An experiment involving slow-moving neutrons dealt a huge blow to the theory of Symmetron Field, which was considered a candidate for explaining the Dark Energy.

Dark Energy is a strange form of energy which is spread anywhere and everywhere in space. According to the Standard Model of Cosmology, it contributes 68.3% of the total energy present in today’s observable universe. Despite the fact that scientists hardly know a thing about it, they believe that it is responsible for the continuous acceleration in the expansion rate of the universe. Researchers have spent years to find a scientific explanation for this form of energy but all of their efforts have gone in vain. Recently, scientists carried out an experiment to unveil a mysterious particle, known as Symmetron, but things didn’t go according to plans.

It becomes impossible to elaborate certain cosmological phenomena, like the expansion of universe, with the current set of particles and the forces associated with them. This gave birth to new entities including Dark Energy and Dark Matter which helped the scientists to explain the existence of those unique concepts. One of the theories, which were associated with the dark energy, is called Symmetron Field and it says that space acts just like a Higgs field.

The researchers at the Vienna University of Technology created an experiment to check the effectiveness of this claim. The experiment was capable of measuring extremely small forces by making use of neutrons. A 100-day campaign was arranged at the Institut Laue-Langevin and the measurements were taken on its PF2 ultra-cold neutron source. The researching team was hoping that they will be guided to the symmetrons but the particles didn’t seem to exist at all. It will not mark the end of this theory but it is certainly a massive blow as it proves that the symmetrons cannot exist at a broad range of parameters and scientists will have to look for a different explanation of dark energy following this experiment. The Lead Scientist of the project, Hartmut Abele, referred to that in the following words:

“We already have proof of the Higgs field, and the symmetron field is very closely related. Nobody can say what the mass of symmetrons is, nor how strongly they interact with normal matter. That’s why it’s so hard to prove their existence experimentally — or their non-existence.”

Due to the fact that nothing can be said about the existence of symmetrons outside a specific range of parameters, scientists are taking their time before switching from one experiment to the next. The symmetrons with very high coupling constants, in a certain range, can be avoided because they would have been detected earlier in other experiments which used Massive Pendula. Similarly, another instance that does not needs to be investigated is to have symmetrons with low coupling constants and high mass. The reason for this is that such particles would have appeared at the atomic level. However, there are still a lot of options left and the researching team investigated them in this experiment.

Neutrons can have two independent quantum physical states and the energies of these states depends on the forces exerted on the neutron. This is the reason why neutrons are considered an extremely sensitive force detector. A stream of extremely slow neutrons was bombarded between two mirror surfaces. If the force on the neutron varies according to its location, this is a strong indication about the existence of a symmetron field. For instance, if the force acting on the neutron just above the surface of the mirror is different from the force further up, a symmetron field could be there. Phillipe Brax of CEA, Guillaume Pignol of LPSC, and Mario Pitschmann of TU Vienna worked on finding the influence of a symmetron field on a neutron. Tobias Jenke, who played an integral part in setting up the experiment at the TU Vienna, praised the precise measurements of the PF2 instrument and said,

“With its unrivalled flux of ultra-cold neutrons, PF2 is practically the only instrument out there for this type of high-precision measurement at extremely low count rates.”

This experiment might have initiated the time bomb for the symmetron field theory but it will be too early to exclude it completely. Scientists will either need to come up with a different theory to explain the dark energy or they will need to improve their measurements even more. For now, the researchers concluded the matter by saying,

“We have excluded a broad parameter domain: if there were any symmetrons with properties in this domain we would have found them.”