Galaxies rarely live alone. Instead, dozens to thousands are drawn together by gravity, forming vast clusters that are the largest objects in the universe.
“Galaxy clusters are one of the most awe-inspiring things in the universe,” said Prof. Emeritus Don Lamb, a University of Chicago astrophysicist and co-author on a new paper published March 9—one that may point the way towards solving a decades-long mystery.
Scientists have long known that the hydrogen gas in galaxy clusters is searingly hot—about 10 million degrees Kelvin, or roughly the same temperature as the center of the sun—which is so hot that hydrogen atoms cannot exist. Instead the gas is a plasma consisting of protons and electrons.
But a puzzle persists: There is no straightforward explanation for why or how the gas stays so hot. According to the normal rules of physics, it should have cooled within the age of the universe. But it hasn’t.
The challenge for anyone trying to solve this puzzle is that you can’t exactly create these kinds of powerfully hot and magnetic conditions in your backyard.
However, there is now one place on Earth where you can: the most energetic laser facility in the world. The National Ignition Facility at Lawrence Livermore National Laboratory is able to create such extreme conditions—though only for a tiny fraction of a second in a volume the size of a dime.
Scientists from UChicago, the University of Oxford, and the University of Rochester worked together to use the National Ignition Facility—located in Livermore, California—to create conditions similar to the hot gas in gigantic galaxy clusters. “The experiments conducted at the NIF are literally out of this world,” said Jena Meinecke, who was the first author on the paper.
The scientists focused 196 lasers onto a single tiny target, creating a white-hot plasma with intense magnetic fields that exists for a few billionths of a second.
This was long enough for them to determine that instead of a uniform temperature, there were hot and cold spots in the plasma.
This dovetails with one of the theories that has been proposed for how heat is trapped inside galaxy clusters. Normally, heat would be easily distributed as electrons collide with each other. But the tangled magnetic fields inside the plasma can affect these electrons, causing them to spiral along the direction of magnetic fields—which can prevent them from evenly distributing and dispersing their energy.
In fact, in the experiment they saw that the conduction of energy was suppressed by more than a factor of 100.
“This is an incredibly exciting result because we’ve been able to show that what astrophysicists have proposed is on the right track,” said Lamb, the Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy and Astrophysics.