An experiment in the United Kingdom has failed to find evidence of a particle meant to explain most of the universe’s mass. But the search isn’t over.
When cosmologists observe the way the universe expands, they find that present-day theories of matter can’t explain most of the universe’s energy. They call the unknown energy “dark energy,” and theorists have tried to explain it by proposing undiscovered particles and corresponding fields. Experiments have failed to find evidence of such particles, but in physics, that’s not necessarily a bad thing.
“We haven’t excluded everything,” Clare Burrage, an associate professor in physics and astronomy at the University of Nottingham in the UK and one of the study authors, told Gizmodo. “There’s still a window of parameters that’s arguably more interesting.”
Two 1998 observations of the most distant supernovae revealed that not only is the universe expanding, but the expansion is accelerating. Explaining this expansion required a new, undiscovered force driving things apart, which physicists call dark energy. Calculations since then have revealed that dark energy should make up more than two-thirds of the universe’s total mass and energy—but we don’t know what the source of the energy actually is.
Physicists understand the forces between regular matter in the universe, like the electromagnetic force, as fields (where you are in the field determines how strongly you feel the force) with corresponding particles (you can understand the interactions between two matter particles as the exchange of force particles). So, some theories of dark energy propose that it’s a new kind of force, too weak to be observed on Earth, with a corresponding particle; these proposed particles have names like the chameleon or the symmteron. Recent computational evidence has shown that the theory of chameleons, so named because their properties depend on the environment they exist in, are a viable theory of dark energy.
Researchers working in the United Kingdom had previously proposed that if these forces existed, they might be detectable using a special kind of experiment akin to Galileo’s dropping two balls from the top of the leaning tower of Pisa. The researchers placed an almond-sized aluminum ball, attached to a rod so it can move around, in an extreme vacuum chamber. Then, they pumped and trapped a pulse of cold rubidium atoms, then released the trap. Using a detection scheme called atom interferometery, based on shining specially turned lasers onto the atoms, the researchers measured how the atoms moved toward the aluminum ball held in various positions, looking for the slightest differences in acceleration from theoretical expectations.
The experiment found that if chameleon or symmetron particles exist, their effects are too slight to be measured by this setup, according to the paper published in Physical Review Letters. This kind of null result is important—it tells theorists and experimentalists to look elsewhere for a dark energy-explaining particle.
These results confirm a similar set of results from a 2017 paper from a team of scientists here in the United States, though with a slightly different detection scheme. This paper “is of very high quality and corroborates our earlier limits,” Holger Müller, leader of the 2017 efforts from the University of California, Berkeley, who was not involved in the new study, told Gizmodo in an email. “They use similar, but not identical techniques, so this is a meaningful strengthening of the experimental data. I should give full credit that it was a theoretical paper by Burrage, Copeland and Hinds,” three of the scientists on this new paper, “that inspired us to look at chameleons.”
And it’s important to keep looking. These experiments leave room for some iterations of chameleons to live, Burrage told Gizmodo. It’s now a matter of increasing the sensitivity of these experiments.