Calculations by RIKEN researchers predict that signatures of gravitational wave signals from merging neutron stars should reveal what happens to matter during the extreme pressures that occur during such mergers. If you take some water and squeeze it with a plunger, the molecules will compress as they get closer together.

If you keep increasing the pressure, you reach a point where the atoms collapse and form a super 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, and this creates an impressive density – a single teaspoon of this type weighs several hundred billion pounds. But what happens if you keep increasing the pressure? Even astrophysicists do not know the answer to this question.

The density at the heart of neutron stars is three to five times greater than that of an atomic nucleus; this is the highest density attainable before a black hole forms. No one knows what happens to matter in such extreme concentrations.

One theory states that the superdense soup of neutrons and protons will decompose into a soup of quarks and gluons, the most fundamental building blocks of matter.

“Some researchers believe that quark phases will appear at the center of neutron stars,” says Shigehiro Nagataki of the RIKEN Big Bang Astrophysics Laboratory. “But that’s speculation.”

One promising way to find out if this strange form of matter exists is to observe the merger of two neutron stars with gravitational wave detectors. There are two possibilities, if any, for how protons and neutrons would decay into the quarks that formed them during fusion. They can undergo a sudden transition similar to liquid water turning into vapor at its boiling point at normal pressure. Or there may be a fuzzy transition, similar to water turning into steam at pressures above the critical point.

Now, Nagataki and his colleagues have warned of the merger of two neutron stars and calculated the gravitational waves they would create to investigate the second possibility.

The frequency of gravitational waves from merging neutron stars often depends on how fast the neutron star is spinning. Larger neutron stars typically spin more slowly and vice versa. The team discovered that it is possible to investigate whether a quark phase exists in a neutron star by analyzing the frequency of gravitational waves. If present, gravitational waves can also reveal what the quark phase looks like.

While current gravitational wave detectors cannot detect this, the next generation of detectors that will come into operation in the next decade should detect it.

“It’s great to think we can identify the transition type by detecting gravitational waves,” Nagataki says.