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Comment by neosat | Hacker News Reader

Can someone with the appropriate background please confirm or refute? "But a simple test suggests that dark matter does not in fact exist. If it did, we would expect lighter galaxies orbiting heavier ones to be slowed down by dark matter particles, but we detect no such slow-down."

Is that hypothesis correct? (i.e. would we expect lighter galaxies orbiting heavier ones to slow down if dark matter exists)

If it is correct, and we don't observe that effect, would it not be a very strong argument refuting the existence of dark matter?

For folks mentioning this as a "crackpot" or "anti-dm" theory, is the strong claim that the hypothesis is incorrect?

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I've read nearly every comment here and no one seems to have an answer... It is a very convincing argument, to be honest.

> and no one seems to have an answer

Especially DM proponents!

There are phenomena in the universe that, for example, general relativity does not predict correctly. Is your conclusion that we should discard any useful theory which ever fails to match an observation?

Would someone who relied on Newtonian dynamics 100 years ago likewise be incorrect to do so because it failed to explain many aspects of planetary motion?

No one is suggesting anything should be discarded. Just that the null hypothesis be tested.

Yup, no one still seems to have an answer to a simple, straightforward question, and the lack of any convincing counterargument makes it even more convincing.

I hesitate to respond to you because your tone seems especially combative.

It is well known that the relevant wake friction calculation ("classical Chandrasekhar") is for a pointlike structureless body. Satellite galaxies and their parents are not this. Importantly satellites can lose matter and thus mass, so one has to correct for the evaporation and tidal loss (especially to the halo) of the satellite galaxy; and the host galaxy's potential grows adiabatically over the lifetime of the system.

If the density of the halo and other sparse matter increases closer to the host galaxy, inward migration of the satellite sees two effects driven by the collision into more particles: more satellite mass is torn off by tidal interactions and more halo particles surrounding the host are swept into the satellite's wake. All that matter stays well above the surface of the host galaxy rather than descending into some inner halo or deep into the host galaxy itself. This serves to "suspend" the sinking satellite, posing a challenge for the "they should sink" reasoning; it also reduces the amplitude of the wake, which moderates the drag on the satellite. The combined result is that the evolution of specific angular momentum may not be enough to clearly support Kroupa's claim in the fine article linked at the top.

There has been a lively dialogue in academic publications about the scale of these effects for some twenty-five years. Kroupa's article touches upon his own position in these, but not those of others (e.g. HS Zhao, Binney) who work and publish in the area (with all of the above citing one another from time to time).

I don't mind this at all, personally, as at worst Kroupa is omitting something an interested reader could hunt down her- or himself, and after all it was "only" published on a general interest site whose present front page (https://iai.tv/articles) has an article by Eric "electric universe" Lerner entitled "The Big Bang Didn't Happen".

Thank you for the informative and thoughtful response. My comment and question was certainly not intended to be combative. I was genuinely curious for a perspective quite like the one you provided above, because it seemed odd that a claim so strong would not have a counter view given the active area of research. I think the original article would benefit from some perspective like the one you provided. While you're aware of the relevant threads of research that balance out the view in the original article, for someone not familiar with that even things that one would consider "well known" will not be so.

The view you provided helps understand the counter argument of why competing factors (e.g. losing mass) result in more sophisticated outcomes than "we would expect lighter galaxies orbiting heavier ones to be slowed down by dark matter particle". I still don't know enough about the subject to understand if the original authors claim and expectation is still likely. But your context helps me understand other factors that are in play.

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