On Random Scattering


Recently, some research (originally published in 2013) is making the rounds about the “largest structure in the Universe”, a conglomeration of quasars some 1.24 gigaparsecs in size, with the result that if the discovery is true, the (observable) universe would not be uniform on a cosmological sense.  That’s an interesting philosophical issue*, and a fairly interesting result because thus far everything has obeyed that cosmological principle.

This article (and more importantly, the paper it summarizes) does a fairly good job of summarizing the big issue with this work: It may not be true.

Or at least, it’s apparent from the work the post-doctoral researcher did that these results are highly (perhaps too highly) sensitive to the method used.

The method, by the way, consisted of drawing spheres with a radius of 100 megaparsecs around a quasar, finding all the other quasars that fall within that sphere, and drawing spheres around THEM.  It works in principle: Once you have identified every quasar object within 100 megaparsecs of another quasar, you have a specific reproduceable reason to say “these objects are connected, these aren’t.”

Except… why 100 megaparsecs?  If you change that exact distance, the size of your large quasar group changes.  Nadathur finds that the actual nearest neighbor distribution for randomly distributed points in the volume the scientist described was around 75 megaparsecs, and the average distance between the quasars in the cluster was also roughly 75 megaparsecs.  That’s already a bad sign, because there’s no sign that this cluster is actually clustered…

Nadathur also tried generating random samples of quasars with the same basic distribution as the real ones, and found that such enormous clusters happened about 8.5% of the time.  That’s not quite low enough if you’re trying to claim what we’re seeing is definitely special.

The reason I bring this up is that studying young stars tends to fall into the same kinds of numerical traps.  A good example is the Castor moving group, first identified by Anosova in 1991. Anosova’s rationale was to check around the most massive and complex star systems – triple stars and up, basically – to see if they were parts of larger, more massive stellar groups.  Whatever cloud formed alpha Centauri, for instance, had enough mass to form two nearly-solar-mass stars AND a rather distant companion that people have argued about for years as to whether it’s actually gravitationally bound to alpha Centauri (current state of the art says yes, it definitely is).

So Anosova looked around Castor, a nearby sextuplet (it’s a pair of A stars and a pair of A stars, orbited at some great distance by a pair of M dwarfs), drew a box 5 km/s wide in velocity space, and tabulated all the known nearby stars whose motions fell into that box.  That was what people did at the time, and it’s essentially a good idea, because it sort-of works: If there IS anything still moving in parallel with Castor since the time they formed, it would be in that box.

The problem is, 5 km/s different is a very, unreasonably large box as it turns out, and people took that list of things moving roughly like Castor as the truth without checking up to make sure they were the same age or anything (which is tricky even now).  When I say that 5 km/s box is unreasonable, I mean that a 1 km/s discrepancy adds up to 1 parsec after 1 million years (roughly), so after 200 million years, a star moving 5 km/s different from Castor could have been 1000 parsecs away, which is, to put it gently, a problem.

So what of Castor?  Yeah, it’s probably not a product of a single star formation event, no matter how useful it would be for it to be real.  Of course, most of the other nearby young moving groups suffer the same general fate, with velocity discrepancies of 2 kilometers per second that reach improbable sizes not too far into the past.  Then there’s Argus (discovered by Torres in 2008), which has a kind of an opposite problem, in that its velocity dispersion HAS to be 5 km/s to explain how stars got from IC 2391 to here near the Sun, in only 40-50 Myr.

What gives?  Well, I suspect once we have Gaia data, we’ll find out that many of these nearby young moving groups aren’t any more real than the giant Huge Luminous Quasar Group.  A lot of the stars currently put in those groups are definitely young, so we’ll see how things reshape.

* It’s not a scientific issue; proving the Cosmological Principle wrong does not imply that all of cosmology is wrong, or that creationism is right… despite what some bloggers have claimed.


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