Dark energy is the enigma at the heart of modern physics: the universe is supposed to be awash with the stuff, but it’s never been seen and its nature is unknown.
When faced with a mystery of such epic proportions, simply eliminating certain options counts as success. This week, such progress, using an ingeniously simple tabletop experiment, was recognized by the prestigious Blavatnik Prize for Young Scientists.
Professor Clare Burrage, from the University of Nottingham and winner of the £100,000 prize, said: “We don’t know what dark energy is. It’s the name we give to something we don’t understand so we can start talking about it. And when so little is known, even ruling things out feels like great progress.
Dark energy was dreamed up to fill a huge gap in theoretical physics. Scientists predicted that due to the internal pull of gravity, the expansion of the universe should be slowing down. But observations of distant stars have shown that the expansion of the universe is accelerating instead.
Dark energy is a proxy for everything that drives this expansion, and to balance the necessary equations, it must account for 70% of the contents of the universe.
A popular theory is that dark energy is a “chameleon force” that adjusts its properties according to its local environment. “In a dense environment, your power becomes very short, but in empty space it becomes very far,” Barrage said.
This may explain how an elusive force can be powerful enough to control the fate of the entire universe, but remain invisible in our own solar system.
Dark energy experiments typically involve space observatories, huge particle accelerators, or detectors buried deep underground. However, Burrage’s theoretical work proved that small and light objects in the near-vacuum environment of Earth can still feel the full force of dark energy. Together with colleagues from Nottingham and Imperial College London, Burrage invented a “chameleon trap” that can be built in a laboratory.
The setup involved dropping ultra-cold atoms into a bowling ball-sized vacuum chamber containing a lump of aluminum. If a chameleon force existed, it should have a higher value in empty space and be “hidden” near the heavy lump of metal.
By precisely tracking the movement of the atoms using pulsed laser light, the team looked for any unexpected accelerations that could be due to a chameleon force.
“You’re looking to see if there’s an additional force pulling the atoms apart,” Burridge said. “Obviously it would have been great to see something.”
Unfortunately, no mysterious powers were revealed, but the experiment was able to narrow down the possible values that a chameleon power could take in a small window. “With one upgrade to the experiment, we hope to close that window,” Barrage said. “It’s definitely technologically achievable.”
The findings have been received positively, Burrage said, although some theorists have spent years developing hypothetical chameleon powers. “When you’re a theorist, you put things out into the world, and a lot of the time people just say, ‘Oh, that’s nice,’ and move on,” she said. “So people were just excited to see testing done.”
Some might be put off by the slim chances of a breakthrough in a field where so little is known, but for Burrage, that’s the appeal of working on dark energy. “I’m very stubborn – it runs in the family,” she said. “I’m a rock climber in my spare time. I like challenges and I don’t give up easily.”
Her current work is focused on using data from the European Space Agency’s (Esa) Gaia mission, which is making detailed measurements of stars in the Milky Way. Esa’s Euclid mission, which is focused directly on the question of dark energy, is expected to launch this year. The mission will look at how the universe has evolved over the past 10 billion years to look for fingerprints of dark energy.
“Euclid is the big one,” Burrage said. “It will map the distribution of galaxies that we can see in the sky.”
The mission will observe up to 2 billion galaxies using infrared and visible light to study their shape and motion. The goal is to get a more accurate picture of the competing forces of gravity that cause galaxies to clump together and dark energy that drives the accelerated expansion of space.
“The fact that so little is known is exciting,” Burrage said. “I feel like you can make a lot of progress somewhere.”
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