Neutron stars are high on the list of the most delicious objects in the universe. First, there were dense “nuclear noodles” under their crusts. Now we have new evidence that the nuclei of the most massive neutron stars consist of an exotic “soup” of subatomic particles called quarks.
Physicists have created new calculations that use data from gravitational waves first discovered in a neutron star collision in August 2017, as well as observations of surprisingly massive neutron stars. Their conclusion points to an exciting result – the nuclei of the most massive neutron stars are so dense that atomic nuclei no longer exist and condense into quark matter.
According to the researchers, this is an important milestone in understanding the strange innards of these extreme objects.
“Confirming the existence of quark nuclei in neutron stars has been one of the most important goals of neutron star physics since this possibility was first considered about 40 years ago,”
Neutron stars are pretty wild. They are actually dead – the collapsed remains of massive stars that would have been between 8 and 30 solar masses (a measurement based on the mass of the sun). When these stars go into supernova, most of their mass is blown up into space. The remaining core collapses into an incredibly dense object.
The resulting neutron stars can be between about 1.1 and 2.3 solar masses, which are packed in a dense, small sphere with a diameter of only 10 to 20 kilometers. Five large neutron stars, each containing more mass than our sun, could fit comfortably along Hadrian’s wall and have space.
When the nuclear collapse supernova occurs, the protons and electrons in the atoms that make up the object are compressed into neutrons and neutrinos. The neutrinos escape and leave the neutrons behind under such high pressure conditions that they fuse together. This essentially turns the neutron star into a large nucleus with a density that is more than 100 trillion times the density of water at the bottom of the crust. (This creates the “nuclear noodle” structures.)
However, density is expected to increase the deeper you go, and that’s where the idea of quark matter cores comes into play. Quarks are basic subatomic particles that combine to form composite particles such as protons and neutrons.
You can probably see where this is going. For several decades, astronomers have been hypothesizing that neutrons decay even further into their quarks at a sufficiently high heat and density and form a kind of quark soup.
However, it is really difficult to find out what is in a neutron star. Therefore, the collision in August 2017 – GW170817 – was very exciting for astronomers as the way the two stars changed when they got close enough to gravitationally deform each other could provide information about their internal structure.
Vuorinen and his team used this gravitational wave signal along with new theoretical and particle physics results to make their enticing calculation. They found that neutron stars towards the upper mass limit of such objects – at least 2 solar masses – have properties that indicate the presence of a huge quark matter nucleus that is more than half the total diameter of the neutron star.
It is not an absolute slam dunk; but the calculations show that if the nuclei of these stars are, something really strange should be going on Not Quark matter.
“There is still a low probability that all neutron stars are made of nuclear matter only,” said Vuorinen.
“However, what we were able to do is quantify what this scenario would require. In short, the behavior of dense nuclear material would have to be really strange. For example, the speed of sound should almost match that of light.”
Not only would the discovery of quark matter in neutron stars be astonishing for its own sake – it could help us learn more about the earliest moments in our universe.
Cosmologists believe that for a few microseconds shortly after the Big Bang, known as the quark epoch, the universe was filled with a hot soup of quark-gluon plasma that quickly merged into hadrons.
Nowadays we can find quark matter only very briefly in particle collider experiments; but some massive neutron stars could also house it. If we can characterize the neutron star conditions under which quark matter forms, this could help us to better understand the quark epoch.
The LIGO-Virgo collaboration has identified a second neutron star fusion since GW170817, and it is only a matter of time before further mergers are added. Analysis of further mergers could help the team further validate their calculations and iron out the uncertainties.
“There is reason to believe that the golden age of gravitational wave disaster physics is just beginning and that we will soon see many more such leaps in our understanding of nature,” said Vuorinen.
The research was published in Natural physics.