The BNSmergers project sought to answer some fundamental questions in modern astrophysics by focusing on the internal composition of neutron stars. Neutron stars are the most compact objects in our universe, which means that they concentrate very high masses within a very small volume.
“Densities inside the core of a neutron star reach an incredible 100 million tonnes per cubic centimetre,” explains project coordinator Chris Van Den Broeck from the National Institute for Subatomic Physics (Nikhef) in the Netherlands. “This makes them ideal ‘laboratories’ for extreme-matter environments. This is particularly true when two neutron stars merge, forming a binary neutron star system. This results in even higher densities than inside a single star.”
In order to study binary neutron star systems, astrophysicists must first find them. Gravitational-wave astronomy, which as its name suggests uses gravitational waves to collect data about distant objects, presents astrophysicists with an opportunity to detect and observe binary neutron star systems like never before.
“This work relies on a detailed understanding of the merger processes,” says Van Den Broeck. “This can usually only be done with highly sophisticated theoretical models that describe the gravitational-wave and electromagnetic signals that are released during and after the merger. The development of such models for generic binary neutron stars was the key objective of BNSmergers.”
Analysing gravitational waves
The project, which was undertaken with the support of the EU-funded Marie Skłodowska-Curie Actions programme, built on recent discoveries that have transformed astronomy. The first direct detection of gravitational waves from the collision of two black holes was detected as recently as 2015, while the first combined gravitational wave and electromagnetic wave observation of a binary neutron star merger was found in 2017.
“Modelling high density matter however remains among the most challenging problems in theoretical physics,” adds Tim Dietrich, Marie Skłodowska-Curie fellow at Nikhef, the Netherlands. “Even a single simulation can run for weeks or up to months on a supercomputer.”
To address this, Dietrich and his colleagues were able to develop a new analytical framework, based on hundreds of collected computational simulations. This enables astrophysicists to work much faster than with existing numerical relativity simulations. “The approximation is also accurate enough to be directly employed to analyse gravitational-wave signals,” says Dietrich.
Database to the stars
These results could help astrophysicists unlock some of the secrets of the universe. “We were able to improve existing gravitational-wave models that are used to describe the electromagnetic signals connected to binary neutron star mergers,” explains Dietrich.
“This has opened up new information about the properties of neutron stars, the state of matter inside them, and even about the expansion rate of the universe. These models also open up the potential to study more exotic compact objects, such as stars that consist only of dark matter. While these scenarios are generally more speculative, theoretical investments are required to rule out or confirm their existence.”
Dietrich recently received the prestigious Heinz Billing Prize for the advancement of scientific computation for his work on the BNSmergers project. The prize is awarded every 2 years by the Max Planck Society in Germany for outstanding contributions in computational physics. “The fact that I received the Heinz Billing Prize for the advancement of scientific computation for my work in numerical relativity is yet further proof of the rising importance of gravitational-wave astronomy,” notes Dietrich.
The project has also resulted in the first gravitational-wave database for binary neutron star systems. Project simulations, together with simulations carried out before the start of the project, have been made publicly available. Already, numerous scientists have made use of this resource to support their research into neutron stars. “We hope that in this way, the entire scientific community can benefit from our scientific work over the last few years,” concludes Van Den Broeck.