From Subatomic to Cosmic Scales: Simulating, Modelling, Analysing Binary Neutron Star Mergers

Project Information

SMArt

Grant agreement ID: 101076369

DOI

10.3030/101076369

EC signature date 15 December 2022

Start date 1 September 2023

End date 31 August 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 499 762,50

EU contribution

€ 1 499 762,50

Coordinated by

UNIVERSITAET POTSDAM
Germany

Periodic Reporting for period 1 - SMArt (From Subatomic to Cosmic Scales: Simulating, Modelling, Analysing Binary Neutron Star Mergers)

Reporting period: 2023-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

The research project investigates two fundamental questions in modern physics: What is the nature and behaviour of matter at extremely high densities, i.e. beyond those found inside atomic nuclei, and how fast is our Universe expanding? Hence, the project connects subatomic studies and nuclear physics studies with the investigation of our Universe on cosmological scales.
To answer these questions, the project studies one of the most compact objects in our Universe: neutron stars. Neutron stars are extremely dense remnants of massive stars, with densities exceeding those found in atomic nuclei.
The particular focus of the research project is the study of the collision of two neutron stars. Such mergers provide a unique opportunity to explore matter under the most extreme conditions in the Universe. When two neutron stars merge, they generate powerful gravitational waves—ripples in spacetime—as well as various forms of electromagnetic radiation (light), which can be detected by advanced instruments and telescopes.
Using high-performance computing, the project creates detailed numerical simulations of the final moments of neutron star mergers. These simulations predict both the gravitational-wave signals and the electromagnetic emissions produced by these cosmic events. By combining these theoretical models with observational data from upgraded gravitational-wave detectors and telescopes covering the electromagnetic spectrum, the research aims to deepen our understanding of matter under extreme conditions and to measure the Universe’s expansion rate with greater precision.
A key focus is on improving the accuracy of these models to reduce systematic uncertainties. High-quality simulations and advanced data analysis techniques enable a unified interpretation of all observed signals, ensuring future discoveries rest on reliable theoretical foundations rather than modeling limitations.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Within the project, we have performed numerical simulations of binary neutron star mergers, covering a wide range of system parameters such as masses, spins, and orbital eccentricities. In addition, we continuously increase the realism of the microphysical treatment, e.g by incorporating realistic physical effects including magnetic fields, neutrino radiation (with novel inclusion of muonic neutrinos), and advanced neutron star equations of state to represent matter under extreme conditions better. By comparing different numerical codes, we identified areas where waveform models require improvement, particularly for highly spinning systems. Machine-learning techniques were developed to accelerate key computational steps and to predict merger outcomes, which supports the modeling of postmerger phases.
Significant progress was made in developing and refining gravitational-wave models that describe tidal interactions of the binary neutron star coalescence, including also higher-order modes in the signals. While all these models were based on numerical-relativity simulations, we have also started to investigate how observational data can be used to calibrate waveform models. To check this ansatz, we have performed a study based on mock observational data, with a view toward future detectors such as the Einstein Telescope.

Since neutron star merger do not only create gravitational waves, we also investigated possible electromagnetic signatures of neutron star mergers. In this regard, we improved numerical tools to model non-thermal emissions from gamma-ray bursts and kilonova afterglows, enabling more precise extraction of astrophysical parameters from observations. Real-time frameworks for transient lightcurve fitting were also advanced to support rapid analysis of new events.
A key achievement was the creation of a unified multi-messenger analysis framework that integrates gravitational-wave data, electromagnetic observations, and nuclear physics constraints. This framework allows for a comprehensive interpretation of neutron star merger signals and is being upgraded to leverage GPU computing for improved efficiency.

Together, our efforts have resulted in a suite of high-accuracy simulations, models, and data-analysis tools that advance our understanding of neutron star mergers, nuclear matter at supranuclear densities, and the expansion rate of the Universe. Beyond the core scientific outcomes, the project fostered interdisciplinary collaborations with nuclear physicists and expanded international scientific networks through dedicated workshops and research visits, enriching the broader astrophysics community.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The most impactful result is the development of the nuclear physics and multi-messenger astrophysics framework NMMA, a unique and publicly available software that enables the simultaneous Bayesian analysis of gravitational waves, kilonovae, and gamma-ray burst afterglows, providing a powerful resource for future multi-messenger discoveries. In addition, we achieved major progress beyond the state of the art in waveform modelling with a new phenomenological model that combines speed and accuracy at an unprecedented level and is likewise publicly available. We also opened an entirely new avenue by demonstrating that gravitational-wave observations themselves can be used to calibrate waveform models, rather than relying solely on analytical or numerical-relativity inputs. Complementing these breakthroughs are significant advances in our numerical simulations, most notably the integration of full radiation magnetohydrodynamics into numerical-relativity simulations of binary neutron star mergers. In this context, we were the first to show that the commonly used approximation of neglecting muonic neutrino interactions can introduce biases in the interpretation of multi-messenger detections. To address this, we performed the first numerical-relativity simulations that explicitly incorporate such muonic interactions, thereby enabling more accurate astrophysical studies.