Raimond Snellings studies quarks, one of the smallest building blocks of matter. For reasons that physicists don’t fully understand, quarks exist in our universe mainly in groups of threes, to form, for example, protons and neutrons. In his research, Snellings conducts experiments to break up groups of quarks—to figure out why and how they bind together to form regular matter via one of the four fundamental forces, the strong force.
To that end, he and his colleagues smash lead ions together at the Large Hadron Collider. These collisions regularly reach temperatures of 5.5 trillion degrees Celsius, or 10,000 times hotter than the core of the sun. The lead ions break up into their constituent quarks—for a moment. If you divided one second into 1023 frames, you'd see the resulting slippery substance, known as a quark-gluon plasma, for just one frame. The entire plasma, which quickly congeals back into ordinary three-quark particles, is small enough to fit easily inside a virus.
Snellings was making good progress on his quark studies when the LHC shut down for two years of hardware upgrades last December. So now the Utrecht University physicist ponders signals produced by different giant machines: the Laser Interferometer Gravitational Wave Observatory and its European counterpart, Virgo, whose three detectors in the US and Italy listen for the ripples in spacetime produced when astronomical bodies collide. In the gravitational waves produced by neutron stars, Snellings suspects he might find clues to the origins of quarks—and of the universe itself.
At first glance, neutron stars seem to have little to do with Snellings’ quest for quarks. The ultracompact cores of dead stars, neutron stars are astronomical in mass, packing a quantity of material about equal to the mass of the sun into a sphere with roughly the cross-sectional area of Chicago. Sometimes, two neutron stars collide and merge to form a black hole. Neutron star material is so dense that a teaspoon of it would weigh 10 million tons. Their surface temperatures average around 600,000 degrees Celsius, positively chilly compared to the quark-gluon plasma produced at the LHC.
But here’s the connection. Researchers think that at the core of a neutron star, particles can get so compressed that quarks no longer act in triplets. Quarks begin to behave independently to form a different phase of matter, dubbed “quark matter.” As in the highly controlled conditions within the LHC, neutron stars should produce exotic phases of quarks not readily accessible on Earth. In this sense, you can think of a neutron star as an astronomical-scale particle collider, says astrophysicist Jocelyn Read of California State University, Fullerton.
Physicists’ broader goal is to map what happens to quarks over wide ranges in pressure and temperature, says physicist Horst Stöcker of the Frankfurt Institute for Advanced Studies. Quarks change phase depending on their environment, similar to how water can take the form of liquid, ice, and vapor. Physicists predict that quarks should experience many phase transitions, but they don’t know the exact conditions.
That’s because researchers only roughly understand the nature of the strong force, which dictates those phase transitions. To deepen this understanding, they compare the outcomes of real experiments and simulated ones based on the theory of the strong force, known as quantum chromodynamics. These simulations consist of math equations that describe how the strong force binds quarks together using particles called gluons. But the theory itself is mathematically cumbersome, and crunching the numbers is computing-intensive: Even relatively simple quark simulations can require supercomputers.
But the collisions of neutron stars could provide clues to these phase changes. Before the neutron stars merge, each one pulls on the other via gravitational attraction, like the moon creating tides on Earth. This forceful gravitational pull deforms the neutron stars, changing their shape. When the gravitational waves from a collision ripple out across the universe and eventually register on an Earth-based detector, researchers can use them to estimate how much the stars have been stretched or squeezed, says Read. Those changes allow scientists to infer material properties of the star, which point to the phases of matter in its interior.
So far, LIGO and Virgo have only confirmed the detection of one gravitational wave from a neutron star source. But since the observatories began their most recent observing run in April, researchers have picked up three other signals that likely come from neutron stars: two possible collisions between pairs of neutron stars, and a signal that looks like a black hole eating a neutron star. As the observatories pick up more neutron star activity, researchers could look for quark phase changes by comparing the various events, says Read. For example, they could study how the physical dimensions of neutron stars vary with their mass. “If lower-mass stars have a bigger radius, and then all of a sudden, the higher-mass stars jump to a smaller radius, that could be an indication that something really dramatic is going on with the matter in their cores,” she says.
Physicists also want to use particle colliders to simulate neutron star conditions on Earth, says Stöcker. To that end, he is working to build the Facility for Antiproton and Ion Research in Germany, expected to start operations around 2024. The facility will house a machine that collides ions at lower temperatures than the LHC, to mimic the cores of neutron stars.
Ultimately, these studies can help reveal how the universe came to be in its current state, says Snellings. Physicists generally agree that some 13.7 billion years ago, the universe consisted of hot and dense quark-gluon plasma. To assemble into the matter that would eventually make up Earth and our solar system, that material would have to experience similar phase transitions as the material his team produced at the LHC. Those phase transitions, in turn, need to follow rules consistent with those that dictate the texture of neutron star matter. By studying fundamental interactions of quarks in the present, physicists could help discover our past.