Hunting for dark energy

It seems an impossible task. How do you detect a force you know next to nothing about? But this is exactly what physicists wishing to understand dark energy – the elusive force that is expanding the Universe – are attempting to do. And by conducting an experiment that recreates the conditions of deep space in the lab, Paul Hamilton from the University of California in Los Angeles and colleagues just helped narrow the search.

University of Queensland cosmologist David Parkinson says the study, published in Science in August, is “fantastic, and really well-executed”. While the team didn’t detect dark energy, the results help establish what dark energy is not.

Until the 1990s, cosmologists thought the Universe was either expanding at a steady rate or slowing down. But Nobel-winning work by two teams in 1998 found that the expansion of the Universe is accelerating. The unknown force behind this phenomenon was dubbed dark energy.

Attempts to detect dark energy in the laboratory have been in vain.

Physicists have since calculated that around 70% of the stuff in the Universe is dark energy. Dark matter constitutes a further quarter. Ordinary matter, made up of atoms, is a paltry 4.9%.

But apart from that, dark energy remains a mystery. It has never been directly detected and cosmologists can only theorise about its properties. One school of thought adapts an idea first proposed by Einstein in an early version of his theory of gravity: that dark energy is a “cosmological constant”, an intrinsic property of space that is pushing galaxies apart.

But while dark energy pushes distant galaxies apart it appears to have no effect closer to home. It doesn’t seem to be pushing the Solar System apart, for example.

That problem inspired an alternative dark energy theory. In 2004, theorist and study co-author Justin Khoury, along with then-Colombia University colleague Amanda Weltman, suggested the strength of dark energy changed according to the density of matter around it. Because of its changeable nature, this particular theory of dark energy has been dubbed a “chameleon field”.

In the huge tracts of empty space between galaxies, the theory goes, chameleon fields exert forces across billions of light-years, pushing matter apart. But in areas where matter is more densely packed – within the solar system, for instance – it has no discernible effect. This would explain why attempts to detect dark energy in the laboratory have so far been in vain.

In 2015 British physicists Clare Burrage, Edmund Copeland and Edward Hinds proposed a method that might catch the chameleon force at work in the lab. They suggested using an ultra-sensitive instrument called an atom interferometer to recreate the low densities of matter found in deep space.

Their instructions were fairly straightforward. Take a vacuum chamber, 20 centimetres in diameter. Place a two-centimetre-wide solid metal sphere inside. Drop a diffuse cloud of atoms from the top of the chamber towards the sphere. Using a laser, measure the acceleration of the atoms during their 10 to 20 millisecond free-fall.

If the chameleon field exists, it should fizzle out in the vicinity of the metal sphere, due to the density of matter found with in the sphere. As the atoms closed in on the sphere, their descent should speed up.

“If we have good enough instrumentation, we should be able to see it in the lab,” Parkinson says, adding that the table-top experiment design “was something very new”.

At the time Burrage didn’t have such an instrument – she and her colleagues planned to build one. But they were beaten by Hamilton who, upon reading the paper in August 2015, realised he already had an atom interferometer with those dimensions built and ready to go.

Hamilton, who was then at the Berkeley campus of the University of California, and co-author Holger Müller specialise in super-sensitive instruments. They built the atom interferometer used in the study five years ago. It can measure forces as weak as one millionth of the Earth’s gravitational force acting on a single atom.

Hamilton chilled a thin cloud of caesium atoms until they were almost completely still. These were dropped from the top of the chamber over the aluminium sphere and the team measured their fall with lasers.

And they found … nothing. The atoms dropped as though there was no chameleon field – their acceleration was exactly as you would predict from gravitational forces alone.

To double-check the result, they also measured the speed of atoms dropped well to the side of the sphere. If a chameleon field existed, they should feel its effects throughout their descent, and so fall slower than those directly over the sphere – but they dropped at the same speed.

One possibility is that the chameleon field was too weak for their instrument to detect. Müller and colleagues are already installing a more sensitive laser and a seismometer that detects and corrects for minute vibrations that may rumble through the atom interferometer. Meanwhile, Burrage and her collaborators are also racing to complete their own super-sensitive device.

Müller says the ultimate experiment would be to send the instrument to space. Thanks to Earth’s gravity, atoms only get a few milliseconds of free fall time. But in zero gravity, a single atom could be positioned to hover over the aluminium sphere and tracked over hours or days to see if it’s eventually tugged along. Their paper has already created “ripples of interest” from NASA, Müller says: “That’s the magic of atomic physics. It’s a field where stuff is moving fast!”