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Initially, a simulation of a uniform beam of electrons and positrons interacting with a plasma. As the beam passes through the background plasma, the positrons (red) are focused and the electrons (blue) are ejected to form the surrounding cloud. This illustrates the physics behind the “current filament instability,” which is believed to play a key role in the propagation and dynamics of cosmic jets. The simulation was performed using the OSIRIS Particle-in-Cell code and is one of the largest ever performed for such a beam-plasma interaction. | Authors: Pablo J. Bilbao and Luís O. Silva (GoLP, Instituto Superior Tecnico, Lisbon and University of Oxford).
In the first experiment of its kind, scientists have recreated “cosmic fireballs” here on Earth in a particle accelerator. The experiment aimed to study the stability of jets of high-temperature gas or plasma blasted to Earth by supermassive black hole-powered galactic engines called blazars. This in turn could solve the mystery of hidden magnetic fields and missing high-energy gamma rays.
Scientists from the University of Oxford and the Science and Technology Infrastructure Council’s (STFC) Central Laser Facility (CLF) have teamed up and turned to CERN’s HiRadMat (High Radiation to Matter) facility to create electron-positron pairs. Then they blew them up matter-antimatter pairs of matches across 3.3 feet (1 meter) of plasma recreate the conditions in the power currents supermassive black holes known as blazers. This allowed them to simulate the most extreme physics in the universe.
“These experiments show how laboratory astrophysics can test theories of the high-energy universe,” team member and University of Strathclyde researcher Bob Bingham said in a statement. “By recreating relativistic plasma conditions in the laboratory, we can measure the processes that shape the evolution of cosmic jets and better understand the origin of magnetic fields in intergalactic space.”
What kind of flames?
Blazars are a subset of active galactic nuclei (AGN), the central regions of galaxies dominated by monstrously feeding supermassive black holes millions or even billions of times the mass of the Sun. These cosmic titans are surrounded by swirling flat clouds of gas and dust called accretion disks, which shine brightly due to the friction caused by the massive gravitational pull of the central black hole.
These accretion disks gradually drop material into the brain of the black hole, but not all of the material surrounding the black hole is consumed. Powerful magnetic fields drive some of the material toward the black hole’s poles, where it accelerates to nearly the speed of light and explodes as two collimated jets of plasma. A blazar is the name given to AFNs that direct one of these jets of plasma directly at Earth. These jets emit intense gamma rays that can be detected here on Earth with ground-based telescopes. But something is missing.
As these gamma rays blast through intergalactic space, they scatter photons in the background light of stars, creating matter in the form of electrons and antimatter in the form of positrons. These matter and antimatter pairs should scattering from the cosmic fossil radiation field that fills the cosmos everywhere, called “cosmic microwave background” or “CMB”, which is the remnant of an event that occurred shortly after the Big Bang.
This scattering should produce lower-energy gamma rays that can be captured by space-based gamma-ray telescopes such as the Fermi spacecraft. However, detection of such low-energy gamma rays has so far eluded these instruments.
Help! Our gamma rays are missing!
There are several theories as to why low energy gamma rays can be “missing”. One idea suggests that electron-positron pairs are deflected by weak intergalactic magnetic fields and that this deflects low-energy gamma rays out of our line of sight. Another suggestion is that these matter-antimatter pairs become unstable as they travel through the ultra-rare material scattered between galaxies. This can cause small fluctuations in the current in these jets, which create magnetic fields that cause further instability. The net result would be a dissipation of the beam’s energy. Another possibility is that there is a relict magnetic field between galaxies, left over from the early universe, that interferes with low-energy gamma rays.
By testing these first two concepts, the research team achieved some very enlightening and surprising results. The team expected the beam to spread out and be disrupted. But what they actually observed was a beam that maintained its narrow shape with little perturbation and no perturbations that create magnetic fields. This means that the instability of the plasma beam is too weak to explain the missing low-energy gamma rays. This could then support the idea of a relict magnetic field existing in the intergalactic medium, the matter that drifts between galaxies.
The Fireball experiment installed in the HiRadMat irradiation area. | Credit: Gianluca Gregori.
The findings raise additional questions. First of all, since the early universe was very uniform, it is not known how such a relic could have been seeded in the primordial cosmos. Answering this conundrum may require looking to physics outside the Standard Model, perhaps with future instruments such as the Cherenkov Telescope Array Observatory (CTAO).
“It was great fun to be part of such a ground-breaking experiment that adds a new dimension to research at CERN – we hope that our impressive result will stimulate the plasma astrophysics community’s interest in exploring fundamental cosmic questions in a ground-based high-energy physics laboratory,” said Subir Sarkar, a scientist at the University of Oxford.
The team’s research was published Monday (Nov. 3) in the journal PNAS.