Liquid Universe

By Jeffrey Winters|Friday, November 01, 1996
In the beginning--or to be precise, a trillionth of a trillionth of a trillionth of a second after the Big Bang--the universe was a seething, unimaginably hot ball of energy. The forces that we know-- gravity, electromagnetism, and the strong and weak forces that govern atoms--were still joined as one. But in an instant, this sea of energy changed. Much as water abruptly turns to ice, the universe crossed a temperature threshold and the universal force fragmented.

As unlikely as it seems, a group of Finnish physicists claims to have re-created these primordial conditions in the lab. They believe they have simulated important features of the newborn universe in a flask of liquid helium chilled to near absolute zero, and their experiments, they say, bear out the predictions of a controversial theory.

As the universe cooled, according to this theory, defects in the fabric of space-time appeared, just as water solidifying to ice develops crystal imperfections. These cosmological wrinkles--called cosmic strings-- would have been far thinner than an atom, perhaps infinitely long and extremely massive, spanning the universe. As strange as these objects seem, they would solve a host of vexing problems in cosmology. Because of their enormous mass, the strings’ powerful gravitational fields would have helped pull together the first galaxies and provided the framework for the large- scale structure of the universe.

What does liquid helium have to do with all this? When cooled to almost the lowest temperature possible--absolute zero, or minus 459.67 degrees Fahrenheit--helium suddenly becomes a frictionless fluid, a strange state of affairs physicists call superfluidity. Many of its atoms flow together in lockstep. In this unusual state, helium in many ways resembles a vacuum--particles can move through the superfluid without resistance, for example, and spontaneous miniature ripples sometimes form that correspond to the virtual particles that pop in and out of existence within a vacuum.

Most important, though, nearly identical mathematics underlies both the transition of liquid helium to the superfluid state and the transformations that physicists believe took place in the early universe as it cooled and formed cosmic strings. Wojciech Zurek, a physicist at Los Alamos National Laboratory in New Mexico, suggested about ten years ago that physicists can take the same equations that describe the creation of cosmic strings and use them to predict what will happen to superfluid helium as it is heated and allowed to cool again. The heated superfluid, it turns out, should give rise to a specific number of vortices, corresponding mathematically to the formation of cosmic strings. If the predicted number of vortices appeared, the mathematics that underlies both cosmic string formation and the odd behavior of superfluid helium would be shown to be sound. This in turn would be a boost for the plausibility of cosmic string theory.

Physicists Grigori Volovik and Matti Krusius at Helsinki University of Technology are the first to have successfully tested Zurek’s proposition. They began by cooling a rotating cylinder of helium to about one-thousandth of a degree above absolute zero, a temperature just below the transition from the normal liquid state to superfluidity. They then shot a neutron through the superfluid, heating a region less than a thousandth of an inch across with just enough energy to change it into a normal liquid for about a millionth of a second. When the region cooled back to the superfluid state, tiny vortices formed. The rotating cylinder reinforced the vortices, keeping them around long enough to be spotted by a magnetic detector. The physicists found as many as 20 vortices per neutron shot--a number consistent with Zurek’s predictions.

The Helsinki researchers’ superfluid helium has turned out to be such a good model for the early universe that they are planning to look at other deep cosmological problems, from the origin of gravity to the scarcity of antimatter. There are a lot of fundamental problems in physics that are related to the properties of the physical vacuum, Volovik says. We can model many of them with our helium.
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