Evidence of universe's oldest state of matter lurking inside neutron stars

Evidence of universe's oldest ...
An artist's concept of the layers of a neutron star, including the quark core
An artist's concept of the layers of a neutron star, including the quark core
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An artist's concept of the layers of a neutron star, including the quark core
An artist's concept of the layers of a neutron star, including the quark core

Astrophysicists have found evidence of a strange substance called quark matter at the heart of compact stars. Combining recent theoretical calculations with measurements of gravitational waves from neutron star collisions, the researchers found that the most massive neutron stars most likely have “quark cores.”

In normal matter, elementary particles called quarks are only ever found inside protons and neutrons. But if that normal matter is subjected to extreme temperatures, or clumped together in very high densities, it can “melt” together, giving the quarks free rein to roam anywhere within that matter. This exotic new state is known as quark matter.

It’s believed that a form of this strange stuff called quark-gluon plasma filled the universe about 20 microseconds after the Big Bang, behaving like an immensely hot liquid before cooling into the regular matter that fills the universe today. Nowadays, the only places you’ll find quark matter are (briefly) in particle collisions at the Large Hadron Collider – and perhaps, at the heart of neutron stars.

When certain stars die, their cores collapse to become either a black hole or a neutron star. In the latter case, this new object crams more than the mass of the Sun into a space the size of a city. Obviously, that creates some extreme density of matter, which has long been theorized to give rise to quark matter.

For the new study, researchers from the University of Helsinki now claim to have all but confirmed quark cores in some neutron stars. Specifically those with the largest masses, of two times the Sun or more.

It was previously thought that two solar masses was the absolute upper limit for neutron stars – any more mass and the original star would have collapsed into a black hole instead. But astronomers have recently found a handful of neutron stars that exceed this “limit.”

And it’s in these stars that quark cores can be found, according to the new study. In some cases, the quarks may even make up more than half of the neutron star itself.

To get to this conclusion, the team calculated the “equation of state” of matter in neutron stars. This equation describes what the matter in a neutron star would be like, based on the relation between the pressure and energy density. General relativity says this can be figured out using a relation between all possible sizes and masses of neutron stars.

And lately, astronomers have been given much clearer ideas of just how big and massive neutron stars can get. Particularly, collisions between neutron stars have been detected by LIGO and Virgo as gravitational waves. Information encoded in these signals can reveal a lot about the objects that produced them.

All together, the waves of new data allowed the study to be as accurate as possible. The researchers say they’ve “almost certainly” discovered quark matter – but they also acknowledge that there’s a chance they’re wrong. Quark matter is merely the most straightforward explanation.

“There is still a small but non-zero chance that all neutron stars are composed of nuclear matter alone,” says Aleski Vuorinen, lead author of the study. “What we have been able to do, however, is quantify what this scenario would require. In short, the behavior of dense nuclear matter would then need to be truly peculiar. For instance, the speed of sound would need to reach almost that of light.”

More data on neutron stars – which is pouring in all the time – will help refine these calculations even further.

The research was published in the journal Nature Physics.

Source: University of Helsinki

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