On Wednesday, scientists announced that a massive particle detector known as the Alpha Magnetic Spectrometer-2, mounted on the International Space Station, may have detected evidence of dark matter.
Dark matter is considered to be the elusive, hidden substance that “holds” the universe together. Although dark matter has thus far been undetected by instruments, it is thought to be nearly six times as prevalent in the universe as ordinary matter, which makes up approximately four percent of our universe. In contrast to ordinary matter, which can be seen easily, dark matter does not emit or interact with light and cannot be detected by telescopes. Scientists know dark matter exists because “something” in the universe is exerting great forces on the things we can see—dark matter’s presence is detected through its gravitational pull on normal matter.
According to Space.com, “Physicists have suggested that dark matter is made of WIMPs, or weakly interacting massive particles, which almost never interact with normal matter particles. WIMPs are thought to be their own antimatter partner particles, so when two WIMPs meet, they would annihilate each other, as matter and antimatter partners destroy each other on contact.”
The discovery recorded by the Alpha Magnetic Spectrometer-2 (AMS-2) consisted of about 400,000 positrons, which are the antimatter partner particles of electrons. Based on physicists’ speculation of what happens when there is a collision between WIMPs, “the energies of these positrons suggest they might have been created when particles of dark matter collided and destroyed each other” according to Space.com.
So are these positrons evidence of dark matter? According to Space.com, “the characteristics of the positrons detected by AMS-2 match predictions for the products of dark-matter collisions…Furthermore, the positrons appear to come from all directions in space, and not a single source in the sky. This finding is also what researchers expected from the products of dark matter, which is thought to permeate the universe.”
Although the positron signal detected could be evidence of dark matter, it could also have come from another source, such as pulsars in our Milky Way Galaxy. Pulsars are highly-magnetized pulsating stars which emit a beam of electromagnetic radiation.
It is hoped that as AMS-2 continues to collect more data, scientists will be able to determine the actual source of the signal. In the meantime, scientists will attempt to detect WIMPs using “underground experiments on Earth such as the Cryogenic Dark Matter Search and XENON Dark Matter Projects” according to Space.com.
The Cryogenic Dark Matter Search (CDMS) “is a series of experiments designed directly to detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials.” The original experiment took place in a tunnel under the campus of Stanford University. The current experiment is “located deep underground in the Soudan Mine in northern Minnesota.”
The XENON Dark Matter Project is attempting to detect WIMPs “through their elastic scattering,” and is located underground at LNGS, Italy. At the time of its inception in 2007, Xenon was considered “to be one of the most important experiments in the world by Discover Magazine.”
So what does this discovery mean and why is dark matter important? Because matter and antimatter annihilate one another on contact, one of the big questions in astrophysics has been “where is all the antimatter?” The discovery by AMS-2 may help scientists finally move from inference to actual evidence of the existence of dark matter.
Understanding dark matter is important because dark matter not only gives our universe its structure, it may also determine our universe’s fate. We know our universe is expanding, and that “gravity will ultimately determine the fate of the expansion.” Because gravity is dependent upon the mass of the universe, measurements regarding mass need to include both light and dark matter. Thus, it is important to know just how much dark matter there is in our universe to know our universe’s ultimate fate.
As cosmologist Dr. Michael Turner of The University of Chicago says, “Without dark matter the universe doesn’t work. Without dark matter our galaxy would fly apart.”
Once a cosmic mystery, dark matter may finally be revealed in light of the recent discovery by AMS-2.
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