Gravitational waves can indicate transitions to strange quark matter


April 10, 2023

(Nanowerk News) The signatures in the gravitational wave signal from the merger of neutron stars should reveal what happens to matter at the extreme pressures generated during the merger, calculations predicted by RIKEN researchers (Physical Review Letter, “Merger and post-merger binary neutron star with quark-hadron crossover equation of state”).

If you take water and compress it with a piston, it will shrink as the molecules are pushed closer together.

If you keep increasing the pressure, you will reach a point where the atoms collapse and form a very dense soup of neutrons and protons. The only places in the universe where this occurs are neutron stars, the burnt-out remnants of stars that produce astonishing densities—one teaspoon of such matter weighs several hundred billions of kilograms.

But what will happen if you continue to increase the pressure even further? Even astrophysicists don’t know the answer.

The density at the heart of a neutron star is three to five times higher than that of the atomic nucleus; it’s the highest density achievable before a black hole formed. No one knows what happens to matter with such extreme densities.

One theory holds that the incredibly dense soup of neutrons and protons will break down into a soup of quarks and gluons — the most basic building blocks of matter.

“Some researchers believe that a quark phase will emerge at the center of a neutron star,” said Shigehiro Nagataki of the RIKEN Astrophysical Big Bang Laboratory. “But that’s a guess.” Image of two neutron stars Figure 1: The gravitational waves generated by a merger between two neutron stars can reveal the creation of free quarks through the merger. (Image: Lagoon Design)

A promising way to find out if this strange form of matter exists is to observe the merger of two neutron stars using a gravitational wave detector.

If there is, there are two possibilities for how the protons and neutrons will disintegrate into their constituent quarks during the combination. They can go through sharp transitions, like liquid water changing to vapor at its boiling point at normal pressure. Or there may be a blurry transition, analogous to how water turns into steam at a pressure above its critical point.

Now, Nagataki and co-workers have stimulated the merger between two neutron stars and calculated the gravitational waves they would produce to explore the second possibility.

The frequency of the gravitational waves from a merger of neutron stars usually depends on how fast the neutron star is rotating. Larger neutron stars usually rotate more slowly, and vice versa.

The team found it possible to investigate whether a quark phase exists in a neutron star by analyzing the frequency of its gravitational waves. If they do exist, gravitational waves could also reveal how the quark phases came to be.

While current gravitational wave detectors can’t detect this, the next generation of detectors, coming online in the next decade, should be able to.

“It’s amazing to think that we can detect this type of transition by detecting gravitational waves,” said Nagataki.


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