Dive into the interior of neutron stars and you'll find, guess what, neutrons. But it's not as simple as that. The deeper the dive, the fuzzier and denser the interior gets. There's no shortage of theories as to what might make up the centre of these cosmic objects. One hypothesis is that it's filled with free quarks, not confined inside neutrons. Another is that it's made of hyperons, particles that contain at least one quark of the "strange" type. Another still is that it consists of an exotic state of matter called a kaon condensate.
In a paper published today in the journal Nature Physics, a quintet of researchers including Aleksi Kurkela from CERN's Theory department provides evidence that massive neutron stars can contain cores filled with free quarks. Such quark matter resembles the dense state of free quarks and gluons that is thought to have existed shortly after the Big Bang and can be recreated at particle colliders on Earth, such as the Large Hadron Collider.
To reach this evidence, the researchers combined information from astronomical observations of neutron stars with theoretical calculations. While astronomical observations provide some information about the stars' interior, they don't reveal their exact make-up.
The theoretical calculations involved describing the state of matter inside a neutron star from the crust all the way down to the centre. To do this, the researchers used so-called equations of state, which relate the pressure of a state of matter to the energy density - the amount of energy packed into a system or region of space per unit volume.
The team then plugged two pieces of information from astronomical data into these calculations: the observation that neutron stars can have masses equivalent to two Suns; and the possible values of a property called tidal deformability for a neutron star with a mass of about 1.4 times that of the Sun. The tidal deformability describes the stiffness of a star in response to stresses caused by the gravitational pull of a companion star, and was previously derived from observations of gravitational waves (ripples in the fabric of spacetime) emitted by the merger of two neutron stars.
From this combination of theory and data, the researchers find that the cores of neutron stars with a mass 1.4 times that of the Sun should be filled with neutrons. By contrast, more massive stars can contain large quark-matter cores. For example, a 2-solar-mass neutron star with a radius of about 12 km could have a quark-matter core with a radius of about 6.5 km - about half of the star's radius.
"Our analysis does not completely rule out the existence of massive stars with neutron cores but it demonstrates that quark-matter cores are not an exotic alternative," says Kurkela. "We can't wait to incorporate new neutron-star data into our analysis and see how they will affect this conclusion."
