Not so fast on modified gravity

An international group of astronomers including Oliver Müller from the University of Basel has debunked another premature claim that a leading modified gravity theory is dead – publishing their results in the prestigious journal Nature.

The earlier claim – also in Nature – neglected a subtle but vital environmental effect in the theory known as Modified Newtonian Dynamics (MOND).  

The claim came from the slowness of the motions within a distant dwarf galaxy. The authors argue that its extra gravity in MOND would speed up these motions, thus the observations seemed to be strongly in disfavour of MOND. However, they did not consider that the gravitational environment of the dwarf could affect motions within it. If it is close to a massive galaxy – which is the case here – then the motions within the dwarf would be slower.

“This effect has been known for a long time,” said Hosein Haghi, a professor in the physics department at the Institute for Advanced Studies in Basic Sciences, Zanjan, Iran. “These Nature authors were unaware of our papers on how to include it.”.

Galaxies rotate so quickly that they should fly apart. This can be avoided in two ways. One is to invoke a halo of dark matter around every galaxy. However, dark matter particles have never been discovered despite many decades of very sensitive searches, often using large detectors. The other way is to modify the law of gravity on astronomical scales, far beyond those where we know for sure what the right law is. This MOND approach explains a huge variety of galactic rotation curves using only their visible mass, without the need to invoke a dark matter particle.

“There have been many premature claims on the death of MOND in very influential journals,” said lead author Pavel Kroupa, a professor at the University of Bonn in Germany and at the Charles University in Prague. “So far, none stand up to detailed scrutiny.”

“Never bet against MOND on galactic scales”, concludes Oliver Müller, a former PhD student from the University of Basel.