Physicists identify unique signature to confirm quark-gluon plasma in Universe

In the first fractions of a second of our Universe’s existence, the energy density was so incredibly high that there were no protons and neutrons, just a hot “quark soup” known as a quark-gluon plasma (QGP). Physicists have successfully recreated this unique state of matter in high-energy laboratories, but those conditions are exceedingly rare in the current cosmos. According to a new paper in the journal Physical Review Letters, German physicists have performed computer simulations indicating that a QGP could form in the immediate aftermath of a binary neutron star merger and that it should produce a telltale detectable signature in the gravitational waves emanating from that event.

“Compared to previous simulations, we have discovered a new signature in the gravitational waves that is significantly clearer to detect,” said co-author Luciano Rezzolla of Goethe University in Frankfurt, Germany. “If this signature occurs in the gravitational waves that we will receive from future neutron-star mergers, we would have a clear evidence for the creation of quark-gluon plasma in the present universe.”

A hot, dense soup

Quarks, the fundamental components of subatomic particles, are bound together by force-carrying gluons to form protons and neutrons. But under the extreme high-energy conditions of the early Universe in its first microseconds of existence, that couldn’t happen. Instead, quarks and gluons mingled freely in a dense soup until things cooled down sufficiently for protons to condense out of the QGP. Before the first second was up, the Universe had gone through its entire inflationary period, sowing the seeds for the large-scale structures we see today.

To create QGPs in the lab—at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider at CERN—physicists must achieve temperatures hundreds of thousands of times hotter than the Sun. They do so via heavy-ion collisions (using gold at RHIC, lead at the LHC) to generate the enormously high energies required to blast the nucleon apart. As Ars’ John Timmer explained in 2012:

The quarks and gluons that spill out of a proton collision tend to have nothing but empty space around them. In a heavy ion collision, the large number of nucleons that are broken apart at once means that, instead of flying into empty space, a given quark or gluon will have the opportunity to interact with those pouring out of nearby nucleons. As a result, for a brief instant, the collisions don’t look much like an explosion; instead, it looks more like the boundaries between nucleons melting, leaving behind a sea of quarks and gluons that are interacting.

Physicists were surprised to discover in 2005 that the QGP isn’t really a gaseous plasma in terms of its behavior. It’s more akin to a superfluid, since there is virtually no viscosity (resistance from friction)—it is, in fact, the most perfect liquid yet observed. Later experiments revealed that particle collisions within the QGP swirl faster (a measure of vorticity) than any other known fluid, too.

A telltale signature?

Where might we find a QGP in the Universe today? Rezzolla and his GUF colleagues thought a binary neutron star merger might be a good candidate, and they hypothesized that we could find evidence for a QGP in the gravitational waves produced by such a merger, thanks to the continued success of LIGO. 

LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. (A third detector came online at VIRGO in 2016.) Each instrument is so sensitive that it also picks up small ambient vibrations, like a rumbling freight train or natural thermal vibrations in the detectors themselves.

On September 14, 2015, at 5:51 AM EST, both detectors picked up signals within milliseconds of each other for the very first time. The waveforms of those signals serve as an audio fingerprint—in this case, evidence for two black holes spiraling inward toward each other and merging in a massive collision event, sending powerful shock waves across spacetime. That first detection snagged the collaboration the 2017 Nobel Prize in Physics.

There have been a number of additional gravitational wave detections since then, most notably 2017’s binary neutron star merger, supported by a simultaneous gamma-ray burst and signals in the rest of the electromagnetic spectrum. And last year LIGO/VIRGO observed five gravitational wave events in a single month. Three were from merging black holes, one was the second neutron star merger, and another may have been the first instance of a neutron star-black hole merger. So the likelihood of detecting additional binary neutron star mergers in the future is strong.

But much of the focus on those early detections has been on the merging point; gravitational wave recordings weren’t made beyond that. Rezzolla et al. used computer simulations to model two merging neutron stars, focusing on the immediate aftermath of that merger. Those simulations showed that a phase transition would occur, whereby the extreme environment would cause hadrons to melt and briefly create a QGP of freely mingling quarks and gluons. That transition should leave a clear, distinct signature in the gravitational wave signals emanating from that event.

Now we just have to wait for LIGO/VIRGO to detect another neutron star merger—this time recording a bit longer to hopefully capture a signal from that telltale phase transition.