Lab-based experiments narrow down the search for dark energy

A new experiment, tracking the motion of single atoms, has failed to detect a fifth fundamental force — placing important constraints on the nature of dark energy.

The atom interferometer that the team used in their experiment to detect a fifth fundamental force of nature (Imperial College London)

A table-top in a London basement may seem like an unlikely place to test for one of the most elusive and mysterious elements of cosmology. But, this is exactly where a team of researchers has managed to place important constraints on the properties of dark energy.

Dark energy, the mysterious force that acts in opposition to the force of gravity — driving the expansion of the universe — remains tantalisingly out of touch for researchers. But, the experiment in question, set about to test one of the more popular theories of dark energy.

Since its inception, some physicists have proposed that dark energy is a ‘fifth’ fundamental force that acts on matter, beyond the four already known — gravitational, electromagnetic, and the strong and weak nuclear forces.

These same proposals suggest that this fifth force may be ‘screened’ or ‘hidden’ for large objects like planets or weights on Earth. Thus, making it difficult to detect. This ‘screening’ effect is the necessary addition to Einstein’s theory of General Relativity that allows it to explain the effect of dark energy.

Enter researchers at Imperial College London and the University of Nottingham. Their goal was to test the possibility that this fifth force is acting on single atoms — and thus far, they have found no evidence that this is the case.

In fact, the findings of their latest experiment — performed at Imperial College London and analysed by theorists at the University of Nottingham — could rule out popular theories of dark energy that modify the theory of gravity and leaves fewer avenues for researchers to search for this elusive fifth force.

Whilst this may not tell us what dark energy is, it places strong constraints on related theories, narrowing down the hunt for the energy — which researchers believe comprises 68% of the universe.

Professor Ed Copeland, from the Centre for Astronomy & Particle Physics at the University of Nottingham, says: “This experiment, connecting atomic physics and cosmology, has allowed us to rule out a wide class of models that have been proposed to explain the nature of dark energy, and will enable us to constrain many more dark energy models.’’

Searching for a fifth fundamental force of nature

The experiment tested theories of dark energy that propose the fifth force is comparatively weaker when there is more matter around — the opposite of how gravity behaves.

This would mean it is strong in vacuum-like space but is weak when in the presence of an abundance of matter. Therefore, an experiment using two large weights, for instance, would mean the force becomes too weak to measure.

The researchers instead tested a larger weight with an incredibly small weight — a single atom — where the force should have been observed if it exists.

The team used an atom interferometer to test whether there were any extra forces that could be the fifth force acting on an atom. A marble-sized sphere of metal was placed in a vacuum chamber and atoms were allowed to free-fall inside the chamber.

The crux of the experiment is, that if there is a fifth force acting between the sphere and atom, the atom’s path should deviate slightly as it passes by the sphere. Thus, causing a change in the path of the falling atom. However, no such deviation in motion was observed. Thus implying, no such fifth force was acting on the atom.

Professor Ed Hinds, from the Department of Physics at Imperial, said: “It is very exciting to be able to discover something about the evolution of the universe using a table-top experiment in a London basement.”

The findings are reported in the journal Physical Review Letters.