304 North Cardinal St.
Dorchester Center, MA 02124
The story so far: Many physicists firmly believe that the entire visible part of the universe is only 5% of all the matter in it. He believes the rest is made up of dark matter and dark energy. Once this was conclusively demonstrated through various indirect observations and calculations, experiments began to be prepared to hunt down these elusive particles. The latest innovation in the field of dark matter is an experiment with a dark matter detector called LUX-ZEPLIN (LZ) in South Dakota, USA. To date, it is the most sensitive dark matter detector in the world. To give an idea of the level of difficulty in measuring evidence of a dark particle, it is said that the chamber of this LZ detector can only contain one gram of dust if it is to detect a dark matter particle. This is the extent to which researchers must go to rule out unwanted signals coming from other entities.
All interactions in the universe are the result of four fundamental forces acting on particles – the strong nuclear force, the weak nuclear force, the electromagnetic force and gravity. Dark matter is made up of particles that have no charge – meaning they do not interact through electromagnetic interactions. So these are particles that are “dark” specifically because they don’t emit light, which is an electromagnetic phenomenon, and “matter” because they have mass like normal matter and therefore interact through gravity.
The gravitational force, in addition to not being fully integrated and understood by particle physicists, is extremely weak. First, a particle that interacts so weakly becomes rather elusive to detect. This is because the interactions of other known particles could drown out the signals of dark matter particles.
There is strong circumstantial evidence for dark matter, and this evidence is reflected at different levels (or distance scales, as physicists would explain). On the shortest distance scale, consider the rotation of galaxies. If you look at the stars all the way from the center of any galaxy to its edge, the way the velocities of the observed stars vary can be plotted. In the laboratory, the same function can be plotted on a graph, assuming that visible matter is all there is. There is a significant difference between the observed plot of stellar velocities and the calculated value as you move from the inner part of the galaxy towards its edge. If you now assume that there is some fraction of matter that exerts a gravitational force on the rest of the stars in the galaxy because it cannot be seen any other way, and recalculate the graph, it fits the observed value. This means that there is a certain amount of dark matter in the galaxy.
One may argue that the model is at fault and that there is some other way to reconcile this discrepancy between the calculated and observed velocities in rotating galaxies. Here, evidence from other distance scales emerges.
The universe can be observed at different levels — at the level of electrons and nuclei or atoms, or galaxies or galaxy clusters, or even at greater distances where the entire universe can be mapped and studied. Cosmologists, the people who study the physics of the universe, usually work at the last three scales, and particle physicists study the lowest and even smaller scales.
In this context, the second piece of evidence came from the observation of the so-called Bullet cluster of galaxies. The Bullet cluster is formed by the merger of two galaxy clusters. Physicists have found from their calculations that the way these mergers occurred cannot be fully explained if we believe that the visible universe is all there is. Therefore, there should be such a thing as dark matter, and also an estimate of how much dark matter there should be in the universe.
Similar arguments exist from space mapping such as the Sloan Digital Sky Survey and studies of the fibrous nature of the universe at a closer look. While correcting the model could help explain one of these discrepancies, not all of them can be explained in the same way. Therefore, physicists now take the concept of dark matter very seriously.
“The neutrino would be an excellent candidate if it were more massive,” says Shrihari Gopalakrishna of The Institute of Mathematical Sciences, Chennai, who has worked on dark matter theory. However, being too light doesn’t quite fit the bill. Other postulated entities include the supersymmetric partner of the Z boson, a particle that mediates the electroweak interaction. Yet another explanation speaks of “hidden sector particles” and Axions, the dark matter boson and condensate. There are many other theories.
The search for one of these candidates continues, because the story is a story that combines gravity, supersymmetry, hidden worlds to form science fiction.
Dark matter is made up of particles that have no charge. These particles are therefore “dark”, specifically because they do not emit light, which is an electromagnetic phenomenon, and “matter” because they have mass like normal matter and interact through gravity.
There is strong circumstantial evidence for dark matter, and this evidence is reflected at different levels (or distance scales, as physicists would explain).
To date, the most sensitive dark matter detector experiment in the world is LUX-ZEPLIN (LZ) in South Dakota, USA.