Scientists may be closer than ever to solving the mystery of what lies deep beneath the surface of extremely dense, dead stars called neutron stars.
A new supercomputer analysis of neutron stars has found that there is between an 80% and 90% chance that these objects have nuclei filled with free quarks, fundamental subatomic particles that are usually only found bound 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 neutron star cores are indeed full of free quarks, they must be composed of an exotic form of matter known as “cold quark matter.” And individual protons and neutrons cannot exist in cold quark matter. So atoms cannot exist. Just quarks.
If true, this would make neutron stars resemble incredibly massive atomic nuclei.
“It is fascinating to see concretely how each new neutron star observation allows us to infer the properties of neutron star matter with increasing precision,” said study lead author Joonas Nättilä, who is about to take up his post as an associate professor at the University of Helsinki. , he said in a statement.
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Neutron stars are born when the fuel required for nuclear fusion, which occurs in the cores of stars with masses 10 to 20 times that of the Sun, runs out. This results in the cessation of outward energy that has kept the star stable against the inward pressure of its own gravity for millions or even billions of years.
With gravity winning this cosmic conflict, the core of a star begins to collapse. As this occurs, the star’s outer material, where nuclear fusion is still ongoing, is blown away in a massive supernova explosion.
This leaves the stellar core concentrated at one to two times the mass of the sun, only up to 12 miles (20 kilometers) across.
This massive reduction in the size of what is now a neutron star creates matter so dense that a block the size of a sugar cube would weigh about 1 billion tons if brought to Earth. This is a sugar cube that weighs as much as 3,000 Empire State Buildings.
Now the question is: What is this incredibly exotic substance, probably found nowhere else in the universe, made from? And more generally, could conditions in the densest regions of these dead stars actually create an entirely new phase of matter, called cold quark matter, devoid of protons and neutrons?
Scientists cannot visit neutron stars to sample this material; Since even the nearest neutron stars are about 400 light-years away, the next best thing is to simulate the conditions beneath the stars’ surfaces using a powerful combination of real astronomical data and supercomputers.
This new research used a type of statistical inference called Bayesian inference, which calculates the likelihood of different model parameters by making direct comparisons with observational data.
This allowed the team to determine the boundaries of neutron star matter and led the crew to conclude that the existence of cold quark matter was highly likely. The mechanism also suggested the existence of a “non-nuclear” state of matter in neutron stars, in which quarks are allowed to exist “deconfined” in protons, neutrons, and other particles.
“The quarks and gluons that compose them were freed from the typical color constraint and allowed to move almost freely,” Aleksi Vuorinen, professor of theoretical physics at the University of Helsinki, said in a statement. said.
The team’s supercomputer simulations suggest that there is less than a 20% chance that matter in neutron stars will experience a rapid phase change from nuclear matter to “quark matter,” almost like water turning into ice. Such a rapid change in matter could destabilize neutron stars to such an extent that even a tiny bit of quark matter would collapse to form a black hole.
The research also revealed 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 is something that may be possible if gravitational wave detectors become sensitive enough to “hear” tiny ripples in space-time resulting from the last moment before two neutron stars. Those orbiting collide.
However, even with improved observational data, better models of neutron star cores will still require large amounts of computing power and time.
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“We had to use millions of CPU 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 University of Helsinki graduate student Joonas Hirvonen. expression.
The team’s research was published in the journal Nature Communications in December.