When it comes to “eradicating” cosmic ghosts, perhaps only the most extreme objects in the universe are up to the task: neutron stars.
Scientists have run simulations of collisions between these ultra-dense and dead stars, showing that such powerful events may be able to briefly ‘capture’ neutrinos, also known as ‘ghost particles’. The discovery could help scientists better understand neutron star mergers as a whole; these are events that create environments turbulent enough to forge elements heavier than iron. Such elements cannot even be created in the hearts of stars – and this 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 small mass. These characteristics ensure that they interact with matter very rarely. To put that in perspective, as you read this sentence, more than 100 trillion neutrinos are flowing through your body at the speed of almost light, and you feel nothing.
These new simulations of neutron star mergers were conducted 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 gets extreme enough to ensnare a bunch of those “cosmic ghosts.”
At least for a short while.
Despite their lack of interaction with matter, neutrinos created in the collision would become trapped at the interface between neutron stars and become much hotter than the relatively cold hearts of the colliding dead stars.
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This is also called the neutrinos being “out of thermal equilibrium” with the nuclei of cold neutron stars. During this hot phase, which lasts about two to three milliseconds, the team’s simulations indicated that neutrinos can interact with merging neutron star matter, restoring thermal equilibrium.
“Neutron stars are actually cold before merger. Although they can be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” said team leader David Radice, assistant professor of physics and astronomy. and astrophysics at Penn State’s Eberly College of Science, a statement said. “As they collide, they can get very hot. The interface of the colliding stars can be heated 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 cool down by emitting neutrinos.”
Cosmic ghost traps set
Neutron stars are born when a massive star with at least eight times the mass of the Sun at its core runs out of fuel needed for nuclear fusion. After the fuel supply ends, the star can no longer support itself against the inward pressure of its own gravity.
This sets off a series of core collapses that cause the merger of heavier elements, which then form even heavier elements. This chain ends when the dying star’s heart is filled with iron, the heaviest element that can be forged in the core of even the heaviest stars. Then the gravitational collapse occurs again, causing a supernova explosion that blows away the star’s outer layers and most of its mass.
Rather than forging new elements, this eventual core collapse forges an entirely new state of matter unique to the interiors of neutron stars. Negative electrons and positive protons are squeezed together, creating an ultra-dense soup of neutrons, which are neutral particles. An aspect of quantum physics called ‘degeneration pressure’ prevents these neutron-rich nuclei from collapsing further, although this can be overcome by stars of sufficient mass collapsing completely – creating black holes.
The result of this series of collapses is a dense dead star, or neutron star, with one to two times the mass of the original star – crammed into a width of about 20 kilometers. For context, the matter that makes up neutron stars is so dense that if a tablespoon of it were brought to Earth, it would weigh about as much as Mount Everest. Maybe more.
However, these extreme stars don’t always live (or die) in isolation. Some binary star systems contain two stars large enough to produce neutron stars. As these binary neutron stars orbit each other, they emit ripples in the fabric of space and time called gravitational waves.
As these gravitational waves bounce off neutron star binaries, they carry angular momentum with them. This results in the loss of orbital energy in the binary star system and causes the neutron stars to gravitate towards each other. The closer they get, the faster they emit gravitational waves – and the faster their orbits continue to narrow. Eventually, the neutron stars’ gravity takes over and the dead stars collide and merge.
This collision creates “sprays” of neutrons, enriching the environment around the merger with free versions of these particles. These can be “grabbed” by the atoms of elements in this environment during 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 of gold, silver, platinum and uranium. The decay of these elements also causes an explosion of light that astronomers call a ‘kilonova’.
The first moments of collisions between neutron stars
Neutrinos also form during the first moments of neutron star mergers, where neutrons are torn apart, the team says, creating electrons and protons. And the researchers wanted to know what might happen during these first moments. To gather some answers, they created simulations that use an enormous amount of computing power to model the merger of binary neutron stars and the physics associated with such events.
The Penn State team’s simulations show for the first time that the heat and density generated by a collision with a neutron star are sufficient for a brief moment to trap even neutrinos, which under all other conditions have earned spooky nicknames.
“These extreme events push the boundaries of our understanding of physics, and by studying them we can learn new things,” Radice added. ‘The period during which the merging stars are out of equilibrium is only two to three milliseconds, but as with temperature, time here is relative; the orbital period of the two stars before the merger may be as little as one millisecond.
“It is in this short out-of-equilibrium phase that the most interesting physics takes place. Once the system is back in equilibrium, the physics is better understood.”
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The team thinks that the precise physical interactions that take place during neutron star mergers could influence the light signals from these powerful events that can be observed on Earth.
‘How the neutrinos interact with the stars’ matter and are ultimately emitted could affect the oscillations of the merged remnants of the two stars, which in turn could affect what the electromagnetic and gravitational wave signals from the merger look like as 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 types of signal differences. In this way, these simulations play a crucial role, allowing us to gain insight into these extreme events while also informing future experiments and observations in some kind of way. feedback.
“There is no way to reproduce these events in a laboratory to study them experimentally, so the best window we have to understand what happens during binary neutron star mergers is through simulations based on mathematics emerging from Einstein’s general relativity.”
The team’s research was published May 20 in the journal Physical Reviews Letters.