Finnish researchers reveal secrets behind neutron stars
A Finnish research group has studied the properties of the nuclear matter found inside neutron stars, the densest matter known in the universe, taking advantage of 130-million-year-old data.

In August 2017, the LIGO and Virgo observatories were the first to detect gravitational waves produced not by colliding black holes but by two merging neutron stars. The hope is that this discovery will increase our understanding of the properties of these extremely dense and small objects, as well as those of the exotic matter inside them.

As neutron stars spin around each other at a dizzying speed just moments before impact, they direct extremely powerful tidal forces at each other. This disrupts the spectrum of gravitational waves emitted in the process, making a lot of information available on the characteristics of the matter found inside the stars. It is this highly exotic phenomenon that the Finnish researchers utilised in their investigation.

Based on this observation, the research group from the University of Helsinki and CERN, the European Organisation for Nuclear Research, was able to derive stringent new bounds on the properties of dense nuclear matter.

“We were among the first to show how the measurement of gravitational waves restricts the thermodynamic properties of extremely dense nuclear matter,” says Aleksi Vuorinen, an associate professor at the University of Helsinki. Vuorinen worked on the project together with Eemeli Annala and Tyler Gorda, members of his research group, as well as with Aleksi Kurkela, currently working at CERN.

The main result of the project was an improved prediction for the equation of state describing the thermodynamic properties of nuclear matter. To achieve this, the group used theoretically known properties of nuclear matter, as well as their own earlier results on quark matter, and combined this theoretical input with the new neutron star observations. As a by-product of this, the researchers were able to constrain the measurable properties of neutron stars, such as their radius. In light of the new findings, the most likely value of this quantity seems to fall between 11 and 13.5 kilometres.

Gravitational waves as tools for particle and nuclear physics

While the idea of gravitational waves providing new insights to the field of particle physics is not new, the researchers were surprised by the dramatic nature of the results obtained using the LIGO and Virgo observations.

“The observation of a single neutron star merger was enough to significantly constrain the nuclear matter equation of state. This is quite a remarkable result that highlights the potential of gravitational wave measurements as a tool of particle and nuclear physics,” describes Aleksi Vuorinen.

More news is already on the horizon, as the gravitational wave observation from last autumn will very likely not be the only one of its kind. In addition, the accuracy of neutron star radius measurements has greatly improved in recent years, and progress in theoretical particle and nuclear physics has also been swift. The topicality of the matter is reflected in the fact that nearly 700 new scientific articles have already been published based on the gravitational wave finding released last October.

Information dating back 130 million years

Some 130 million years ago, two neutron stars collided in a far-away galaxy. In addition to a gamma-ray burst detectable on Earth, the collision produced gravitational waves, which were measured by the LIGO and Virgo observatories in August 2017. Being the first measurement of its kind, this has opened a new window on both the properties of neutron stars and the very exotic matter inside them.

”Neutron stars can contain entirely new states of matter, from superfluid nuclear matter to quark matter, in which neutrons and protons have been squeezed inside each other,” tells Aleksi Vuorinen.

Neutron stars are products of supernova explosions, and contain the densest matter in the known universe. This is why both astronomers and nuclear and particle physicists have long been interested in them.

The Finnish research group’s results are prominently displayed in the latest issue of the Physical Review Letters journal, which included the article in its Editor's Suggestion category.

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Aleksi Vuor­inen looks into the heart of neut­ron stars