez (University of Cape Town) [[email protected]], Andrea Maselli (Sapienza University of Rome) [[email protected]], Sabino Matarrese (University of Padova) [[email protected]], Nikolaos E. Mavromatos (King’s College London) [[email protected]], Eugenio Meg´ıas (University of Granada, Spain) [[email protected]], Cole Miller (University of Maryland) [[email protected]], Bishop Mongwane (University of Cape Town) [[email protected]], David F. Mota (University of Oslo) [[email protected]], Francesco Muia (Deutsches Elektronen-Synchrotron Hamburg) [[email protected]] Germano Nardini (University of Stavanger) [[email protected]] Hiroyuki Nakano (Ryukoku University) [[email protected]], Zainab Nazari (Bogazici University, and ICTP) [[email protected]], David Neilsen (Brigham Young University) [[email protected]], Savvas Nesseris (Instituto de Fisica Teorica UAM-CSIC) [[email protected]] Octavio Obregon (De- partamento de Fisica, Universidad de Guanajuato) [octavio@ﬁsica.ugto.mx] Maria Okounkova (Flatiron Institute Center for Computational Astrophysics) [mokounkova@ﬂatironinstitute.org], Roberto Oliveri (CEICO - Institute of Physics of the Czech Academy of Sciences) [[email protected]],

9 Giorgio Orlando (University of Padova) [[email protected]], Nestor Ortiz (National Autonomous University of Mexico) [[email protected]], Paolo Pani (Sapienza University of Rome) [[email protected]], Francesco Pannarale (Sapienza University of Rome) [[email protected]], George Pappas (Aristotle University of Thessaloniki) [[email protected]], Sohyun Park (CERN) [[email protected]], Richard O’Shaughnessy (Rochester Institute of Technology) [[email protected]], Vasileios Paschalidis (University of Arizona) [[email protected]], Harald Pfeiffer (Max Planck Institute for Gravitational Physics) [[email protected]] Geraint Pratten (University of Birmingham) [[email protected]], David Radice (Pennsylvania State University) [[email protected]], Sebastien´ Renaux-Petel (Institut d’Astrophysique de Paris) [[email protected]], Stephan Rosswog (Stockholm University, Sweden) [[email protected]], Milton Ruiz (University of Illinois at Urbana-Champaign) [[email protected]], Mairi Sakellariadou (King’s College London) [[email protected]], Nicolas Sanchis-Gual (Instituto Superior Tecnico,´ Lisbon, Portugal) [[email protected]], B.S. Sathyaprakash (Pennsylvania State University) [[email protected]] Patricia Schmidt (University of Birmingham) [[email protected]], Erik Schnetter (Perimeter Institute for Theoretical Physics) [[email protected]], Olga Sergijenko (Taras Shevchenko National University of Kyiv) [[email protected]], Lijing Shao (Peking University) [[email protected]], Stuart L. Shapiro (University of Illinois at Urbana-Champaign) [[email protected]] Deirdre Shoemaker (University of Texas at Austin) [[email protected]], Daniel M. Siegel (Perimeter Institute for Theoretical Physics & Department of Physics, University of Guelph) [[email protected]], Carlos F. Sopuerta (Institute of Space Sciences (ICE, CSIC and IEEC)) [[email protected]], Thomas P. Sotiriou (University of Nottingham) [[email protected]], Ulrich Sperhake (University of Cambridge) [[email protected]], Leo Stein (University of Mississippi) [[email protected]] Nikolaos Stergioulas (Aristotle University of Thessaloniki) [[email protected]], Chris Stevens (University of Otago) [[email protected]], Stanislav Fisenko (MSLU,Russia) [stanislavfi[email protected]], Sourabh Nampalliwar, (University of Tubingen)¨ [[email protected]], Sorin Dragomir (Universita` degli Studi della Basilicata, Dipartimento di Matematica, Informatica ed Econo- mia) [[email protected]], Pawel Szewczyk (Astronomical Observatory, University of Warsaw) [[email protected]], Magdalena Szkudlarek (Janusz Gil Institute of Astronomy, University of Zielona Gora) [[email protected]], Nicola Tamanini (Max Planck Institute for Gravitational Physics) [[email protected]], Gianmassimo Tasinato (Swansea University) [[email protected]], Antonios Tsokaros (University of Illinois at Urbana-Champaign) [[email protected]], Sergey Tsygankov (University of Turku) [sertsy@utu.fi], Cora Uhlemann (Newcastle University) [[email protected]], Caner Unal (CEICO Institute of Physics of the Czech Academy of Sciences) [[email protected]], Maarten van de Meent (Max Planck Institute for Gravitational Physics) [[email protected]], Elias C. Vagenas (Kuwait University) [[email protected]], Daniele Vernieri (University of Naples “Federico II”) [[email protected]], Filippo Vernizzi (Insitut de Physique Theorique,´ CEA Saclay) [fi[email protected]], Anzhong Wang (Baylor University) [anzhong [email protected]],

10 Barry Wardell (University College Dublin) [[email protected]], Scott Watson (Syracuse University) [[email protected]], Amanda Weltman (University of Cape Town) [[email protected]], John Ryan Westernacher-Schneider (University of Arizona) [[email protected]], Kinwah Wu (Mullard Space Science Laboratory University College London, UK) [[email protected]] Kadri Yakut (University of Ege) [[email protected]], Stoytcho Yazadjiev (University of Soﬁa) [[email protected]ﬁa.bg], Nicolas´ Yunes (University of Illinois Urbana-Champaign) [[email protected]] Silvia Zane (Mullard Space Science Laboratory, University College London, UK) [[email protected]], Ivonne Zavala (Swansea University) [[email protected]], Miguel Zilhao˜ (Instituto Superior Tecnico´ Lisbon, Portugal) [[email protected]], Aaron Zimmerman (University of Texas at Austin) [[email protected]], Yosef Zlochower (Rochester Institute of Technology) [[email protected]], Miguel Zumalacarregui (Max Planck Institute for Gravitational Physics) [[email protected]], Ashkbiz Danehkar (University of Michigan) [[email protected]]