Early universe recreated in LHC was superhot liquid - physics-math - 25 November 2010 - New Scientist

The early universe was an extremely dense and superhot liquid, according to the surprise first findings of the ALICE experiment at the Large Hadron Collider near Geneva, Switzerland.

The experiment to probe the early moments of the universe started up on 7 November, smashing together the nuclei of lead atoms inside the LHC's circular tunnel to produce incredibly dense and hot fireballs of subatomic particles at over 10 trillion °C. The idea behind ALICE is to recreate the exotic, primordial "soup of particles" known as quark-gluon plasma that appeared microseconds after the universe's birth. Gluons and quarks went on to become the constitutive "bricks" of neutrons and protons inside atomic nuclei.

Many models have suggested that the flow of particles from these subatomic fireworks produced in high-energy nuclear collisions should behave like a gas and not a liquid. "These observations keep surprising us," says David Evans of the University of Birmingham, UK, a member of the ALICE team.

A further surprise was the density of subatomic particles created by the smash. One major school of thought suggests there is an upper limit on how many interacting gluons can be packed into a given volume: when this saturation point is reached during a collision, no more new debris particles should therefore be produced.

But to the surprise of the ALICE scientists, the lead ions' mini big bang produced more subatomic particles than expected. "This means that if an upper limit exists, it has not yet been reached at the energies used at LHC," says Evans.

Gold soup

This is especially significant, because ALICE has used the highest energies yet. It used energies 13 times higher those used in similar experiments in 2005 at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York.

The Brookhaven experiments smashed nuclei of gold atoms together to probe the same questions as ALICE. They delivered the puzzling finding that the primordial soup formed after the collision appeared to flow like a near-perfect liquid with almost no viscosity.

Energies up

Some models had indicated that if higher energies were used, viscosity values – resistance to flow of a fluid – would indicate the presence of a gas. "At the time, the RHIC results already surprised us," says Evans. "But our data still seem to show that the quark-gluon plasma flows like a superhot liquid."

He adds, however, that it is too early to draw any new interpretation about the structure of the early universe.

"Those results are impressive," comments John Ellis, a theorist at CERN who was not involved in the experiment. "But these are early days. We knew it would be very difficult to find hints of a gaseous form of quark-gluon plasma. Maybe it all started as a gas, and then its expansion, and therefore cooling, led it to become a liquid."

"We have to look deeper into the details of the collisions to be sure," admits David Evans, who is already looking forward to the LHC's next phase in 2013, when the possible collision energies will be doubled.

Journal references: arxiv.org/abs/1011.3914; arxiv.org/abs/1011.3916

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