Hacker News Reader

Comment by HardlyCurious | Hacker News Reader

Interestingly, YouTuber and physicist Sabine Hossenfelder claims that same cluster is actually evidence against dark matter.

View on HN

Well, that's swell. Can you point to the specific video? I see a few about dark matter and want to have a look.

Now that I thought I finally got the point....

http://backreaction.blogspot.com/2021/05/dark-matter-situati...

> Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.

That's funny. The parent's NASA link uses the high relative velocities as "direct evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity." The lack of friction seems obvious with this view of limited interaction between dark matter particles.

That's also how I understood it. No photons, no electrical interaction, no friction.

Chandrasekhar dynamical friction is just a function of gravity:

https://en.wikipedia.org/wiki/Dynamical_friction

Well, ok, but that means there's still _less_ friction with those particles because they don't interact electrically, right? So I still would expect the same result of the 4 blobs in different positions in space.

What's wrong with that view?

I'll take a look and see if I can remember which one. Would be great if there was a way to search videos CC to skip to timestamps...

If a video has manmade captions and not auto-generated, you actually can check the transcription which has timestamps.

Ah found it.

https://youtu.be/4_qJptwikRc

6:50 or so for relevant piece.

Reply on news.ycombinator.com