The use of condensed matter systems as a way of modelling difficult-to-access quantum systems – the high-energy state of the universe seconds after the Big Bang, for example, when there was a rapid period of exponential expansion, known as inflation – has become well accepted in particle physics.
And a new paper by a team of US researchers in the journal Nature Physics presents observations of quantum phase transitions that suggest our current theories are on the right track.
In ordinary matter, we are familiar with phase transitions, the moment in which matter changes in some abrupt and significant way – for example, water freezing to ice at zero degrees. Similar things occur at the quantum level, and understanding the conditions and effects of quantum phase transitions can help answer some of the most fundamental questions of physics, potentially up to how the cosmos came into being.
In the currently accepted model of the cosmos, the burst of inflation was followed by the formation of “topological defects” which deformed the uniform early universe leading to the formation of the features we see today, such as galaxies, stars, and planets.
“How inflation is initiated and evolves into topological defects remains a hot topic of debate,” note the researchers, led by Lei Feng from the James Franck Institute, Enrico Fermi Institute and the University of Chicago demonstrate a model that may provide a mathematical analogue.
To probe the mystery, Feng and his colleagues used a Bose-Einstein condensate – an ultra-cold atomic gas, cooled to close to absolute zero. In this experiment, the condensate comprised just 30,000 caesium atoms, held in place with an optical trap, with close to zero energy in the system. Then, to put it simply, the researchers shook the atoms back and forth.
This modulation of the system created two new low energy states, until waves began to form in the condensate – and those waves grew exponentially once the shaking reached a critical point.
This suggests a process similar to what we see as inflation – or, as the paper puts it, “These observations suggest that atoms occupy a coherent superposition of well-defined momentum states and the density wave emerges from their interference.”
What’s more, the fact that the atoms moved into two different ground states rather than shifting all at the same time to a single new one shows a way in which the uniformity of a stable quantum state could be broken.
The researchers do not suggest that this is how the universe may have come into being, but offer the results as evidence of one way quantum phase transitions actually behave in the physical world.