Crooked star clusters may be a sign that Einstein’s gravity is wrong

The Hyades star cluster has a strange distribution of stars that could be explained by an alternative theory of gravity NASA, ESA, and STScI

Astronomers have found an unexpected asymmetry in stars escaping from their clusters, and it can't be easily explained by our standard theories of gravity.

A strange effect in clusters of stars is bringing our ideas of how gravity works into question. These star clusters seem to have an unexpected asymmetry, which fits better under an alternate theory of gravity called modified Newtonian dynamics (MOND) than under Albert Einstein’s widely accepted theories.

Clusters of stars, which orbit the centre of their galaxy, typically look a bit like a two-armed pinwheel with opposing tails – though they don’t spin. Their tails are formed when stars bouncing around within the cluster begin to travel either slightly faster or slightly slower than the cluster as a whole. The leading tail in front of the cluster is made up of stars that are slightly closer to the galaxy’s centre, and the trailing one in the back is made of stars that are slightly further from the galactic centre and fall behind. In standard, or Newtonian, gravity, we would expect these two tails to be roughly equal – as the stars bounce around within the cluster, they should be equally likely to be thrown into either tail.

But when Pavel Kroupa at the University of Bonn and his colleagues examined three stellar clusters, they found that wasn’t true. The leading tails all had more stars than the trailing ones. The researchers performed a series of simulations and found that this asymmetry was a match to the predictions of MOND.

“It’s like there are two doors to escape the cluster, and the stars can only pass through the doors if they have the right direction and the right energy – otherwise they will just bounce around within the cluster,” says Kroupa. “In MOND, the front door is simply bigger.” Because of the way gravitational effects compound with one another in MOND, the forces pulling stars towards the centre of the galaxy, and therefore towards the leading tail, are stronger than in Newtonian gravity.

In the simulations, this phenomenon had broader effects for entire clusters. More stars bouncing to the leading tail made the cluster spin and slow down. Eventually, the simulated clusters fell apart. Under MOND, the clusters had lifetimes between 20 and 50 per cent the length of clusters experiencing Newtonian gravity. This would roughly match what astronomers have observed, Kroupa says.

However, with only a few clusters tested for this effect, it’s not yet time to bin Newtonian gravity. “It’s somewhat promising, but it does not provide completely definitive evidence for MOND,” says Indranil Banik at the University of St Andrews in the UK. “This asymmetry does make more sense in MOND, but in any individual cluster there could be other effects that are causing it – it’s a bit unlikely that would happen in all of them, though.”

To prove that this asymmetry is caused by MOND, we will have to observe many more star clusters in detail, and it will have to pass other cosmic tests as well. Many cosmologists are sceptical of it, partially because it doesn’t seem to fit with observations of the universe on the largest scales, and partially because of the enormous consequences if it is real.

“If MOND is correct, then all calculations regarding galaxies, galaxy formation, galaxy interaction, and much of the whole universe, are completely wrong,” says Kroupa. “We would have to reset – reinvent cosmology, basically.” The evidence required to do that would be more than a few crooked clusters.


Pavel Kroupa et al. Asymmetrical tidal tails of open star clusters: stars crossing their cluster’s práh† challenge Newtonian gravitation. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stac2563

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