2010/04/11

Flat rotation curves and Facebook

In one of the more bizarre meta-experiments that have come out of the latter-day social web, Trieste astrophysicist Paolo Salucci is trying to use Facebook to spread some astrophysics, not to the public, but within the astronomical community.

Specifically, he’s trying to “eliminate the deep-routed [sic] wrong misconception [sic] of Flat Rotation Curves of Spiral Galaxies”. A little scientific background: rotation curves are simply a measurement of how fast the stars are moving around the center of their host galaxies (plotted as a function of distance from the center to make a curve). If gravity is responsible for the motion of the stars, we can use the curve to determine the amount of mass in the galaxy. And when we do this, we find that there appears to be much more mass than the luminous matter — the stars — is responsible for. This is one of the strongest pieces of evidence for dark matter.

Salucci has a fairly specific axe to grind: the evidence is often caricatured as “flat rotation curves”. However, when considered in detail, the rotation curves are not completely flat, but do indeed seem to rise and (in the rare cases we can measure far enough out from the center) fall. More specifically, galaxy rotation curves do appear to take a very simple form, each of them being one of a very limited family of possibilities. This is much less variation than might have been naively surmised, but does seem to be borne out by massive numerical simulations of the Universe (including dark matter) as well as the observations.

Nonetheless, I think Salucci misses the point (or perhaps I miss his): indeed we do say “galaxies have flat rotation curves” but this meme (let’s not call it a “wrong misconception”) isn’t about the detailed shape of the rotation curves — rather, it is shorthand for “galaxies are dominated by dark matter”. Yes, we probably should be more precise in our language, but I don’t think we are spreading quite as gross a misconception as Salucci (who works directly in this field and so is admittedly more attuned to it than I) worries.

Of course the real interest in this experiment may just be the attempt to use a consumer social network to foster real discussion (or right thinking) within a specialized and technical community. Sarah Kendrew has an excellent dissection of the methodological side of Salucci’s attempt: can we actually measure how big a problem this “wrong misconception” is? How quickly should we expect Facebook to solve this problem? Would this be a good or a bad thing, outside of the usual professional channels of peer-review and conferences? I completely agree with Sarah that this experiment per se may not teach us much, but that the broader presence of professional astronomers, and scientists more generally, in the world of the web has already begun to prove itself useful as a tool for communication to the public and within the professional community.

I think Andrew's point is well take and it's true that when we speak of "flat rotation curves" we simply imply they are dark matter dominated. I think Salucci's take is that behind this assumption others also make the one that all rotation curves behave the same why. Salucci, in his years of work together with Persic, has shown that there is an "universal rotation curve" which describes in detail the characteristics of even the non-flat rotation curves by simply characterizing the degree to which they are dark matter dominated.

Posted via web from Traction Lobe