Utrecht University- New light on neutron stars and the universe itself

Researchers from Utrecht University and Nikhef, among others, are publishing a study today in the leading journal Science that addresses no less than two essential questions in astrophysics. First, they give a sharp estimate of the size of a typical neutron star, the remnant of a collapsed burnt-out star several times the mass of the Sun. Second, they have arrived at a new estimate of the speed at which the universe is expanding. This speed is currently the subject of intense scientific debate.

The researchers found the two answers by combining a wide range of neutron star observations using radio telescopes, telescopes and gravitational wave detectors.

Lead author Tim Dietrich is a former postdoc at Nikhef in Amsterdam, and currently Professor at the University of Potsdam. In this study, he works closely together with a team that includes gravitational experts from Utrecht University, such as Nikhef PhD candidate Peter Pang.

Multi-messenger astronomy
In modern astrophysics, scientists combine different types of signals from space to study fundamental questions about the cosmos, ranging from light and other electromagnetic radiation to cosmic particles and, more recently, gravitational waves. This so-called multi-messenger astronomy is a fast-growing branch of science.

A new era of multi-messenger astronomy began in 2017, when scientists observed the signals of a collision between two neutron stars. Light and other radiation were captured from that event, as well as gravitational waves: small measurable ripples of Einstein’s space-time that occur when extremely compact objects swallow each other up. Even at that time, the measurements showed that such collisions are the place where heavy chemical elements such as gold are forged.

Since then, the gravitational waves of a handful of such neutron star collisions have been measured with detectors in the US and Europe, sometimes combined with telescope observations. The detectors also regularly pick up fusions of black holes, an even more violent process in which no light is emitted.

By analysing these measurements and other observations of neutron stars, the researchers conclude that a neutron star typically has a diameter of only 12 kilometres. The mass of such a small super-compact neutron sphere is unimaginably large: hundreds of thousands of times the mass of the Earth. Just one teaspoon of the star’s matter would weigh millions of tonnes.

Because of this more precise knowledge of the neutron star radius, the team was able to rule out some of the existing theoretical models that describe nuclear forces. Tim Dietrich: “The observation of colliding neutron stars allows us to understand the behaviour of matter on scales smaller than an atom. So our study literally ranges from cosmic to sub-atomic scales.”

Much discussion
From the observations, Dietrich’s team also made a new, independent estimate of the so-called Hubble constant. “This is a measure of the rate at which the universe itself has been expanding since the Big Bang,” says Dietrich. “In recent years there has been a lot of discussion about this constant, because there are two measurement methods that give very different results.”

The value that has now been found is closest to the Hubble constant, which is the result of precision satellite measurements of cosmic background radiation; a remnant of the Big Bang more than 14 billion years ago. This also supports the most common models for the course of the Big Bang, called lambda-CDM. “A mild preference for the previous Cosmic Microwave Background measurement,” said Peter Pang.