The universe is generally believed to be expanding at an accelerating rate, thanks to a mysterious force dubbed dark energy. But how exactly does this force work? Researchers in the UK have conducted an experiment to test a hypothesis that this fifth fundamental force works on single atoms – and found no trace of it.
Dark energy was "discovered" in 1998, when physicists realized that the expansion of the universe wasn't slowing down, as they expected. Instead, it seemed to be doing the exact opposite. Further observations over the years have backed up the theory, and now it's a widely accepted explanation that explains a lot of weirdness in the standard model of cosmology.
But what is dark energy, exactly? Nobody really knows, but it appears to be a fifth fundamental force, with the known four being gravity, electromagnetism and the strong and weak nuclear forces. Whatever dark energy is, it seems to act almost like the opposite of gravity, strongly pushing matter away from all other matter instead of pulling it together.
One branch of theories suggests that dark energy gets weaker the more matter is around it, meaning it's at its strongest out in the vacuum of space. That could also explain why some previous tests to detect it came up empty – they were using large weights, which would have reduced the effect of the force.
So a team from the University of Nottingham and Imperial College London devised new experiments to test this theory. They set out to investigate the interactions between a large and a small weight. In this case, the "large" weight was a metal ball the size of a marble, while the small weight was a single atom.
The researchers placed the ball in a vacuum chamber, then dropped atoms around it. The idea was that if there was a fifth force acting between the two weights, and it behaved the way this theory suggested, then the path of the atom should bend slightly as it passed the marble.
After running these tests, the team found no trace of any such force. That effectively rules out this particular theory, which would also have required modifications to the theory of gravity. Similarly, another recent study ruled out "chameleon" particles from the Sun as carriers of the force.
But a null result isn't confirmation that dark energy itself doesn't exist – it just means it could be hiding elsewhere, and we've now narrowed the search.
"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," says Ed Copeland, lead author of the study.
The new experiments could also add to a pile of increasing evidence that the dark energy theory itself is wrong. Other alternative explanations include "Tardis" regions of spacetime that are bigger on the inside, a "dark fluid" with negative mass, or that by just accounting for the changing structure of the cosmos, the need for dark energy as a stand-in vanishes. Some even call into question the validity of the original discovery that the expansion of the universe is accelerating.
With so many questions surrounding this mysterious force, no doubt many more studies will be conducted to try to unravel the enigma.
The research was published in the journal Physical Review Letters.
Source: Imperial College London
