Professor Nicola Spaldin – Cosmic strings
Story by MacDiarmid Institute Principal Investigator Shaun Hendy
What do the origins of the early universe have to do with materials science? More than you might think, according to Nicola Spaldin and her colleagues at the ETH in Zurich, Switzerland. Spaldin has found an unusual material that mimics aspects of the way the universe might have been shortly after the Big Bang. I was lucky enough to see Spaldin speak about this in February when she travelled to Nelson to address the MacDiarmid Institute’s Conference on Advanced Materials and Nanotechnology. In her talk she described how threads of electric charge in a material called yttrium manganite behave like cosmic strings, long filaments of energy that some physicists think may have formed as the universe cooled after expanding from the Big Bang. Now if that sounds to you a bit like a pair of seven year olds re-enacting the fishing up of Te Ika-a-Māui in their paddling pool, you have my sympathy. Surely she is exaggerating when she says she can recreate the early universe in her lab? Well, sort of. The genius of Spaldin’s experiment relies on something physicists call “universality”. This is the observation that under certain circumstances behaviour in one physical system is very similar to that in other, apparently quite different, systems. If you heat a permanent magnet up, for instance, it will lose its magnetic properties once it gets hot enough. But close to the temperature at which magnetism completely disappears, the magnet starts to behave in much the same way as a liquid does when it boils and loses it structure to form a gas. Spaldin has exploited this principle of universality to construct a crystalline material that will support filaments of electric charge that behave like cosmic strings might have ten billion years ago.
This is rather exciting for cosmologists, who—try as they might—have yet to see any sign of cosmic strings. Some physicists, especially those of us who twenty years ago may have written PhD dissertations on how to detect cosmic strings (okay, so I may be the only person who fits this category), have been waiting a very long time for news. But the real pay-off is that Spaldin has been able to use her material to test some of the properties that cosmic strings and the string-like filaments of charge in her material likely share. Spaldin has designed her crystals so that strings form as the crystals are cooled, to mimic the cooling and expansion of the early universe. And by cooling the yttrium manganite crystal faster, more of Spaldin’s strings form. Likewise, the number of cosmic strings that formed early on in our universe will depend on how fast it cooled after the Big Bang. By measuring the relationship between cooling rate and the number of strings that form in her crystals, and applying the principle of universality, Spaldin has been able to test theories about the number of cosmic strings that might be out there in the universe. What she has found is that her strings behave as theory predicts, but only up to a point. If the crystal is cooled very rapidly, Spaldin finds that the number of strings that form drops off in a way not predicted by theory. This tantalising result is food for thought for cosmologists. If fewer cosmic strings formed than we thought, this might explain why they have been so hard to find. Spaldin and her colleagues plan to continue their investigations by refining the design of their crystals to make their strings even more like those of the cosmic variety. Materials science is relevant to many aspects of our life, but as Spaldin has shown, it can also be used to give insight into the very early universe. Such is the beauty of science.