Particle physics experiments

Experiments point the way to solving the mystery that keeps the clusters hot

Galaxies rarely live alone. Instead, tens to thousands are pulled 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 Professor Emeritus Don Lamb, a University of Chicago astrophysicist and co-author of a new paper published March 9, which could point the way to solving a decades-long mystery.

Scientists have long known that hydrogen gas in galaxy clusters is extremely hot – about 10 million degrees Kelvin, or about the same temperature as the center of the sun – which is so hot that hydrogen atoms cannot cannot exist. Instead, the gas is a plasma composed of protons and electrons.

But one enigma persists: there is no simple explanation for why or how the gas stays so hot. According to the normal rules of physics, it should have cooled over the age of the universe. But this is not the case.

The challenge for anyone trying to solve this puzzle is that you can’t exactly create those kinds of powerfully hot, magnetic conditions in your garden.

However, now there is a place on Earth where you can do it: the most energetic laser installation in the world. The National Ignition Facility at Lawrence Livermore National Laboratory is capable of creating such extreme conditions – but only for a tiny fraction of a second in a volume the size of a penny.

Scientists from UChicago, the University of Oxford and the University of Rochester have worked together to use the National Ignition Facility – located in Livermore, California – to create hot gas-like conditions in gigantic galaxy clusters . “The experiments at NIF are literally out of this world,” said Jena Meinecke, who was the paper’s first author.

Scientists focused 196 lasers on a single small target, creating a white-hot plasma with intense magnetic fields that exist for billionths of a second.

It was long enough for them to determine that instead of a uniform temperature, there were hot and cold spots in the plasma.

This agrees with one of the theories that have been proposed for how heat is trapped inside galaxy clusters. Normally, heat would be easily distributed when electrons collide with each other. But the entangled magnetic fields inside the plasma can affect these electrons, causing them to spiral in the direction of the magnetic fields, which can prevent them from evenly distributing and dispersing their energy.

In fact, in the experiment, they saw that energy conduction was suppressed by more than a factor of 100.

“This is an incredibly exciting result because we were able to show that what astrophysicists have come up with is on the right track,” said Lamb, Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy and Astrophysics.

“This is indeed an amazing result,” added study co-author Professor Petros Tzeferacos of the University of Rochester, who supervised computer simulations of the complicated experiment. “The simulations were key to unraveling the physics at play in the turbulent, magnetized plasma, but the level of thermal transport suppression was beyond what we expected.”

The simulations were performed with computer code called the FLASH codes, which was developed at the University of Chicago and is now housed at the Flash Center for Computational Science at the University of Rochester, led by Tzeferacos. The code allows scientists to simulate their laser experiments in exquisite detail before doing them, so they can get the results they seek.

This is essential because scientists only get a few valuable snapshots from the facility – if something goes wrong, there’s no redoing. And because the conditions of the experiment only last a few nanoseconds, scientists need to ensure that they are making the measurements they need at exactly the right time. This means that everything must be precisely plotted well in advance.

“It’s a challenge when you’re at the extremes of what can be done, but that’s where the line is,” Lamb said.

However, other questions remain about the physics of galaxy clusters. Although the hot and cold spots are strong evidence for the impact of magnetic fields on cooling hot gas in galaxy clusters, more experiments are needed to understand exactly what is going on. The group is planning its next set of experiments at NIF later this year.

For now, however, they are happy to have shed some light on why the gas in galaxy clusters is still hot even after billions of years.

Galaxies rarely live alone. Instead, tens to thousands are pulled 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 Professor Emeritus Don Lamb, a University of Chicago astrophysicist and co-author of a new paper published March 9, which could point the way to solving a decades-long mystery.

Scientists have long known that hydrogen gas in galaxy clusters is extremely hot – about 10 million degrees Kelvin, or about the same temperature as the center of the sun – which is so hot that hydrogen atoms cannot cannot exist. Instead, the gas is a plasma composed of protons and electrons.

But one enigma persists: there is no simple explanation for why or how the gas stays so hot. According to the normal rules of physics, it should have cooled over the age of the universe. But this is not the case.

The challenge for anyone trying to solve this puzzle is that you can’t exactly create those kinds of powerfully hot, magnetic conditions in your garden.

However, now there is a place on Earth where you can do it: the most energetic laser installation in the world. The National Ignition Facility at Lawrence Livermore National Laboratory is capable of creating such extreme conditions – but only for a tiny fraction of a second in a volume the size of a penny.

Scientists from UChicago, the University of Oxford and the University of Rochester have worked together to use the National Ignition Facility – located in Livermore, California – to create hot gas-like conditions in gigantic galaxy clusters . “The experiments at NIF are literally out of this world,” said Jena Meinecke, who was the paper’s first author.

Scientists focused 196 lasers on a single small target, creating a white-hot plasma with intense magnetic fields that exist for billionths of a second.

It was long enough for them to determine that instead of a uniform temperature, there were hot and cold spots in the plasma.

This agrees with one of the theories that have been proposed for how heat is trapped inside galaxy clusters. Normally, heat would be easily distributed when electrons collide with each other. But the entangled magnetic fields inside the plasma can affect these electrons, causing them to spiral in the direction of the magnetic fields, which can prevent them from evenly distributing and dispersing their energy.

In fact, in the experiment, they saw that energy conduction was suppressed by more than a factor of 100.

“This is an incredibly exciting result because we were able to show that what astrophysicists have come up with is on the right track,” said Lamb, Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy and Astrophysics.

“This is indeed an amazing result,” added study co-author Professor Petros Tzeferacos of the University of Rochester, who supervised computer simulations of the complicated experiment. “The simulations were key to unraveling the physics at play in the turbulent, magnetized plasma, but the level of thermal transport suppression was beyond what we expected.”

The simulations were performed with computer code called the FLASH codes, which was developed at the University of Chicago and is now housed at the Flash Center for Computational Science at the University of Rochester, led by Tzeferacos. The code allows scientists to simulate their laser experiments in exquisite detail before doing them, so they can get the results they seek.

This is essential because scientists only get a few valuable snapshots from the facility – if something goes wrong, there’s no redoing. And because the conditions of the experiment only last a few nanoseconds, scientists need to ensure that they are making the measurements they need at exactly the right time. This means that everything must be precisely plotted well in advance.

“It’s a challenge when you’re at the extremes of what can be done, but that’s where the line is,” Lamb said.

However, other questions remain about the physics of galaxy clusters. Although the hot and cold spots are strong evidence for the impact of magnetic fields on cooling hot gas in galaxy clusters, more experiments are needed to understand exactly what is going on. The group is planning its next set of experiments at NIF later this year.

For now, however, they are happy to have shed some light on why the gas in galaxy clusters is still hot even after billions of years.

“It’s a reminder that the universe is full of amazing things,” Lamb said.

The experiment’s principal investigator was Professor Gianluca Gregori of the University of Oxford. Team members also included Professor Alexander Schekochihin from Oxford, Archie Bott from Princeton and James Steven Ross from Lawrence Livermore National Laboratory.