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SYFY WIRE galaxy clusters

Some 200 lasers recreated the heat and magnetism of a monster galaxy, right here on Earth

Something about galaxy clusters almost broke the laws of physics. Almost.

By Elizabeth Rayne
Liz Coma Galaxy NASA

Plasma in ginormous galaxy clusters can get beyond scorching, which seems like nothing uncommon in space — except the reason it has no chill was unknown until now.

Galactic clusters are full of hydrogen gas about as hot as the guts of the Sun. Meaning, temperatures are too high for hydrogen atoms to form, so the hydrogen exists as a superhot plasma of electrons and protons. Previously, it was nearly impossible to figure out why gobs of plasma have stayed this hot for so long. Physics says the entire universe expanded and cooled from its inception. Shouldn’t all that plasma have cooled with it?

To find out why, physicist Gianluca Gregori of Oxford University and his research team needed to blast heat on the question. Using the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory, they aimed 196 lasers at a tiny target to recreate the conditions in the center of a galaxy cluster for only a fraction of a second, but that was enough to reveal something. Gregori led a study that was recently uploaded to the preprint server arXiv.

“Lasers going as fast as possible and smashing into each other created the turbulent plasma conditions that we were looking for,” he told SYFY WIRE. “The main point is not that there are hot and cold spots in galaxy clusters, but that they are still there after such a long time.”

The thing about electrons is that they like to travel along a magnetic field line, but refuse to move to another one. This has to do with thermal conduction. Think of a sword being forged in any fantasy movie or video game. The tip closest to the fire seems to glow orange because it is so hot, and the heat of the flame excites electrons in that metal, which begin to vibrate. More and more of the blade turns orange as those vibrating electrons perturb electrons higher up, which explains why blacksmiths — elven, orcish, or human — have to wear such thick gloves.

Now imagine that blade, which is being held horizontally, has cooled and there is another already hot blade positioned perpendicular to it (which makes no sense in a fantasy realm but this is for science). Say the previous blade is one magnetic field and the new blade is another. The electrons in the hot new blade don’t want to go anywhere else. They only keep vibrating within that sword, which means they don’t perturb electrons in the cool sword, so it doesn’t heat up. This explains why superhot plasma has stayed trapped in galaxy clusters for eons.

“If magnetic fields are all tangled up, the electrons cannot easily conduct heat and regions that were initially hot will stay hot,” said Gregori. “When we carried out the experiment, we did not expect magnetic fields to play such an important role in thermal conduction.”

The bombardment of lasers created plasma searing enough to recreate plasma with powerful magnetic fields, even if it was a dime-sized volume that only held for billionths of a second. It was a flash of revelation. Wherever magnetic fields were seriously messed up in galaxy clusters, there were regions where plasma stayed as hot as it is since the universe awoke, but there are still more charged particles where that came from, and they come in the form of cosmic rays. Gregori and his team want to find out more about how they interact with the plasma.

“Cosmic rays are affected by turbulent magnetic fields,” he said. “How they propagate and exchange energy with the bulk of the plasma is an open question we would like to explore.”

So another mystery of the universe has been solved. Now if only someone could figure out how the cosmos really came into being before it spewed out the stuff that created galaxy clusters and everything else, that would possibly answer things we might have not even imagined yet.