There has been another attempt to explain away the radial acceleration relation as being fine in ΛCDM. That’s good; I’m glad people are finally starting to address this issue. But lets be clear: this is a beginning, not a solution. Indeed, it seems more like a rush to create truth by assertion than an honest scientific investigation.

Sam: This looks strangely familiar. Frodo: That’s because we’ve been here before. We’re going in circles! Last year, Oman et al. published a paper entitled “The unexpected diversity of dwarf galaxy rotation curves”. This term, diversity, has gained some traction among the community of scientists who simulate the formation of galaxies.

I have been musing for a while on the idea of writing about Naturalness in science, particularly as it applies to the radial acceleration relation. As a scientist, the concept of Naturalness is very important to me, especially when it comes to the interpretation of data. When I sat down to write, I made the mistake of first Googling the term. The top Google hits bear little resemblance to what I mean by Naturalness.

Vincent: Want to talk about MOND? Jules: No man, I don’t consider MOND. Vincent: Are you biased? Jules: Nah, I ain’t biased, I just don’t dig MOND, that’s all. Vincent: Why not? Jules: MOND is an ugly theory. I don’t consider ugly theories. Vincent: MOND makes predictions that come true. Fits galaxy data gooood.

So the always humorous, unabashedly nerdy xkcd recently published this comic: {.alignnone .size-full .wp-image-2940 loading=“lazy” attachment-id=“2940” permalink=“http://tritonstation.com/2016/11/12/xkcdd/astrophysics/” orig-file=“https://tritonstation.files.wordpress.com/2016/11/astrophysics.png” orig-size=“579,314” comments-opened=“1”

There has already been one very quick attempt to match ΛCDM galaxy formation simulations to the radial acceleration relation (RAR). Another rapid preprint by the Durham group has appeared. It doesn’t do everything I ask for from simulations, but it does do a respectable number of them. So how does it do? First, there is some eye-rolling language in the title and the abstract.

After writing the commentary on the latest fin du MOND, it occurred to me that there are many issues that I consider to be obvious. But I’ve been thinking about them for a quarter century, so perhaps they may need to be clearly elucidated for those who don’t share that background. I am thinking, in particular, of galaxy formation modelers and theorists.

Last time, I addressed some of the problems posed by the radial acceleration relation for galaxy formation theory in the LCDM cosmogony. Predictably, some have been quick to assert there is no problem at all. The first such claim is by Keller &

So the radial acceleration relation is a new law of nature. What does it mean? One reason we have posed it as a law of nature is that it is interpretation-free. It is a description of how nature works – in this case, a rule for how galaxies rotate. Why nature behaves thus is another matter.

Flat rotation curves were the first clear evidence that the dynamics of galaxies do not follow the same rules as planetary systems. But they do follow rules. These include asymptotic flatness, Tully-Fisher, the luminosity-size-rotation curve shape relation (aka the `universal’ rotation curve), Renzo’s rule, and the central density relation.

Previously I noted how we teach about Natural Law, but we no longer speak in those terms. All the Great Laws are already know, right? Surely there can’t be such things left to discover! That rotation curves tend towards asymptotic flatness is, for all practical purposes, a law of nature. It is tempting to leap straight to the interpretation (dark matter!), but it is worth appreciating the discovery for itself.