Researchers in the UK and Finland claim to have simulated some possible aspects of the early universe in a test tube of liquid helium-3. The experiment, which involves “colliding” the boundaries between two different phases of superfluid helium, is analogous to what may have happened shortly after the Big Bang, when some physicists believe that membranes or “branes” permeating multidimensional space collided with each other.

Physicists believe that a rapid expansion of the universe occured about 10–35 s after the Big Bang. Proponents of the string theory of particle physics have suggested that this “inflation” was brought about by the collision and subsequent annihilation of a D-brane and an anti-D-brane. D-branes are a mathematical consequence of string theory and are hypersurfaces in multidimensional space — in our familiar 3D world, for example, a 2D-brane would be a membrane like the skin of a balloon.

String-theorists believe that D-branes have zero thickness but a huge mass, and would therefore give off a tremendous amount of energy when they annihilate with each other. But string theory has yet to be proven experimentally and it remains an extremely contentious issue amongst some particle physicists and cosmologists.

Now, Richard Haley and colleagues at Lancaster University and the Helsinki University of Technology have come up with an experiment that they claim is the closest analogue system to colliding branes and anti-branes in the early universe (Nature Physics doi:10.1038/nphys815). While their system could shed some light on what might happen during such collisions, Haley told that their experiment is by no means a test of string theory.

Phase boundaries collide

The team cooled several millilitres of liquid helium-3 to about 150 µK in a test tube-like sample holder. A magnetic-field gradient was applied to the sample, which created a sandwich of two different phases of superfluid helium: a region of phase “A” in the middle of the test tube and regions of phase “B” at both ends. The A and B regions were separated by phase boundaries, with the boundary from B to A being analogous to a brane and the boundary from A to B analogous to an anti-brane.

The team then changed the field gradient on the sample to reduce the thickness of the A region until the “brane” and “anti-brane” collided and annihilated, leaving just the B phase.

Helium-3 becomes a superfluid at very low temperatures because individual atoms pair up to form “Cooper pairs”. To study the remaining B phase the team used a heater at the bottom of the tube to create a stream of broken Cooper pairs that flowed up the tube.

While the B phase should be isotropic — in other words, have no structure — the broken pairs appeared to scatter off structures as they passed through the test tube. Haley and colleagues think these structural “defects” in the B phase are vortices of swirling helium created during the annihilation, similar to the defects in the form of “cosmic strings” that string theorists think were created when D-branes collided in the early Universe.

This is not the first time that helium-3 has been proposed as an experimental analogue of the early universe. One of the leading proponents of such analogies — Grisha Volovik of the Helsinki University of Technology — told that Haley and colleagues have managed to simulate brane collision and test whether strings will be created.

Do branes really exist?

Volovik, who was not involved in the experiment, is adamant that the work is neither a test of string theory nor a test of whether brane annihilation was responsible for inflation. “[The experiment] confirmed the possibility of the creation of topological defects in the process of brane collision, but this does not mean that branes really exist,” he said.

Haley, who is currently rebuilding the experiment to allow the team to better characterize the defects, agrees that the team has not come up with a test of string theory. Indeed, the researchers originally set out to study the phase transition between A and B and only began speaking to the cosmologists when they realized that there may be a connection between defects in the B phase and cosmic strings.