Originally, physicists believed dark matter could interact with itself and with ordinary matter. Now, a new theory puts some constraints on the nature of this relationship.
Most of the Universe is “dark”.
80 to 85 percent of the matter in the Universe is dark matter, which is accumulated mainly in clusters of galaxies and plays a crucial gravitational role in the mechanics of our Universe.
The fundamentality of dark matter to astrophysics and cosmology is proportional to its omnipresence and pervasive effects on the rest of matter.
Particle physicists believe that not only is dark matter a dominant constituent of the cosmos, it also interacts with visible matter.
They also think that beyond its “visible” gravitational manifestation, dark matter could be forever interacting with ordinary matter in other ways.
The Hunt for Wimpy WIMPs Continues
The difficulty scientists face in their study of dark matter stems from its very nature; it either doesn’t interact at all with ordinary matter, or it does but at undetectable levels.
Gravitational effects of dark matter on galaxies and matter therein are scientifically glaring.
But, finding how it interacts with itself and the rest of matter in the visible Universe will help astrophysicists unlock its mystery as well as many secrets of the mechanisms of the cosmos.
There are a number of models that describe dark matter. The most accepted by the scientific community is based on WIMPs, or Weakly Interacting Massive Particles, also known as theoretical dark matter particles.
Decades after it was first proposed, the WIMP paradigm can’t be considered canon until WIMPs existence is proved.
All experiments aiming at spotting WIMPs, which don’t emit any light, have yet to succeed.
One of these experiments is using the Large Underground Xenon (LUX), a highly sensitive detector of dark matter particles, which lies nearly one mile underground in South Dakota to hunt for WIMPs.
PandaX-II is another ongoing WIMP experiment that uses Pandax (Particle and Astrophysical Xenon Detector), a detector buried underground in the Sichuan Province of China.
At about 1.5 miles deep inside earth, Pandax is the deepest underground dark matter detector.
Physicists Challenge the WIMP Model
To directly detect dark matter particles, Pandax uses 500 kg of liquid xenon that, should a dark matter particle pass through it, would generate two signals at the same time.
Besides just gravitational interaction, scientists working on the Pandax-II experiment assume dark matter can interact with ordinary matter in other ways. So, they also focus on identifying these signals.
The WIMP paradigm suggests the existence of “mediator particles”, hypothetical particles that express the interaction between dark matter and ordinary matter
Now, an international team of particle physicists has analyzed two-years worth of data (2016-2017) from the Pandax-II and think this experiment is more likely to validate another dark matter model.
This model, known as SIDM, for Self-Interacting Dark Matter Model, was first proposed in 2000.
Hai-Bo Yu, a particle physicist at the Department of Physics and Astronomy at the University of California Riverside, is one of the study’s co-leaders.
Yu explained that:
“The WIMP paradigm assumes this mediator particle is very heavy — 100 to 1000 times the mass of a proton — or about the mass of the dark matter particle… The SIDM model, on the other hand, assumes the mediator particle is about 0.001 times the mass of the dark matter particle… The presence of such a light mediator could lead to smoking-gun signatures of SIDM in dark matter direct detection, as we suggested in an earlier theory paper. Now, we believe PandaX-II, one of the world’s most sensitive direct detection experiments, is poised to validate the SIDM model when a dark matter particle is detected.”
As to the significance of the study’s findings, Flip Tanedo, a dark matter scientist at UC Riverside said:
“This is a particle physics constraint on a theory that has been used to understand astrophysical properties of dark matter. The study highlights the complementary ways in which very different experiments are needed to search for dark matter.”