Quarks and gluons have formed a unexpected liquid. Click here to see animation.© RHIC/BN

The Universe consisted of a perfect liquid in its first moments, according to results from an atom-smashing experiment.

Scientists at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, New York, have spent five years searching for the quark-gluon plasma that is thought to have filled our Universe in the first microseconds of its existence. Most of them are now convinced they have found it. But, strangely, it seems to be a liquid rather than the expected hot gas.

Quarks are the building blocks of protons and neutrons, and gluons carry the strong force that binds them together. It is thought that these particles took some moments to condense into ordinary matter after the intense heat of the Big Bang.

To recreate this soup of unbound particles, the RHIC accelerates charged gold atoms close to the speed of light before smashing them together. Previous experiments have shown that these collisions create something the size of an atomic nucleus that reaches 2 trillion degrees Celsius, about 150,000 times hotter than the centre of the Sun.

"This stuff was last seen in the Universe 13 billion years ago," says Sam Aronson, a director of high energy research at Brookhaven.

Now experiments have revealed that this hot blob is a liquid, which lives for just 10-23 seconds. "This was completely unexpected," says Wit Busza of the Massachusetts Institute of Technology, one of the team of researchers who reported their discovery on 18 April at the American Physical Society conference in Tampa, Florida.

Hot water

"The surprising thing is that the interaction between the quarks and gluons is much stronger than people expected," says Dmitri Kharzeev, a theoretical physicist at Brookhaven. The strength of this binding keeps the mixture liquefied despite its incredible temperature. "It's as much a fluid as the water in this glass," Kharzeev says, pointing to his drink.

The researchers worked out the liquid's structure by tracking the particles that spray out as the droplet falls apart and quarks team up to form normal matter. "It's a very complicated thing," says Busza. "But we've been amazed at how simple the results are."

The resulting liquid is almost 'perfect': it has a very low viscosity and is so uniform that it looks the same from any angle.

This may help to explain why the deepest parts of the Universe seem similar wherever astronomers look, says Kharzeev. If the primordial liquid had been as viscous as honey, the Universe could have turned out much more lumpy, he explains. "We can be certain this will change our picture of the early Universe," he says.

The researchers now hope to measure the heat capacity, viscosity and even the speed of sound in the quark liquid. But the RHIC has been hit by cuts in the recent US budget, forcing it to reduce its operating time from 30 to 12 weeks next year. Further investigations will inevitably take years to complete, says Aronson. 

Brookhaven National Laboratory