Strange Quark Matter: Gravitational Waves Hold Clues to the Universe’s Densest Matter

Gravitational waves could reveal whether the quark soup that existed in the early Universe is created in neutron-star mergers. 

RIKEN researchers hope that gravitational wave signals from merging neutron stars could reveal the existence of ultra-dense quark-gluon matter. By simulating these mergers and analyzing the resulting gravitational waves, they suggest that next-generation detectors due within the next decade could support this theory. .

Calculations by RIKEN researchers predict that telltale signatures contained in gravitational wave signals from neutron star mergers will reveal what happens to matter under the extreme pressures produced by the merger. there is

If you take water and compress it with a piston, the molecules will contract as they get closer to each other.

Further pressure reaches a point where atoms collapse to form an ultra-dense soup of neutrons and protons. The only place in the universe where this happens is in neutron stars, the collapsed remnants of burnt-out stars, which create incredible densities – a teaspoon of this material weighs hundreds of billions of kilograms.

But what if we increase the pressure even more? Even astrophysicists don’t know the answer.

The density in the core of a neutron star is 3-5 times higher than in the atomic nucleus. This is the highest density achievable before black holes form. No one knows what happens to matter at such extreme densities. One theory is that an ultra-dense soup of neutrons and protons breaks down into a soup of quarks and gluons, the most basic building blocks of matter.

“Some researchers believe that quark phases occur at the core of neutron stars,” says Shigehiro Nagataki of the RIKEN Big Bang Laboratory of Astrophysics. “But that’s a guess.”

A promising way to find out if this odd-shaped material exists is by using gravitational-wave detectors to observe the merger of two neutron stars.

If it exists, he has two ways that protons and neutrons can coalesce and decay into quarks. It can undergo rapid changes in the same way that water, which is liquid at normal pressure, turns into vapor at its boiling point. Alternatively, there may be ambiguous transitions, similar to how water turns to steam at pressures above its critical point.

Now Nagataki and his colleagues are investigating the second possibility, where he stimulated the merger of two neutron stars and calculated the gravitational waves they produced.

The frequency of gravitational waves from neutron star mergers usually depends on the rotation speed of the neutron star. Typically, larger neutron stars rotate slower and vice versa.

The research team found that by analyzing the frequency of gravitational waves, it is possible to investigate whether quark phases exist in neutron stars. If gravitational waves exist, they can also reveal what phases of quarks are like.

Current gravitational wave detectors cannot detect this, but next-generation detectors that will be put into practical use in the next decade or so should be able to detect it.

“It is surprising to think that we should be able to detect the nature of transitions by detecting gravitational waves,” says Nagataki. 

Reference: “Merger and Postmerger of Binary Neutron Stars with a Quark-Hadron Crossover Equation of State” by Yong-Jia Huang, Luca Baiotti, Toru Kojo, Kentaro Takami, Hajime Sotani, Hajime Togashi, Tetsuo Hatsuda, Shigehiro Nagataki and Yi-Zhong Fan, 26 October 2022, Physical Review Letters. 
DOI: 10.1103/PhysRevLett.129.18110