Comment by quantum_mcts | Hacker News Reader

Comment by quantum_mcts | Hacker News Reader

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I've long noticed that this idea - that scientists just "jumped to conclusion" that there must be some dark matter to resolve some observational inconsistencies - is very prevalent among non-specialists. It could not be further from the truth, of course - as soon as the rigorous statistical study of galaxy rotation curves was provided by Vera Rubin et. al. in the 80s, the MOND classes of alternatives to gravity started to appear.

I tried to understand why this narrative is so prevalent, and it looks like it is due to its very appealing nature - both to the listener and to the speaker: "all those egghead scientists can't see an obvious solution to a problem they are fighting with for decades". I also noticed that when you get deeper into it, then you find that the source - the "speaker" part in the whole narrative - is some kind of science popularizer. Who at the same time (surprise) is a proponent of his own flavor of modified gravity theory.

Truth is - everyone, including me, had this idea as soon as they first heard about the DM. If you actually study all the evidence and discussions around the subject, you'll see that introducing an extra "dark" particle is the most obvious solution to the whole collection of observational inconsistencies. Modifying gravity in such a way so that it is self-consistent and consistent with all the observations... I actually don't think anyone completely finished that project so far.

Everyone got drilled into them in school the stories of scientists and philosophers clinging to outdated models and introducing extraneous elements to extend their life in light of contradicting evidence, the piling up of epicycles to explain celestial movements by circular motions, or luminiferous aether for light to be wave in a fluid medium.

So of course when you first hear "so astronomers found out the equations don't work out with the matter we can detect, so they figured that there must be uhhh... more matter, invisible matter! That exists but you can't detect no matter what!" it sounds like those examples.

If you dig in deeper, then you realize that it's not just fudging of equations and that there being mass that's very sparse and unaccounted-for makes sense.

When Sabine Hossenfelder and Sean Caroll agree on something, I pay attention.

I do take some humility there. We have stunningly brilliant contrarians who would be the first to dismantle dark matter on the public stage, and instead they tell us, “no, it’s a real thing. We just haven’t seen it in a lab on earth much yet.”

Now if Eric Weinstein started saying dark matter is actually the consequence of a massively egocentric MOND, and everyone else is a WIMP, I might start believing that too ;)

What do Hossenfelder and Carroll agree on? It was my understanding that Hossenfelder thinks we need both DM and MoND while Carroll is firmly in the conventional DM camp.

You caught me, in that she’s 100% open to new evidence. Kinda why she is so effective.

So, over the past decade, Hossenfelder made a series of videos and talks about “dark matter is real.” And then “is dark matter real?”

Her opinion evolves. But the conclusions are generally that the data points to a measurable thing along many lines of evidence.

I find the cosmic web the most compelling, personally. Like, what else could that be besides a mystery? You can’t draw the web in some other configuration, it’s a real thing somehow.

From my understanding. Her hesitation that she expresses is more linked to an underlying intuition: the universe is completely deterministic—we just can’t see all of the rules being computed, and the initial conditions are intractable.

I also find that pretty compelling.

can it be possible that the galaxy just appears to be rotating faster due to time dilation, so from the POV of the galaxy itself it doesn't seem like there is anything out of the ordinary?

[It's not my branch, so I have to guess.] They are using special and general relativity to make a lot of corrections, like red shift and gravitational lens, so there is 0% chance that all the community forgot to add that correction.

Galactic rotational velocities are on the order of 100 km/s, which, though blazingly fast by human standards, is so slow compared to the speed of light that special relativistic effects are completely negligible.

Moreover, it's not just the absolute magnitude of the velocity that is the problem, it's also the shape of the curve. Newtonian mechanics (even with the extremely tiny relativistic corrections taken into account) predict that the rotational speed of a galaxy would decrease the further you go out from the center, and we in fact find that this speed remains more or less constant.

except its not consistent with all the observations, which is why it's not accepted consensus.

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