Magnetic field lines tangled like spaghetti in a bowl, as found in black holes, might be behind the most powerful particle accelerators in the universe. That’s the result of a new computational study by researchers from the Department of Energy’s SLAC National Accelerator Laboratory, which simulated particle emissions from distant active galaxies. SLAC scientists have found a new way to explain how these black hole plasma jets boost particles to the highest energies observed in the universe. The results could prove useful for fusion and accelerator research on Earth.
At the core of these active galaxies, supermassive black holes launch high-speed jets of plasma – a hot, ionized gas – that shoot millions of light years into space. This process may be the source of cosmic rays with energies tens of millions of times higher than the energy unleashed in the most powerful manmade particle accelerator.
“The mechanism that creates these extreme particle energies isn’t known yet,” said SLAC staff scientist Frederico Fiuza, the principal investigator of the new study. “But based on our simulations, we’re able to propose a new mechanism that can potentially explain how these cosmic particle accelerators work.”
The results could also have implications for plasma and nuclear fusion research and the development of novel high-energy particle accelerators.
Simulating cosmic jets
Researchers have long been fascinated by the violent processes that boost the energy of cosmic particles. For example, they’ve gathered evidence that shock waves from powerful star explosions could bring particles up to speed and send them across the universe.
Scientists have also suggested that the main driving force for cosmic plasma jets could be magnetic energy released when magnetic field lines in plasmas break and reconnect in a different way – a process known as “magnetic reconnection.”
However, the new study suggests a different mechanism that’s tied to the disruption of the helical magnetic field generated by the supermassive black hole spinning at the center of active galaxies.
“We knew that these fields can become unstable,” said lead author Paulo Alves, a research associate working with Fiuza. “But what exactly happens when the magnetic fields become distorted, and could this process explain how particles gain tremendous energy in these jets? That’s what we wanted to find out in our study.”
To do so, the researchers simulated the motions of up to 550 billion particles – a miniature version of a cosmic jet – on the Mira supercomputer at the Argonne Leadership Computing Facility (ALCF) at DOE’s Argonne National Laboratory. Then, they scaled up their results to cosmic dimensions and compared them to astrophysical observations.
From tangled field lines to high-energy particles
The simulations showed that when the helical magnetic field is strongly distorted, the magnetic field lines become highly tangled and a large electric field is produced inside the jet. This arrangement of electric and magnetic fields can, indeed, efficiently accelerate electrons and protons to extreme energies. While high-energy electrons radiate their energy away in the form of X-rays and gamma rays, protons can escape the jet into space and reach the Earth’s atmosphere as cosmic radiation.
“We see that a large portion of the magnetic energy released in the process goes into high-energy particles, and the acceleration mechanism can explain both the high-energy radiation coming from active galaxies and the highest cosmic-ray energies observed,” Alves said.
Roger Blandford, an expert in black hole physics and former director of the SLAC/Stanford University Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), who was not involved in the study, said, “This careful analysis identifies many surprising details of what happens under conditions thought to be present in distant jets, and may help explain some remarkable astrophysical observations.”
Next, the researchers want to connect their work even more firmly with actual observations, for example by studying what makes the radiation from cosmic jets vary rapidly over time. They also intend to do lab research to determine if the same mechanism proposed in this study could also cause disruptions and particle acceleration in fusion plasmas.
This work was also co-authored by Jonathan Zrake, a former Kavli Fellow at KIPAC, who is now at Columbia University. The project was supported by the DOE Office of Science through its Early Career Research Program and an ALCC award for simulations on the Mira high-performance computer. ALCF is a DOE Office of Science user facility.
SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science.
Black Holes Have High-Speed Jets – But Do They Also Have a Back Door?
One of the biggest problems when studying black holes is that the laws of physics as we know them cease to apply. The conventional wisdom is that in a black hole, large quantities of matter and energy concentrate in an infinitely small space (known as a gravitational singularity), space-time curves towards infinity, and all matter is destroyed... Or is it?
New research at the Institute of Corpuscular Physics in Valencia, Spain suggests that if the singularity is treated as an imperfection in the geometric structure of space-time, matter may indeed survive its foray into the black hole and come out the other side -- And by doing so, resolve the problem of the infinite, space deforming, gravitational pull.
"Black holes are a theoretical laboratory for trying out new ideas about gravity," says Gonzalo Olmo, a researcher at the Universitat de Valencia (UV) in Spain. Together with Diego Rubiera from the University of Lisbon in Portugal and Antonio Sanchez, a PhD student also at the UV, Olmo analyzes black holes using theories besides General Relativity.
Specifically, in this work he has applied geometric structures similar to those of a crystal or graphene layer, not typically used to describe black holes, since these geometries appear to better match what happens inside a black hole. "Just as crystals have imperfections in their microscopic structure, the central region of a black hole can be interpreted as an anomaly in space-time, which requires new geometric elements in order to be able to describe them more precisely. We explored all possible options, taking inspiration from facts observed in nature."
Using these new geometries, the researchers obtained a description of black holes whereby the center point becomes a very small spherical surface. This surface is interpreted as the existence of a wormhole within the black hole.
"Our theory naturally resolves several problems in the interpretation of electrically charged black holes," Olmo explains. "In the first instance we resolve the problem of the singularity, since there is a door at the center of the black hole, the wormhole, through which space and time can continue."
This study is based on one of the simplest known types of black hole: rotationless and electrically charged. The wormhole predicted by the equations is smaller than an atomic nucleus, but gets bigger as the charge stored in the black hole increases. So, a hypothetical traveler entering a black hole of this kind would be stretched to the extreme, or "spaghettified," and would be able to enter and slide through the wormhole. Upon exiting they would be compacted back to their normal size.
Seen from outside, these forces of stretching and compaction would seem infinite, but the traveler himself, living it first hand, would experience only extremely intense, but not infinite, forces. It is unlikely that the star of the movie "Interstellar" would survive a journey like this, but the model proposed by the researchers posits that matter would not be lost inside the singularity, but rather would be expelled out the other side through the wormhole at its center to another region of the universe.
Another problem that this interpretation resolves, according to Olmo, is the need to use exotic energy sources to generate wormholes. In Einstein's theory of gravity, these "doors" only appear in the presence of matter with unusual properties (for example, a negative energy pressure or density), something which has never been observed. According to Olmo, "In our theory, the wormhole appears out of ordinary matter and energy, such as an electric field."
The interest in wormholes for theoretical physics goes beyond generating tunnels or doors in space-time to connect two points in the Universe. They also help explain phenomena such as quantum entanglement or the nature of elementary particles. Thanks to this new interpretation, the existence of these objects could be closer to science than fiction.