The standard model of physics describes quarks, the building blocks of protons and neutrons, as very sociable creatures. In almost all environments they will only ever be observable as pairs or triples. Within these groups, it is mathematically impossible to differentiate between or separate them, since the energy required to separate the quarks is high enough to create a new pair of quarks from the quantum foam. Where they cannot form these separate and small groups, either because they possess too much thermal energy to stay permanently connected to each other or because an external force is squeezing them together, they form what is called quark matter. This exotic state of matter somewhat resembles a liquid, in which none of individual quarks are discernible from each other.
One fairly well studied occurrence of quark matter is the quark-gluon plasma formed in the extreme environments of high energy particle collisions, such as those in Large Hadron Collider. Here quark particles are thrown away from each other by intense kinetic energy. Similarly to how electrons can’t hold on to atomic nucleus in the traditional kind of plasma, here the quarks cannot hold onto each other and move freely through space. Another kind of quark matter, that has been hypothesized to exist by physicists for a long time, is so-called “cold and dense quark matter”. This type would form from the enourmous gravitational pressure inside stellar remnants. Traditionally these objects have been called neutron stars, as the gravitational force acting on protons and electrons is extreme enough, to squish them together into neutrons. However, under intense enough pressure, these neutrons get further compressed to form quark matter, in which the boundaries between individual quarks are no longer existant.
Proving the existence of this hypothetical densest state of matter was thought to be impossible, since it would hide within large neutron stars, one of the most extreme environments in the universe. It is unknown if the pressure inside a stellar remnant can be high enough to form quark matter without being so high as to collapse the neutron star into a black hole. But researchers from the University of Helsinki may have found a way to show its existence by observing its structural effects on the gravitational behaviour of the star.
They use mathematical tricks from string theory that have already helped describe the quark-gluon plasma in order to build a mathematical model of compact stellar remnants. This model has one free unknown parameter they could vary, which corresponds roughly to the constituent masses of the quarks from which the quark matter is formed. Varying this parameter, they get four different solutions for the structure of a compact star: a traditional neutron star, a star made entirely out of quark matter and two different kinds of hybrid stars: one with a crust made from quark matter and one with a mantle made of quark matter.
Gravitational wave measurements from LIGO and VIRGO have put certain restraints on the tidal deformability of neutron stars. These gravitational wave detectors can detect the ripples in space time left by the merger of two neutron stars. How much the neutron star squishes and stretches under the gravitional influence of the other star has a subtle effect on the resulting gravitational wave amplitude and frequency spectrum. The researchers can use their mathematical models to calculate this tidal deformability depending on the choice of quark mass. They find that models representing hybrid stars fit the experimental observations closer than models which describe traditional neutron stars. Traditional neutron stars are however well within the error bars of the gravitational wave data.
While the scientists admit that their models are based on heavy simplifications of the underlying theory of quantumchromodynamics their results may prove to be the first signs that we need to rethink our understanding of neutron stars.
Paper:
Annala , E , Ecker , C , Hoyos , C , Jokela , N , Rodriguez Fernandez , D & Vuorinen, A. 2018 , ‘ Holographic compact stars meet gravitational wave constraints ‘ , Journal of High
Energy Physics , vol. 2018 , no. 12 , 078 . https://doi.org/10.1007/JHEP12(2018)078
Author: Jeff Schymiczek
University of Helsinki
Jeff – I love the image of neutron stars squishing and stretching! It’s amazing how much is left to discover in astronomy and cosmology 🙂
-Edie