by When it comes to “breaking” Cosmic ghosts, only the most extreme things in the universe can be up to the task: neutron stars.
Scientists have carried out simulations of collisions between these ultradense and dead stars, showing that such powerful events can “trap” short 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 create elements heavier than iron. Such elements cannot be created even in the hearts of the stars – and this includes the gold on your finger and the silver around your neck.
Due to their lack of charge and extremely small mass, neutrinos are considered the “ghosts” of the particle zoo. These characteristics mean that they rarely interact with matter. To put that into perspective, as you read this sentence, there are more than 100 trillion neutrinos streaming through your body at near-light speed, and you can’t feel a thing.
Penn State University physicists performed these new simulations of neutron star mergers, and finally showed that the point where these dead stars meet (the interface) becomes extremely hot and dense. In fact, it becomes big enough to ensnare a bunch of “cosmic ghosts.”
At least for a while, anyway.
Despite their lack of interaction with matter, neutrinos created in the collision would become trapped at that neutron-star merger interface and become much hotter than the relatively cold cores of the colliding dead stars.
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This is referred to as the neutrinos being “out of thermal equilibrium” with the cold neutron star cores. During this hot phase, which lasts about two to three milliseconds, the team’s simulations showed that neutrinos can interact with the material of a merging neutron star, helping to restore thermal equilibrium.
“Neutron stars before the merger are effectively cold. Although they may be billions of degrees, Kelvin, their incredible density means that this heat contributes significantly to the energy of the system,” team leader David Radice, assistant professor of physics , astronomy. and astrophysics in the Eberly College of Science at Penn State, said in a statement. “When they collide, they can become very hot. The interface of colliding stars can heat up to temperatures in the trillions of degrees Kelvin. However, they are so dense that photons cannot escape to dissipate the heat; instead, we think they have cooled down by emitting neutrinos.”
Setting Cosmic ghost traps
Neutron stars are born when a massive star with at least eight times the mass of the sun runs out of the fuel needed for nuclear fusion at its core. After that fuel supply ends, the star can no longer support itself against the inward push of its own gravity.
This initiates a series of cores that initiate the fusion of the heavier elements, which then become even heavier elements. This chain ends when the core of a dying star is filled with iron, the heaviest element that can be forged in the core of even the largest stars. Then, the gravitational collapse occurs again, triggering a supernova explosion that blows away the star’s outer layers and most of its mass.
Instead of creating new elements, this final core assembly creates a 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 known as “degeneracy pressure” prevents these neutron-rich cores from collapsing any further, although stars can overcome this within enough mass that they collapse completely – until birth. on black holes.
The result of this series of collapses is a dead compact star, or neutron star, that has between two and two times the mass of the parent star — and is about 12 miles (20 kilometers) across. For context, the material that comprises neutron stars is so dense that if a tablespoon of it were brought to Earth, it would be the size of Mount Everest. Maybe more.
These big stars don’t always live (or die) alone, however. Some binary star systems contain two stars massive enough to give birth to neutron stars. As these binary neutron stars orbit each other, they release ripples in the very fabric of space and time called gravitational waves.
As these gravitational waves exit neutron star binaries, they send angular momentum with them. This results in a loss of orbital energy in the binary system and fuses the neutron stars together. The closer they orbit, the faster they emit gravitational waves — and the faster their orbits become tighter. Eventually, the neutron stars’ gravity takes over, and the dead stars collide and merge.
This collision creates a “spray” of neutrons, which enriches the environment around the fusion with free versions of these particles. These can be “grabbed” by the atoms of the elements in this environment during a phenomenon called called the “quick capture process” (e-process) . This creates heavier elements that undergo radioactive decay to form lighter elements that are still heavier than iron. Think gold, silver, platinum, and uranium. The decay of these elements also creates bursts of light astronomers call “kilonova.”
The first moments of neutron star collisions
Neutrons are also created during the first moments of a neutron star merger as neutrons are ripped apart, the team says, creating electrons and protons. And the researchers wanted to know what could be happening during these initial moments. To find some answers, they created simulations that use enormous amounts of computing power to model binary neutron star mergers and the physics involved in such events.
The Penn State team’s simulations showed for the first time, for a brief moment, that the heat and density generated by a collision with a neutron star is enough to capture even neutrinos, which in all other cases deserve their ghostly nicknames.
“These extreme events stretch the limits of our understanding of physics, and studying them allows us to learn new things,” said Radice. “The period in which the merging stars are out of equilibrium is only two or three milliseconds, but like temperature, time is relative here; the orbital period of the two stars before the merger can be as little as one millisecond.
“This short step outside of equilibrium is where the most interesting physics happens. When the system returns to equilibrium, the physics is better understood.”
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The team thinks 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 the neutrinos interact with the star’s material and are ultimately emitted can affect the oscillations of the merger remnants of the two stars, which can affect the appearance of the wave signals electromagnetic and gravitational forces of the merger 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 vital role, allowing us to gain insight into these extreme events while informing future experiments and observations in a kind of feedback loop.
“There is no way to reproduce these events in a laboratory to study them experimentally, so our best window into understanding what happens during binary neutron star mergers is through mathematically based simulations which derives from Einstein’s theory of general relativity.”
The team’s research was published on 20 May in the journal Physical Reviews Letters.
