You’re sweet as a honey bee
But like a honey bee stings
You’ve gone and left my heart in pain
All you left is our favorite song
The one we danced to all night long
It used to bring sweet memories
Of a tender love that used to be
Now it’s the same old song
But with a different meaning
Since you been gone –The Four Tops
Those of you who’ve been with me since the start of our current series on Dark Matter, including parts I, II, III, and 3.5, know that I’m a big proponent of dark matter. I think, based on everything that we know, that it is the simplest, easiest, and most likely explanation for why — on galactic scales and larger — the Universe behaves as it does.
One of the key observations we have, though, supports another conclusion as well. You see, when we look out at a spiral galaxy, like this:
We can measure how quickly it rotates. We can look at the central part of the galaxy, the intermediate parts, and the outskirts. If there were only stars and gas (i.e., normal matter), we’d expect the outskirts to rotate more slowly than the inner parts, the same way that the outer planets orbit our Sun more slowly than the inner ones.
But it doesn’t do that. The outskirts move at the same speed as the inner portion. The difference between the expected (A) and the observed (B) is easy to see.
Now, there are two possible reasonable explanations for this. The first one is dark matter. In addition to all of the other things that dark matter explains, a diffuse dark matter halo around a galaxy could very easily explain the rotation of these galaxies, and we have very simple models that can accurately explain them.
But the other possibility is that Newton’s laws of gravity get modified for very small accelerations. How small? Below changes in velocity of about one nanometer per second over a time interval of a second. So if you modify Newton’s F = ma to add this minimum acceleration, you can also fix these rotation curves. It works for practically every spiral galaxy.
But then we can take this theory, MOND (MOdified Newtonian Dynamics), and try to apply it to other things.
1.) We can try to apply it to low-surface-brightness galaxies, like above, and see if it gives us a reasonable prediction.
2.) We can try to apply it to galaxies moving around superclusters, and we can see if it gives us a reasonable prediction.
3.) And we can apply it to the structure that the Universe forms, and we can see if we get a reasonable answer out.
As you may have guessed, on counts 1, 2, and 3 above, MOND completely fails! It simply makes a prediction that doesn’t match up with observations at all.
So you’d think that would be the end of the story, right? I mean, MOND has been singing the “we predict galaxy rotation curves” song since 1983, and it’s been failing to predict these other things for just as long.
Because someone (my guess is HongSheng Zhao, one of the authors of this paper who’s fond of press releases and modifying gravity) is pimping this piece of evidence like it tells us something. Guess what? Galaxy rotation curves are the only thing MOND has ever been good for! MOND is lousy for everything else, and dark matter — which is good for everything else — is good for this too!
So thanks to a number of people for bringing these to my attention, because the record needs to be set straight. Dark matter: still fine. MOND: still horribly insufficient. Now, maybe we can get the editors and referees of journals like this to not only do quality control on the data, but also on the reasonableness of the conclusions drawn.
Because I’ll start listening to the MOND song again, just as soon as it starts explaining these things it currently doesn’t, but not until then. I don’t understand why anyone would think otherwise.