Neutron stars, which are the result of a supernova explosion, are the smallest and densest stars in the universe, with a radius of approximately 10 km, have about 1.4-2.2 solar masses and represent nature close to the limit of what is possible. Its own gravity is so powerful that it crushes protons and electrons until they fuse, leaving an incredibly dense material.
When a supernova explodes, it spews out most of what the star once was, leaving behind an almost unimaginably dense core. If what remains is more than twice the mass of the Sun, it will collapse into a black hole. The somewhat less massive nuclei have enough gravity to squeeze matter into a state even beyond the extremes of the white dwarfs, leaving only neutrons to survive.
Physicists have in the past considered this to be the densest and most exotic matter possible , but other experts pondered the possibility of something even more extreme, quark matter where subatomic particles that normally comprise electrons, protons, and neutrons behave like an exotic liquid.
Now, a team of Finnish scientists has found pretty solid evidence for the existence of this type of matter, which they have been theorizing about for 40 years. It is a matter outside the laws of physics: the quark matter found inside the largest neutron stars that exist.
Physicists performed new calculations using gravitational wave data first detected from a neutron star collision in August 2017, along with observations of surprisingly massive neutron stars. Their conclusion suggests an exciting result: the nuclei of the most massive neutron stars are so dense that atomic nuclei cease to exist and condense into quark matter.
It is, according to the researchers, an important milestone in understanding the strange insides of these extreme objects.
“Confirming the existence of quark nuclei within neutron stars has been one of the most important goals of neutron star physics since this possibility was first raised about 40 years ago,” says Aleksi Vuorinen of the University of Helsinki and the Helsinki Institute of Physics.
Its existence is very likely
Even with large-scale simulations performed on supercomputers unable to determine the fate of nuclear matter within neutron stars, the physicists came up with a new approach to the problem. They realized that by combining recent findings of theoretical particles and nuclear physics with astrophysical measurements, it might be possible to deduce the characteristics and identity of the matter residing within neutron stars.
They found that the matter within the cores of the most massive stable neutron stars looks much more like quark matter than ordinary nuclear matter.
Uncertainties
Of course, Vuorinen acknowledges that “there is still a small but different probability that all neutron stars are composed only of nuclear matter “, due to some uncertainty about the way sound operates under such extreme conditions. “However, what we have been able to do is quantify what this scenario would require. In short, the behavior of dense nuclear matter should be really peculiar. For example, the speed of sound should reach almost that of light.”
The discovery of quark matter within neutron stars would not only be surprising in and of itself, but could help us learn more about the earliest moments of our universe.
We will have to wait for the new observations that are planned for the coming months and years that will surely be able to further refine the results and point in the right direction regarding quark matter.
Referencia: Eemeli Annala et al, Evidence for quark-matter cores in massive neutron stars, Nature Physics (2020). DOI: 10.1038/s41567-020-0914-9
