I read this paper out of sheer spite. Being told, "anyone believing in ΛCDM isn't a physicist" at a recent seminar, as well as receiving an unnecessarily insulting comment on an old post, tends to wind me up the wrong way.
Anyway, "El Gordo", which Wikipedia says means "the fat one", here refers to a monster galaxy cluster in the early Universe. It's so massive that it's a bit of a puzzle how such a gargantuan structure managed to form so quickly after the Big Bang.
But haven't we heard such claims before ? Indeed. The Bullet Cluster is most famous as an example of two colliding clusters clearly demonstrating that the dynamics of the systems does not follow the baryonic matter : i.e., while the X-ray gas clearly gets stuck in the middle during the collision, lensing measurements show that the dark matter doesn't really notice the collision much. But it's only slightly less famous because its sheer mass and the collisional velocity of its sub-clusters were thought to be a problem for ΛCDM, though that prospect receded when it was shown that the collision velocity wasn't as high as initially thought. Even the authors of the current paper appear to concede that point, if reluctantly.
This paper covers quite a bit of ground, so I'm going to strictly limit my comments here as to whether El Fatso does indeed contradict standard cosmology. The MONDian stuff they also present is very interesting, but I think it would have been better as a separate paper (the protracted other arguments against ΛCDM, by contrast, are wearing very thin, and in my view are long since discredited). But, while it's entirely possible that this is a case of boy-who-cried wolf, as it stands I find the evidence that the Fat One is worryingly large considerably more compelling than for the Bullet Cluster.
Essentially, what they do is use an enormous, pure dark matter simulation to search for analogues of El Gordo. This "Jubilee" simulation is the largest to date, covering about the same volume as the entire visible Universe at the distance in question (a fact which is annoyingly buried on page 13). What they try to do is find how often such analogues occur, and given the survey size in which El Gordo was found, estimate if this discovery is compatible with expectations or not.
Of course, pure dark matter simulations are necessarily limited in what they can tell you. But unlike other cases, where the baryonic physics is almost certainly responsible (in my opinion) for any discrepancy between theory and observation, it's harder to see this being the case for El Gordo. Its two interesting features are its sheer mass and the infall velocity of its two major components. Both of these shouldn't be affected much at all by observations being restricted to the baryonic components. So using a pure dark matter model ought to be perfectly valid in this case.
I do have a slight quibble that their search criteria may be excessively strict : rather than search, say, for objects within some mass/velocity range, they search for objects which are either as extreme or more extreme than El Gordo. That's probably placing a bit too much faith in the observations, but this is largely negated because they show in their figures more detailed distributions of what they found.
Which is : no clusters at all as massive as this one at any infall velocity. Given the well-defined mass and velocity distribution, they can extrapolate to see exactly how rare El Gordo is (or in other words how large the volume would have to be to contain such a behemoth), and the answer is, "very" : they expect to find of order 0.0000000001 such clusters, so finding even one is very surprising indeed.
Does this mean cosmology is all wrong then ? Possibly yes, though I wouldn't bet on it. I have a lot of issues with the language of the paper - I don't think it makes any sense to use the term "falsify" in the probabilistic sense they do, something that is "false" cannot be false with some given probability value. Likewise they take the 5σ threshold as something magical for some reason, and quote distances and timescales which seem worryingly accurate (e.g. kpc scales when describing something Gpc away; "559 Myr" - I find it very unlikely that such precision is justified). And I'm biased because I'm also continually annoyed that other people continually get away with making extremely grand claims while I get routinely pulled up on things which shouldn't be controversial at all.
But this is all by-the-by. More importantly, the mass dropoff in the simulations is extremely steep : go down a factor three and such clusters do exist in their simulations. So I do wonder just how secure that observational mass estimate really is. Likewise for the infall velocity, which was based on more detailed, smaller simulations. In the seminar they claimed that you couldn't use a much lower velocity and still get the same (quite distinctive) morphology as the observations, but I couldn't really see much difference between the high and low velocity cases in the figures they showed. But I would have to check that more carefully - perhaps I missed something. In any case, however detailed and extensive the previous simulations were, it's always worth remembering that there's more than one way to skin a cat.
What I think has the potential to become very much more interesting is that this monster was found in a relatively small survey. We can argue about probabilities till the cows come home, but for single objects this won't do much good. It is perfectly valid to posit a weird, unlikely occurrence to explain a weird object. Indeed, if the mechanism at work wasn't in some way unusual, we ought to see such oddities everywhere. So however unlikely their simulations say a giant cluster at this distance might be, so long as it is physically possible, a single object is nothing very worrying. After all, it's pretty unlikely that we happen to live on a planet with total solar eclipses, but we don't hold that as evidence against cosmology. You're bound to get some unusual features - flaw of averages, and all that.
But if you can show, as they heavily imply, that they expect such objects to actually be quite common in observations... then you've got something really interesting. Then you're really dealing with physics, not statistics : you need a mechanism that must occur quite frequently. And so far as I know, ΛCDM just doesn't have that.
If I can find the time, I'll try and do a proper paper chase on this one. To be honest, because of certain people's tendency to make exaggerated claims about how obviously ΛCDM is some kind of weirdly dull cult, I view such claims with far more skepticism than I otherwise would. It's hugely counterproductive. And that's a shame, because disproving it would be truly spectacular. I, for one, would quite like to know when their really is a wolf that's come along to gobble up the standard model's carefully-tended fluffy sheep.
A massive blow for ΛCDM - the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology
Such a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter (ΛCDM) cosmology at such high z. However, this is required for non-cosmological hydrodynamical simulations of the merger to match its observed properties. Here, we determine the probability of finding a similar object in a ΛCDM context using the Jubilee simulation box with a side length of 6h−1 Gpc.
