Published online 21 December 2007 | Nature | doi:10.1038/news.2007.399

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Helium-3 experiment replicates colliding-brane theory of cosmology.

Can you model what happened after the Big Bang in your lab?

Can you model what happened in the early Universe in the laboratory? Yes, according to one group of physicists. A team at Lancaster University in the United Kingdom has used liquid helium and a magnetic field to build a finger-sized representation of the early cosmos. Their findings, published today in Nature Physics1, could help string theorists to refine their models.

It is thought that shortly after the Big Bang, the nascent Universe underwent a very rapid period of expansion, known as inflation. But theorists are still debating what drove the rapid growth.

A popular theory amongst string theorists is that inflation was caused by the collision of two 'branes'. A brane (derived from the word 'membrane') is a three-dimensional object suspended in a higher-dimensional space, in the same way that a sheet of paper is a two-dimensional object in a three-dimensional space. One idea within string theory is that our entire Universe sits on a single such brane, and that it was our brane’s collision with another that drove inflation.

The question is how to test this theory: finding two Universe-sized objects that you can crash into each other might be slightly difficult. But a good analogue can be found in a strange concoction of liquid helium, according to Richard Haley, who led the team at Lancaster University.

Superfluid

Haley and his group used an 8mm by 45mm cylinder filled with helium-3, an isotope of helium that contains two protons and a single neutron. When cooled to just 150 microkelvin above absolute zero, helium-3 becomes a superfluid and begins to take on some odd traits: ghostly 'quasi-particles' are formed that can flit effortlessly through the frigid liquid. And the entire system can undergo 'symmetry breaking' — a phenomenon also thought to have led to the creation of every force we see today except gravity. It also tends to settle into one of two phases, which physicists label A and B.

The team used a magnetic field to create an A-phase slice of helium-3 sandwiched between two sections of B-phase liquid. They then decreased the field and watched as the two B-phases collided.

The colliding phases were, believe it or not, good analogues for colliding branes, Haley says. While helium-3 is radically different from the vacuum of space, the maths governing the two systems are similar.

Spot the defects

Haley and his team found evidence of defects — similar to those predicted by brane theory — left over from the collision.

Normally, quasi-particles can move with no resistance through superfluid helium-3, but have trouble crossing between A and B phases. Haley’s team found the quasi-particles still encountered resistance even after the A-phase was completely removed. The most likely explanation is that their flow was impeded by strange, quantum-mechanical vortices left over from the collision, the team says.

Universe-sized brane collisions are thought to leave behind a tangle of defects called 'cosmic strings' — massive, spaghetti-like objects that would criss-cross the entire cosmos. None have been seen to date, but theorists believe that their gravity waves might one day be detected by specialized instruments.

This lab model of brane theory is far from perfect, says Cliff Burgess, a theorist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. For example, branes on the universal scale move at close to the speed of light and crinkle more easily than helium-3 branes.

But nevertheless, he says, the liquid helium-3 analogue could provide some clues about how cosmic defects form and what they might look like. "Just having these two communities [condensed-matter physicists and string theorists] talking may suggest mechanisms to the theorists," Burgess says.

Haley says his team will now begin trying to characterize the exact nature of the defects created when their little branes collide.