|A smear with dots, |
also known as NGC 5264-HST.
And if you had to ask, why did you have to ask me?
You sent me like two million messages and comments and emails asking what I think about NGC 1052-DF2, that galaxy which supposedly doesn’t contain dark matter. Thanks. I am very flattered by your faith.
But I’m not an astrophysicist, I’m a theorist. I invent equations and then despair over my inability to solve them. That’s what I do. I know about as much about telescopes as penguins know about cacti. And until last week I thought a globular cluster is a kind of glaucoma. (Turns out it’s not.)
But since you ask.
If nothing else, I have the benefit of a university account with subscriptions to all major journals, so at least I could look at the Nature paper in question. We can read there:
“the existence of NGC1052–DF2 may falsify alternatives to dark matter. In theories such as modified Newtonian dynamics (MOND) and the recently proposed emergent gravity paradigm, a ‘dark matter’ signature should always be detected, as it is an unavoidable consequence of the presence of ordinary matter.”This paragraph is decorated with two references, one of which is to Milgrom’s MOND and one is to Erik Verlinde’s emergent gravity. And, well, I’m a theorist. Therefore I can tell you right away those people don’t know a thing about the theories they try to falsify.
It is beyond me why so many astrophysicists believe that modified gravity is somehow magically different from particle dark matter, or indeed all other theories we have ever heard of. It’s not.
For both modified gravity and particle dark matter you have additional degrees of freedom (call them fields or call them particles) which need additional initial conditions. In a universe in which you have a large variety of initial conditions (seeded by quantum fluctuations), you will get a large variety of structures. Same thing for modified gravity as for particle dark matter.
Another way to put this is that you can always cook up exceptions to the rule. The challenge isn’t to explain the exception. The challenge is to explain the rule. Modified gravity does that. Particle dark matter doesn’t.
Of course you don’t see the exceptions in Milgrom’s or in Verlinde’s paper. The reason is that both merely contain equations which describe time-independent situations. These equations derived in Milgrom’s and Verlinde’s papers are not theories, they are certain limits of theories, approximations that work in some idealized circumstances, such as equilibrium. The full theories are various types of “modified gravity” and if you want to rule those out, you better find out what they predict first.
But we don’t even have to stoop so low. Because, interestingly enough, the authors of the Nature study rule out MOND without even making a calculation for what that limit would predict. Stacy had to do it for them. And he found that MOND is largely compatible with the upper value of their supposed measurement results.
Having said that, let us have a look at their data.
So the galaxy in question is an “ultra-diffuse galaxy” with “globular clusters.” For all I can tell that means it’s a smear with dots. The idea is that you measure how fast the dots move. Then you estimate whether the visible mass suffices to explain the speed by which the dots move. If it does, you call that a galaxy without dark matter. Have I recently mentioned that I am not an astrophysicist?
There are ten of the globular clusters, and here is the data. With the best-fit Gaussian.
|Figure 3b of Dokkum et al, Nature 555, p. 629–632|
In case that looks a little underwhelming, some nice words must be added here about how remarkable an achievement it is to make such a difficult measurement and how brilliant these scientists are and so on. Still, ehm, that’s some way to fit data.
And it’s not the only way to analyze the data. Indeed, they tried three different ways which gives the results 4.7 km/s, 8.4 km/s and 14.3 km/s. All I learn from this is that it’s not enough data to make reliable statistical estimates from. But then I’m a theorist.
Michelle Collins, however, is an actual astrophysicist. She is also skeptical. She even went and applied two other methods to analyze the data and arrived at mean values of 12 +/-3 km/s or 11.5+/-4 km/s, which is well compatible with Stacy’s MOND estimate.
Michelle also points out that globular clusters are often not good representatives to measure what is going on in a whole galaxy, because these clusters might have joined the galaxy at a late stage of formation. In other words, even those estimates might be totally wrong because the sample is skewed.
When I factor all this information together, I arrive at a 95 percent probability that this supposedly dark-matter-less galaxy will turn out to contain dark matter after all and it will be well compatible with modified gravity.
I give it an equally high probability that five years after the claim has been refuted, astrophysicists will still say modified gravity has been ruled out by it. Like the Nature paper refers once again to the Bullet cluster but fails to mention that the Bullet cluster can well be explained with modified gravity, but is difficult to explain with particle dark matter.
A recent Nature editorial praised a “Code of Ethics for Researchers” that was proposed by the World Economic Forum Young Scientists Community. In this Code, you can read that scientists are supposed to pursue the truth and
“Pursuing the truth means following the research where it leads, rather than confirming an already formed opinion. This is particularly challenging but necessary when questioning current beliefs [...] Results must be represented accurately without over- or understatement, hiding facts and/or drawbacks, or misleading the reader in any way.”Maybe the editors at Nature should read what they recommend.
Update April 11: The paper’s first author has some comments on the various points of criticism that have been raised here.
Update April 14: A group of astrophysicists has a response on the arXiv: “Current velocity data on dwarf galaxy NGC1052-DF2 do not constrain it to lack dark matter.”
Update April 16: Another paper on the arXiv today, this one showing that the observations aren’t in conflict with modified gravity: MOND and the dynamics of NGC1052-DF2.