Would discovery of a galaxy without dark matter disprove MOND?

[lamdar]

I’ve been reading about LCDM and MOND recently. And there have been reports of galaxies with little dark matter. I know the lack of dark matter in NGC1052-DF2 was shown not true afterwards. But if such a galaxy without dark matter is actually discovered, would this be sufficient to disprove MOND?

 

[Vanadium 50]

I don't think this can be answered in the abstract. How many examples? How well measured are they? How far do they deviate from the MOND prediction? What signs are there for previous interactions with other galaxies? Are these galaxies typical or is there some other feature that makes them unusual? How many estimates for the DM component are there and do they agree?

And so on and so on.

 

[PeroK]

I’ve been reading about LCDM and MOND recently. And there have been reports of galaxies with little dark matter. I know the lack of dark matter in NGC1052-DF2 was shown not true afterwards. But if such a galaxy without dark matter is actually discovered, would this be sufficient to disprove MOND?

There was a thread about this not that long ago. On the face of it yes - even a single galaxy that obeys the default Newtonian dynamics would disprove MOND. The proponents of MOND, however, believe that secondary MOND effects from neighbouring galaxies may explain this.

You could do a bit of research on this online. I only recall the gist of the argument.

 

[berkeman]

There was a thread about this not that long ago.

Is it maybe in the "Similar Threads" list at the bottom of this page?

 

[PeroK]

I can't immediately see the thread, but the term I believe was "external field effect". That explains the Newtonian rotation curve of a galaxy with no dark matter.

 

[Ibix]

Yes. IIUC, MOND says that there's non-Newtonian behaviour when the modulus of the Newtonian acceleration would be below a certain value. Much of the gravitational interaction between stars in a galaxy is below this value, so galaxy rotation curves are non-Newtonian. But if there's a large mass nearby a galaxy it can push the gravitational acceleration above that critical value across the whole galaxy. So MOND proponents say you can get Newtonian behaviour (what a dark matter proponent would call a dark matter free galaxy) in rare circumstances with MOND.

 

[PeroK]

... a modified MOND, perhaps!

 

[Ibix]

There's also the point V50 makes about the quality of evidence. One measurement being enough to falsify a theory is an idealisation. It has to be one measurement by skilled observers with high confidence that possible measurement errors have been controlled and possible confounding factors eliminated. Otherwise we'd falsify Newtonian gravity every time someone did a parachute jump.

 

[PeroK]

There's also the point V50 makes about the quality of evidence. One measurement being enough to falsify a theory is an idealisation. It has to be one measurement by skilled observers with high confidence that possible measurement errors have been controlled and possible confounding factors eliminated. Otherwise we'd falsify Newtonian gravity every time someone did a parachute jump.

Not if the parachute didn't open!

 

[Ibix]

Not if the parachute didn't open!

Terminal velocity for a human isn't very high (Wikipedia says 55-90m/s depending on whether you spread out or curl up), so even without a working chute you don't fall according to a naive application of Newtonian gravity. And we're way off topic here!

 

[Vanadium 50]

<splat!>

While data quality is part of what I was commenting on, there's also statistics. If you see a galaxy that has only a 0.1% of satisfying the null hypothesis, whatever that may be, you would draw different conclusions if you saw it after 5, 1000 or 1 000 000 galaxies.

 

[Walrus]

I’ve been reading about LCDM and MOND recently. And there have been reports of galaxies with little dark matter. I know the lack of dark matter in NGC1052-DF2 was shown not true afterwards. But if such a galaxy without dark matter is actually discovered, would this be sufficient to disprove MOND?

Dark matter has not been discovered, it is predicted, there is a difference.