Scientists may be closer than ever to solving the mystery of what lies deep under the surface of extremely dense dead stars called neutron stars. A new supercomputer analysis of neutron stars has shown that there is an 80% to 90% chance that these objects have nuclei filled with free quarks, which are fundamental subatomic particles that are usually only bonded together in other particles such as protons and neutrons.

Protons and neutrons come together to form atomic nuclei with electrons around them. But according to the team, if the cores of neutron stars are indeed filled with free quarks, they would consist of an exotic form of matter known as “cold quark matter.” And separate protons and neutrons cannot exist in cold quark matter. Therefore atoms cannot exist. Just quarks.

If true, this would make neutron stars appear to be incredibly large atomic nuclei.

“It is exciting to see concretely how each new observation of a neutron star allows us to infer the properties of the neutron star’s matter with increasing precision,” said lead study author Joonas Nettila, who is about to become an associate professor at the university. of Helsinki. , the message says.

Neutron stars are born when the cores of stars 10 to 20 times the mass of the Sun run out of fuel to carry out their own nuclear fusion. This results in the end of the external energy that has kept the star stable against the internal pressure of its own gravitational force for millions, even billions of years.

In this cosmic tug of war, gravity wins and the star’s core begins to collapse. When this happens, the star’s outer material, where nuclear fusion is still ongoing, is blown away in a powerful supernova explosion.

Thus, the stellar core has a mass of one to two times that of the Sun and is concentrated to a width of only about 12 miles (20 kilometers).

This massive reduction in the size of what is now a neutron star makes matter so dense that a block the size of a sugar cube would weigh about 1 billion tons if sent to Earth. A sugar cube weighing as much as 3000 Empire State Buildings.

So the question is: What is this incredibly exotic substance that may not be found anywhere else in the universe? And more generally, could conditions in the densest regions of these dead stars actually create an entirely new phase of matter, devoid of protons and neutrons, called cold quark matter?

Scientists cannot visit neutron stars to sample this material; Since even the nearest neutron stars are about 400 light-years away, it is best to simulate conditions beneath the surface of stars using a powerful combination of real astronomical data and supercomputers.

This new study used a type of statistical inference called Bayesian inference, which calculates the probability of various model parameters by directly comparing them with observational data.

This allowed the team to define the boundaries of neutron star matter and allowed the crew to infer the presence of cold quark matter with a high probability. The mechanism also suggested that a “nuclear-free” state of matter exists in neutron stars, where quarks can be found “separately” in protons, neutrons, and other particles.

“The quarks and gluons that make them up have been freed from the typical color constraint and can move almost freely,” Aleksi Vuorinen, professor of theoretical physics at the University of Helsinki, said in a statement. said.

The team’s supercomputer simulations show that the probability that matter inside neutron stars rapidly transforms from nuclear matter into “quark” matter is less than 20%, just as water turns into ice. This rapid change of matter can destabilize neutron stars so much that even the smallest quark matter can collapse to form a black hole.

The study also showed that the existence of quark matter nuclei could be fully confirmed with further analysis in the future.

The key to this will be determining the strength of the phase transition from nuclear matter to quark matter; This may be possible if gravitational wave detectors become sensitive enough to “hear” tiny ripples in space-time that have emerged from the last moment. two neutron stars. Bumping into each other.

However, even with improved observational data, better models of neutron star cores will still require huge amounts of computational resources and time.

“We had to use millions of processor hours of supercomputer time to be able to compare our theoretical predictions with observations and constrain the possibility of quark matter nuclei,” said team member and PhD student at the University of Helsinki, Joonas Hirvonen. The team’s research was published in the journal Nature Communications in December.