When it comes to “destroying” cosmic ghosts, perhaps only the most extreme objects in the universe are up to the task: neutron stars.
Scientists have simulated collisions between these ultradense, dead stars, showing that such powerful events could briefly “trap” neutrinos, otherwise known as “ghost particles.” The discovery could help scientists better understand neutron star mergers as a whole, which are events that create environments turbulent enough to forge elements heavier than iron. Such elements cannot even be created in the hearts of the stars, and that includes the gold on your finger and the silver around your neck.
Neutrinos are considered the “ghosts” of the particle zoo due to their lack of charge and incredibly low mass. These characteristics mean that they very rarely interact with matter. To put this into perspective, as you read this sentence, over 100 trillion neutrinos are passing through your body at close to the speed of light, and you feel nothing.
These new neutron star merger simulations were carried out by physicists at Penn State University and ultimately showed that the point where these dead stars meet (the interface) becomes incredibly hot and dense. In fact, it becomes extreme enough to trap a number of these “cosmic ghosts.”
At least for a short while, anyway.
Despite their lack of interaction with matter, the neutrinos created during the collision would be trapped at this neutron star merger interface and become much hotter than the relatively cold cores of the colliding dead stars.
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This is what we call neutrinos “out of thermal equilibrium” with the cold cores of neutron stars. During this hot phase, which lasts about two to three milliseconds, the team’s simulations indicated that neutrinos can interact with the matter of molten neutron stars, helping to restore thermal equilibrium.
“Neutron stars before they merge are indeed cold. Although they can reach billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” David Radice, team leader and assistant professor of physics, astronomy and astrophysics in Penn State’s Eberly College of Science, said in a statement. “When they collide, they can get very hot. The interface of colliding stars can be heated to temperatures of several trillion degrees Kelvin. However, they are so dense that photons cannot escape to dissipate the heat; we think they cool by emitting neutrinos.”
Setting Cosmic Ghost Traps
Neutron stars are born when a massive star, whose mass is at least eight times that of the Sun, no longer has the fuel in its core, necessary for nuclear fusion. Once this fuel is exhausted, the star can no longer resist the push of its own gravity.
This triggers a series of core collapses that trigger the fusion of heavier elements, which then provide even heavier elements. This chain ends when the core of the dying star is filled with iron, the heaviest element that can be forged in the core of even the most massive stars. Then gravitational collapse occurs again, triggering a supernova explosion that carries away the star’s outer layers and most of its mass.
Instead of creating new elements, this final collapse of the core creates an entirely new state of matter unique to the interior of neutron stars. Negative electrons and positive protons are forced together, creating an ultradense soup of neutrons, which are neutral particles. An aspect of quantum physics called “degeneracy pressure” prevents these neutron-rich nuclei from collapsing further, although this can be overcome by stars whose mass is large enough to collapse completely, giving rise to black holes.
The result of this series of collapses is a dense dead star, or neutron star, with a mass between one and two times that of the original star, packed together to a width of about 20 kilometers. As a reminder, the material that makes up neutron stars is so dense that if a tablespoon were brought to Earth, it would weigh about as much as Mount Everest. Maybe more.
However, these extreme stars do not always live (or die) in isolation. Some binary star systems contain two stars massive enough to give rise to neutron stars. As these binary neutron stars orbit each other, they emit ripples in the very fabric of space and time called gravitational waves.
As these gravitational waves impact the neutron binary stars, they carry angular momentum with them. This causes the binary system to lose orbital energy and causes the neutron stars to move closer together. The closer they get, the faster they emit gravitational waves and the faster their orbits tighten. Eventually, the gravity of the neutron stars takes over and the dead stars collide and merge.
This collision creates “sprays” of neutrons, enriching the environment around the fusion with free versions of these particles. These can be “captured” by atoms of elements in that environment in a phenomenon called the “rapid capture process” (r-process). This creates superheavy elements that undergo radioactive decay to create lighter elements that are still heavier than iron. Think gold, silver, platinum, and uranium. The decay of these elements also creates a burst of light that astronomers call a “kilonova.”
The first moments of neutron star collisions
Neutrinos are also created during the first moments of a neutron star merger, the team explains, when neutrons are split apart, creating electrons and protons. The researchers wanted to know what might be happening during those first moments. To get answers, they created simulations that use massive amounts of computing power to model binary neutron star mergers and the physics associated with such events.
The Penn State team’s simulations revealed for the first time that, for a brief moment, the heat and density generated by a neutron star collision are enough to trap even neutrinos, which under all other circumstances have earned their ghostly nickname.
“These extreme events push the boundaries of our understanding of physics, and studying them allows us to learn new things,” Radice added. “The period during which molten stars are out of equilibrium is only two to three milliseconds, but like temperature, time here is relative; the orbital period of the two stars before the merger can be as short as a millisecond. »
“This brief out-of-equilibrium phase is where the most interesting physics occurs. Once the system returns to equilibrium, the physics is better understood.”
The team believes that the precise physical interactions that occur during neutron star mergers could influence the light signals from these powerful events that could be observed on Earth.
“How neutrinos interact with stellar matter and are ultimately emitted can impact the oscillations of the merged remnants of the two stars, which in turn can impact how the electromagnetic and gravitational wave signals from the merger appear when they reach us here on Earth,” team member Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, said in the statement. “Next-generation gravitational wave detectors could be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role, allowing us to gain insight into these extreme events while informing future experiments and observations in a way that provides a feedback loop.”
“There is no way to reproduce these events in the laboratory to study them experimentally. The best way to understand what happens during a binary neutron star merger is therefore to carry out simulations based on mathematics from Einstein’s theory of general relativity.”
The team’s research was published May 20 in the journal Physical Reviews Letters.
Originally published on Espace.com.