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A team of Finnish researchers has found strong evidence of the existence of a type of matter that until now had been merely theoretical. It is “exotic matter of the quark” and is located on the inside of the biggest stars of neutrons that exist. The finding, which has just been published in “Nature Physics”, is produced by combining recent results of studies of particle physics and nuclear measurements of gravitational waves generated, precisely, by this kind of stellar corpses.

All the matter around us consists of atoms, with dense nuclei in their centers made of protons and neutrons.

The atomic nuclei are surrounded by a cloud of electrons of negative charge, and in the vast majority of the materials there is a considerable distance between an atom and the adjacent. But that is just the “normal” state of the art. In extreme conditions, the gravity can reach to make the atoms approach each other to the point of losing their electrons. And even beyond that, even to get the own cores, now naked, coming together to give shape to a subject-matter extremely dense.

One of the most extreme places in the Universe

What is certain is that in the Universe there are few places more extreme than neutron stars. These are genuine “stellar corpses” that are formed when a star which in origin was several times larger than the Sun exhausts its fuel, which will permte continue burning. The energy generated by nuclear fusion in the cores of stars is the only force capable of opposing the gravitational pressure, that is compressed. And when the fuel that keeps the furnace of fusion power ends, nothing can oppose gravity, it begins to crush the star.

at The end, which was a ball of burning gas with a radius of millions of kilometers (the radius of the giant Betelgeuse, for example, is 617 million km) is crushed and reduced to a dense ball of matter super compressed of close to a dozen kilometres. A teaspoon of coffee full of that stuff would weigh more than mount Everest.

An atomic nucleus of tens of kilometers

scientists know that within neutron stars, atomic matter collapses in nuclear matter immensely dense, in which protons and neutrons are so close together that the whole star, or what’s left of it, could be considered as a huge and single atomic nucleus.

Many, however, spent decades wondering if in the depths of the neutron star matter could collapse even more , until a hypothetical and exotic state called “the field of the quark”, in which the own protons and neutrons have been divided into its fundamental pieces (each one is formed by three quarks), forming a soup ultradensa in which not even the particles that formed the atomic nuclei may already exist. And now, a team of researchers from the University of Helsinki says that the answer to that question is a resounding yes .

“Confirm the existence of nuclei, quark within the neutron star – ensures Aleksi Vuorinen , one of the authors of the article – it has been one of the most important goals of the physics of neutron stars since this possibility was raised for the first time about 40 years ago”.

A totally new approach

today, there are computer simulations a large scale capable of determining the fate of nuclear matter within a neutron star. But even so, the Finnish researchers were able to obtain results thanks to an approach to problem completely new. In fact, they realized that by combining recent findings on particle theory and nuclear physics with measurements, observational, it was posibe to deduce the characteristics and the identity of the subject that resides in the heart of the neutron star. Or at least in the more stable and massive.

In them, in effect, an internal matter of the star is much more to the subject of quarks which nuclear matter ordinary. The calculations indicate that in these stars the core diameter identified as a subject of quark may prove to be more than half of the total diameter of the entire neutron star. However, Vuorinen points out that there are still many uncertainties associated to the exact structure of the neutron star. No one, in fact, has been able to see until now, the interior of no.

What does it mean then to claim that he has discovered the field of quark? According to Vuorinen “there is still a possibility, small but not null, of which neutron stars are made up only of nuclear matter normal. However, what we have done is to quantify the conditions that would be necessary for this scenario. In summary, the behavior of nuclear matter dense (but conventional) would have to be truly peculiar. For example, the speed of sound would need to reach almost to the light”.

The message of the gravitational waves

One of the key factors that contributed to the new finding was the appearance of the results of two recent studies of astrophysics observational: the measurement of gravitational waves generated by the merger of two neutron stars and the detection of neutron stars in very massive, with masses near two solar masses.

In the fall of 2017, in effect, the observatories LIGO and Virgo have detected, for the first time, gravitational waves generated by two neutron stars were merging. This observation established an upper limit rigorous for a number called “deformability of tide”, which measures how sensitive it can be the structure of a star into the gravitational field of a companion in orbit. Such a result was later used to obtain an upper limit for the radii of these two stars of neutrons in collisions, which were found to be approximately 13 km

in The same way, and although the first observation of a neutron star dates back to 1967, the precise measurements of their masses have not been possible until the last two decades. And most of the neutron stars whose masses are known with high accuracy within a window that oscillates between 1 and 1.7 solar masses . But the last decade has been witness of three neutron stars that reached, and even exceeded slightly, the two solar masses.

The equation of state of matter

In the new analysis, these astrophysical observations are combined with theoretical results from cutting edge particle and nuclear physics. Which allowed to derive a precise prediction of what is known as ” the equation of state of stellar matter neutron “, which refers to the relationship between its pressure and energy density. An integral component in this process was a result well-known from general relativity, which relates the equation of state with a relationship between the possible values of radii and masses of neutron stars.

Since that first detection of 2017, we have already noticed several mergers of neutron stars, and the LIGO and Virgo have quickly become an essential part of the research of these stellar corpses. And it is precisely this rapid accumulation of new information that has allowed to improve the accuracy of the findings of the Finnish team, confirming the existence of matter-quark within the neutron star.

According to the researchers, the new observations planned for the coming months and years will refine the results even more and eliminate or greatly reduce the current uncertainties.

“There are good reasons to believe -concludes Vuorinen – the golden age of the astrophysics of gravitational wave is barely in its beginning, and that soon we will witness many more breaks like this in our understanding of nature”.