Focus: Cosmology in the Lab

Published May 19, 2006  |  Phys. Rev. Focus 17, 18 (2006)  |  DOI: 10.1103/PhysRevFocus.17.18
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Left behind. Cooling rapidly through a phase transition leaves behind so-called topological defects, such as the “knots” in this liquid crystal that has been cooled into a colorful phase. The Universe also cooled rapidly, and experiments with a superconductor support a theory of topological defect formation in the cosmos.

The hot, young Universe cooled so rapidly that not all regions ended up in exactly the same state, according to theory. Experimenters have explored this phenomenon by rapidly cooling controlled, laboratory samples, and in the 12 May PRL a multinational team reports the most detailed measurement yet. They cooled a superconducting sample many times to learn precisely how frequently it ends up in such a heterogeneous state. The results support a theory on the consequences of rapid cooling that should apply to many situations, including the first violent microsecond of the Universe.

If you freeze water quickly, you don’t get a single crystal but many crystals, perhaps with different orientations, separated by “defects” where the crystals meet. In theory, cooling extremely slowly would allow all regions to “agree” and create a single smooth crystal. When the Universe cooled rapidly through a series of phase transitions, different regions should have frozen into different states–not states of matter, but abstract states of the fundamental forces such as electromagnetism and the weak force. According to theory, the Universe’s defects where different regions meet can be points that are fundamental particles, or they can be lines called cosmic strings. These two kinds of so-called topological defects act something like knots on an infinite string–you can move them around, but you can never get rid of them.

Thirty years ago, Tom Kibble, of Imperial College London, proposed a theory that predicts a precise relationship between cooling rate and the number of topological defects. Later Wojciech Zurek, now at Los Alamos National Laboratory in New Mexico, added to the theory and pointed out that it should apply to any rapid phase transition–in the lab or in the Universe. The prediction depends on details like the material properties and the cooling rate–which aren’t well-known for the Universe–but lab experiments can test the theory’s basic principles.

A team led by Roberto Monaco of the University of Salerno in Italy has now done the most precise quantitative test yet of the Zurek-Kibble theory. They used a pair of niobium rings separated by a thin layer of a nonconducting material in an arrangement that resembled two bagel halves with cream cheese between them. Cooling the rings forces them into the superconducting state. With rapid cooling, a quantum mechanical quantity called phase may adopt different values at different places around the rings, which creates a type of topological defect called a vortex. This defect, which is detectable through simple voltage and current measurements, is quantized–in each cooling cycle, only a single vortex can be created or none at all.

The researchers heated and cooled the rings over 100,000 times, varying the cooling time from milliseconds to tens of seconds and looking for a vortex after each cooling step. The probability of vortex creation grew roughly as the square root of the cooling rate, consistent with the Zurek-Kibble theory as applied to this experiment. Four years ago the team reported a somewhat different dependence [1] that they now believe was caused by problems with their previous experimental setup. “Now we’ve got a really good system,” says coauthor Ray Rivers of Imperial College London.

This experimental result “looks very impressive,” says Zurek. He notes that some previous experiments had not seen the expected defects, though, so “there’s lots of room” for further tests of the theory, especially under conditions where more than one defect occurs.

–Don Monroe


References

  1. R. Monaco, J. Mygind, and R. J. Rivers, “Zurek-Kibble Domain Structures: The Dynamics of Spontaneous Vortex Formation in Annular Josephson Tunnel Junctions,” Phys. Rev. Lett. 89, 080603 (2002).

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