The Southern African Large Telescope (SALT), based at Sutherland, in the Karoo region, in the Northern Cape province, is promising to become an important instrument for research into dark matter. “SALT is shaping up to be very important for answering questions about dark matter in the cores of galactic clusters,” affirmed Michigan State University astronomer Professor Megan Donahue. “SALT has given us information we haven’t be able to get any other way.”

Donahue is a member of a major astronomical research programme, designated (Galaxy) Cluster Lensing and Supernova survey with Hubble (CLASH). Observations from SALT have contributed to this programme, although the main sources of data have been space telescopes.

CLASH has been made possible by the complementarity of the different types of space telescope now in operation. In addition to the Hubble Space Telescope, operating in the visual wavelengths, observations have also been taken from the Chandra X-ray Observatory, the XMM (X-ray Multi-Mirror)-Newton Observatory and the Spitzer space telescope (in the infrared wavelengths). Other, ground-based telescopes have also assisted.

Among many other important discoveries, CLASH has confirmed important concepts regarding dark matter. This substance, about which almost nothing is known, makes up about 27% of our universe (the even more mysterious dark energy comprises some 68% of the universe) while conventional matter is responsible for less than 5%.

“CLASH is giving us the best maps of dark matter ever,” she reported. “There was some worry that maybe we were missing something regarding dark matter in our earlier studies. There were some tensions between models and observations. What we wanted to test was if observations of a properly selected sample of galactic clusters would resolve these tensions. With a well-chosen random sample, we are getting results closer to what we expected. So the theory has survived the experimental test!”

Currently, dark matter – so called because it neither reflects nor absorbs light, and so is invisible – is conceived as being composed of slow-moving, therefore “cold”, particles and so is referred to as CDM (cold dark matter) for short. When dark matter was first conceived, there was no indication what it would be like: it could, for example, have been ‘hot’ (fast-moving).

Initial, pre-CLASH, observations revealed that galaxy clusters are composed of 85% CDM and 13% normal matter. These results differed from the predictions derived from dark matter simulations. A number of explanations for this discrepancy were proposed. One was that the simulations took only CDM into account and ignored the effects of more conventional matter made up of protons, neutrons and electrons, including intergalactic gas, stars and planets. Another was that these first observations simply suffered from sample bias – only a limited number of galaxy clusters had been studied, and these were all prominent and powerful gravitational lenses. (Gravitational lensing happens when the gravitational field of a star, galaxy or galaxy cluster bends the light from a more distant object behind it, creating double or multiple images of that object, allowing it to be seen; otherwise, it would not be detectable.) A more extreme suggestion was that there was an error in the generally accepted conception of the universe.

The results of CLASH have established that, while conventional matter is a factor in the discrepancy between the simulations and the early observations, they are only a small factor, responsible for about 30% of the discrepancy. The real problem was revealed to be sample bias. The relationship between the CLASH observations and the predictions of the mass concentrations of clusters in dark-matter-only simulations is 0.96 plus or minus 0.18 (statistically consistent with 1.0).

The behaviour of conventional matter differs from that of CDM. CDM primarily interacts with other matter through the force of gravity, while more conventional matter also interacts with other matter through photons. For example, conventional matter can collide with itself and lose energy by radiating light, but CDM cannot. The complex behaviour of conventional matter, much more computationally taxing and difficult to calculate, alters the predicted distribution of matter in the universe, even though conventional matter comprises a small fraction of the total mass budget. Comparisons with more complete simulations including the effects of conventional matter are planned to check this result and to test the more difficult simulations.

“The concept of CDM has survived all the experiments and observations. The alternative concepts of dark matter – for example, hot dark matter – have not,” explained Donahue. “CLASH results are essentially confirming that dark matter is made of cold particles. We’ve never found one in a laboratory – yet.”

Donahue presented a plenary session paper at the recent 2014 South African Institute of Physics conference at the University of Johannesburg.