Galaxies in the early universe were merging up to 10 times more than is predicted by theories of modified Newtonian dynamics, known as MOND. This rate of mergers is instead in line with theories that take into account dark matter, the invisible and currently undetected stuff thought to make up about 85 per cent of all matter.
at the University of Manchester, UK, and his colleagues analysed observations of half a million galaxies from the local universe out to 18.6 billion light years away, which means many of them formed almost 12 billion years ago when the universe was much more compact.
Because of that, those galaxies met and merged more frequently than they do in the local universe, said Conselice. He and his team counted galaxies in the early universe that looked like they were about to merge, as well as irregular galaxies that were clearly the product of a recent merger.
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He estimates that the merger rate in the early universe was five to 10 times higher than would be predicted by MOND theories that tweak the laws of gravity instead of assuming the presence of dark matter. He presented the work at a virtual meeting of the American Astronomical Society on 9 June.
Both ideas were put forward to explain why galaxies rotate faster than would be expected based on the mass of their stars and gas.
“But one of the things that really hasn’t been considered very much is how dark matter would affect galaxy evolution,” said Conselice. When galaxies orbit each other closely, dark matter makes a difference because it acts as a brake.
“Think of the dark matter as a pool that the two [galaxies] are located in together” said Conselice. “As each galaxy moves through it, like a boat, the dark matter particles produce a wake, and that will decelerate them and eventually lead to the two galaxies merging.”
This effect had already been shown in computer models – in models without dark matter but with modified gravity, the merger rate was seen to be lower. Now there is observational evidence that galaxies do indeed follow the dark matter scenario.
Conselice doesn’t claim that this clinches the case, but it comes close. “You have to really detect the particles for dark matter to be absolute proven. But there are other ways of showing it probably does exist; we have many already, and this is another one,” he said.
at the University of Strasbourg in France, an astronomer who compares dark matter and MOND theories, calls for caution. He would like to wait for computers to become fast enough to simulate the evolution of the universe under the MOND assumption, in order to see if the merger rates really are too low.
“We need to make sure the competition is fair,” he says. “When we know for sure it is fair and it’s disfavouring MOND, that would be highly intriguing.”
Müller says that, on the scale of galaxies, MOND generally fits our observations better than dark matter theories, so “seeing [MOND] losing on its turf, where it’s supposedly best, is exciting”.
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