Two neutron stars almost about to merge, emitting gravitational waves and light.
Collision of two neutron stars showing the electromagnetic and gravitational waves emitted during the merger process. The combined interpretation of multiple messengers allows us to understand the internal composition of neutron stars and the properties of matter under the most extreme conditions in our Universe. Image credit: Tim Dietrich

An international team, including young researchers from Germany, the Netherlands, Sweden, France and the USA, recently developed an interdisciplinary framework to address some fundamental questions connected to the collisions of neutron stars. In a study published this week in Science (Multi-messenger constraints on the neutron-star equation of state and the Hubble constant), the team used a “multi-messenger” approach to constrain the typical radius of neutron stars and the expansion rate of our Universe. In a multi-step procedure, the work starts with theoretical inputs describing the nuclear matter in the core of neutron stars and then adds observations of neutron stars and of binary neutron star mergers in both the gravitational-wave and electromagnetic domain. 

It is like seeing these spectacular collisions with multiple glasses. Each pair of glasses can capture only some details of these events, while combining all glasses gives you a truly comprehensive description,” explains Mattia Bulla who works at Nordita in Stockholm and is an expert in modelling the electromagnetic signal from neutron-star collisions. “I was thrilled to be part of this study and contribute with one piece of the jigsaw puzzle,” says Lina Issa, a Master student at the École normale supérieure Paris-Saclay who did a nine-month internship at Nordita and developed some of the electromagnetic models used in the work.

With such a comprehensive multi-messenger framework, the team was able to constrain the “city-size” radius of a typical neutron star to 11.7 kilometers, with an uncertainty of only 800 meters. At the same time, the authors were able to measure the so-called Hubble constant, a fundamental constant describing the expansion rate of our Universe. They found that our Universe is expanding at a local rate that is consistent with two independent estimates from the Cosmic Microwave Background radiation and supernova explosions but cannot arbitrate the current tension between these two probes.

The multi-messenger framework developed in this work is general and can be easily extended to include more sophisticated models and future data from neutron stars and binary neutron star mergers. “Adding new data to our framework is straightforward and will allow us to draw even stronger constraints on the physics of these events,” adds Mattia.
The future is bright - so stay tuned!