It is now well established through observations that the universe is expanding which has been speeding up over time. However, the cause of the expansion is not yet known. It is assumed to be a mysterious energy acting opposite to gravity causing this expansion.
This energy is termed as “dark energy” and it is estimated that it makes up to 72 percent of the constituents in the universe. A new experiment has confirmed that it is not a new force. The study appears in the Physical Review Letters journal. Ed Copeland, a physicist from the University of Nottingham said that the experiment related to cosmology and atomic physics helped in ruling out several models which have been proposed for the explanation of dark energy and will eliminate many more models.
There are four fundamental forces in the Universe which are gravity, electromagnetic force, strong nuclear force and weak nuclear force. Some anomalies do not fit into the current model of how the Universe functions. So they are clubbed under a fifth hypothetical force.
These anomalies have helped in putting constraints on the fifth force and helped in constructing alternatives to general relativity for explaining the increasing rate of expansion of the Universe. Some of the alternatives are chameleon theory and symmetron theory.
If another force present in the Universe was responsible for almost 75 percent of its mass-energy density then it should be easy to detect. But if it acts opposite to gravity i.e. a repulsive force then it should be weakest in the area where gravity is the strongest force. This should hold true for objects with significant mass such as a planet.
An experiment was designed to observe this anti-gravitational force. Within a sealed vacuum chamber, a large aluminium sphere and small weights – atoms of rubidium-87 were placed and atom interferometry was used for measuring the acceleration of the smaller weight towards the larger one. If a fifth force was present then the atom should have been veered off its path as it moved towards the sphere. But no such deviation was observed which clearly explains that no such anti-gravity force was present in the chamber. It does not mean that there is no fifth force at all. However, the force can be defined by several constraints which also means that similar constraints can be placed on chameleon and symmetron theories.
There may not be always a straightforward answer to fundamental questions of this nature so the correct questions are asked to narrow down on the correct answer. Answering this question would give significant answers about the evolution of the universe.
Journal Reference: Physical Review Letters