By Peter Weiss
When matter was new in the universe, it was an exotic gas whose components later congealed into the more-ordinary matter made of atoms. At least, that was the story. Now, physicists trying to re-create that gas in an accelerator say that the universe’s original stuff appears to have been a liquid.
Like the gas that had been expected, the ur-liquid the physicists made is ultrahot and ultradense—up to 150,000 times as hot as the sun’s core and 100 times as dense as ordinary atomic nuclei.
It’s essentially a sample of primordial matter from the explosive birth of the universe, Samuel Aronson of Brookhaven National Laboratory in Upton, N.Y., said at a press conference Monday in Tampa, Fla., at a meeting of the American Physical Society. “We think we’re looking at a phenomenon last seen in the universe more than 13 billion years ago,” he says.
Such material presumably permeated the universe during the first microseconds after the Big Bang. Then, it cooled and differentiated into a host of particles, including the protons and neutrons in all the matter that now exists (SN: 8/26/00, p. 136: Seeking the Mother of All Matter). The finding that it might have been a liquid could influence cosmological models of how the universe evolved, adds Dmitri E. Kharzeev, head of Brookhaven’s theory group.
Scientists refer to the original material, predicted to be a gas, as the quark-gluon plasma. Quarks are the building blocks of protons, neutrons, and more-exotic entities, whereas gluons are massless particles that glue together quarks.
For more than 20 years, scientists in Europe and the United States have used accelerators to search for the quark-gluon plasma (SN: 2/19/00, p. 117: Melting nuclei re-create Big Bang broth). Since it began operating in 2000, only Brookhaven’s Relativistic Heavy Ion Collider, or RHIC, has been employed in this quest. The new analysis relies on 2000–2003 data.
To make a quark-gluon plasma, RHIC physicists slam together gold ions accelerated to nearly the speed of light. The protons and neutrons of the colliding ions transform into energy, out of which quarks and gluons emerge, the RHIC scientists say.
“They’ve found a system of quarks and gluons that’s very different from normal nuclear matter,” says Joseph Kapusta of the University of Minnesota, Twin Cities.
What’s more, the quarks and gluons that the RHIC has been producing for years undergo collisions, interactions, and motions more characteristic of liquids than of gases, Aronson reports. Although some evidence of liquid behavior in the RHIC-made matter had been recognized for years, scientists gripped by long-standing theoretical predictions of a gas or plasma were slow to give it credence (SN: 6/21/03, p. 387: Hot Mama: Has matter’s mother paid a call?).
“What we’ve struggled with is that it’s not quite the quark-gluon plasma that we had predicted,” says Brookhaven’s Thomas W. Ludlam.
However, that resistance has largely melted. Four reports representing the consensus of RHIC researchers on the newly identified liquid state will appear in an upcoming Nuclear Physics A. They follow more than a year of vigorous debate, mainly between theorists convinced that the quark-gluon plasma had been found and experimentalists wary that other interpretations of the data were still possible.
Scientists are eagerly awaiting future experiments that will determine specific properties of the newfound liquid state, such as its viscosity, temperature, and heat capacity, says Kapusta.
Even though the RHIC scientists have become hesitant to identify the universe’s first matter as a plasma, they’re mum on what a new label might be.
