It’s no secret that the universe is expanding. Astronomers wonder if, at some distant point in the future, the universe will cease to expand and begin contracting. The problem is that there isn’t enough matter in the universe to prevent it from expanding into infinity, or flying apart. If this is the ultimate fate of the universe, what has prevented it from happening in the 13.7 billion years that the universe has already existed? This question, as well as others, only has one possible answer. The universe must contain more matter than what is actually visible.
Scientists have named this “missing mass” dark matter, and they estimate that it accounts for over 80 percent of the total mass of the universe. The problem is that no one really knows for certain whether it exists, because it is invisible. If dark matter exists, its gravitational effects would distort astronomical data. Discovering it would enable scientists to gather more accurate data in their quest to understand the universe and its ultimate fate.
A project is underway deep inside the abandoned Soudan mine in northern Minnesota to prove the existence of dark matter. The project, known as the Cryogenic Dark Matter Search, aims to provide evidence for the existence of particles called WIMPs. WIMP is an acronym for weakly interacting massive particle, and they are a byproduct of the radioactive decay of dark matter. Scientists have predicted that WIMPs occasionally interact with atoms of normal matter, although such interactions are very rare and weak. When they do happen, these interactions generate energy in the form of a small amount of heat and also create ions.
The CDMS uses an array of crystalline detectors that are cooled to a temperature very near absolute zero to enable detection of the minute amount of heat generated by WIMP interactions. The signals created by particle interactions within the detectors are amplified and recorded by a computer for later analysis. By comparing properties such as the size and relative timing of these signals, researchers are able to determine whether the particle that interacted with the crystal was a WIMP or one of the other numerous particles that fall to Earth as cosmic radiation. Several layers of material as well as a half-mile thick layer of rock above the detectors help to filter out normal cosmic rays. Unfortunately, neutrons produced within the Earth could interact with the detectors to produce signals similar to those that would be produced by WIMP interactions. These neutron interactions are known as background events. Typically, the chance of a signal being due to background events rather than actual WIMP interaction must be less than one in one thousand to be considered an actual discovery.
In 2010, two events were reported that may have led to the discovery of WIMPs. These events, however, turned out to be indistinguishable from other background events with a one-in-four chance of the signals' being caused by WIMP interactions; much too high to fulfill the requirement for particle discovery. Due to the inconclusive nature of these findings, the team decided to replace the germanium detectors with silicon, a more sensitive crystal, to create the CDMS II.
On April 13 of this year, three more events were reported, this time with a much higher level of confidence and a signal-to-background ratio of only 0.19 percent, much lower than the 25 percent ratio obtained from the CDMS. Furthermore, the CDMS II results suggested a particle much lighter than what had been predicted but remained consistent with other theories regarding dark matter. Some scientists on the project still don’t believe that the CDMS II findings indicate a true discovery, but they do believe that further investigation is warranted. The SuperCDMS, which uses more sensitive detectors, is now running in the mine and is expected to yield further insight into the existence of dark matter. In addition, a second facility will be opened in Canada to gather more data.