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Neutron stars generate matter 10 billion times harder than metal

– News of September 25, 2018 –

Neutron stars are the remains of gigantic explosions that mark the end of life of massive stars. These explosions, the supernovas, are triggered by the gravitational collapse of the heart of these stars. This generates incredibly dense small bodies, the neutron stars. Neutron stars are surprising stars for several reasons. If they were a bit denser, they would collapse to create black holes.

One cubic centimeter taken from the surface of a neutron star would have a mass of several hundred million tons. This extraordinary density brings matter to states that are not well understood. The more you plunge into the heart of a neutron star, the denser the matter becomes, the more difficult it becomes to model it. It is nevertheless important to understand these states of matter that could for example inform us about the gravitational signature of a neutron star, a very interesting data for the recent science of the observation of gravitational waves.

A trio of researchers have tried to simulate what is happening just below the crust of a neutron star, and their results are very surprising. They tried to calculate how 983 thousand protons and 2.3 million neutrons would behave in a very high density environment. It sounds simple but it would take about 2 million computing hours for a computer to do this simulation. Indeed, each particle interacts permanently with all those around it. Previous simulations have already led to the hypothesis that under these conditions, the material creates strange structures, a model that today bears the name of nuclear pasta.

The degenerate matter of a neutron star would thus spontaneously be organized into balls, tubes or leaves, depending on the depth. Researchers have naturally decided to name these states gnocchi, spaghetti or lasagna. These are the nuclear pasta. The new study goes even further in their simulations. The American-Canadian trio sought to determine the breaking point of these nuclear pasta. They wanted to know if the material at depth is denser and more rigid than the material at the surface. And the answer is yes. These nuclear pasta are very likely the hardest material in the universe. The hardness of this material would be about 10 billion times that of the metal.

This result also means that neutron stars could emit gravitational waves continuously. The creation of nuclear pasta would be irregular under the crust of these stars, and neutron stars are known for their rapid rotation. But an enormous irregular mass in rapid rotation also means the presence of gravitational waves. The nuclear pasta model would be difficult to verify. It is unthinkable to hope to recreate them in a laboratory because the necessary pressure greatly exceeds the capacities of the human genius. In reality, these structures can not exist outside a neutron star. Perhaps a fairly precise gravitational wave observatory could show that neutron stars do emit gravitational waves, but our detectors still have some progress to make before allowing this kind of observation.

Image by NASA / CXC / ASU / J. Hester et al.

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