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The ALICE detector at the Large Hadron Collider.

A picture of the LHC's ALICE detector, which helped observe the densest matter.

Photograph courtesy Mona Schweizer, CERN

Ker Than

for National Geographic News

Published May 24, 2011

A superhot substance recently made in the Large Hadron Collider (pictures) is the densest form of matter ever observed, scientists announced this week.

Known as a quark-gluon plasma, the primordial state of matter may be what the entire universe was like in the immediate aftermath of the big bang.

The exotic material is more than a hundred thousand times hotter than the inside of the sun and is denser than a neutron star, one of the densest known objects in the universe.

"Besides black holes, there's nothing denser than what we're creating," said David Evans, a physicist at the University of Birmingham in the U.K. and a team leader for the LHC's ALICE detector, which helped observe the quark-gluon plasma.

"If you had a cubic centimeter of this stuff, it would weigh 40 billion tons."

Densest Matter Acts Like Perfect Liquid

By triggering hundreds of thousands of high-speed collisions each second, physicists using the LHC hope to break subatomic particles into even more basic forms of matter, which can be used to study what the universe was like a trillionth of a second after the big bang.

LHC scientists made the quark-gluon plasma last year by smashing together lead ions—lead atoms that have been stripped of their electrons—at nearly the speed of light.

As the name suggests, quark-gluon plasma is made up of quarks and gluons. Quarks are the elementary building blocks of positively charged protons and neutral neutrons, which make up atomic cores. Gluons are particles that "glue" quarks together using the so-called strong force.

It's thought that, as the universe cooled, the quark-gluon plasma that existed after the big bang coalesced to form matter as we know it today. (Related: "Strange Particle Created; May Rewrite How Matter's Made.")

The quark-gluon plasma created at the LHC is about twice the amount and about twice as hot as quark-gluon plasma previously made using the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York.

Still, the plasmas created by the two machines are very similar, scientists said this week during the Quark Matter 2011 Conference in Annecy, France. For example, scientists have now confirmed that both versions behaved like so-called perfect liquids, with nearly zero friction.

"If you stir a cup of tea with a spoon and then take the spoon out, the tea stirs for a while and then it stops. If you had a perfect liquid and you stirred it, it would carry on going around forever," Evans explained.

Some theories predict that, in the extreme heat of the very early universe, quarks and gluons would have been even more widely spaced, creating a quark-gluon plasma that behaved like a gas. The ALICE team is therefore looking for evidence of gas-like behavior in the early stages of their quark-gluon plasma formation.

"There are slight differences between our measurement and RHIC's," Evans said.

"It could well be that in the very early stages [of our quark-gluon plasma], it's behaving more like a gas, and then as it cools it turns into a liquid, but we will need to investigate this further."

Highs and Lows of Making Matter

If this gas-to-liquid transition has indeed been observed, it would be surprising, since theory predicts that it should occur at much higher temperatures than those currently being produced at the LHC, said Thomas Ludlam, chair of the physics department at Brookhaven.

"I would regard the ALICE claim that they may be seeing hints of this as very interesting, but rather speculative at this stage," said Ludlam, who was not involved in the project.

The results are nevertheless very exciting, he added. "They show that the LHC"—which went online in 2009 after more than a year's delay due to mechanical problems—"is squarely in the game now."

(Related: "LHC Gets First Results; Step Toward 'God Particle'?")

Also, by comparing the lower energy quark-gluon plasma created at the RHIC with the higher energy version from the LHC, scientists could gain a better understanding of how and when the substance changed as the universe cooled, Ludlam said.

"I think we're now at a point where, with these two machines, we can look over a very wide energy range at the properties of the quark-gluon plasma as it evolves with temperature and density," Ludlam said.

With this goal in mind, he added, RHIC scientists have been trying for the past year to create a quark-gluon plasma at even lower energies, to find the temperature at which quarks and gluons come together to form protons and neutrons.

Meanwhile, the LHC is still operating at only half of its maximum energy, and the ALICE team expects to create even denser forms of quark-gluon plasma as the machine ramps up in the future.

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