A group of researchers from Nanjing University and the University of Science and Technology of China developed an experiment that tested a model for dark energy. The results of the experiment ruled out the model in question that predicted a fundamental fifth force in nature
Dark energy surrounds quite a bit of mystery. It is simple to describe but full of question marks. Its nature has been discussed by many, and it is likely that more will be discussed in the future. The study of dark energy began with the discovery of Einstein’s equation and observations of the expanding universe. Broadly speaking, it is energy that fills the universe that generates negative pressure, that is, it creates expansion and not contraction, unlike the other known gravitational sources. This is an explanation that can satisfy the appetite of many, but physicists do not stop there. They continue to wonder about the source of dark energy and its repulsion mechanism. They know that physical objects with defined energy are gravitationally attracted to each other and do not repel each other. In quantum language, objects are described as fields and energy as an excitation of the field. For example, the light particles are nothing more than excitation, or waves on the electromagnetic field. Similarly, the electrons and quarks appear as waves on matter fields.
This principle has led physicists around the world to discuss the possibility that the source of dark energy stems from a quantum field that creates gravitational repulsion. The next question is of course which field, and what are its properties.
Over the years, a number of more or less plausible options have been put forward, the most prominent of which is the chameleon theory. This theory takes into account that dark energy is more significant when the density of matter is low. The reason for this is, of course, observational, after all, dark energy is able to repel entire galaxies but is not able to break them up. The chameleon model is so named because the mass of the particle associated with the chameleon field changes as a function of the density of the normal matter in its environment. This may sound like an extreme assumption that breaks the principle of equivalence, but thanks to it it is possible to calibrate the medium of interaction between the chameleon field and the rest of the matter in the universe. If the chameleon field is massive, the interaction medium is very short, similar to the weak and strong nuclear forces. If the mass of the chameleon is negligible, its interaction medium is very long, similar to photons that can mediate the electromagnetic forces even over long distances. Therefore, the gravitational repulsion in our environment is very weak because the mass of the chameleon is large and its range of influence is low. On the other hand, in sparse areas, the chameleon field is not massive and its effect is mediated over large distances.
In order to test the chameleon model, researchers from China developed an experiment that should be sensitive to the force exerted by this field on bodies with mass. Using a rotating gear and a floating test mass, the researchers hoped to observe a cyclic force resulting from the approach and distance of the mass on the gear relative to the test mass. After they removed the influence of external forces, including gravitational and electromagnetic forces, no indication of a fifth force was observed from the hypothesized chameleon field. The researchers published the results of the experiment in the prestigious journal Nature Physics a few days ago. According to the researchers, the experimental system should be sensitive to the range of strengths of the chameleon field if it does explain the effect of dark energy on the universe. Since the field effect was not observed as predicted, the researchers believe that this explanation is completely ruled out. At the same time, they think that the experimental system can be used in the future for the benefit of examining additional models of dark energy and for the benefit of sensitive gravitational measurements.
Although this experiment joins the collection of “failed” experiments that did not yield positive results, the advantage of the chameleon model was that it could be tested experimentally. Let’s hope that in the future efforts will continue to rule out or verify such models, similar to the chameleon and symmetron models.
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