The Invisible Giant: Dark Matter is Quietly Reshaping the Universe

Dark matter has been one of the greatest astronomical mysteries for a very long time. It was first speculated as far back as the 1600’s, not long after Newton presented his theory of gravity, as astronomers considered objects that may emit little or no light but could instead be observed by their gravitational effect on other, visible objects. With the discovery of dark nebulae, which can only be identified by the light they absorb from bright objects behind them, it seemed clear that there was much more to the Universe than could be seen by visible light.

Our current understanding of dark matter may be attributed, in part, to Fritz Zwicky, who, in 1933, studied the motion of galaxies in the Coma Cluster. Basically, the Coma Cluster is a super-cluster of more than 1,000 galaxies, all gravitationally bound together. With the ability to measure the galaxies’ speeds, it is possible to determine the mass of the cluster. However, Zwicky’s result was around 500 times greater than Hubble’s prediction, which estimated the existence of 800 galaxies, each containing around a billion stars. This led to the conclusion that dark matter is present in far greater quantities than luminous matter.

Credit: Third Party Reference

Later on, from the 1950s onwards, studies of very distant spiral galaxies revealed a very unexpected characteristic. Scientists expected the galaxies to be made up of fast-moving central objects, with more slowly moving objects in the arms. This, however, was not the case. They instead found that objects moved at the same speed at all distances from the center. This result suggested that the galaxies contained more mass than the scientists could see.

So far, dark matter may only be observed through its interaction with other visible objects. On July 12th, a report was released stating that researchers of the international team using the PandaX detector in China have new experimental data showing that dark matter interacts with ordinary matter in more ways than just through gravity.

They have imposed constraints on the manner in which dark matter interacts with ordinary matter, which will aid us in finally identifying the dark matter particle* and even detect it here on Earth.

“Despite these hypotheses, the term ‘WIMP’ has been popularly bandied about – WIMP standing for ‘Weakly Interacting Massive Particle’, and pointing to a non-baryonic particle. These particles, however, are currently hypothetical. “

Dark matter (simply non-luminous space material) is considered to make up 85% of the matter in the Universe (this percentage includes dark energy). The favourite, WIMP theory, has been challenged by the international research team at PandaX.

They propose a new theory: the self-interacting dark matter model (SIDM).

This new model takes into account all of the dark matter effects thus far observed and was used by the PandaX-II team (PandaX is the detector – Particle and Astrophysical Xenon Detector, whereas PandaX-II refers to the name of the experiment) in their search for dark matter interactions. If a dark matter particle were to collide with the PandaX detector’s liquified xenon, the result would be two simultaneous signals: one of electrons and one of photons. The assumption is made for this experiment that dark matter interacts with ordinary matter through more than just the gravitational interaction, and that the mediator of this force is much less massive than predicted in the WIMP model.

Credit: Third Party Reference

With this change in the theory from a massive mediator particle to a light mediator particle, it seems it could be very possible to pick out extremely clear dark matter signatures using the PandaX detector. This will be one of the world’s most sensitive direct detection experiments, and it seems highly probable that PandaX-II will be the first experiment to discover the elusive dark matter particle.

Reference: https://www.space.com/