Scientists are always looking to understand the nature i.e., cosmos (universe), its existence and working. In this regard, energy and matter in terms of fundamental particles and forces controlling them are always at the forefront of scientific research. The discovery of dark energy has greatly changed how we think about the laws of nature. Concepts of dark matter and dark energy have always intrigued and puzzled scientific community. Physics textbooks teach that there are four fundamental forces of nature: gravity, electromagnetism, and the strong and weak nuclear forces. Scientists have unified all the forces of nature to four basic forces in order to explain most of the universal consequences but now they are thinking about a fifth or unknown force to understand and explain the concepts of dark matter and dark energy. An experiment to test a popular theory that dark energy –the unknown force that is causing the universe to expand at an accelerating rate– is a ‘fifth’ force that acts on matter has found no evidence of its existence. Some physicists propose dark energy is a ‘fifth’ force beyond the four already known forces. However, researchers think this fifth force may be ‘screened’ or ‘hidden’ for large objects like planets or weights on Earth, making it difficult to detect. Astronomers have found an unexpected link between mysterious ‘dark matter’ and the visible stars and gas in galaxies that could revolutionize our current understanding of gravity. The finding suggests that an unknown force is acting on dark matter. Such a force might solve an even bigger mystery, known as ‘dark energy’, which is ruling the accelerated expansion of the Universe. Dark energy is a mysterious universal force with a fantastical name. First predicted by a version of Einstein’s general theory of relativity, support for its existence was only uncovered near the end of the twentieth century. Even today, relatively little is known about it.
According to scientists, only 4% of the universe is made of known material and rest is dark matter. Stars and gas in galaxies move so fast that astronomers have speculated that the gravity from a hypothetical invisible halo of dark matter is needed to keep galaxies together. However, a solid understanding of dark matter as well as direct evidence of its existence has remained elusive. Now, scientists believe that the interactions between dark and ordinary matter could be more important and more complex than previously thought, and even speculate that dark matter might not exist and that the anomalous motions of stars in galaxies are due to a modification of gravity on extragalactic scales. Scientists explain that the dark matter seems to ‘know’ how the visible matter is distributed. They seem to conspire with each other such that the gravity of the visible matter at the characteristic radius of the dark halo is always the same. This is extremely surprising since one would rather expect the balance between visible and dark matter to strongly depend on the individual history of each galaxy. It is possible that a non-gravitational fifth force is ruling the dark matter with an invisible hand, leaving the same fingerprints on all galaxies, irrespective of their ages, shapes and sizes. The implications of the new research could change some of the most widely held scientific theories about the history and expansion of the universe.
A supernova is a massive explosion of a star and the immense release of light and energy from the event against the dark galactic backdrop outshining entire galaxies. The brightness and colour of the light observed from a supernova provides an estimate of our distance from the dead star. Using this information to calculate the distances of dying stars, researchers are able to study the size of the universe and the speed of its expansion. Deep space observations of supernovae, especially those made using the Hubble space telescope suggested that the universal expansion was instead picking up speed. Dark energy functions as an antagonist to gravitational force, as gravity is described to bring matter together, dark energy pushes it apart.
The prevailing theory for much of the past century suggested that after the Big Bang, the universe was expanding, but that gravity was causing the rate of expansion to slow down. However, there was a discrepancy in the expected and observed brightness from the most distant supernovae. Knowing the rate for the expansion of the universe, dark energy is quantified to currently comprise roughly 68 per cent of the universe. It has a density of approximately 10-29 grams/centimetre3, which can be analogized as the mass of a feather roughly divided by the volume of Earth - adding to the challenging endeavour of studying its characteristics. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative. The 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 darker energy models. The experiment tested theories of dark energy that propose the fifth force is comparatively weaker when there is more matter around – the opposite of how gravity behaves. This would mean it is strong in a vacuum like space, but is weak when there is lots of matter around. Therefore, experiments using two large weights would mean the force becomes too weak to measure. Now, researchers have tested the possibility that this fifth force is acting on single atoms, and found no evidence for it in their most recent experiment. This could rule out popular theories of dark energy that modify the theory of gravity, and leaves fewer places to search for the elusive fifth force. The team used an atom interferometer to test whether there were any extra forces that could be the fifth force acting on an atom. A marble-sized sphere of metal was placed in a vacuum chamber and atoms were allowed to free-fall inside the chamber. The theory is, if there is a fifth force acting between the sphere and atom, the atom’s path will deviate slightly as it passes by the sphere, causing a change in the path of the falling atom.
Acknowledgement: The use of information retrieved through various references/sources of internet in this article is highly acknowledged.
