Comment by autocorr | Hacker News Reader

Comment by autocorr | Hacker News Reader

I find this essay very curious, especially the strong view against the Bullet Cluster supporting the model of Dark Matter. This is the first time in professional astronomy I've seen this brought up. The details in the linked essay say as much as a few studies find that it's improbable that the two clusters collided at the speed that would be required. It was funny to see my same concern raised in the second comment on the blog:

> I don't get it. How does modified gravity explain displacement of visible and gravitating matter in the case of Bullet Cluster?

> The improbability of conditions that are needed to set up the collision is another question.

I didn't find the reply especially convincing, or the arguments for the likelihood of small numbers raised in the main essay. Attachments to certain "elegant solutions" by theorists have posed significant hurdles to scientific progress. Einstein removing the cosmological constant for an expanding universe is a famous example.

> I didn't find the reply especially convincing, or the arguments for the likelihood of small numbers raised in the main essay.

The author isn't trying to convince you of her counterargument to doctrine. The author is trying to convince you to be less convinced by hand-waviness in lieu of rigorous deduction. The author doesn't even believe that inflation is incorrect, nor does she claim to believe MOND over standard dark matter models; she is pointing out that taking one good, well-evidenced, well-reasoned theory and using it as a prop to chock up a piece of shaky, though attractive, supposition is bad science. She's pointing out that even highly regarded thinkers can allow simplification for the purpose of communication and impact to erase complexity in reasoning and teaching.

So yeah, her arguments may not convince you. She's not trying to convince you, she's trying to show you reasonable doubt. Different standard of evidence.

> I find this essay very curious, especially the strong view against the Bullet Cluster supporting the model of Dark Matter. This is the first time in professional astronomy I've seen this brought up.

I've heard it in a few places, starting not too long after the bullet cluster lensing results were published. I thought some revised LCDM studies found the velocities were less discrepant than originally argued, but I think it's still a few sigma off.

Regarding the question of the offset in visible versus total inferred matter, I think MOND proponents would note that MOND doesn't work in clusters with only the baryonic mass we see. At those large scales MOND still requires some form of dark matter, though I believe it is argued a light non-WIMP particle can do it (neutralinos?) and is at a factor of ~2 level, not a factor of 5 or 8 like dark matter. But I have never heard if you can get that type of displacement with those types of particles.

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