

Video: Satellite galaxies Video: Satellite galaxies

Something missing? (Image: NASA/ESA/STScI/AURA/A. Aloisi) Have you seen these galaxies? (Image: NASA)

LIKE moths about a flame, thousands of tiny satellite galaxies flutter about our Milky Way. For astronomers this is a dream scenario, fitting perfectly with the established models of how our galaxy’s cosmic neighbourhood should be. Unfortunately, it’s a dream in more ways than one and the reality could hardly be more different.

As far as we can tell, barely 25 straggly satellites loiter forlornly around the outskirts of the Milky Way. “We see only about 1 per cent of the predicted number of satellite galaxies,” says Pavel Kroupa of the University of Bonn in Germany. “It is the cleanest case in which we can see there is something badly wrong with our standard picture of the origin of galaxies.”

It isn’t just the apparent dearth of galaxies that is causing consternation. At a conference earlier this year in the German town of Bad Honnef, Kroupa and his colleagues presented an analysis of the location and motion of the known satellite galaxies. They reported that most of those galaxies orbit the Milky Way in an unexpected manner and that, taken together, their results are at odds with mainstream cosmology. There is “only one way” to explain the results, says Kroupa: “Gravity has to be stronger than predicted by Newton.”

Challenging Newton’s description of gravity is controversial. But regardless of where the truth lies, the Milky Way’s satellite galaxies have become the latest battleground between the proponents of dark matter and theories of modified gravity.

Our standard picture of the universe comes from many decades …