Shining a light on dark matter

David Heath
Monday, 21 December 2009 10:17

Science - Space

First postulated in the 1930s by Astronomer Fritz Zwicky, dark matter was simply a description for matter, the existence of which could only be inferred by calculating the amount of matter required to fully create galaxies and their gravitational effects and from that deduct the visible material.  The difference, or 'missing mass' was obviously 'dark.' 

Most calculations suggested that dark matter makes up around 80% of the known universe and for a very long time, there have been no observations, just speculation as to its make-up.

Enter an abandoned iron ore mine in Minnesota. 

For some years, astrophysicists have hoped to gain an insight into the structure of dark matter by observing interactions between it and more 'normal' matter.  To this end, the Cryogenic Dark Matter Search (CDMS) experiment has maintained 30 super-cooled detectors 800m below ground at the abandoned Soudan iron mine in northern Minnesota since 2003.

Data from 2007-2008 shows two clear events that seem to be an interaction between a Weakly Interacting Massive Particle (WIMP) - the hypothetical candidate for dark matter - and a nucleus of the material in the detector.  According to the results summary, "two events with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made. The ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be fewer than one chance in a thousand of the signal being due to background. In this case, a signal of about five events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs."

However, the scientists are still excited.  What next?