Source: cnBeta
The Milky Way rarely exists alone. Instead, dozens to thousands of Galaxies will be gravitationally drawn together to form a massive galaxy cluster that becomes the largest celestial body in the universe. "Galaxy clusters are one of the most awesome things in the universe," said Professor Don Lam, an astrophysicist at the University of Chicago and co-author of a new paper published in Science on March 9, 2022.
The paper could point the way to unlocking a decades-long mystery.
Scientists have long known that the temperature of hydrogen in the galaxy cluster is frighteningly high — about 10 million kelvin degrees, or about the same temperature as the center of the sun, and that hydrogen atoms can't exist under such high temperatures. Instead, these gases are plasmas made up of protons and electrons.
But a conundrum remains. There is no direct explanation for why or how the gas is kept at such a high temperature. According to normal rules of physics, it should cool down within the age of the universe. But it didn't.
The challenge for anyone trying to solve this conundrum is that you can't fully create this powerful thermal and magnetic condition in your backyard.
But now there's a place on Earth where you can do it: the world's most energetic laser facility. The National Ignition Facility at lawrence Livermore National Laboratory was able to create such extreme conditions — albeit only for a fraction of the time in a dime-sized volume.
Scientists from the University of Chicago, the University of Oxford and the University of Rochester collaborated to create conditions similar to hot gas in massive galaxy clusters by utilizing a national ignition facility in Livermore, California. Jena Meinecke, the paper's first author, lamented: "The experiments conducted at the National Ignition Facility were simply out of the ordinary. ”
The scientists focused 196 lasers on a tiny target and created a white-hot plasma with a strong magnetic field — a billionth of a second in its existence.
This was long enough for them to determine that there was no uniform temperature in the plasma, but rather hot and cold spots.
This doesn't dovetail with one of the theories about how heat is trapped in galaxy clusters. Normally, heat is easily distributed as electrons collide with each other. But the tangled magnetic field inside the plasma affects these electrons, causing them to rotate in the direction of the magnetic field — which may prevent them from evenly distributing and dispersing their energy.
In fact, the researchers saw in experiments that the conduction of energy was suppressed more than 100 times.
"This is an incredible result because we have been able to prove that the astrophysicist's proposal is on the right track," Lamb said.
Study co-author Petros Tzeferacos, a professor from the University of Rochester, added: "This is truly an amazing result. Simulation is key to unlocking the physics that play a role in turbulent, magnetized plasma, but the level of heat transfer inhibition exceeds our expectations. ”
The simulations were done using a type of computer code called FLASH code, which was developed by the University of Chicago and is now hosted by the Flash Center for Computational Sciences at the University of Rochester, led by Tzeferacos. The code allows scientists to delicately simulate laser experiments before doing them so that they can achieve the results they seek.
This is critical because scientists have only a few precious opportunities — and if something goes wrong, there's no chance to redo it. And since the experimental conditions last only a few nanoseconds, scientists must ensure that they make the required measurements at the right time. This means that everything has to be precisely planned a long time in advance.
"It's a challenge when you're at very extremes where you can do it, but that's the frontier," Lamb said.
However, there are more questions about the physics of galaxy clusters. Although hot and cold spots are conclusive evidence of the effect of magnetic fields on the cooling of hot gases in galaxy clusters, further experiments are needed to understand exactly what is going on. The team is planning the next round of experiments at NIF later this year.
For now, though, they're happy to have revealed why the gas in the galaxy cluster is still hot even after billions of years.
"It reminds us that the universe is full of magical things," Lamb said. ”