Cosmologists have revealed intruiging new ways to probe the mystery of whether dark energy exists and how it might be accelerating the universe’s growth

Physicists are homing in on exactly what dark energy does (Image: Picturegarden/Getty)

A LIGHT in the darkness can come from unexpected places. Unusual experiments for probing dark energy seem set to revolutionise our understanding of this mysterious force.

In Chicago last week, the world’s largest meeting of cosmologists debated two of the forces that could push the universe apart: inflation, the proposed period of exponential expansion that the universe went through immediately after the big bang; and dark energy, the present-day force thought to be responsible for pushing the cosmos outward at an ever increasing rate.

The announcement in March that gravitational waves had been seen should essentially prove that inflation happened. But the results are on ice. The BICEP2 telescope team, which did the work, may have underestimated the impact of galactic dust on the signal. If real, the pattern of the waves they saw in the cosmic microwave background – the earliest light emitted in the universe – is the fingerprint of the universe’s rapid expansion.


Astronomers and cosmologists at the International Conference on Particle Physics and Cosmology (COSMO) duked it out over how their models for the universe would be affected in two futures: one in which the results hold, the other in which dust blows them away.

“Everyone wants BICEP2 to be right,” Will Kinney of the University at Buffalo, New York, told a packed auditorium. “Because if it is, we are going to be doing incredibly precise physics on the inflationary model within the foreseeable future. And it’s going to be really cool.”

For now, physicists will have to wait. New data from the Planck satellite, which could clear up BICEP2’s problems, is not due to be released until November, but rumours swirled at COSMO that at least one paper based on Planck data within BICEP2’s field of view will be published any day.

In the calm before that storm, much of the attention is on dark energy, and some big steps have been made. Dark energy is a theoretical necessity, exerting a repulsive force that explains how the speed of our universe’s expansion is accelerating. But we know almost nothing about it.

COSMO saw novel work for exploring dark energy in Earthly laboratories (see “Chameleon screen“), an experiment that could show that the expansion is a fundamental property of space-time itself (see “A quantum of space-time“), and new constraints on the most devastating model of dark energy, which would see our universe tear itself apart, atom for atom (see “Phantom menace“).

“Everybody and their mother is constraining dark energy,” says Dragan Huterer at the University of Michigan in Ann Arbor. “That’s the name of the game: you’re measuring the expansion history of the universe.”

Everybody and their mother is constraining dark energy. That’s the name of the game

Chameleon screen Many physicists think dark energy is shoving the universe apart by countering gravity. If that’s true, why have experiments never seen it? One hypothesis is that the force adapts to its environment and is only active in a near vacuum, while the dense matter of the solar system “screens” it from view. Now Clare Burrage at the University of Nottingham, UK, and her colleagues have begun work on a laboratory test to find these screened “chameleon” forces. If dark energy’s effects can be felt only across a space as empty as the universe, Burrage says, then the same effect may show up in a vacuum chamber containing only a small ball of stable material and a cloud of a mere 1000 atoms. The team plans to use a laser to move the atoms 1 millimetre across the chamber. As they travel, the atoms will feel the gravitational pull of Earth as well as that of the confounding ball of material, and the experiment will measure which forces are affecting them. If the disguising force of the ball is acting on the atoms, they should take a slightly different path, which will be visible in their final quantum states. The team has not yet done the experiment, but has requested a special laser from a quantum GPS system from the UK Ministry of Defence, which should arrive in the next few months. Finding chameleon-like effects won’t necessarily mean they’ve found dark energy, says Adrienne Erickcek of the University of North Carolina at Chapel Hill. But it will show that screening mechanisms are a plausible explanation for our failure to measure the effects of dark energy in the local universe. “This is very exciting,” Erickcek says. “I had always assumed that the chameleon force would be screened no matter what, but they showed really convincingly that it need not be. It’s amazing.”

A quantum of space-time An experiment in a shed in the suburbs of Chicago could show that dark energy is simply an emergent property of space-time, much as fluid dynamics emerges from how water molecules interact. The goal of the Holometer experiment is to find the fundamental units of space and time. These would be a hundred billion billion times smaller than a proton. Like matter and energy at the quantum scale, these bits of space-time would act more like waves than particles. “The theory is that space is made of waves instead of points, that everything is a little jittery, and never sits still,” says Craig Hogan at the University of Chicago, who runs the experiment. The Holometer is designed to measure this “jitter”. It directs two powerful laser beams through tubes 40 metres long. The lasers measure the positions of mirrors along their paths at two points in time. If space-time is smooth and shows no quantum behaviour, then the mirrors should remain perfectly still. But if both lasers measure an identical, small difference in the mirrors’ position over time, after all other effects are ruled out, that could mean the mirrors are being jiggled by fluctuations in the fabric of space. Taking this idea a step further, Hogan says the quantum states of space-time and matter could be entangled, so you can’t measure one without affecting the other. Our best current theories describe space-time in terms of geometry, and matter in terms of quantum fields, but struggle to unite the two. If the Holometer sees something, Hogan says, it could point to a way of unifying them. At the tiny scales at which the two properties are connected, the geometry of space-time alone should force the universe to expand. Hogan told the COSMO meeting that initial results show that Holometer can measure quantum fluctuations, if they are there, and could collect enough data for an answer within a year.

Phantom menace Most models of dark energy hold that the amount of it remains constant. But about 10 years ago, cosmologists realised that if the total density of dark energy is increasing, we could be headed for a nightmare scenario – the “big rip”. As space-time expands faster and faster, matter will be torn apart, starting with galaxy clusters and ending with atomic nuclei. Cosmologists called it “phantom” energy. To find out if this could be true, Dragan Huterer at the University of Michigan in Ann Arbor turned to type Ia supernovae. These stellar explosions are all of the same brightness, so they act as cosmic yardsticks for measuring distances. The first evidence that the universe’s expansion is accelerating came from studies of type Ia supernovae in the late 1990s. If supernovae accelerated away from each other more slowly in the past than they do now, then dark energy’s density may be increasing and we could be in trouble. “If you even move a millimetre off the ledge, you fall into the abyss,” Huterer says. Huterer and colleague Daniel Shafer have compiled data from recent supernova surveys and found that, depending on which surveys you use, there could be slight evidence that the dark energy density has been increasing over the past 2 billion years, but it’s not statistically significant yet (Physical Review D, doi.org/vf9). Phantom energy is an underdog theory, but the consequences are so dramatic that it’s worth testing, Huterer says. The weakness of the evidence is balanced by the fact that the implications are huge, he says. “We will have to completely revise even our current thinking of dark energy if phantom is really at work.”

This article appeared in print under the headline “Shots in the dark”