The problem with theories about the universe in the first crucial fractions of a second after the Big Bang is they are very hard to test. It’s not like we can create a whole new universe and watch it to explode to see how things work. At least, that is what one might think, but a team from the Universität Heidelberg have done something a little like that, recreating the early universe’s expansion on a very small scale.
One of the favored models for the formation of the universe is called Inflation Theory. It proposes that the universe expanded unimaginably fast – around 1026-fold – before it was 10-32 seconds old. This solves a lot of problems with the structure we see in the universe today, but it still has plenty of skeptics.
Given the early universe was staggeringly hot, a set of atoms cooled to temperatures a few billionths of a degree above absolute zero might seem a strange way to model it. Nevertheless, that is what PhD student Celia Viermann and co-authors have done, inspired by a forty-year-old observation of the parallels between sound waves in fluids and quantum fields.
The team used the fact that, at those temperatures, potassium-39 becomes a superfluid, something that flows with zero viscosity. Soundwaves within superfluids, known as phonons, can only occur at discrete energy levels, analogous to the geometry of gravitational fields moving through space-time.
The team generated short bursts of sound waves in the center of 23,000 potassium atoms and then manipulated their speed with magnetic fields. These manipulations created conditions that parallel an inflationary just-born universe.
To further the comparison, the authors investigated the behavior of quantum waves within the atoms. Quantum waves are the smallest excitations allowed by quantum physics within a superfluid cold enough to suppress other excitations. In an accompanying News and Views, Professor Silke Weinfurtner of the University of Nottingham writes; “The commonly held theory is that the Universe was devoid of everything except quantum excitations at the beginning of inflation.”
The pattern imprinted on the superfluid matched theories of the inflationary universe. Among other things, it saw the spontaneous appearance of pairs of particles, something thought to have been common in those crazy first trillionths of a femtosecond. Such pair-production was reported for the first time in quantum-field experiments earlier this year.
The importance of the work lies not just in the observations made, but the precise control the authors were able to demonstrate over their system, indicating it can be adjusted in future to explore differing cosmological models.
As Weinfurtner notes, the gap between the map and the terrain is vast. The growth during the inflationary period was equivalent to expanding from the size of an atom to a sphere a light-year across. Viermann’s miniature universe tripled in size during the experiment, so it’s not really the same thing.
Nevertheless, the team have far exceeded Blake’s aspiration to “See a world in a grain of sand”, modelling the entire universe in something far smaller than we can see.