We've encountered "El Gordo" (The Fat One) a couple of times before. First there was a claim that this merging cluster is just too big, too early in the Universe, and colliding at a speed too high to be compatible with Standard Model predictions. I noted that such claims do seem legitimate, but I wouldn't jump on the bandwagon just yet : the chance of finding such a cluster seemed to be very non-linearly dependent on the parameters, so I'd bet observational errors could have quite a part to play here.
Next there was a rebuttal which said that everything was fine. The mass was actually only about half the earlier estimates, and the infall velocity way smaller. But while I'm strongly in favour of the Standard Model and naturally suspicious of these claims that it's been debunked, I have to say I didn't like this particular paper. Their mass estimates seem fine so far as I can tell, but I found the justification for their much lower collision velocity very badly expressed.
Along comes a counter-rebuttal (hence the ping-pong) that, oh joy, does the same thing but for the opposite side of the argument.
The authors of this latest study don't seem to have any beef with the revised lower mass estimate. Instead their attention turns to the infall/collision velocity. They maintain there's also still a problem with the more famous Bullet Cluster, but the studies I read a while back found that there isn't, so I'm going to ignore that aspect.
They start with quite a nice overview of previous studies but immediately fail to learn any lessons from them. Noting that different authors have come up with radically different values and interpretations, they seize on the latest measurements as being of unimpeachable accuracy and precision. Inclination angles have varied from 30 to 75 degrees or so, infall velocity estimates have varied from 1200 to 2500 km/s, the current stage of the merging has been interpreted differently... all this to me suggests that there's really quite a wide margin for error here, and we just don't have good enough observational data or numerical simulations to constrain anything very much,
I also dislike their whole interpretation of the low probabilities of finding such objects in the Standard Model. It's a one-in-ten-billion, they say. This is a big red flag by itself. It doesn't make any sense to me that the Standard Model could get things so fabulously right in such a plethora of circumstances and then fail so utterly miserably in others, where the forces at work are supposedly the same. Of course, to give them their due, they would say that the Standard Model fails miserably elsewhere too, but they're wrong about that.
On a more pragmatic level, I wish they'd define the velocity terms more clearly. What exactly is meant by the infall, peculiar, and observed velocity ? Oh I'm sure they're simple enough (I can certainly make an educated guess what they mean), but without having them spelled out and rigorously defined, this becomes just as confusing as the previous paper.
And where this really becomes critical is their underlying methodology. The previous authors, they say, deliberately disregard high infall velocities because that contradicts the Standard Model. They say this means we can't use that analysis to say how likely it is that such a feature arises in the SM, but I think this is daft. If we can show that such a scenario works, that this agrees with observations, then the circular nature of the argument doesn't matter : "we looked for SM-compatible solutions and found one" is a perfectly valid approach, and while it might not tell you about probabilities, demonstrating compatibility is much more important.
And then they do the exact same bloody thing but in reverse, insisting that they must search simulations only for objects with infall velocities which are incompatible with the SM ! This is rank hypocrisy and exasperatingly silly.
They make similar daft and blunt claims about other previous analyses too. They could have just said "we improved the analyses" rather than stating that the others were outright wrong. Saying there's no reason to assume the cluster components are gravitationally bound in no way implies that they're not. And sure, it could be that LCDM is incorrect and it could be that this would result in a high infall velocity, but what exactly is the reason to assume with such certainty that this must be the case ? When is a factor of a mere 1.5 grounds for an unsolvable discrepancy ?
It's all just weird and I don't like it at all. If you're going to say that some infall velocities must be excluded, then say why. Saying that you exclude them just because they're compatible with the SM makes not a lick of sense. That's committing exactly the sin they were trying to avoid. I liked their earlier paper much more than this. As it stands, I'm now far less convinced that El Gordo is the CDM-killer they think it is.
