This Week’s Science News from SWTG

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Bad News for MOND

Image: ESA

A group of astrophysicists has presented an analysis of binary star systems and claims that they’ve ruled out the most popular alternative to dark matter, modified Newtonian dynamics, MOND for short. While this isn’t evidence for dark matter in and of itself, it’s basically kicking the main competitor off the market, leaving dark matter as the only game in town. 

You see, we know very well how gravity works on earth and in the solar system. We can’t just throw the theory out. The idea of modified gravity is therefore that the law of gravity only changes at small accelerations, which loosely speaking means at weak gravitational fields. 

Now, it might seem like the gravitational field of a galaxy should be stronger than that of only one sun, but that isn’t so. It’s because what matters for the strength of gravity is how much the matter is concentrated, not just how much there is in total. The matter in our sun is very concentrated, that in the Milky Way is very spread out. Therefore, you wouldn’t see a modification of gravity in the solar system, but you’d see it in galaxies.

This is why the wide binaries are so interesting. These are cases where two stars orbit around each other, but at a big distance. If they’re close together, you should not see a modification of gravity. If they’re far apart, you should see it. The data come from ESA’s Gaia mission.

We already talked about this in June. Back then, there was a workshop on which several groups presented analyses from wide binary systems. The first paper appeared in April and said that the data are consistent with unmodified gravity for not-so-wide binaries, but the really wide ones seem to prefer modified gravity. This is what you’d expect if MOND was right. The second paper appeared in May and agreed with that. It said that the data provide evidence for the breakdown of standard gravity. 

The third was a presentation at the workshop which said there’s no evidence for modified gravity in wide binaries, but rather 16-sigma evidence against it. This means MOND has been ruled out at an incredibly high level of confidence. The news is now that this result has been peer reviewed and published.

The published paper contains an extensive discussion about why the two groups find so wildly different results. To make a long story short, it’s because they use different samples of the data. 

The paper that finds evidence for MOND uses pretty much the entire available data. The paper that finds evidence against it throws out data with high uncertainty. They show in the new paper that including this high uncertainty data brings back the evidence for MOND.

I’ve found that to be very interesting because we’ve seen the same thing in data from galaxy rotation curves, that the higher the uncertainty of the data, the better MOND seems to work. This raises the very real possibility that MOND is a systematic artifact coming from data interpretation. I don’t want to jump to conclusions here, but I am sure there’ll be more papers about this in the near future because I know someone who’s working on it, so stay tuned. 

For now, little Albert is pleased that he’s been right, once again.